User Manuals

Click the links below to access User's Manuals (aka Operating Hints).

Adjusting Press Cake Solids

January 31, 2007

Frequently, there is a need to adjust the percentage of solids in press cake from a screw press. This is done by changing the air pressure on the cake discharge cone of the press. The higher the air pressure, the drier the cake. For example, at maximum air pressure (60 to 100 psi) some specific material in a paper mill may produce cake with 40% to 50% solids. This controls well down to where, with a minimal air pressure of 5 to 10 psi, the press will make cake with 25% to 30% solids.

To get the solids lower than this, it is necessary to actuate the air cylinder so that it withdraws entirely from the cake discharge end of the press. Thus the screw press starts to work like an internally fed rotary drum screen. With close flow control, the press can be made to make "cake" with 10% or 20% solids. However, there is a tendency for the press to purge, with negligible dewatering. The same thing happens with a rotary drum screen if the flow into that screen is more than it can take.

With a given screw press, it may be difficult to control solids content of the press cake at either extreme. For this reason Vincent offers the Series CP and VP presses for high solids cake and the Series KP presses for lower solids content. The differences are achieved mostly by changing the screw shaft configuration, flight pitch changes, the number of stages of compression, and L/D ratio of the screen length to screw diameter.

Another common way to increase the solids content of the press cake is to reduce the speed (rpm) of the screw. This allows more residence time for the material in the press, allowing more liquid to drain out.

It should be kept in mind that there are adverse effects of increasing the solids content of press cake. At higher cone pressures, the amount of suspended solids that are driven through the screen into the press liquor will increase. At the same time, high cone air pressures tend to reduce the overall throughput capacity of the press. Similarly, the amps drawn by the press drive motor will increase as the air pressure is increased. Also, dry press cake is more abrasive than wet cake, which will increase the frequency at which the screw must be rebuilt.

Issue 183

Capacity Measurement

April 20, 2006

There are many ways to measure the throughput capacity of a screw press. As emphasized in brochures, press capacity can vary a great deal, depending on the nature of the material being fed to it.

Generally, it is hard to measure the pounds per hour (pph) of material being fed into a screw press. Instead, we focus on measuring the flow rates of press liquor and press cake, and then adding them to get the total.

Steady flow conditions must be established before measuring flow rates. Specifically, it is a poor practice to measure the time required for a given sample of material to feed through a press. Instead, establish steady flow operation and then, during this period, capture timed samples of press cake and press liquor.

Press liquor flow is best measured by diverting the flow into either a 5-gallon pail or, in larger presses, a 55-gallon drum. Timing the seconds to fill the container allows calculation of gallons per minute (gpm). Multiplying this figure by 500 gives the flow in pph in most cases.

At times the press liquor flow goes into a tank or pit. This makes it easy to calculate the flow. Simply measure the cross section dimensions of the pit or tank, and time the seconds or minutes required for a change in level of liquor in the tank.

Other times, the press liquor drain line is close to floor level, making it impossible to capture the flow in a 5-gallons pail. In this case, a heavy duty plastic bag can be used to collect a timed sample of press liquor.

The flow rate of press cake is frequently best measured by using a tarpaulin. This requires up to five people: four to hold the corners of the tarp, and one to run the stop watch. The process is to swing the tarp into the cake discharge, allowing it to fill until a sample in the range of 50 to 150 pounds is captured. With the elapsed seconds and sample weight known, the pph rate is easily calculated.

In some applications, screw press capacity is measured in tons per day of dry solids. Therefore, when catching and weighing press cake, it is important to save a sample of this cake and measure its solids content.

There are cases where only one flow rate can be timed and weighed. For example, maybe press liquor can be captured, but press cake is inaccessible. In this situation, samples of inbound material, press cake, and press liquor should be collected for moisture content analysis. Knowing the three moisture contents and one flow rate, the other two flow rates can be calculated. It involves five unknowns and five equations, so it is best if you liked algebra class.

When running tests like these, it is convenient to also measure the bulk density of the press cake. This is easily done by weighing a 5-gallon pail full of cake. Knowing that five gallons is two-thirds of a cubic foot makes the calculation easy. Alternatively, a cardboard box can be measured, filled, and weighed. Questions requiring bulk density data arise from time to time.

Issue 177

Fabric Sleeve Replacement

January 29, 2002

The most important item in changing a fabric sleeve is to note that the seam is either a lap or double hook joint. Arrows are printed on the sleeves to denote the direction in which the rotor should go past (sweep over) the joint.

The arrows assure that the seam is installed so that the waves of liquid sweep over the blunt edge of the seam, not against it. That is, sleeves should be installed so that the blunt edge of the lap joint is not facing into the waves of liquid that are pulsed by the rotor. While it is harder to visualize, the same effect is true with the double hook seams.

If the sleeve is installed the other way around, the wave of liquid created by the moving rotor paddle will hit an edge of fabric. This leads to early seam failure. This is true although there may be a film adhesive, which laps over the joint of fabric in a smooth manner.

It is important to take a few extra minutes when changing a sleeve. The seam should be straight with the main axis of the machine; the two hems should be uniform, without any pinches; there should be no dips or ripples in the fabric surface.

To properly install a new sleeve, first position it reasonably uniformly and clamp it tight. Next slightly tension the sleeve temporarily with the tensioning springs. This will make evident any non-uniformly tensioned areas. Loosen the sleeve clamp at one end and tap it so as to pull the fabric tight; then re-tighten the sleeve clamp.

The location of the seam of each sleeve should be noted. The options available include at the top, at the bottom, and next to an inspection door. Most commonly the seam is placed where it can be seen through the inspection door.

Safety tip: when inspecting sleeves with the machine in operation care must be taken. There is a natural tendency to poke a finger through a suspected hole. If this is done with the machine in operation, the rotor will surely sever the finger.

REMOVING AND RE-INSTALLING THE FABRIC SLEEVE ASSEMBLY

To remove the assembly that holds the fabric sleeves, first shut off the flow into the Fiber Filter and then turn off the machine. It will help to operate the backflush system before disassembly. Set the level of the rotor at an attitude, most generally the horizontal position, which will be convenient for removing the discharge head and sleeve assembly. An allen head wrench is used to loosen the setscrews that hold the inner race of the bearing that is mounted to the discharge head. Next the springs that tension the fabric should be removed, and the clamps (or bolts) that hold the discharge head to the main body should be released. This will allow the discharge housing to slip off the shaft of the rotor.

Vertical legs on its underside support the sleeve assembly. There is a pin welded at the top center of the inlet head, at the far end of the machine, down by the drive motor. This pin goes through hole in a plate of the sleeve assembly, to prevent it from rotating.

Look through the open end of the sleeve assembly to make sure the fabric is not dragged or pushed into the rotor. Do this during both disassembly and re-assembly operations.

The Fiber Filter has a collar with a setscrew that is located on one of the four tie rods of the sleeve assembly. It is used to hold the sleeve assembly reasonably tight during re-assembly operations. This will prevent the telescoping parts of the sleeve assembly from coming loose and allowing the fabric sleeves to sag. Upon tensioning the springs as a final re-assembly step, the telescoping piece will pull away from this collar. Normally this set collar is never moved during routine maintenance.

Following re-assembly of the Fiber Filter, check the fabric sleeves through the inspection panels. This should be done before putting power to the machine, as a loose sleeve will become entangled in the rotor.

Installation Hints

July 6, 2006

The first thing to take into account in a screw press installation is to make provision for both bypassing the press and handling overflow from the inlet hopper. Solutions can take a wide variety of forms. Questions to be asked include: What will happen to excess material if the press will not take the flow being fed to it? What will happen if, over time, screw press capacity declines? This can be due to things like screw wear or changes in the material being fed to the press.

Spill containment is another consideration, especially in agricultural installations. Under certain conditions, it is possible that the flow being fed to a screw press will all purge out the cake discharge, at a consistency unchanged from what it was when it went into the press.

Making provision for future maintenance is very important. Room should be left at the cake discharge end of the machine so that the screw can be removed. The screw comes out through the cake discharge hole.

When it comes to rigging, the first consideration must be safety. Also, be sure to properly support the press when lifting it from the truck. Do not lift just one end, as it is possible for the frame to deflect, which in turn can shift the screw positioning within the press. Interference between the screw and the screens can result.

As for mounting a screw press, do not bolt or weld the press down to a level foundation! Instead, first set the press where it is to be installed. Next, place shims between the press frame and the steelwork (or concrete pedestals or foundation) to fill any gap where the press is to be anchored. Only after this shimming should the press be pulled down tight. Doing otherwise will likely rack the frame of the press, and this can cause screw-to-screen interference.

The press should be mounted solidly to a foundation or structural steel. If a press goes into a jammed condition while operating, without the press being anchored in place, the frame of the press can twist. If tramp metal is big enough, it can cause such a jam.

To suit individual situations, the press can be installed with the cake discharge tilted upwards a few degrees. In fact, this is the preferred installation position, as it improves press liquor drainage. When the angle is more than 15º, consult the factory as it is likely that the oil level in the gearbox will have to be adjusted.

In order to get the motor out of the way, hollow shaft gearboxes can be rotated 90º or 180º. This, too, requires adjustment of the oil level, as indicated in the operating manual. If the press is too long to be installed in a given space, the drive can be changed. Presses with hollow shaft gearboxes can be changed to right-angle drive gearboxes. Presses with in-line drives (motor direct coupled to a concentric gearbox) can be changed to a belt drive, with the motor mounted either above or to one side of the gearbox. In extreme cases, we go to a parallel shaft gearbox with the input and output shafts on the same side. This puts the motor next to the inlet hopper of the press. In all cases where the drive is to be changed in this manner, the change must be made at the time of order entry, not after we have built the press.

Issue 175

Operating Hints - Fiber Filter

(Scroll to bottom for pdf version)

INSTALLATION

The Fiber Filter may tend to vibrate, so for a permanent installation you will want to anchor all the feet of the machine to supporting structure.

Note that, since the angle of inclination to be adjusted, flexible hoses must be used for the inlet flow and, possibly, the filtrate flow.  Be sure to allow enough hose length for full travel of the tilting mechanism without interferences.

Changing the filter sleeve assembly is required periodically.  To accomplish this, convenient floor space must be left at the sludge discharge end of the machine.

Operating the optional sleeve flushing mechanism requires space at the drive end of the Fiber Filter so that the flush liquid supply hose and tube can move in and out.

The filtrate flow can be allowed to empty into a tank or collection pan mounted under the machine.  (Such a pan is useful to the operator for detecting a torn filter sleeve.)

It is important that provision be made to immediately shut off the flow of liquid going into the Fiber Filter in the event that the drive motor is stopped or trips out.  The inbound flow going into the Fiber Filter will purge through the sludge discharge chute if the rotor stops turning.  Provision should be made for this eventuality.

It is best to make provision to by-pass both part and all of the inbound flow to the Fiber Filter.

Spill containment is a consideration.

 

RIGGING

The Fiber Filter may have a lifting eye that is positioned at the top of the machine, near the center of gravity; a chain can be slung through this hole for lifting.  Alternatively the machine can be lifted from below with a forklift.

 

START-UP

Before putting power to the Fiber Filter, the rotor should be turned by hand to make sure that it turns freely.  This can be done by taking the cover off the end of the motor and turning the cooling fan by hand.  Also, it can be done by reaching through the sludge discharge chute to grip and turn the rotor.  Be sure the motor starter is locked out when performing this procedure.

The rotor of the Fiber Filter turns in a counter-clockwise direction, when viewed from the drive end of the machine.

Be sure that the spring tension is set correctly tight prior to start up. 

 

INSTRUMENTATION

In almost all cases satisfactory operating conditions can be achieved by adjusting the elevation of the Fiber Filter.  However in some cases, driving the Fiber Filter with a Variable Frequency Drive can be useful in optimizing performance.  The use of a VFD can be valuable during initial start-up for establishing the best pulley ratio for future fixed speed operation (if a belt drive is used).

A useful instrument for testing a Fiber Filter is an ammeter.  This is particularly true if there are high concentrations of solids in the flow to the Fiber Filter.

 

SPRING TENSION

The Vincent Fiber Filter features an External Fabric Tensioner.  These springs can be adjusted with the machine in operation.  Usually spring tension is adjusted only once after initial stretch has occurred in a new sleeve.

The effect of spring adjustment on capacity and filtration is minimal.  However, tight spring adjustment is vital for achieving long sleeve life.  The fabric must be kept taught; otherwise it will flutter and fail in a few days.

On the Models FF-12 and FF-30, the springs have a free length of 6".  These should be compressed 1", to 5" length, for proper tension on the fabric sleeves.  In the case of the FF-6, the springs are 4" long and they should be compressed to 2-1/2" length.  (Some FF-6 machines have 6" orange springs; these should be compressed to 5-1/2".)

 

FF-6 TIGHTENED TO 5-1/2" FF-12/30 TIGHTENED TO 5
FF-6 TIGHTENED TO 5-1/2"   FF-12/30 TIGHTENED TO 5"

The springs will normally stretch out by 3/8" to 3/4" during initial operation of the Fiber Filter.  The spring compression should be re-set after this occurs.

 

OPERATION

The Fiber Filter separates a flow containing dilute solids into a stream of filtrate and sludge.  This sludge will be quite wet; however, it will contain the majority of the insoluble (fiber) solids.  The Fiber Filter is not a press, so it does not separate the solids into a cake.  When starting with a clean, empty Fiber Filter, it can take several minutes for the first sludge to appear at the discharge.  This is because the fabric sleeves can hold up to five minutes worth of sludge.

With a very dilute feed, one would expect around 95% of the flow fed to the Fiber Filter to come out as filtrate and 5% as sludge.  This can vary significantly in applications where there is high solids consistency in the feed to the Fiber Filter.

If excessive vibration or high motor amps are evident, it is likely that the solids are not discharging from the sleeve.  This can be remedied by either (a) lowering the elevation angle of the machine or (b) increasing the flow of liquid into the machine.  (It can also be an indication that sludge has bridged and is accumulating in the sludge discharge spout.)

In some applications the operation of the Fiber Filter is cyclical.  The operating cycle can range from a few seconds up to two minutes.  The cycle starts with a longer period of minor vibration and minimum solids (sludge) discharge.  This is followed by a shorter period of stronger vibration and a heavy discharge of sludge.  This occurs with a constant, uniform inbound flow and a steady discharge of filtered liquid.

It must be anticipated that the Fiber Filter may purge.  Under this condition the inbound flow will discharge, unfiltered, through the sludge discharge chute.  This condition will occur if the electrical power to the Fiber Filter is interrupted without the inbound flow being shut off.  It can also occur if the sleeve becomes blinded (coated over); if the elevation angle is too low; or if inbound flow conditions change.

The Fiber Filter will overload and trip out if excessive solids accumulate within the sleeves.  This occurs if there is insufficient liquid flow through the machine.  The condition can be avoided by reducing the angle of inclination of the Fiber Filter.  The overload condition will occur either if the machine is set to operate with a thick inbound flow and this flow is significantly reduced, or if the solids content of the inbound flow is significantly increased, without lowering the angle of inclination.  Alternate rotor configurations are available that avoid this problem.

fiber filter fiber filter
STEEP INCLINATION   LOW LEVEL INCLINATION

Excessive splashing of liquid from the sludge discharge chute is corrected by backflushing the sleeves, by increasing the angle of elevation, or by reducing the inbound flow (gpm).

The rotor of the Fiber Filter turns in a counter-clockwise direction, when viewed from the drive end of the machine. 

However, it has been found that operation in the reverse direction, with certain rotor designs, may produce improved results.  Reverse operation may be appropriate if excessive splashing of liquid from the sludge discharge chute occurs.  Try this before you give up.

 

VARIABLES

The five variables, in order of importance, that affect Fiber Filter operation are:

This assumes that solids consistency in the inbound flow is not a controllable variable.

 

FEEDING

Fiber Filters require that the feed to the machine be at a constant flow rate.  The feed must be at low pressure.  Good performance of the Fiber Filter can be achieved by gravity feeding from above.  An overflow line can be used to maintain constant head and flow.

The Fiber Filter can also be fed by pumping a flow directly into the machine.  A variable speed pump, especially a diaphragm pump, works best.  A regulating type valve, such as a globe valve, may be required to adjust the inbound flow.

 

INCLINATION

The principal adjustment of the Fiber Filter is made by changing the angle of inclination.  In general, with a steeper angle, greater dewatering is achieved.  Usually greater throughput capacities can be achieved with a more gentle angle.  The fluttering cycle and vibration intensity are also affected.

If the angle of inclination is too great, or if the inbound flow is too little, it is possible that no sludge will be produced.  In this situation the suspended solids in the flow are being disintegrated and beaten through the sleeves by the action of the rotor.  This condition is normally accompanied by mild to severe fluttering of the sleeves.

Not uncommonly, with certain rotor designs it will be necessary to aim the Fiber Filter downward, with the discharge below the horizontal.  This occurs with either very thick flows or very low gpm flows.  It occurs when there is not enough liquid present to flush the slurry/sludge from the Fiber Filter.  (It may be necessary to place a block under the elevation pivots or the feet of the machine in order to achieve a downward angle.)

If the angle of inclination is too high for a given flow, or if the inbound flow is too low, the fabric sleeve will fill with solids.  This can lead to severe fluttering of the fabric, and the Fiber Filter may trip out on electrical overload or the sleeves may fail.

 

FABRIC SLEEVES

The fabric sleeves of the Fiber Filter are made of tweed woven monofilament fabric.  The synthetic polymer fabric is selected according to a rating of micron size.  In addition, the fabric is selected for its rating in terms of tensile strength (circumferential as well as axial) and chemical and temperature resistance.

Standard polyester sleeves are good to 220o F and are resistant to both caustic and acid cleaning solutions.  PEEK sleeves are good for 350o F. 

Micron ratings of 20 to 190 are typical.  These relate to the size of the particle that will pass through the sleeve, not to the passage size through the filaments of the fabric.

The following chart lists common sleeves:

Micron Rating Air Perm Mesh Nominal Opening Material
RLX 572 80 0.007" 0.190 mm Polyester
155 260 100 0.006" 0.150 mm PEEK
132 299 110 0.005" 0.118 mm Polyester
118 501 120 0.005" 0.118 mm Polyester
86 234 170 0.0035" 0.086 mm Polyester
50 72 270 0.002" 0.050 mm Polyester
43 208 325 0.002" 0.045 mm PEEK
31 39 500 0.001" 0.030 mm Polyester
20 24   0.0008" 0.020 mm Polyester
12 19   0.0005" 0.012 mm PEEK

 

PEEK sleeves are seven times stronger that polyester, but cost twice as much.  This fabric is more chemical resistant, also.

The sleeves quiver while the Fiber Filter is in operation.  This action keeps the sleeve from blinding.  A quivering action is normal and will result in very long sleeve life.  Fluttering, on the other hand, reduces sleeve life.  It ultimately results in the failure of the fabric.  Replacement is simple.

 

BACKFLUSH SYSTEM

In certain applications the fabric sleeve of the Fiber Filter may become blinded with usage.  This may be corrected by using the backflush system.  The frequency at which this is necessary varies considerably: it can be as often as twenty times an hour.  Or, operation once a shift or once a day may be all that is required.

Some sanitary applications require that the machine be flushed for CIP purposes.  This is done with the backflush system. 

This backflush system consists of an internal spray rings with nozzles directed to spray the outside of the fabric sleeves.  The spray ring is moved back and forth inside the Fiber Filter either by hand or by an air cylinder mounted on the outside.  A booster pump is included to increase the pressure of the spray fluid.  A canister filter is included to prevent plugging the spray nozzles.  A control panel is included that allows setting the frequency and duration of the spray cycle.

The backflush system is designed to operate with 200 to 250 psi at the nozzles.  It can be operated with or without the rotor in operation, and with or without flow through the machine.  The preferable mode is with the machine in normal operation (full flow through the machine at normal rotor rpm).

Backflush fluid can be water or CIP solution.  In some cases it is necessary to use chemical cleaners such as caustic or acid in addition to a water flush cycle.

 

RPM:  FIBER FILTER SPEED

Optimal operation of the Fiber Filter is almost alwaus achieved by adjusting the elevation.  With consistent material being fed to the machine, especially with an elevated tank for gravity feed, constant speed should be perfectly adequate.  In a few cases superior performance may be achieved by driving the unit with a Variable Frequency Drive (VFD).  In such case the Fiber Filter speed (rpm) can be set later by changing drive sheaves (if a belt drive is used).

In general, higher speed results in higher throughput capacity.  It can also result in splashing of wet material from the discharge and reduced solids concentration in the sludge/slurry discharge.  High speed operation increases the power draw of the motor.

In applications involving filtration of a variety of solutions, or inconsistent inbound flow, the Fiber Filter is best equipped with a VFD.

 

SLEEVE LIFE

The most common sleeve failure is a tearing at the hem.  The second most common is a failure of the seam.  These failures generally occur because the machine is allowed to operate with the fabric in a fluttering condition.  When this occurs the machine will be seen to vibrate, and rattling noise will be heard from the spray rings inside the machine.

To prevent fluttering: 

  1. Be sure the springs are quite tight (not loose).
  2. Lower the inclination so that wetter sludge is produced. 
  3. Increase the flow into the machine so that solids do not accumulate inside.
  4. Switch to a heavier, preferably PEEK, fabric sleeve.

 

CAPACITY MEASUREMENT

The best way to measure capacity of a Fiber Filter is to collect timed samples of filtered liquid and discharge sludge.  Allowing the filtered liquid to accumulate in a tank, and measuring the change in depth over time, works well.  Similarly, a 5-gallon pail is suitable for collecting discharge sludge.

 

PERFORMANCE MEASUREMENT

For a quick performance measurement of the Fiber Filter it is convenient to collect samples of inbound and filtered liquid.  These should be equal size samples (one fluid ounce is typical).  These samples are poured into the center of a piece of cotton cloth; making a ball and twisting the cloth will force the liquid through the cloth.  The fiber will remain on the cloth, allowing a visual comparison between inbound and outbound.

Similarly, samples of inbound and outbound liquid can be collected in Imhoff cones or jars and allowed to settle.  The differences noted give an idea as to the effectiveness of the machine.

Use of a laboratory centrifuge on inbound and outbound samples permits a more quantitative measurement of performance.  Similarly, oven drying of filter samples permits a quantitative analysis of suspended (insoluble) solids.  If the fluid being filtered contains dissolved solids (sugars), the samples should be washed to zero Brix as part of the testing procedure.

 

SLEEVE CLEANING

In some sanitary applications it is necessary to remove the filter sleeve assembly from the Fiber Filter and soak it in a cleaning solution such as caustic.  This may have the benefit of shrinking the fiber back to a taught condition, depending on the fabric material being used.  Purchase of a spare sleeve assembly is recommended for this type of operation.

 

SHUT-DOWN

When shutting down the Fiber Filter, the sleeve assembly should be cleaned.  If practical, this should be done by admitting fresh water to the inlet of the machine. 

Also, the backflush system should be used.  This is done by leaving the machine in operation (rotor spinning) but with no flow being admitted.  It will help to lower the angle of inclination of the Fiber Filter.  Operate the back flush system to clean the fabric sleeves.  This will prevent solids either from overloading the machine on start-up or from crusting on the fabric.

 

SLEEVE FAILURE DETECTION

A metal screen can be installed over the filtrate drain in the barrel of the Fiber Filter.  Should a sleeve fail, fiber will blank over this screen and cause liquid to drain from the discharge head, alerting an operator.  This type of screen is illustrated below.

screen failure detection fiber filter alert fiber filter screen

 

LUBRICATION

There are up to four grease lubrication fittings on the Fiber Filter.  These are on the shaft seal housing and flanged bearing at the inlet as well as the shaft seal housing and flanged bearing in the discharge head.  Normal bearing grease is suitable for the shaft seals. 

 

FABRIC SLEEVE REPLACEMENT

The most important item in changing a fabric sleeve is to note that the seam is either a lap or double hook joint.  Arrows are printed on the sleeves to denote the direction in which the rotor should go past (sweep over) the joint (seam).

The arrows assure that the seam is installed so that the waves of liquid sweep over the blunt edge of the seam, not against it.  That is, sleeves should be installed so that the blunt edge of the lap joint is not facing into the waves of liquid that are pulsed by the rotor.  While it is harder to visualize, the same effect is true with the double hook seams. 

If the sleeve is installed the other way around, the wave of liquid created by the moving rotor paddle will hit an edge of fabric.  This leads to early seam failure.  This is true although there may be a film adhesive, which laps over the joint of fabric in a smooth manner.

It is important to take a few extra minutes when changing a sleeve.  The seam should be straight with the main axis of the machine; the two hems should be uniform, without any pinches; there should be no dips or ripples in the fabric surface.

To properly install a new sleeve, first position it reasonably uniformly and clamp it tight.  Next, slightly tension the sleeve temporarily with the tensioning springs.  This will make evident any non-uniformly tensioned areas.  Loosen the sleeve clamp at one end and tap it so as to pull the fabric tight; then re-tighten the sleeve clamp.

The location of the seam of each sleeve should be noted.  Most commonly the seam is placed where it can be seen through the inspection door.

Safety tip:  when inspecting sleeves with the machine in operation care must be taken.  There is a natural tendency to poke a finger through a suspected hole.  If this is done with the machine in operation, the rotor will surely sever the finger.

 

REMOVING AND RE-INSTALLING THE FABRIC SLEEVE ASSEMBLY

To remove the assembly that holds the fabric sleeves, first shut off the flow into the Fiber Filter and then lock out the machine.  It will help to operate the backflush system before disassembly.  Set the level of the rotor at an attitude, most generally the horizontal position, which will be convenient for removing the discharge head and sleeve assembly. 

An Allen head wrench is used to loosen the setscrews (usually four) that hold the inner race of the bearing that is mounted to the discharge head. 

At this point, there are two options:  (1) The springs can be left in place and the entire discharge head and sleeve assembly can be removed as one piece, or (2) the springs can be removed, followed by the discharge head.  Once this is done, the sleeve assembly can be slipped out of the barrel of the Fiber Filter. 

Note that the discharge bearing comes off with the head; there is no need to loosen the four bolts which hold the bearing.

Axial rails support the sleeve assembly.  The inlet end slides over a spout ring in the inlet head of the machine.

Look through the open end of the sleeve assembly to make sure the fabric is not dragged or pushed into the rotor.  Do this during both disassembly and re-assembly operations.

Following re-assembly and tightening the springs of the Fiber Filter, check the fabric sleeves through the inspection panels.  This should be done before putting power to the machine, as a loose sleeve will become entangled in the rotor.

 

ROTOR CONFIGURATION

Several rotor designs are available for Vincent Fiber Filters.  Most designs have straight paddles with ribbon flighting added to direct fiber toward the discharge.  Contrary to normal screw conveyor logic, a tight pitch ribbon flight is used to move large quantities of solids, while a long pitch ribbon flight is used when the solids flow is minimal.  Switching to a long pitch rotor can reduce excessive water being present in the discharge sludge.

 

ROTOR INSTALLATION

The rotor is supported by two spherical self-aligning roller bearings.  These are mounted at the drive end and on the discharge head. 

Special care must be taken when re-installing a rotor.  The drive end of the rotor must be slipped through the shaft seal housing.  To do this properly, remove the seal housing and insert the end of the rotor through the hole in the inlet head.  The rotor should be pushed in just far enough that the seal housing can be slipped onto the end of the shaft.  Then the rotor shaft can be pushed through the drive-end bearing until the thrust shoulder on the rotor shaft seats against the bearing.  The seals can be damaged if the seal housing is not loosened during rotor installation.

Care must be taken that the rotor spins freely after assembly.  Rarely, this may require relocating one or both of the two main bearings.  Both of these bearings are self-aligning.  The rotor shaft should not ride heavily on the seal housing when assembly and alignment are complete.

 

SHAFT SEALS

Fiber Filters use Johns Manville JM Clipper lip seals.  These are the c-cup style.  There will be a pair in the housing at the drive (inlet) end of the machine, mounted on the inlet head.  These seals have internal springs to hold the lip against the shaft. 

 

BACKFLUSH SYSTEM

Most Fiber Filters are supplied with a backflush system.  This consists of a pressure boosting pump, a solenoid valve to open the water line when the pump is in operation, a filter for filtering the flush water, and a control panel.  The control panel has a timer with two clocks:  the top clock sets the time interval between flushings, and the lower clock sets the time duration that the pump will run during the flush cycle.  There is also a solenoid valve which supplies air to the air cylinder which is used to move the spray ring assembly along the length of the sleeves.

fiber filter

 

REPLACEMENT PARTS

The most common wear parts in the Fiber Filter are the sleeves, the sleeve clamps, the seals, and the bearings.  These are stocked by Vincent.  Be sure to specify the Serial Number of your machine when ordering replacement parts.  The seals and bearings, like the sheaves and belts, are standard OEM components that can be purchased from the original equipment manufacturer (OEM).  The specification of these items is included in the O&M Manual.

 

SAFETY

These Operating Hints have left unstated the obvious safety hazard:  a Fiber Filter, like any rotating machine, is unforgiving.  If clothing or limb gets caught in the rotor it will not stop until damage has been done.

The easiest way to get hurt with a Fiber Filter is to reach inside the sludge discharge while the machine is operating.  There has already been one minor injury as a result of this, so do no let yourself become the second.

A second way to get hurt is to press your hand or finger against the fabric sleeve while the rotor is in operation.  If you push your finger through a hole, the rotor will cut off the finger.

The use of common sense is all that is required.

 

Robert Johnston, P.E.

fiber filter parts list

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Operating Hints - CP-4 Press

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August 16, 2014

START-UP
The CP-4 press runs on single phase current, 110 volts. It is provided with a reversing switch so that the direction of rotation can be selected. The screw of the press turns in a counterclockwise direction, when viewed from the drive end of the press.

Alternatively, the press can be supplied with motor options such as 208-220 volt single phase, three phase, 380-580 volt, explosion proof. Extended length inlet hoppers and 316 stainless steel construction are available.

To suit individual situations, the press can be installed tilted upwards. Also, the gearbox can be rotated 90o or 180 o. Consult the factory in these situations as the oil level in the gearbox may have to be adjusted.

Before putting power to the screw press it is advisable to bump the motor or even rotate the screw by hand. This will prevent damage to the press in case the screw has shifted so as to hit the screen. Also, the presence of tramp material in the press will be revealed. To turn the screw by hand, remove the fan guard on the motor and turn the fan blades.


INSTRUMENTATION
The most useful instrument to have when testing a press is an ammeter. The load drawn by the drive motor of the press is indicative of how much work the press is doing. The higher the amps, the better the dewatering. Also, the higher the amps, the closer the press is to jamming, and the greater is the abrasive wear. Very low amps indicate little dewatering is being done; the screen is blinded; low compression is taking place; or the flow into the press has stopped.

A moisture balance is valuable for measuring the moisture content of the inbound material and of the press cake. If an oven is used to dry samples, be sure it is set at 160o F or less if there are sugars in the sample. Samples should be left in the oven overnight. The tare weight of the pan should be much less than the weight of the sample which is being dried.

Although it is unusual for a CP-4, level controls can be useful in operating a press. With a signal providing the depth of material in the inlet hopper, the speed of the press can be varied to match the flow going into the press. With egg shells, a simpler level control is used to only signal when a high level is reached; its signal will increase the screw speed. In special cases the press can be turned off when a low level is reached and re-started when a higher level is reached.

In the case of pressing liquids that contain dissolved sugars, a refractometer is valuable for assessing press performance. The Brix of the inbound flow, the press cake, and the press liquor will all be the same figure. The higher the Brix, the higher will be the solids content of the press cake.

If dissolved (soluble) solids are present, the suspended (insoluble) solids (fiber) in the press liquor are generally measured by filtering and washing a sample and drying the filter paper in an oven. Dissolved solids will be washed from the sample during the washing process.


INVERTER VFD & PLC CONTROL
While it is not necessary for a CP-4 press, it can be handy to have an inverter VFD to start, protect, and operate the screw press. (A three phase motor is required if a VFD is used, although the power supply can be single phase; have someone at the factory explain this to you.) With a VFD it is possible to establish the optimal combination of screw speed and discharge cone air pressure. The VFD also can be used to reverse the press in case of a jam or to slow it down during upset conditions.

Nine presses out of ten will operate unattended, indefinitely, and just fine at line frequency of 50 or 60 Hertz. However, there are exceptions, especially with larger screw presses:

In some cases the press will tend to jam, overload, and trip out on high amps. In this situation it may be necessary to program the controls so that the cone automatically goes open on high amps, re-closing at a lower set point. This arrangement requires a solenoid operated 4-way air valve, replacing the manual valve which is provided with the press.

In other cases of jamming, a simpler arrangement is to install a Cone Timer. A timer is used to periodically open the cone. The closed period is determined by the amount of time required for press cake to accumulate in the press. The duration of the "cone open" period is long enough to dump most of the press cake that has formed. This type of operation may be used if the press periodically experiences jamming or overload due to fluctuations in the amount of material being fed into the press. Alternatively, it may be used with slippery or slimy press cake that cannot be dewatered to sufficient firmness to force the cone open. Cone Timer panels are available from Vincent at no charge.

Some applications require the use of a specially programmed variable frequency drive. In this case the VFD is not used to change the speed of the press, but, rather to set it for auto-reversing operation. By having the screw run backwards for three or four turns every few minutes, some difficult-to-dewater materials can be pressed much more effectively. This operation can help a great deal with material which tends to blind (cover over) the openings in the screen. Vincent has loaner VFD's if you want to give it a try. The technique works well on bar screens; care must be taken with perf screens so that the screw does not snag the screen during the reverse cycle.

In some applications a press is sized for handling upset conditions of large flow, while the normal flow is quite small. In these cases a level control is used, and the PLC can be programmed to turn off the press when a low level is reached in the inlet hopper, and the press re-started when a higher level is signaled.

Once through start-up, the cone is almost always permanently left in the closed position at whatever air pressure has been found to be effective. A plug of cake will be left around the cone whenever the press is turned off; this will clear on its own accord on restarting the press.

However, some materials may set up and become hard or freeze within the press when the press in turned off. This is especially true in the case of pressing wet coffee grounds or outdoor installatons. For these applications it is advisable to open the cone for a period of one minute before turning off the press. This allows the press to partially empty itself, fluffing the material left in the press. Vincent can provide information for automating this procedure.


FEEDING
Material can be fed into the press many ways. Pouring from a 5-gallon pail is most common for the CP-4 press. In pilot or production runs, screw conveyors, pumps, transition chutes, or cyclone separators are used. Consider making provision for overflow return of material in the event that more is fed to the press than it can take. Spill containment is a consideration.

Sometimes either a static (sidehill or parabolic) screen or a rotary drum screen (RDS) is mounted over the inlet hopper to prethicken the flow ahead of the press; the tailings (solids) from the screen are funneled into the press. This arrangement is desirable when the feed to the press is dilute.

Also, material can be dropped from a shredder or cyclone separator into the press. A shredder is used to increase capacity and dewatering in the case of low bulk density materials like lettuce leaves, alfalfa, onion peel, and cornhusk, or to prevent blockage.

Most commonly, the best screw press performance is achieved if the material in the inlet hopper stays just over the top edge of the screw. Usually presses work the best with only atmospheric pressure in the inlet hopper. In order to minimize static head, press headboxes are kept short, and level controls are used to minimize the depth.

When a pump is used to feed a press, the system can be either open or closed. We recommend the open system where little or no pressure exists in the inlet hopper, thus preventing the press from being force-fed. In this arrangement either there is an open return line allowing flow back to the source feeding the press, or level is controlled in the inlet hopper. It is best to have a line that allows material to recirculate past the press inlet. This will prevent pressurizing the inlet of the press, which can cause both blinding of the screen and purging from the cake discharge.

A port on the side of the inlet hopper is frequently provided on larger Vincent presses. It is used to view the level of material over the screw. It has a bolted cover because it is rarely used.

If a fluid flow is piped through a sealed cover which is bolted to the inlet hopper, force-feeding is possible. A by-pass tee should be provided so that the pressure in the inlet hopper is minimized. In addition, a 2" vent line, open to the atmosphere, must be provided to prevent siphoning material in the inlet hopper out through the recirculation line.

Inlet hopper pressure over one to four psi can force fibrous material against the screen so as to blind off the screen, resulting in unsatisfactory performance.

At pressures above 10 to 15 psi in the inlet hopper, it is possible to blow the "plug" of press cake that forms at the discharge of the press. Unscreened liquid will purge from the cake discharge. Exercise caution if either hot or hazardous material is being pumped into a press.

At inlet hopper pressures of 40 psi and above, the shaft seals will be blown out of their housing. At pressures around 60 psi the screen will start to separate from its support plates, resulting in bypassing of feed material directly into the press liquor flow.


BUILDING A PLUG
In order for the press to work, a plug of cake must form between the cake discharge opening and the pressure cone. The press will almost always do this on its own accord as material is fed into the press.

In the case of sloppy materials like manure and DAF sludge, it may be advisable to start off by first packing the discharge of the press with any available fibrous material.

Alternatively, the press can be turned on and the feed pump allowed to run just long enough to fill the feed line and the press. Then shut off the pump, leaving the press running, and wait until no more liquid drains from the screen of the press. Repeat this process until a plug of cake starts to open the cone.


PRE-THICKENING

Almost always, the thicker a flow going into a press, the better it will work.

If the flow into a screw press is too dilute, the high volume of liquid going through the press screen can cause either of two problems. The flow may either flush most of the solids through the screen, or it may plaster solids against the screen, thus blinding (covering over) the screen.

To prevent these things from occurring, it may be necessary to pre-thicken the flow ahead of the screw press. This is commonly done with a static screen (sidehill or parabolic) or a rotary drum screen (RDS). In the case of very dilute feed to the press, a Vincent Fiber Filter can be used.


AIR CYLINDER REGULATOR
To regulate the air pressure of the discharge air cylinder, presses are supplied with an air pressure regulator along with a Parker four-way reversing valve. These should be installed near the cone end of the press. (Until recently FRL (Filter, Regulator, Lubricator) sets were provided to regulate air pressure. Most air cylinder manufacturers now recommend against the use of lubricators.)

The Parker valve allows manual selection of the shut, open, or "neutral" position. This valve connects air supply from the regulator to one end of the air cylinder, while simultaneously opening the other end to atmosphere. The vent line on the 4-way valve allows air to escape when pressure is switched from one end of the air cylinder to the other.

Continuous air flow from the Parker vent line indicates a leak inside the air cylinder, or possibly a faulty 4-way valve.

Once material is going through the press, set the 4-way valve so that the discharge cone goes shut in the "in" or closed position. Start with a low air pressure, working your way up until the desired performance is obtained.

The neutral position of the Parker valve is used only in testing. If left in the neutral position, the cone will not move unless it is pushed open by press cake. If, later, the flow of press cake is diminished, the cone will remain in the position to which it was pushed, and purging can occur.



FRL AIR REGULATOR WITH 4-WAY REVERSING VALVE

DISCHARGE CONE
The principal adjustment of the press is made with the discharge cone. The cone is the component at the cake discharge end of the press that acts as a door or stopper plug to restrict material from leaving the press. The more pressure exerted by the discharge cone, the drier the cake material will be leaving the press. Also, the motor amps can be expected to increase with added pressure, and throughput may decrease.

In the small CP-4 press, the cone is usually actuated by an air cylinder, although lever arm actuated models used to be available.

Presses are generally started up with the cone in the withdrawn position. Once material is going through the press, the cone is set to the closed position. With many materials it is satisfactory to start the press with the cone in the closed position. However, with materials that are dry to begin with, such as swarf or plastic wash tank sludge, it becomes more important to start with the cone in the open position in order to prevent an unnecessary jam. Alternatively, very thin or soupy materials, like pumped pig manure or clarifier underflow, might tend to purge right through the press if the press is started with the cone in the open position; with these materials it is frequently better to start up with the cone nudged into the closed position.

As the pressure on the cone is increased, not only will the cake become drier, but the flow through the press may also be reduced. With very slimy inbound material it may be possible to apply enough cone pressure to stop the flow altogether.

High cone pressures can result in increased quantities of suspended solids in the press liquor.

Typical air cylinder pressures to actuate the discharge cone are in the range of 30 to 60 psi. Some materials will press only in a low range, say 10 to 20 psi. Other materials may press best with a pressure of 60 to 100 psi. Air consumption is minimal in all models, 1 to 2 cfm.

On models without an air cylinder, typical weights used actuate the discharge cone vary considerably. When juicing fruit there may be a need to minimize the amount of solids being forced through the screen. At the other end of the scale, sometime applications require an extension arm with a very heavy weight.

It is acceptable to open the discharge cone, in most cases, during normal operating conditions. This allows inspection, while in operation, of the discharge end of the screw and screen. This will give the operator a chance to observe operation with minimum dewatering and maximum throughput. It is also a good technique for purging bad material i.e., either jammed or spoiled material, from the press. (Do not try this trick if you are pressing hot or chemically aggressive materials.)

With some feed materials, the press can be operated with the cone in the withdrawn position. The screw itself will do enough compressing and dewatering to produce a cake at the discharge. If the material being pressed dewaters very readily, the press can jam even with the cone in the withdrawn position. If this happens, a lower compression screw is called for.

An unusual technique is to set the air pressure so that the cone normally stays completely shut. A timer is used to periodically open the cone. The closed period is determined by the amount to time required for press cake to accumulate in the press. This type of operation is used with slippery or slimy press cake that cannot be dewatered to sufficient firmness to force the cone to open. The duration of the "cone open" period is long enough to dump the press cake that has been formed. Cone Timer panels are available from Vincent at no charge.

Care must be taken if a press is to be left running at a very low pressure like 10 psi. If some fiber enters between the cone bushing and the screw shaft, it will take more than that much air pressure to close a cone which has have been pushed open by a heavy flow of cake. The result will be either high moisture content in the cake or, worse, purging.

Where very low air pressures are required for proper operation, it may be practical to put the 4-way valve in a neutral position, half way between open and closed. (Keep in mind that a slug of cake will push the cone open, and it will not re-close on its own afterwards.)

Once through start-up, the cone is almost always left in the closed position at whatever air pressure had been found to be effective. A plug of cake will be left around the cone whenever the press is turned off; this will clear on its own accord on restarting the press.

There are a few applications where the air cylinders are removed and replaced with a jacking bolt. This is used if the cone pushes completely closed even with the lowest air pressure. It results in operating the press with a fixed discharge annulus.
Air cylinders with linear actuators are available.


INTERMITTENT OPERATION

In the case of intermittent operation, it is recommended that the control panel for the feed pump or conveyor which feeds the press should have a timer. This timer should be set to have the press run for three minutes after the feed pump (or conveyor) shuts off. This will partially clear the press so that it will not trip out on overload when it is re-started. (This applies in high torque applications or in installations where the material in the press dries out or freezes.)

An extreme case occurs when pressing spent coffee grounds and some paper mill fibers. Each time the press is turned off, the cone must first be opened for a minute. If this precaution is not taken, nasty damage to the screw or screen can occur when the press is re-started.

(See the previous section, INVERTER VFD & PLC CONTROL.)

Minimize the time that the screw press is run with no material being fed into it. The last material admitted to the press will dry to powder, and it can cause severe accelerated abrasive wear.

Initially the press will likely be run empty in order to check rotation. Even though some rubbing may be heard, negligible wear will occur so long that this period is kept to a minimum. Also, since the screw is supported to some extent by the material inside the press, running dry may allow the screw to rub the screen.


DOUBLE PRESSING
Some processes benefit from what is called double pressing. This means that the cake coming from the press is run through the press a second time (or through a second press). If little moisture is removed in the second (double) pressing, then it is known that the liquid removed in the first pressing is all of the free liquid that there is to be pressed out.

Sometimes water is added to the cake in between the first pressing and second pressing. This is done to enhance the recovery of dissolved sugars in the original press cake.

Molasses can be added to press cake between the first and second pressing. This is used to infuse dissolved sugar into the cake, increasing the solids content of the final press cake.

Capital-effective double pressing can be achieved by using an inexpensive Soft Squeeze Series KP screw press for the first pressing, following with a tighter-pressing Series CP or VP in the second position.


MOISTURE CONTENT

A screw press separates free water. This will leave organic water in the press cake. The organic water is either bound to, or part of, the animal or vegetable molecules. Mechanical pressure alone will not remove organic water; it takes heat or chemistry. Frictional heat from the press can remove organic water, but this obviously should be avoided. For chemistry, see the Hydrated Lime, Gypsum and Alum section. For heat, see the Fluid Injection section.

To determine the moisture content of a material (feed to the press, press cake, or press liquor), a sample should be weighed and dried overnight at a temperature slightly less than 100o C. (If sugars are present use less than 70º C to prevent caramelizing.) The sample should weigh six or more times the tare weight of the sample tray or cup.

The moisture content of press cake varies considerably. Tomato press cake will be 90% moisture. Orange peel will be 80%, unless it is reacted with hydrated lime, in which case it will go down to 72% moisture; add molasses and it will go to 65%. Dairy and hog manure will come out at 70% moisture, unless there is sand or sawdust in the sample, which will reduce the moisture content. Cellulose fiber from a paper mill (knots, screen rejects, primary clarifier underflow) will come out about 50%. However, if secondary (biological) sludge is added, then the moisture content of the cake will go up considerably. With high ash content in paper mill samples, moisture may go down to 40%. Moisture contents of only 25% can be achieved pressing things like eggshell, glass, sand, and plastic chips.

The heat from steam injection can change the chemistry of the material being pressed so that cake with lower moisture content is produced. This blanching or parboiling effect works with fish and orange peel, for example.

A quick approximation of what to expect from a screw press is to squeeze as much water out with your fist, and figure that the press will do a little bit better. A better way is to twist a ball of the material in a cotton cloth.


COMPRESSION
A screw press achieves compression using several methods: (1) The discharge cone of the press causes back-pressure on the material being dewatered. The higher the cone pressure, the greater the liquid removal. (2) The pitch of the flights of the screw tightens as the material is conveyed through the press. This forces liquid to go through the screen. (3) The diameter of the shaft of the screw may be increased progressively, forcing material outward, against the screen. This is a
tapered shaft design.


SCREW WITH TAPERED SHAFT (Note tightening pitch)

Force-feeding (supercharging) the press and applying a vacuum to the outside of the screen are two additional methods which may achieve compression. These two are used infrequently because the performance results are uncertain.


PRESS SPEED (RPM)
In general, the slower the screw speed, the greater the dewatering. Longer residence time in the screened area results from lower screw speed, which allows time for more thorough dewatering. Unfortunately, it also goes with reduced throughput capacity.


Screw press speed (rpm) can be changed by using a Variable Frequency Drive (VFD). Alternatively, the drive motor can be switched to a different pole motor (900, 1200, or 3600 versus the standard 1800 rpm). Most modern motors are good for permanent 120 Hertz operation; they are always good for a test at this high speed.

The smaller Nord gearboxes are all rated for 4,000 rpm input, which makes it easy to switch to a 3000/3600 rpm motor. It is best to switch to synthetic lubricant if this change is permanent. With other gearboxes, the higher speed can result in premature gearbox failure. Consult the factory for assistance.

Low screw speeds are used for cooker crumb, potato peel, many sludges, and low freeness materials in general.

It has become normal for a variable speed drive (frequency inverter VFD) to be used with Vincent presses.

A small change in screw speed, like 20%, will generally not result in a measurable change in performance of the press.


CAPACITY MEASUREMENT

The best way to measure capacity of a press is to collect timed samples of press cake and of press liquor. This should be done during a period of sustained, stable operation, rather than by timing a batch through the press.

Press cake is generally captured in a tarpaulin, and press liquor in a 5-gallon pail or 55-gallon drum. When the drain is at floor level, a 3-mil plastic bag can be used to catch press liquor. If the press liquor goes to a pit or tank, the change in depth can be timed.

Sometimes it is possible to collect only one flow, either press cake or press liquor. In these cases it is possible to estimate the press throughput if the solids content of the inbound material and press cake are measured. It is assumed that there are zero suspended solids in the press liquor, although this is never really the case.


A-B-C PLATES
There are four vertical plates making up the frame of the press. Starting from the drive end of the press, the first one is the Adaptor Plate. The gearbox is bolted to the adaptor plate. Through four spacers, the adaptor plate is welded to the A Plate. This A Plate forms one wall of the inlet hopper; the shaft seal housing is bolted to the A Plate.

The next plate is the B Plate. It forms the downstream wall of the inlet hopper. The screen starts at the B Plate. There is a notch, called a Cord Cutter, in the B plate. Also, there may be a bar called Brian's Stripper welded to the B Plate, inside the inlet hopper; it is in a position to kiss the edge of the screw flight as it passes. These two features prevent long fiber pieces from balling up at the exit of the inlet hopper. See the section ahead on Cord Cutters.

The final plate, the C Plate, supports the discharge end of the screen. The cone lightly touches the C Plate when in the closed position. These plates are detailed in the Nomenclature schematic at the end of this manual.


SCREW LIFE
If a press loses its previous throughput capacity, or if cake moisture content increases, it can be a sign of a worn screw.

A screw can last anywhere from six months to twenty years. It depends on the material being pressed and how hard it is being pressed.

Premature screw failure can arise from several causes. The two main ones are:
(a) If a press is allowed to run continuously even when no material is being fed into it, the screw can wear out in one or two months. The same can happen if a very low flow is consistently fed into the press.
(b) If abrasive material is dewatered with high cone air pressure, rapid wear will occur. This condition is avoided if a few drops of water can always be squeezed from a fistful of press cake.

Two effective ways to extend screw life are:
(a) Various grades of hardsurfacing rod can be used to protect the flights of a screw. The best hardsurfacing will have Tungsten Carbide impregnated in it.
(b) Using a VFD or lower speed motor to reduce the screw rpm will extend screw life

If a worn screw is suspected, the thing to do is to shut down the press, open the cone, and dig out the cake until the tips of the last two flights can be seen or felt. Check how badly the tips are worn. If the there is 3/8” between the tips and the screen, wear is evident. It is also an indication that the sharp edges of the flights throughout the press may have worn, becoming rounded. This can cause the flights to act like a putty knife, plastering solids against the screen, preventing water from coming through.

Worn screws are either restored locally or returned to Vincent for rebuilding. The maximum cost of a screw rebuild is around one third the cost of a new screw.

SCREW CONFIGURATION
Almost all Vincent screw presses use the Interrupted Screw Flight design. The interruptions leave room for four stationary resistor teeth that are mounted outside of the screen. These teeth go through the screen and reach almost to the shaft of the screw. This design of screw press stands in contrast to a Continuous Screw design. The main advantage of the interrupted design is that solid material must accumulate in the interruptions until sufficient consistency is reached for the solids to be pushed toward the cake discharge. There is a reduced tendency for the material being pressed to co-rotate with the screw. Also, there is more agitation within the press and, consequently, quicker and more thorough dewatering.

The screw starts with a feeder section of continuous flight. This picks up material in the inlet hopper and pushes it into the screen section. The feeder section ends at the first resistor tooth. The feeder section of the screw is followed by compression stages where the flights have reduced pitch. The reduction in pitch of the flights results in compression of the material going through the press. CP-4 presses are made with four stages of compression.

A screw configuration referred to as Sterile Butterfly is available. There are a reduced number of flights on this screw, and the flights do not wrap as far around the shaft as is normal. This design screw is good for high throughput of materials that are easily dewatered. Glass and sand are examples. This configuration is rarely used in the CP-4 press.

 

INTERRUPTED FLIGHTS, RESISTOR BARS, and RESISTOR TEETH


 

LARGER PRESS                                                        CP-4 PRESS


PIE CUTTING

Sometimes the compression of a screw is reduced, in the field, in an operation called "pie cutting". This involves cutting pie-shaped segments from certain flights of the screw, leaving a butterfly (end view) configuration. This modification is done to avoid excessive compression and jamming. The "sterile cut" is more dramatic. Consult the factory for assistance before making this modification.

 

 
4" PIE CUT                                                       PIE CUT


 
NOT PIE CUT                                                             BUTTERFLY CUT


JAMMING
Should a press trip out on overload because it has become jammed, a series of steps can be taken to un-jam the press. Generally the easiest thing to do is to reverse the direction of rotation. This will cause the screw to feed material backward into the inlet hopper.

Generally jamming is caused by over-pressing excessively dry material. Running the press backwards will break up this material. If the jamming was caused by tramp material, hopefully this can be found and retrieved from the inlet hopper following operation in the reverse direction.

Having a reversing starter greatly facilitates this operation. These cost little more than a standard starter; they come with a forward-reverse switch. (Having a VFD with a reverse button can be even handier.)

If a press has had extensive use in an abrasive application, the flights can be worn away at the discharge of the press. Radial wear of 1" to 4" in larger presses will lead to serious jamming and, possibly, a burst screen.

When a press is operated in the reverse direction it is possible that solid material in the press will be forced against the A Plate. This can damage the shaft seal. For this reason, care should be taken when running the press backwards.

Usually three or four revolutions of the screw are sufficient to clear a press. If running the press backwards does not clear the jam, the screen should be removed so that the cause of the jam can be determined. Look for a bent flight. Before going to the trouble of removing the screen, shut down the press and try clearing the end of the press with a long screwdriver.

Sometimes when a press is jammed, a flight on the shaft of the screw will fold. This can happen if the press overloads on dry cake or if tramp metal is caught between a flight and a resistor tooth. The weld at the shaft may tear. When this happens flow through the press is greatly impeded.

    

FOLDED FLIGHTS

SCREEN BLINDING
A common problem is for the screen of the press to become blinded (covered over). When this occurs, the flow of press liquor coming through the screen diminishes. The level in the inlet hopper will fill up to where it overflows.

In some cases, the screen can be cleared by periodically reversing the direction of rotation of the screw. This can be programmed with many VFD’s, so that the press runs forward for a given period and then reverses direction briefly for three or four turns when the screen starts to blind. This is one of the easiest possible solutions to test. Sometimes it is the only one that is effective.

Many other methods are used to address blinding: (1) Adding notches to the screw, (2) Reducing or eliminating the pressure in the inlet hopper, (3) Adding press aid to the flow, (4) Changing to a different screen selection, (5) Reducing the screw-to-screen clearance, and/or (6) Employing a screen flush with caustic solution, acid, or high pressure spray.

If blinding occurs after an extended period of satisfactory operation, it is usually due to wear of the screw. Rounded edges of the flights will contribute to blinding.


CHANNELING
A condition somewhat similar to purging can occur with slimy materials, like concord grapes, pineapple pulp, or spent brewer’s grain. These may tend to channel or squirt out from one side of the cone. Two ways to eliminate channeling are to lower the air pressure on the discharge cone and to slow down the speed of the press. Channeling can also be reduced by adding press aid to the material being dewatered, or by reducing the inbound flow to the press.

An option NOT available with the CP-4 press is called the Rotating Cone. A tendency for material to channel can be reduced with this option. The cone is caused to rotate so that the relative motion between it and the (stationary) screen breaks the channeling.

To break up channeling, pieces called wing feeders can be welded to the end tips of the last two flights of the screw. See the Wing Feeder section of this manual.


PURGING

An undesirable condition can occur when the material being admitted to the press purges, without liquid-solid separation, from the cake discharge. This can occur especially if pressure exists in the inlet hopper.

Mechanically, purging occurs when a dry lump of press cake holds open the discharge cone. Un-pressed material will flow around this partial plug.

Purging may occur when there is a much reduced, small flow of cake coming from the press. Usually this is a sign of blinded (covered over) screens. This can be caused by a worn screw. Liquid from the inlet hopper will wick into the press cake, making it soft enough to blow out. Sometimes this condition is avoided by mounting the press inclined at about 5o above horizontal; the simplest way to do this is to place a block under the cone end of the press.

A drop in operating amps can be an indicator that a purging condition has begun. An ammeter circuit can be installed to alarm or trip the system when a reduction in motor amps occurs. This is rarely done.

Purging is prevented with the rotating cone option. This option is very unusual in the Series CP presses, and it may not be available.


BRIDGING
Sometimes bridging will occur at the inlet hopper, stopping the flow of material into the press. This is a common problem with CP-4 presses because the inlet hopper narrows down from the 8" inlet to the 4" screw. Poking with a paint-stir or broom stick is the easiest way to solve this during a simple test.

If a surge hopper is mounted over the inlet to the press, it should have at least one, preferably two or three, vertical walls. This will minimize bridging.

Bonding Teflon sheets to the inlet hopper of the press is a remedy that has been used to reduce bridging of bulky materials which allow free-draining of water.

Another way of overcoming bridging is to direct a stream of water or press liquor into the inlet hopper so as to break the bridge. The nature of the screw press is that essentially all of this added liquid will be removed in the pressing operation. This is rarely done.

A vibrator, mounted on the side of a feed hopper, may also alleviate bridging.

 

RESISTOR TEETH
The interrupted screw design press has stationary teeth that protrude into the flow of material as it passes through the press. These fit into the gaps of the screw where there is no flighting. They stop just short of the shaft of the screw.

Not infrequently the resistor teeth are drilled so that fluid can be injected into the press during operation. See the next section.

Rarely the resistor teeth are shortened, usually by half, to increase the capacity of the press. Removing the teeth altogether will result in co-rotation and jamming.


FLUID INJECTION
Resistor teeth can be drilled so to permit injection of steam, solvent or water while the press is in operation. Also, these modified resistor teeth can be used for CIP cleaning, without the need of removing the screen from the press.

Commonly alcohol injection is used to achieve in-line washing to remove sugars. Hot water injection is used to recover dissolved solids in juice production. Steam injection is used in dewatering raw organic materials.

The moisture reduction that results from steam injection is related to a chemical change that comes with blanching, or parboiling, a material. Steam injection works well on pineapple skin, citrus waste, and raw fish. Tests run with steam injection in a Vincent press at Anheuser-Busch showed little benefit. The material being pressed, spent grain, had already been "cooked" before steam was added.

Injection is achieved by drilling holes through the resistor teeth and piping these holes to a manifold outside of the screen. Photos and drawings are available from the factory. Vincent does not charge for providing a drilled resistor bar.


 
STEAM INJECTION                                                      LAB PRESS
IN TWIN SCREW PRESS                                           STEAM INJECTION


CORD CUTTER AND STRIPPER

Sometimes long stringy material will be pinched where the feeder portion of the screw goes through the hole in the B Plate. This material will co-rotate with the screw and build into a bundle which reduces the flow through the press.

A groove, like a keyway that is 3/8" deep, is cut half way through the hole in the B plate. We call this a Cord Cutter. Material trapped between the flight and the hole in the B plate will pop up slightly as it passes the Cord Cutter. The result is that the material is sheared loose.

Very rarely in a CP-4 press there may be a part called Brian's Stripper welded to the B Plate, inside the inlet hopper. It is welded in a position so that the flight lightly kisses the stripper as it goes past. This strips the material away. Strippers are made of square bar. If your press has one, be sure everyone knows to keep his fingers away from it.

 

STRIPPER AND CORD CUTTER


POLYMER

In rare applications, the addition of polymer is indispensable in achieving adequate screw press performance. Polymers are added to dilute waste streams, especially to those containing very small size suspended solids. The long chain molecules of the polymer will flocculate the solids, agglomerating them to the point where they can be pressed. Under the right conditions, drastic improvement can be observed in press throughput, press cake moisture, and press liquor clarity.

Nalco and GE are the leading polymer suppliers. Their sales engineers are anxious to recommend the product best suited for your application.

Low speed operation of the press is usually required in order to achieve good performance.


NOTCHES
Sometimes it is necessary, during press operation, to have the screw wipe the screens clear of blinding material. This is best achieved by having notches in the outer edge of the screw. Fibrous material accumulates in the notches and brushes away slimy material which may be blinding the screens. Shallow notches (1/8" wide by 1/8" deep, 1-1/2" apart) in the outer edge of the screw flights work well. Typically, notching is done from the B plate to the second resistor tooth. Most Vincent presses are supplied with notches, although they are not common in CP-4 presses.


GRINDING NOTCHES IN THE FLIGHTS

WIPERS
Before the advent of notches, wipers, made of UHMW strips or nylon brushes, were (rarely) mounted to the outer perimeter of the screw flights. If at all possible we advise against the use of such wipers because (a) they tend to cause material to co-rotate and jam in the press, (b) they are difficult to replace, and (c) they wear rapidly, resulting in unacceptably frequent shutdowns for replacement. Wipers tend to improve dewatering performance for the first week or so. After that the wiper material wears and press performance reverts to being just a little better than if no wipers were used. Wipers are made either of 1/4" thick high durometer polyurethane or of nylon bristle brush. Wipers are preferably mounted on the downstream edge of the flight.


PRESS AID
Some materials press best if a press aid is mixed into the material to be pressed. Typical press aids are rice hulls, cottonseed hulls, cellulose fiber from a paper mill, and ground newspaper. Ground wood is the best, but most expensive, press aid.

Press aids are most commonly used in producing juice from deciduous fruit. The press aid gives the press something to get a bite on. Press aids also tend to hold back fines (short fibers) and prevent them from going through the screen with the press liquor. If apples are fed into a press, apple sauce will come through the screen. However, if a press aid is added to the apples, then apple juice will come through the screen.

Typically, the amount of press aid used is only 1% to 3% by weight of the flow going through the press. This will look like more than such a small percentage because press aids have a much lower bulk density than the wet materials that are pressed.


HYDRATED LIME, GYPSUM, AND ALUM
Lime (calcium hydroxide) must be added to citrus peel before it can be pressed. The lime breaks down the pectins or cell walls so that the press can remove moisture. Less than 1% by weight is used. A reaction time of several minutes must be allowed prior to pressing. Lime has been used successfully in the same manner with potato, onion, tomato, carrot, and pineapple waste. It works well on acidic materials such as onions, strawberries, and coffee bean pulp. Vincent offers lime dosing equipment.

Gypsum and alum salts are also effective chemical press aids. They are typically used in dewatering sugar beet pulp, and they have rarely been found effective on other materials.


VACUUM EFFECT
In some applications, increased screw press capacity can be obtained if the area outside of the screen is under a vacuum. This can be achieved by mounting the press at a high elevation, with the press liquor drain line dropping below the surface of a drain tank or pit.

That is, the drain line from the press should go below the surface of the pit or pond into which it drains. If this line is relatively small in diameter and has a steady downward slope, a vacuum will be induced around the screen of the screw press. The mass and velocity of press liquor flowing through the drain line create this vacuum. To draw air bubbles downwards with the press liquor, the velocity of the fluid must be greater than five feet per second.

The cover over the screen of the press will have to be sealed, usually with Silicone.

The amount of vacuum is a function of the elevation between the press and the drain pond. For good results, the press should be mounted on a stand that is 20' tall or higher.


PRESS LIQUOR
A Screw Press produces relatively "dirty" press liquor as compared to a Filter Press. Suspended solids will pass through the screen of the screw press along with the liquid being expressed from the inbound material.

If suspended solids need to be removed from the press liquor, the most common method is to pump the press liquor either over a static (sidehill) screen or through a rotary drum screen. Generally, the screen tailings (sludge solids) are fed back into the screw press along with the flow of inbound material. Most of these solids will be captured with the solids of the inbound material and end up in the press cake. Although some of these fines will once again go through the press screen with the press liquor, equilibrium of recirculating solids is reached and stable pressing operation is realized.

If the press liquor is to be concentrated in an evaporator, better screening than can be achieved with a static screen may be required. Another Vincent machine, the Fiber Filter, provides premium performance. Decanters or centrifuges may be required.


SCREEN

The screen of the CP-4 press is made of slotted wedgewire. Wedgewire screens are one piece weldments that must be replaced when excessive wear has occurred. The standard screen used in the CP-4 has a slot width of .015" to .020". With the advent of recovering cooking oil from cooking crumb, a 0.008" screen has been introduced; this has proven to also work well in alcohol.

The most common damage to a wedgewire screen is for part of the surface to be smeared over by rubbing the screw. This rarely is bad enough to affect press performance.

In the case where a few slots get spread apart by tramp material, TIG welding is used to close the gaps.

Minor rubbing between the screw and screen is normal, although, obviously, hard rubbing will cause wear and premature failure of the screen. With a clearance greater than 3/16", the dewatering performance of the press can start to deteriorate; this depends a lot on the nature of the material being dewatered.

In cases of severe wear or damage, it is common to patch a screen. Stainless sheetmetal is used for this. The reduction in drainage surface is of little consequence as the screens have ample open area.

Perforated stainless steel sheet screens are rarely used in the CP-4 press. In other Vincent presses, perforated screens with hole sizes ranging from .020" to .095" are common. Surprisingly, there is little difference in the degree of filtration achieved by changing hole size. Where the holes are smaller than .3/32", the screen material is so thin that a reinforcing back-up layer of thicker screen is employed. This back-up material generally has 3/8" holes.

The most common cause of screen failure ties to failure of the outboard support bushing. If the bushing holding the end of the screw wears out, it can let the screw move enough to rub against the screen.


 
PROFILE BAR (WEDGEWIRE) SCREEN


  
PERFORATED SCREEN WITH REINFORING SHEET

 

SCREW-TO-SCREEN CLEARANCE
Generally the clearance between the screw and the screen is 1/32", plus or minus 1/32". The screw should not rub the screen hard, as it can cause wear and premature failure of the screen. Tighter clearance is used with materials that blind the screen, such as onion skins. Greater clearance, 1/16", is used with eggshell, pectin, xanthan gum, and corn husk. With a clearance greater than 3/16", the dewatering performance of the press can start to deteriorate; this depends a lot on the type of material being dewatered.

To measure the s2s clearance in wedgewire screens, a feeler gauge can be slipped through and along a slot until it hits the edge of the screw. Measure from the outside of the screen to the edge of the flight; then subtract the thickness of the wedgewire (0.156") from the measured depth in order to calculate the clearance.

If a screw rubs against the screen in a given area, it may be best to grind some off the OD of the screw. Prussian Blue can be useful in finding the spot that is rubbing.

 

WEDGEWIRE DEPTH GAGE              PERFORATED DEPTH GAGE


CONE BUSHING

The cone, which rides on the shaft of the screw, is made of bronze or UHMW to protect the journal surface of the screw shaft. "Cone Sleeve" is the name given to the portion of the screw on which the cone rides. Normally the Cone Sleeve journal area is lubricated with liquid from the material being pressed, such as the juice from apples or water in pectin peel.

The grease fitting is provided to minimize leakage of press liquor through the cone. It is also used for lubricating when materials that are quite dry (paper mill screen rejects) are being pressed. By the time such materials reach the discharge of the press, they do not have enough free moisture left in them to adequately lubricate the cone sleeve.

Automatic grease systems are available. These should be the high pressure (900 psi) electric or battery variety. Vincent provides these for critical applications, such as pulp & paper and alcohol extraction.


WING FEEDERS
Very rarely in the CP-4 press, there are blades welded to the outside tips of the last two flights of the screw. Called "Wing Feeders", these are mounted parallel to the discharge screen surface. Care must be taken that wing feeders are not made so long that they hit the face of the cone when the cone is in the closed position.

Wing feeders can serve two purposes. (1) In the case of materials that want to channel out the discharge of the press, like pineapple and spent brewer's grains, long wing feeders break up the channeling flow. (2) For abrasive applications, short knobby wing feeders are provided as sacrificial wear elements.

When certain materials are fed through a screw press, clumps of dry material may pack in front of the wing feeders. This buildup can cause wear of the screen. Should the problem occur, grind off the wing feeders.

 

 
LONG WING FEEDER                                        KNOBBY WING FEEDER


CLEANING

Commonly material is cleared from a press by stopping the inbound flow, setting the cone in the withdrawn position, and running the press for several minutes until no further material is discharged. This will leave some material inside the press, which can be handy for forming a plug at the cake discharge when the press is restarted.

Material will leave a Vincent interrupted flight press only if there is additional inbound material forcing it out. This makes it difficult to clear all material from a press without removing the screen. One technique used successfully is to feed crushed ice into the press. Water must be fed along with the ice to prevent jamming. When the ice melts, the press will be relatively clean inside.

There are applications in which the press must be cleaned frequently, such as once a shift, in order to meet sanitary regulations. At these installations, the screen is removed from the press in order to remove residual material. A spare screen may be kept on hand, submerged in cleaning solution, in order to minimize downtime.

Cleaning the inside of the screen can be achieved, at least to some extent, by injecting water through the resistor teeth. Holes must be drilled in the resistor teeth to make this possible.

In laboratory applications, the press may be disassembled for cleaning. The various components can be dipped in an appropriate solution or placed in an autoclave.

There is a YouTube video on the Vincent web site showing disassembly of the CP-4 press.

It is unusual that the outside of the screen needs to be cleaned. A pressure washer or swabbing with acid solution can be used. Spray systems for this external cleaning can be built into a press at the Vincent factory.


SHAFT SEAL
The Seal Plate is bolted to the A Plate. This may be solid UHMW (ultra high molecular weight polypropylene or polyethylene) or it may contain one or two Johns Manville (JM Clipper) lip shaft seals. There may be a grease fitting on this plate; the grease is used to reduce leakage and to help prevent fiber material from entering the seal and damaging the screw shaft.

Generally, seals are allowed to drip once they start leaking. They are replaced only in conjunction with major maintenance, as when the screw is removed from the press.

In some cases we have found that leakage from a shaft seal can be stopped by simply selectively loosening or tightening the four bolts holding the seal housing to the A plate.

  
SEAL PLATE                     SPLIT SEAL PLATE

 

SCREW REMOVAL
The screw is removed through the hole in the C Plate, at the cake discharge end of the press. The operation can be difficult the first time, so we recommend consulting with the factory before getting started.

First of all, remove the four bolts holding the shaft seal housing to the A Plate. This will prevent damage when the screw drops out of the gearbox.

The bar with the resistor teeth must be removed before the screw can be removed.

Series CP screw presses use a key and keyway to couple the screw inside the hollow bore of the gearbox. It may be that a great deal of force will be required to push the screw from the gearbox.

The basic procedure is to push or pull the screw out of the gearbox. If the screw is not tight, the screw is removed easiest by pulling, or by prying with a pry bar.

If it is tight, the screw is removed using a heavy fine-threaded rod to push the screw out of the gearbox. This rod is threaded through a heavy nut Vincent has provided (only in larger sized gearboxes) inside the hollow bore of the gearbox. This nut has a lug that fits in the keyway, to prevent the nut from turning. The nut backs up against an internal snap ring in the hollow bore of the gearbox. The result is that the rod pushes against the end of the screw. Liberal use of Anti Seize or Never-Seez is absolutely required to protect the threads. Loaner tool kits are available in Tampa; see Screw Change Kit Drawing B-94693.

Alternatively, the screw can be removed by constructing a gear puller, allowing the use of a hydraulic jack, combined with a cross bar and two long shafts threaded into the gearbox housing. The holes in the gearbox have metric threads. To avoid the need for metric all-thread rod, a length of all-thread rod with English threads is welded to the head of a metric bolt.

If the press has been badly jammed, the key may have rolled inside its pocket. In these cases it can be impossible to press the screw from the gearbox. It then becomes necessary to cut the screw in half, between the gearbox mounting plate and the A plate. The stub shaft can then be bored out in a large boring mill, and the screw can be repaired at the Vincent factory.


JACKING ROD


SCREW REPLACEMENT

When reinstalling a screw, the screw must be pushed in until the step in the shaft seats against the thrust bearing of the gearbox. This will position the flights of the screw so as not to hit the resistor teeth. Be careful when guiding this step in the shaft through the shaft seals.

During re-assembly of a keyed shaft, be sure to apply Never-Seez or Anti Seize to the portion of the screw shaft that goes into the gearbox.

When installing a screw into the press, it may become necessary to use a long allthread rod (English threads). This rod is screwed into the threaded hole on the end of the screw. The screw is slid into the press far enough that the allthread rod goes through the hollow bore of the gearbox. A large washer is slid along the allthread rod to form a brace against the gearbox. Running a nut on the rod, against this large washer, will pull the screw into the gearbox.

Excessive rubbing between a screw and the screen can be corrected by grinding the high spots of interference off the edge of the screw. Prussian Blue, applied to either the screw or the screen, can be useful.


GEARBOX BASICS
Model CP-4 presses use Nord hollow-shaft gearboxes, with C-face mounted motors.

Gearboxes are rated, and sold, by their torque rating. The manufacturers generally offer their designs in progressively larger sized castings, or boxes. The larger the box, the larger the torque rating. Each box size will be available with different gear ratios. In order to keep the torque fairly constant, larger horsepower motors are used with the high speed boxes. Similarly, reduced horsepower motors must be used when a low output speed is selected.

Screw presses are designed around the size of the gearbox that is selected.

 

PRESS LUBRICATION
Lubrication is something we generally review with customer personnel during start-up. It is pretty straight forward:

CONE BUSHINGS: Once a shift
BEARINGS: Weekly
BUSHINGS: Weekly
SHAFT SEAL: Weekly
GEARBOX: Annually
AIR REGULATOR: Whenever empty (if a lubricator is being used)
MOTORS: Never

The most critical lubrication item has to do with the cone bushings. Before starting up a new press, the cone should be run in and out a few times to spread the grease around.

Lubrication of the cone bushings depends a lot on what is being pressed. With orange peel, there is enough press liquor juice acting as a lubricant that the bushings are lubricated only at the end of the processing season (to keep them from locking up on dried-out peel juice). The other extreme is at a paper mill where boiler fuel is made out of reject fiber. There is no free water left in the press cake, so everything is very hot and dry. We automatically supply a 900 psi ATS autolube on paper mill jobs. Another tough application is with vapor tight presses where there is apt to be solvent getting into the cone bushings. The solvent can dissolve and wash out the grease, so we like to see frequent lubrication of the cone bushings. (Food grade grease is used in this application since food ingredients are being produced by the press.)

If Vincent supplies an autolube for the cone bushings, it will be either battery powered or require a hard wired power supply. This should be actuated when the press is first placed in service. It should be set to give one small shot of grease every couple hours. We provide autolubes with large grease reservoirs, so they will go at least two weeks at the maximum greasing schedule. Once operations are stabilized, it may be practical to reset the timer dip switches so that it gets one shot every shift or once a day.

In any case, we tell the operators to run the cone open and closed once a shift because this will spread the grease. This is done with the press in operation.

If Vincent does not provide an autolube for a critical operation, we generally tell the operators to manually grease the cone bushings once a shift. We also tell them to run the cone in and out when they do the greasing, in order to spread the grease around.

The bearings and/or bushings holding the screw get greased on the customer's normal schedule for that type of bearing, maybe once a week, or once a day, or once a month. Whatever grease the customer normally uses will be fine.

The shaft seal housing may have a grease fitting. This grease is to prevent fiber from getting into the seal. The seal should be given a shot of grease whenever the screw support bearings or bushings are greased.

The gearbox oil should be changed once a year. Use mineral oil for a normal 1800/1500 rpm input. Use the same grade oil, but synthetic, for input speeds of 2,400 rpm or more.

Some Nord gearboxes have an autolube canister located in the motor adaptor portion of the gearbox. It is located under a cast iron cap. This autolube should be actuated when the press is placed in service. Nord recommends replacing this autolube once a year.

The air regulator used with the discharge cone air cylinders may have a lubricant jar. If so, Vincent includes a can of light oil along with the air regulator which comes with our screw presses. The jar should be filled when placing the press in service and when the jar is empty, about once a year. It takes very light (sewing machine) oil. The oil helps prevent corrosion inside the air cylinders. (Most air cylinder manufacturers no longer recommend the use of lubricators.)

We have never seen nor heard of anyone greasing the motor bearings.


REPLACEMENT PARTS
Most replacement parts are standard OEM components which are be purchased from their original manufacturer. The specification of these items (gearbox, seals, air cylinder, etc.) is included in the O&M Manual.

The most common wear parts in the Vincent CP-4 Press are the screen, the screw, the cone, and the outboard support bushing. Vincent stocks these. Screws are generally rebuilt at the Vincent factory. Be sure to specify the Serial Number of your press when ordering replacement parts or repairs. If work is required on the screw, it may be more convenient to obtain a loaner press and return your machine to the factory.


SAFETY
These Operating Hints have left unstated the obvious safety hazard: A screw press, like any screw conveyor, is totally unforgiving. If clothing or a limb gets caught in a rotating screw, the screw will not stop. Vincent Corporation has heard of only one injury of this nature with a screw press; do not let yourself become the second. The use of common sense is all that is required.


Robert B. Johnston, P.E.



 

 

AttachmentSize
HINTS-CP4.pdf1.01 MB

Operating Hints - KP Presses

(Scroll to bottom for pdf version)

August 16, 2014


RIGGING
Be sure to properly support the press when lifting it from the truck. Usually a sling positioned under the inlet hopper, on the side closest to the gearbox, finds the center of gravity. On big presses, do not lift just one end of the press, as it is possible for the frame to deflect, shifting the screw within the press. Interference between the screw and the screen can result.


INSTALLATION

Be careful to not to rack the press when tightening the mounting bolts. In the case of large presses, do not just bolt or weld the press down to a level foundation! Instead, first place the press where it is to be installed. Next, place shims between the press frame and the steelwork (or floor, concrete pedestal or foundation) to fill any gap where the press is to be anchored. Only after shimming (or grouting) should the press be pulled down tight. Doing otherwise can rack the frame of the press, and this can cause screw-to-screen interference.

A large press must be mounted solidly, preferably to a foundation or structural steel. If a press draws its full rated horsepower without the press being anchored to the floor, the frame of the press can twist.

For maintenance, the screw is removed through the cake discharge end of the press. Allow the space required.

To suit individual conditions, a hollow bore gearbox can be rotated 180o, even 90o either way. Consult the owner's manual or factory in these situations as it is likely that the oil level in the gearbox will have to be adjusted.

Material can be fed into the press many ways. You may need to allow for return feed of overflow material in the event that more is fed to the press than it can take.

Spill containment is a consideration which should be taken into account, because it may be possible for un-pressed material to purge from the cake discharge of a press.

We recommend that a manual disconnect, for killing power to the motor, be installed close to the press.


When material is piped to a press in a closed piping system, it is important to have a 2" vent line open to the atmosphere, along with an overflow return line. The vent is necessary to prevent a siphon effect which can induce a vacuum in the inlet hopper and reduce press capacity. The return line should empty above the surface of the return pit. The overflow should fill less than half of the cross section of the return line.

A variable frequency drive (VFD) programmed for either variable speed or auto-reversing may be required, depending on the application. If not, the use of a reversing starter switch is recommended; they cost only a little extra. Such a switch is handy for clearing a jammed press.

It order to provide space for a cake take-away conveyor, the press can be installed tilted with an elevated discharge.

Similarly, it is possible that, when operating intermittently with very wet material left in the press between runs, the liquid may loosen the cake plug at the discharge. This might cause the plug to blow out upon start-up. To minimize this condition, the press can be installed with the discharge tilted upwards.


INVERTER VFD & PLC CONTROL

It is always recommended that an inverter VFD be used to start, protect, and operate the screw press. With a VFD it is possible to establish the optimal combination of screw speed and discharge cone air pressure. The VFD also can be used to reverse the press in case of a jam or to slow it down during upset conditions.

Nine presses out of ten will operate unattended, indefinitely, and just fine at line frequency of 50 or 60 Hertz. If two screw presses are mounted in parallel, they are usually fed with a screw conveyor which drops to fill the first press, with the rest going to the second press; this is followed with a drop-out for overflow.

However, we need to address the exceptions:

Use of level controls is becoming more and more common. These are used to regulate either the flow going into the press or to regulate the screw speed.

In some applications a press is sized for handling upset conditions of large flow, while the normal flow is quite small. In these cases a level control is used, and the PLC can be programmed to turn off the press when a low level is reached in the inlet hopper, and the press re-started when a higher level is signaled.

In some cases the press will tend to jam, overload, and trip out on high amps. In this situation it may be necessary to program the controls so that the cone automatically goes open on high amps, re-closing at a lower set point. This arrangement requires a solenoid operated 4-way air valve, replacing the manual valve which is provided with the press.

In other cases of jamming a simpler arrangement is to install a Cone Timer. A timer is used to periodically open the cone. The closed period is determined by the amount of time required for press cake to accumulate in the press. The duration of the "cone open" period is long enough to dump most of the press cake that has formed. This type of operation may be used if the press periodically experiences jamming or overload due to fluctuations in the amount of material being fed into the press. Alternatively, it may be used with slippery or slimy press cake that cannot be dewatered to sufficient firmness to force the cone open. Cone Timer panels are available from Vincent at no charge.

Some applications require the use of a specially programmed variable frequency drive. In this case the VFD is not used to change the speed of the press, but, rather to set it for auto-reversing operation. By having the screw run backwards for three or four turns every few minutes, some difficult-to-dewater materials can be pressed much more effectively. This operation can help a great deal with material which tends to blind (cover over) the openings in the screen. Vincent has loaner VFD's if you want to give it a try. The technique works well on bar screens; care must be taken with perf screens so that the screw does not snag the screen during the reverse cycle.

Once through start-up, the cone is almost always permanently left in the closed position at whatever air pressure has been found to be effective. A plug of cake will be left around the cone whenever the press is turned off; this will clear on its own accord on restarting the press.

However, some materials may set up and become hard, or freeze, within the press when the press in turned off. This is especially true in the case of pressing wet coffee grounds or outdoor installations. For these applications it is advisable to open the cone for a period of one minute before turning off the press. This allows the press to partially empty itself, fluffing the material left in the press. Vincent can provide information for automating this procedure.


VACUUM EFFECT
In some applications, increased screw press capacity can be obtained if the area outside of the screen is under a vacuum. This can be achieved by mounting the press at a high elevation, with the press liquor drain line dropping below the surface of a drain tank or pit.

That is, the drain line from the press should go below the surface of the pit or pond into which it drains. If this line is relatively small in diameter and has a steady downward slope, a vacuum will be induced around the screen of the screw press. The mass and velocity of press liquor flowing through the drain line create this vacuum. To draw air bubbles downwards with the press liquor, the velocity of the fluid must be greater than five feet per second.

The cover over the screen of the press will have to be sealed, usually with Silicone.

The amount of vacuum is a function of the elevation between the press and the drain pond. For good results, the press should be mounted on a stand that is 20' tall or higher.


INSTRUMENTATION

The most useful instrument to have when testing a press is an ammeter. The load drawn by the drive motor of the press is indicative of how much work the press is doing. The higher the amps, the better the dewatering. Also, the higher the amps, the closer the press is to jamming, and the greater is the abrasive wear. Very low amps indicate little dewatering is being done; the screen is blinded; low compression is taking place; or the flow into the press has stopped.

A moisture balance is valuable for measuring the moisture content of the inbound material and of the press cake. If an oven is used to dry samples, be sure it is set at 160o F or less if there are sugars in the sample. Samples should be left in the oven overnight. The tare weight of the pan should be much less than the weight of the sample which is being dried.

As mentioned previously, level controls can be useful in operating a press. With a signal providing the depth of material in the inlet hopper, the speed of the press can be varied to match the flow going into the press. With egg shells, a simpler level control is used to only signal when a high level is reached; its signal will increase the screw speed. In special cases the press can be turned off when a low level is reached and re-started when a higher level is reached.

In the case of pressing liquids that contain dissolved sugars or salts, a refractometer is valuable for assessing press performance. The Brix of the inbound flow, the press cake, and the press liquor will all be the same figure. The higher the Brix, the higher will be the solids content of the press cake.

If dissolved (soluble) solids are present, the suspended (insoluble) solids (fiber) in the press liquor are generally measured by filtering and washing a sample and drying the filter paper in an oven. Dissolved solids will be washed from the sample during the washing process.


START-UP

Before putting power to the screw press, it is advisable to bump the motor or even rotate the screw by hand. This will prevent damage to the press in case tramp material has been left in the press. Also, the screw may have shifted so as to hit the screen. (Minor rubbing is normal; it will go away once there is material in the press.) To turn the screw by hand, remove the fan guard on the motor and turn the fan blades.

The screw of the press turns in a counterclockwise direction, when viewed from the drive end of the press.

If problems are encountered, they are apt to be blinding, jamming, channeling or purging. There is a section for each of these ahead in this manual.


FEEDING

Material can be fed into the press many ways. Commonly, screw conveyors, pumps, transition chutes, pre-thickener screens or cyclone separators are used. Consider making provision for return of overflow material, in the event that more is fed to the press than it can take. Spill containment should be considered.

Sometimes either a static (sidehill or parabolic) or rotary drum screen (RDS) must be mounted over the inlet hopper to prethicken the flow ahead of the press; the tailings (solids) from the screen can be funneled into the press. This arrangement is desirable when the feed to the press is dilute.

Also, material can be dropped from a shredder or cyclone separator into the press. A shredder is used to increase capacity and dewatering in the case of low bulk density materials like lettuce leaves, alfalfa, onion peel, and cornhusk, or to prevent blockage.

Most commonly, the best screw press performance is achieved if the material in the inlet hopper stays just at the top edge of the screw. Usually presses work best with only atmospheric pressure in the inlet hopper. In order to minimize static head, press headboxes are kept short, and level controls are used to minimize the depth.

When a pump is used to feed a press, the system can be either open or closed. We recommend the open system where little or no pressure exists in the inlet hopper, thus preventing the press from being force-fed. In this arrangement either there is an open return line allowing flow back to the source feeding the press, or level is controlled in the inlet hopper. It is best to have a line that allows material to recirculate past the press inlet. This will prevent pressurizing the inlet of the press, which can cause both blinding of the screen and purging from the cake discharge.

A port on the side of the inlet hopper is frequently provided on larger Vincent presses. It is used to view the level of material over the screw. It has a bolted cover because it is rarely used.

If a fluid is piped through a sealed cover which is bolted to the inlet hopper, force-feeding is possible. A by-pass tee should be provided so that the pressure in the inlet hopper is minimized. In addition, a 2" vent line, open to the atmosphere, must be provided to prevent siphoning material in the inlet hopper out through the recirculation line.

Inlet hopper pressure over one to four psi can force fibrous material against the screen so as to blind off the screen, resulting in unsatisfactory performance.

At pressures above 10 to 15 psi in the inlet hopper, it is possible to blow the "plug" of press cake that forms at the discharge of the press. Unscreened liquid will purge from the cake discharge. Exercise caution if either hot or hazardous material is being pumped into a press.

At inlet hopper pressures of 40 psi and above, the shaft seals will be blown out of their housing. At pressures around 60 psi the screen will start to separate from its support plates, resulting in bypassing of feed material directly into the press liquor flow.


BUILDING A PLUG
In order for the press to work, a plug of cake must form between the cake discharge spout and the pressure cone. The press will almost always do this on its own accord as material is fed into the press.

In the case of sloppy materials like manure and DAF sludge, it may be advisable to start off by first packing the discharge of the press with any available fibrous material.

Alternatively, the press can be turned on and the feed pump allowed to run just long enough to fill the feed line and the press. Then shut off the pump, leaving the press running, and wait until no more liquid drains from the screen of the press. Repeat this process until a plug of cake starts to open the cone.


PRE-THICKENING
Almost always, the thicker a flow going into a press, the better it will work.

If the flow into a screw press is too dilute, the high volume of liquid going through the press screen can cause either of two problems. The flow may either flush most of the solids through the screen, or it may plaster solids against the screen, thus blinding (covering over) the screen.

To prevent these things from occurring, it may be necessary to pre-thicken the flow ahead of the screw press. This is commonly done with a static screen (sidehill) or a rotary drum screen (RDS). In the case of very dilute feed to the press, a Vincent Fiber Filter can be used.


AIR CYLINDER REGULATOR
To regulate the air pressure of the discharge air cylinder, presses are supplied with an air pressure regulator along with a Parker four-way reversing valve. These should be installed near the cone end of the press. (Until recently FRL (Filter, Regulator, Lubricator) sets were provided to regulate air pressure. Most air cylinder manufacturers now recommend against the use of lubricators.)

The Parker valve allows manual selection of the shut, open, or "neutral" position. This valve connects air supply from the regulator to one end of the air cylinder, while simultaneously opening the other end to atmosphere. The vent line on the 4-way valve allows air to escape when pressure is switched from one end of the air cylinder to the other.

Continuous air flow from the Parker vent line indicates a leak inside the air cylinder, or possibly a faulty 4-way valve.

Once material is going through the press, set the 4-way valve so that the discharge cone goes shut in the "in" or closed position. Start with a low air pressure, working your way up until the desired performance is obtained.

The neutral position of the Parker valve is used only in testing. If left in the neutral position, the cone will not move unless it is pushed open by press cake. If, later, the flow of press cake is diminished, the cone will remain in the position to which it was pushed, and purging can occur.


FRL AIR REGULATOR WITH 4-WAY REVERSING VALVE


DISCHARGE CONE

The principal adjustment of the press is made with the discharge cone. The cone is the component at the cake discharge end of the press that acts as a door or stopper plug to restrict material from leaving the press. The more pressure exerted by the discharge cone, the drier the cake material will be leaving the press. Also, the motor amps can be expected to increase with added pressure, and throughput may decrease.

The discharge cone is moved in (actuated) either by an air cylinder or, rarely, by weights. Typical air cylinder pressures are in the range of 30 to 60 psi. Some materials will press only in a low range, say 10 to 20 psi. Other materials may press best with a pressure of 60 to 100 psi. Air consumption is minimal in all models, 1 to 2 cfm.

During initial, first-time, start up, presses with air cylinder actuators are generally started up with the discharge cone in the withdrawn position. This will avoid an unnecessary jam.

With the air cylinder models, the discharge cone mechanism can readily be positioned in the "open" (withdrawn or "out") position.

Note that with many materials it is necessary to start the press with the discharge cone in the closed position at low air pressure. Thin or soupy materials, like pumped manure or clarifier underflow, can tend to purge right through the press if the press is operated with the discharge cone open (in the withdrawn ("out") position).

However, with materials that are dry to begin with, such as sawdust or plastic wash tank sludge, it becomes more important to start with the discharge cone in the open position. This is because these materials may tend to jam or overload the press. Similarly, high freeness materials, from which the water falls away freely, will have a tendency to jam in a press. Be sure to start the press with the cone open, and gradually close it with low air pressure, when running such materials for the first time.

Once you are through the initial start up, it will be unlikely that your press should have the cone opened before starting. Most operators rarely open or shut the cone once it is set.

As the pressure on the discharge cone is increased, not only will the cake become drier, but the flow through the press may also be reduced. With very slippery or slimy feed material it may be possible to apply enough discharge cone pressure to stop the flow altogether.

High discharge cone pressures can result in increased quantities of suspended solids in the press liquor.

Care must be taken if a press is to be left running at a very low pressure like 10 psi. If some fiber enters between the cone bushing and the screw shaft, it will take more than that much pressure to close a cone which has been pushed open by a heavy flow of cake. The result will be either high moisture content in the cake or, worse, purging.

On models without air cylinders, weights used to actuate the discharge cone vary considerably. When dewatering food waste there may be a need to minimize the amount of solids being forced through the screen. At the other end of the scale, some KP-10’s used on dairy manure may use a 5' extension arm with a hundred pounds of weight.

With some feed materials, the press can be operated with the discharge cone in the withdrawn position. The screw alone may do enough compressing and dewatering to produce a cake at the discharge.

It is acceptable to open the discharge cone, in most cases, during normal operating conditions. This allows inspection, while in operation, of the discharge end of the screw and screen. This will give the operator a chance to observe operation with minimum dewatering and maximum throughput. It is also a good technique for purging bad material i.e., either jammed or spoiled material, from the press. (Do not try this trick if you are pressing hot or chemically aggressive materials.)

Where very low air pressure is required for proper operation, it may be practical to put the 4-way valve in a neutral position, half way between open and closed. A press can not be left permanently in this condition: keep in mind that a slug of cake will push the cone open, and it will not re-close on its own afterwards.

An unusual technique is to set the air pressure so that the cone normally stays completely shut. A timer is used to periodically open the cone. The closed period is determined by the amount to time required for press cake to accumulate in the press. This type of operation is used with slippery or slimy press cake that cannot be dewatered to sufficient firmness to force the cone open. The duration of the "cone open" period is long enough to dump the press cake that has been formed. Cone Timer panels are available from Vincent at no charge.

Once through start-up, the cone is almost always left in the closed position at whatever air pressure had been found to be effective. A plug of cake will be left around the cone whenever the press is turned off; this will clear on its own accord on restarting the press.

There are a few applications where the air cylinder Is removed and replaced with a jacking bolt. This is used if the cone pushes completely closed even with the lowest air pressure. It results in operating the press with a fixed discharge annulus.
Air cylinders with linear actuators are available.


ROTATING CONE OPTION

Most KP screw presses are offered with an optional feature which makes the cone rotate and strip away the press cake. The rotation is driven by two large pins mounted on the back of the cone. These pins engage with a collar which is clamped onto the screw shaft. Press cake is stripped away by studs on the face of the cone. These studs are in an axial position parallel to the screw shaft.

The rotating cone can serve multiple functions. By stripping the cake away it can prevent either jamming or purging. Its use generally results in wetter press cake and lower motor amps. Most commonly it is used when all the press cake tends to channel out past one side of the cone. It is invaluable in situations where press cake props open the cone and allows un-pressed material to purge.

The cake will tend to co-rotate with the screw when the cone is rotating. A spin-stop bar, welded to the inside-bottom of the cake discharge spout, prevents this.

Positioning the drive collar limits the maximum opening of the cone. If a large amount of cake comes form the press, the cone can run into the drive collar and for it to slip along the screw shaft.

If the cake comes out too wet, shorten the length of the studs on the face of the cone.

Disconnect the rotating cone by removing the drive pins and/or drive collar.

 
DRIVE COLLAR AND DRIVE PINS FOR KP STYLE ROTATING CONE

INTERMITTENT OPERATION
In the case of intermittent operation, it is recommended that the control panel for the feed pump or conveyor which feed the press should have a timer. This timer should be set to have the press run for two minutes after the feed pump (or conveyor) shuts off. This will partially clear the press so that it will not trip out on overload when it is re-started. (This applies in high torque applications or in installations where the material in the press dries out or freezes.)

An extreme case occurs when pressing spent coffee grounds and some paper mill fibers. Each time the press is turned off, the cone must first be opened for a minute. If this precaution is not taken, nasty damage to the press screw or screen can occur when the press is re-started.

(See the previous section, INVERTER VFD & PLC CONTROL.)

Minimize the time that the screw press is run with no material being fed into it. The last material admitted to the press will dry to powder, and it can cause severe accelerated abrasive wear.

Initially the press will likely be run empty in order to check rotation. Even though some rubbing may be heard, negligible wear will occur so long that this period is kept to a minimum. Also, since the screw is supported to some extent by the material inside the press, running dry may allow the screw to rub the screen.


DOUBLE PRESSING

Some processes benefit from what is called double pressing. This means that the cake coming from the press is run through the press a second time (or through a second press). If little moisture is removed in the second (double) pressing, then it is known that the liquid removed in the first pressing is all of the free liquid that there is to be pressed out.

Sometimes water is added to the cake in between the first pressing and second pressing. This is done to enhance the recovery of dissolved sugars in the original press cake.

Molasses can be added to press cake between the first and second pressing. This is used to infuse dissolved sugar into the cake, increasing the solids content of the final press cake.

Capital-effective double pressing can be achieved by using an inexpensive Soft Squeeze Series KP screw press for the first pressing, following with a tighter-pressing Series CP/VP in the second position.


MOISTURE CONTENT

A screw press separates free water. This will leave organic water in the press cake. The organic water is either bound to, or part of, the animal or vegetable molecules. Mechanical pressure alone will not remove organic water; it takes heat or chemistry. Frictional heat from the press can remove organic water, but this obviously should be avoided. For chemistry, see the Hydrated Lime, Gypsum and Alum section. For heat, see the Fluid Injection section.

To determine the moisture content of a material (feed to the press, press cake, or press liquor), a sample should be weighed and dried overnight at a temperature slightly less than 100o C. (If sugars are present use less than 70º C to prevent caramelizing.) The sample should weigh six or more times the tare weight of the sample tray or cup.

The moisture content of press cake varies considerably. Tomato press cake will be 90% moisture. Orange peel will be 80%, unless it is reacted with hydrated lime, in which case it will go down to 72% moisture; add molasses and it will go to 65%. Dairy and hog manure will come out at 70% moisture, unless there is sand or sawdust in the sample, which will reduce the moisture content. Cellulose fiber from a paper mill (knots, screen rejects, primary clarifier underflow) will come out about 50%. However, if secondary (biological) sludge is added, then the moisture content of the cake will go up considerably. With high ash content in paper mill samples, moisture may go down to 40%. Moisture contents of only 25% can be achieved pressing things like sand, eggshell, glass, and plastic chips.

The heat from steam injection can change the chemistry of the material being pressed so that cake with lower moisture content is produced. This blanching or parboiling effect works with fish and orange peel, for example.

A quick approximation of what to expect from a screw press is to squeeze as much water out with your fist, and figure that the press will do a little bit better. A better way is to twist a ball of the material in a cotton cloth.


COMPRESSION
A screw press achieves compression using several methods: (1) The discharge cone of the press causes back-pressure on the material being dewatered. The higher the cone pressure, the greater the liquid removal. (2) The pitch of the flights of the screw tightens as the material is conveyed through the press. This forces liquid to go through the screen. (3) The diameter of the shaft of the screw may be increased progressively, forcing material outward, against the screen. This is a tapered shaft design.

 


SCREW WITH TAPERED SHAFT


Force-feeding (supercharging) the press and applying a vacuum to the outside of the screen are two additional methods which may achieve compression. These two are used infrequently because the performance results are uncertain.


PRESS SPEED (RPM)

In general, the slower the screw speed, the greater the dewatering. Longer residence time in the screened area results from lower screw speed, which allows time for more thorough dewatering. Unfortunately, it also goes with reduced throughput capacity.

Screw press speed (rpm) can be changed by using a Variable Frequency Drive (VFD). Alternatively, the drive motor can be switched to a different pole motor (900, 1200, or 3600 versus the standard 1800 rpm). Most modern motors are good for permanent 120 Hertz operation; they are always good for a test at this high speed.

Higher speed can result in premature gearbox failure. Switching to synthetic oil, replacing the normal mineral oil, is recommended. Consult the factory for assistance.

A small change in screw speed, like 15%, will generally not result in a measurable change in performance of the press.

Low screw speeds are used for cooker crumb, potato peel, many sludges, and low freeness materials in general.

It has become normal for a variable speed drive (frequency inverter VFD) to be used with Vincent presses.

The smaller Nord gearboxes are all rated for 4,000 rpm input, which makes it easy to switch to a 3000/3600 rpm motor. It is best to switch to synthetic lubricant if this change is permanent. With other gearboxes, the higher speed can result in premature gearbox failure. Consult the factory for assistance.

A small change in screw speed, like 20%, will generally not result in a measurable change in performance of the press.

It has become normal for a variable speed drive (frequency inverter VFD) to be used with Vincent presses.

Low screw speeds are used for cooker crumb, potato peel, many sludges, and low freeness materials in general.


CAPACITY MEASUREMENT
The best way to measure capacity of a press is to collect timed samples of press cake and of press liquor. This should be done during a period of sustained, stable operation, rather than by timing a batch through the press.

Press cake is generally captured in a tarpaulin, and press liquor in a 5-gallon pail or 55-gallon drum. When the drain is at floor level, a 3-mil plastic bag can be used to catch press liquor. If the press liquor goes to a pit or tank, the change in depth can be timed.

Sometimes it is possible to collect only one flow, either press cake or press liquor. In these cases it is possible to estimate the press throughput if the solids content of the inbound material and press cake are measured. It is assumed that there are zero suspended solids in the press liquor, although this is never really the case.


A-B-C-D PLATES

There are four or five vertical plates making up the frame of the press, called out in the Nomenclature schematic at the end of this manual. Starting from the drive end of a press with a hollow bore gearbox, the first one is the Adapter Plate. The gearbox is bolted to the adapter plate. Through four spacers, the adapter plate is welded to the A Plate. This A Plate forms one wall of the inlet hopper. The shaft seal housing is bolted to the A Plate.

The next plate is the B Plate. It forms the downstream wall of the inlet hopper. The screen starts at the B Plate. There is a notch, called a Cord Cutter, in the B plate. Also, there may be a bar called Brian's Stripper welded to the B Plate, inside the inlet hopper; it is in a position to kiss the edge of the screw flight as it passes. These two features prevent long fiber pieces from balling up at the exit of the inlet hopper. See the section ahead on Cord Cutters.

The final plate, the C Plate, supports the discharge end of the screen. The cake discharge spout is bolted to the C Plate. The discharge cone touches the spout when the cone is in the closed position.

KP-24 and KP-30 presses have a fifth plate, called the D Plate, on which air cylinders are mounted.

 

SCREW LIFE
If a press loses its previous throughput capacity, or if cake moisture content increases, it is can be sign of a worn screw.

A screw can last anywhere from six months to twenty years. It depends on the material being pressed and how hard it is being pressed.

Premature screw failure can arise from several causes. The two main ones are:
(a) If a press is allowed to run continuously even when no material is being fed into it, the screw can wear out in one or two months. The same can happen if a very low flow is consistently fed into the press.

(b) If abrasive material is dewatered with high cone air pressure, rapid wear will occur. This condition is avoided if a few drops of water can always be squeezed from a fistful of press cake.

Two effective ways to extend screw life are:
(a) Various grades of hardsurfacing rod can be used to protect the flights of a screw. The best hardsurfacing will have Tungsten Carbide impregnated in it.
(b) Using a VFD or lower speed motor to reduce the screw rpm will extend screw life

If a worn screw is suspected, the thing to do is to shut down the press, open the cone, and dig out the cake until the tips of the last two flights can be seen or felt. Check how badly the tips are worn. If the there is 3/8” between the tips and the screen, wear is evident. It is also an indication that the sharp edges of the flights throughout the press may have worn, becoming rounded. This can cause the flights to act like a putty knife, plastering solids against the screen, preventing water from coming through.

Worn screws are either restored locally or returned to Vincent for rebuilding. The maximum cost of a screw rebuild is around one third the cost of a new screw.


SCREW CONFIGURATION
Almost all Vincent screw presses use the Interrupted Screw Flight design. The interruptions leave room for stationary resistor teeth that are mounted outside of the screen. These teeth go through the screen and reach almost to the shaft of the screw. This design of screw press stands in contrast to a Continuous Screw design. The main advantage of the interrupted design is that solids material must accumulate in the interruptions until sufficient consistency is reached for the solids to be pushed toward the cake discharge. There is a reduced tendency for the material being pressed to co-rotate with the screw. Also, there is more agitation within the press and, consequently, quicker and more thorough dewatering.

The screw starts with a feeder section of continuous flights. This picks up material in the inlet hopper and pushes it into the screen section. The feeder section ends at the first resistor tooth. This feeder section of the screw is followed by compression stages where the flights have reduced pitch. The reduction in pitch of the flights results in compression of the material going through the press.

A screw configuration referred to as Sterile Butterfly is common. There are a reduced number of flights on this screw, and the flights do not wrap as far around the shaft as is normal. This design screw is good for high throughput of materials which are easily dewatered. Glass and sand are examples.



INTERRUPTED FLIGHTS, RESISTOR BARS, and RESISTOR TEETH


PIE CUTTING

Sometimes the compression of a screw is reduced, in the field, in an operation called "pie cutting". This involves cutting pie-shaped segments from certain flights of the screw, leaving a butterfly (end view) configuration. The modification is done to avoid excessive compression and jamming. (The "sterile cut" is more drastic.) Consult the factory for assistance before making this modification.


 
4" PIE CUT                                                      PIE CUT


 
NOT PIE CUT BUTTERFLY CUT


JAMMING

Should a press trips out on overload because it has become jammed, a series of steps can be taken to un-jam the press. Generally, the easiest thing to do is to reverse the leads on the electric motor drive. This will cause the screw to feed material backward into the inlet hopper.

Generally jamming is caused by over-pressing excessively dry material. Running the press backwards will break up this material. If the jamming was caused by tramp material, hopefully this can be found and retrieved from the inlet hopper following operation in the reverse direction.

Having a reversing starter greatly facilitates this operation. These cost little more than a standard starter; they come with a forward-reverse switch. (Having a VFD with a reverse button can be even handier.)

If a press has had extensive use in an abrasive application, the outer diameter of the flights will be worn away at the discharge of the press. Radial wear of 1" to 4" in larger presses will lead to serious jamming and, possibly, a burst screen.

When a press is operated in the reverse direction it is possible that solid in the press will be forced against the A Plate. This can damage the shaft seal. For this reason, care should be taken when running the press backwards.

Usually three or four revolutions of the screw are sufficient to clear a press. If running the press backwards several cycles does not clear the jam, the screen should be removed so that the cause of the jam can be determined. Look for a bent flight. Before going to the trouble of removing the screen, shut down the press and try clearing the end of the press with a long screwdriver.

Sometimes when a press is jammed, a flight on the shaft of the screw will fold. This can happen if the press overloads on dry cake or if tramp metal is caught between a flight and a resistor tooth. The weld at the shaft may tear. When this happens flow through the press is greatly impeded.

 

FOLDED FLIGHTS


SCREEN BLINDING

A common problem is for the screen of the press to become blinded (covered over). When this occurs, the flow of press liquor coming through the screen diminishes. The level in the inlet hopper will fill up to where it overflows.

In some cases, the screen can be cleared by periodically reversing the direction of rotation of the screw. This can be programmed with many VFD’s, so that the press runs forward for a given period and then reverses direction briefly for three or four turns when the screen starts to blind. This is one of the easiest possible solutions to test. Sometimes it is the only one that is effective.

Many other methods are used to address blinding: (1) Adding notches to the screw, (2) Reducing or eliminating the pressure in the inlet hopper, (3) Adding press aid to the flow, (4) Changing to a different screen selection, (5) Reducing the screw-to-screen clearance, and/or (6) Employing a screen flush with caustic solution, acid, or high pressure spray.

If blinding occurs after an extended period of satisfactory operation, it is usually due to wear of the screw. Rounded edges of the flights will contribute to blinding.


CHANNELING
A condition somewhat similar to purging can occur with slimy materials, like concord grapes, pineapple pulp, or spent brewer’s grain. These may tend to channel or squirt out from one side of the cone. Two ways to eliminate channeling are to lower the air pressure on the discharge cone and to slow down the speed of the press. Channeling can also be reduced by adding press aid to the material being dewatered, or by reducing the inbound flow to the press.

The rotating cone option is very useful in breaking up channeling. The cone is caused to rotate so that the relative motion between it and the stationary screens breaks the channeling. A pin on the face of the cone will strip away the press cake, breaking up the channeling. If the cake comes out too wet, shorten the length of the pin.

To break up channeling, pieces called wing feeders can be welded to the end tips of the last two flights of the screw. See the Wing Feeder section of this manual.


PURGING

An undesirable condition can occur when the material being admitted to the press purges, without liquid-solid separation, from the cake discharge. This can occur especially if pressure exists in the inlet hopper.

Mechanically, purging occurs when a dry lump of press cake holds open the discharge cone. Un-pressed material will flow around this partial plug.

Purging is prevented with the rotating cone option. To use it, it is necessary for the cone drive to be engaged so that the cone spins with the screw. Pins on the face of the cone will strip away the press cake, preventing it from holding the cone open. If the cake comes out too wet, shorten the length of the pins.

Purging may occur when there is a much reduced, small flow of cake coming from the press. Usually this is a sign of blinded (covered over) screens. This can be caused by a worn screw. Liquid from the inlet hopper will wick into the press cake, making it soft enough to blow out. Sometimes this condition is avoided by mounting the press inclined at about 5o above horizontal; the simplest way to do this is to place a block under the cone end of the press.

A drop in operating amps can be an indicator that a purging condition has begun. An ammeter circuit can be installed to alarm or trip the system when a reduction in motor amps occurs. This is rarely done.

The fact that the screw shaft extends beyond the cake discharge results in the formation of a donut of cake, rather than a plug. Donuts break up easier than plugs.


BRIDGING
Sometimes bridging will occur at the inlet hopper, preventing material from flowing into the press. If an independent surge hopper is mounted over the inlet of the press, it should have at least one, preferably two or three, vertical walls. This will minimize bridging.

Bonding Teflon sheets to the inlet hopper of the press is a remedy that has been used to reduce bridging of bulky materials which allow free-draining of water.

A vibrator, mounted on the side of a feed hopper, may also alleviate bridging.

One way to overcome this is to direct a stream of water into the inlet hopper to break the bridge. The nature of the screw press is that essentially all of this added liquid will be removed in the pressing operation. (It may be convenient to pump a jet of the press liquor into the inlet hopper to break the bridging.) This is rarely done.


RESISTOR TEETH
The interrupted screw design press has stationary teeth that protrude into the flow of material as it passes through the press. These fit into the gaps of the screw where there is no flighting. They stop just short of the shaft of the screw.

Rarely, the resistor teeth are shortened, usually by half, to increase the capacity of the press. Removing the teeth altogether will result in co-rotation and jamming.

Not infrequently the resistor teeth are drilled so that fluid can be injected into the press during operation. See the next section.


KP RESISTOR BAR WITH THREE TEETH


FLUID INJECTION

Resistor teeth can be drilled to permit injection of steam, solvent or water while the press is in operation. Also, these modified resistor teeth can be used for CIP cleaning, without the need of removing the screen from the press.

Commonly alcohol injection is used to achieve in-line washing to remove sugars. Hot water injection is used to recover dissolved solids in juice production. Steam injection is used in dewatering raw organic materials.

The moisture reduction that results from steam injection is related to a chemical change that comes with blanching, or parboiling, a material. Steam injection works well on pineapple skin, citrus waste, and raw fish. Tests run with steam injection in a Vincent press at Anheuser-Busch showed little benefit. The material being pressed, spent grain, had already been "cooked" before steam was added.

Injection is achieved by drilling holes through the resistor teeth and piping these holes to a manifold outside of the screen. Photos and drawings are available from the factory. Vincent does not charge for providing a drilled resistor bar.

 
STEAM INJECTION                                           LAB PRESS

IN TWIN SCREW PRESS                                   STEAM INJECTION


CORD CUTTER AND STRIPPER

Sometimes long stringy material will be pinched where the feeder portion of the screw goes through the hole in the B Plate. This material will co-rotate with the screw and build into a bundle which reduces the flow through the press.

A groove, like a keyway that is 3/8" deep, is cut half way through the hole in the B plate. We call this a Cord Cutter. Material trapped between the flight and the hole in the B plate will pop up slightly as it passes the Cord Cutter. The result is that the material is sheared loose.

A part called Brian's Stripper may be welded to the B Plate, inside the inlet hopper. It goes in a position so that the flight lightly kisses the stripper as it goes past. This strips the material away. Strippers are made of square bar.


STRIPPER AND CORD CUTTERS

POLYMER
In rare applications, the addition of polymer is indispensable in achieving adequate screw press performance. Polymers are added to dilute waste streams, especially to those containing very small size suspended solids. The long chain molecules of the polymer will flocculate the solids, agglomerating them to the point where they can be pressed. Under the right conditions, dramatic improvement can be achieved in press throughput, press cake moisture, and press liquor clarity.

Nalco and GE are the leading polymer suppliers. Their sales engineers are anxious to recommend the product best suited for your application.

Low speed operation of the press is usually required in order to achieve good performance.


NOTCHES

Sometimes it is necessary, during press operation, to have the screw wipe the screens clear of blinding material. This is best achieved by having notches in the outer edge of the screw. Fibrous material accumulates in the notches and brushes away slimy material which may be blinding the screens. Shallow notches (1/8" wide by 1/8" deep, 1-1/2" apart) in the outer edge of the screw flights work well. Typically, notching is done from the B plate to the second resistor tooth. Most Vincent presses are supplied with notches.


GRINDING NOTCHES IN THE FLIGHTS


WIPERS

Before the advent of notches, wipers, made of UHMW strips or nylon brushes, were (rarely) mounted to the outer perimeter of the screw flights. If at all possible we advise against the use of such wipers because (a) they tend to cause material to co-rotate and jam in the press, (b) they are difficult to replace, and (c) they wear rapidly, resulting in unacceptably frequent shutdowns for replacement. Wipers tend to improve dewatering performance for the first week or so. After that the wiper material wears and press performance reverts to being just a little better than if no wipers were used. Wipers are made either of 1/4" thick high durometer polyurethane or of nylon bristle brush. Wipers are preferably mounted on the downstream edge of the flight.


PRESS AID

Some materials press best if a press aid is mixed into the material to be pressed. Typical press aids are rice hulls, cottonseed hulls, cellulose fiber from a paper mill, and ground newspaper. Ground wood is the best, but most expensive, press aid.

Press aids are most commonly used in producing juice from deciduous fruit. The press aid gives the press something to get a bite on. Press aids also tend to hold back fines (short fibers) and prevent them from going through the screen with the press liquor. If apples are fed into a press, apple sauce will come through the screen. However, if a press aid is added to the apples, then apple juice will come through the screen.

Typically, the amount of press aid used is only 1% to 3% by weight of the flow going through the press. This will look like more than such a small percentage because press aids have a much lower bulk density than the wet materials that are pressed.


HYDRATED LIME, GYPSUM, AND ALUM
Lime (calcium hydroxide) must be added to citrus peel before it can be pressed. The lime breaks down the pectin or cell walls so that the press can remove moisture. Less than 1% by weight is used. A reaction time of several minutes must be allowed prior to pressing. Lime has been used successfully in the same manner with potato, onion, tomato, and pineapple waste. It works well on acidic materials such as strawberries and coffee bean pulp. Vincent offers lime dosing equipment.

Gypsum and alum salts are also effective chemical press aids. They are typically used in dewatering sugar beet pulp, and they have rarely been found effective on other materials.


PRESS LIQUOR
A screw press produces relatively "dirty" press liquor as compared to a Filter Press or Belt Press. Suspended solids will pass through the screen of the screw press along with the liquid being expressed from the inbound material.

If suspended solids need to be removed from the press liquor, the most common method is to pump the press liquor either over a static (sidehill) screen or through a rotary drum screen. Generally, the screen tailings (sludge solids) are fed back into the screw press along with the flow of inbound material. Most of these fines will be captured with the solids of the inbound material and end up in the press cake. Although some of these tailings will once again go through the press screen with the press liquor, equilibrium of recirculating solids is reached and satisfactory pressing operation is realized.

If the press liquor is to be concentrated in an evaporator, better screening than can be achieved with a static screen may be required. Another Vincent machine, the Fiber Filter, provides premium performance. Decanters or centrifuges may be required.


SCREEN SELECTION

The screen of the press is made either of wedgewire (slotted screen) or perforated stainless sheet (round holes).

Screens made of wedgewire come standard with 0.015" to 0.020" slot width; they are also available with slots that are 0.008" to 0.060" wide. With slot widths less than 0.012" there is a tendency for the screen to blind (be covered over) with the material being pressed. However they work well in alcohol and oil separation. Changing the slot width generally has little impact on the clarity of the press liquor or the dewatering capacity of the press.

Perforated metal screens are usually a simple sleeve which is held in the screen assembly. These are less durable but inexpensive to replace.

In contrast, wedgewire screens have slots rather than holes. They are expensive long-life weldments that must be replaced when excessive damage or wear has occurred.

The most common damage to a wedgewire screen is for part of the surface to be smeared over by rubbing the screw. This rarely is bad enough to affect press performance. Profile bar screens generally work satisfactorily with 30% or even more of their surface smeared over.

Smeared screens can be remedied by running a box cutter blade through the slots.

Wedgewire screens in Vincent presses can be reversed in order to achieve double life. That is, wear starts at the cake discharge end of the press. When this occurs, the screen can be turned 180o so that the fresh inlet section is then located in the discharge area.

TIG welding is used to close the gaps in the case where either a few wedgewire slots get spread apart by tramp material or a perforated screen gets torn.

In cases of severe wear or damage, it is common to patch a screen. Stainless sheetmetal is used for this. The reduction in drainage surface is of little consequence as the screens have ample open area.

Standard perforated screens have a hole size of 3/32" diameter, although material with 0.060", 0.040", down to 0.023" holes can be supplied. Standard wedgewire screens have slots that are 0.015" to 0.020" wide (except in cooker crumb and alcohol, where 0.008" to 0.010" are used). Surprisingly, usually there is little difference in the degree of filtration achieved by either changing hole size or going to a slotted profile bar screen.

Frequently, increased press capacity can be achieved by changing a perforated screen to one with smaller holes. This unexpected result arises from a combination of factors: (1) smaller hole screens are made of thinner sheetmetal, so that the press liquor has a shorter distance to travel before it falls free from the screen, reducing the chance of sponging backwards through the screen and (2) particles which fall into and plug a larger hole will roll over a smaller hole. Minor rubbing between the screw and screen is normal, although, obviously, hard rubbing will cause wear and premature failure of the screen. With a clearance greater than 3/16", the dewatering performance of the press can start to deteriorate; this depends a lot on the nature of the material being dewatered.

The most common cause of screen failure ties to failure of the outboard support bushing. If the bushing holding the end of the screw wears out, it can let the screw move enough to rub against the screen.


 
PERFORATED SCREEN WITH REINFORING SHEET


 
PROFILE BAR (WEDGEWIRE) SCREEN


SCREW-TO-SCREEN CLEARANCE

Generally the clearance between the screw and the screen is 1/32", plus or minus 1/32". The screw should not rub the screen hard, as it can cause wear and premature failure of the screen. Tighter clearance is used with materials that blind the screen, such as onion skins. Greater clearance, 1/16", is used with eggshells, pectin, xanthan gum, and corn husk. With a clearance greater than 3/16", the dewatering performance of the press can start to deteriorate; this depends a lot on the type of material being dewatered.

On KP-24 and KP-30 presses, the screw-to-screen clearance is larger. It can be checked by removing one half of the screen and bolting the other half tight to the resistor bars. Inspection is made from the side from which the screen half has been removed.

To measure the s2s clearance in wedgewire screens, a feeler gauge can be slipped through and along a slot until it hits the edge of the screw. Measure from the outside of the screen in to the edge of the flight; then subtract the thickness of the wedgewire (generally either 0.250" or 0.375") from the measured depth in order to calculate the clearance.

In the case of perforated screen installations, a depth gauge can be used to measure the screw-to-screen clearance. This is done by first finding an area where the screw flight is next to the screen; poking a straightened paper clip through the screen is handy for this purpose. The depth from the outside of the screen to the edge of the flight is measured, and then the thickness of the screen is subtracted from that measurement. [3/32" perf is 0.075" thick; 0.050" perf is 0.050 thick; 1/32" perf is 0.024" thick; 0.023" perf is 0.015" thick; 3/8" perf back up screen is 0.120" thick.]

If a screw rubs against the screen in a given area, it may be best to grind some off the OD of the screw. Prussian Blue can be useful in finding the spot that is rubbing.

 

WEDGEWIRE DEPTH GAGE PERFORATED DEPTH GAGE


DISCHARGE SPOUT

The press cake emerges from the end of the screen through a discharge spout. If a press tends to jam, it may be necessary to shorten this spout. Conversely, the spout can be lengthened if the cake does not come out dry enough.

There is usually a bar, called a Spin Stop, welded inside this spout. This will prevent cake from co-rotating with the screw. There is a tendency for this to happen when the rotating cone feature is in service. In other cases, performance with materials like corn silage is improved by removing this spin-stop.


SPIN STOP IN SPOUT


CONE BUSHING

The cone rides on the shaft of the screw. "Cone Sleeve" is the name given to the portion of the screw on which the cone rides. There may be a bushing in the cone to support and guide it, and to protect the Cone Sleeve journal surface of the screw shaft. Sometimes the bushing is lubricated with liquid from the material being pressed, such as the juice from apples or water from pectin peel.

Sometimes there is a grease fitting is provided for lubricating the bushing or to minimize leakage of press liquor through the cone bushing.

Bushing lubrication is extremely important when materials that are dry (like paper mill screen rejects) are being pressed. By the time such materials reach the discharge of the press, they do not have enough free moisture left in them to adequately lubricate the cone bushings. In these applications the operator should, at the start of each shift, pump grease in until it comes out between the cone bushing and the screw shaft. Then he should open and shut the cone three times in order to spread the grease.

Rarely, presses are supplied with additional lubrication fittings so that water, in addition to grease, can be metered to the bushings as a lubricant.

Automatic grease systems are available. These should be the high pressure (900 psi) electric or battery variety. Vincent provides these for critical applications, especially pulp & paper.

Liquid leaking past the cone bushings drains out the back of the cone (at the air cylinder end of the press). Almost always it is minimal compared to the flow of press cake. However, a pan can be provided to collect this liquid and drain it into the main flow of press liquor.


WING FEEDERS
Sometimes there are blades welded to the outside tips of the last two flights of the screw. Called "Wing Feeders", these are mounted parallel to the discharge screen surface. Care must be taken that wing feeders are not made so long that they hit the face of the cone when the cone is in the closed position.

Wing feeders can serve two purposes. (1) In the case of materials that want to channel out the discharge of the press, like pineapple and spent brewer's grains, long wing feeders break up the channeling flow. (2) For abrasive applications, short knobby wing feeders are provided as sacrificial wear elements.

When certain materials are fed through a screw press, clumps of dry material may pack in front of the wing feeders. This buildup can cause wear of the screen. Should the problem occur, grind off the wing feeders.


 
LONG WING FEEDER KNOBBY WING FEEDER

CLEANING
Commonly, material is cleared from a press by stopping the inbound flow, setting the discharge cone in the withdrawn position, and running the press for a few minutes until no further material is discharged. This will leave some material inside the press, which can be handy for forming a plug at the cake discharge when the press is restarted.

Material will leave a Vincent interrupted flight press only if there is additional inbound material forcing it out. This makes it difficult to clear material from inside a press without removing the screen. One technique used successfully is to feed crushed ice into the press. Water must be fed along with the ice to prevent jamming. When the ice melts, the press will be relatively clean inside.

There are applications in which the press must be cleaned frequently, such as once a shift, in order to meet sanitary regulations. In these cases, the screen can be removed from the press in order to remove residual material. A spare screen assembly may be kept, submerged in cleaning solution, in order to minimize the downtime required.

Once the screen is removed, the screw and screen are scrubbed with caustic solution.
In one case with a KP-16 press, the liquid discharge drain is blocked shut so that caustic solution can be allowed to fill the collection pan. This cleaning is performed without removing the screen from the press.

Cleaning the inside of the screen can be achieved, at least to some extent, by injecting water through the resistor teeth. Holes must be drilled in the resistor teeth to make this possible.

In laboratory applications, the press may be disassembled for cleaning. The various components can be dipped in an appropriate solution or placed in an autoclave.

It is unusual that the outside of the screen ever needs to be cleaned. Spray systems for this can be built into the press at the Vincent factory. Alternatively, a pressure washer or swabbing with acid solution can be used.


SCREEN REMOVAL AND REPLACEMENT

Wedgewire screens in Vincent presses can be reversed in order to achieve double life. That is, wear starts at the cake discharge end of the press. When this occurs, the screen can be turned 180o so that the fresh inlet section is then in the discharge area.

In the case of screen failure, frequently a solid patch can be welded onto the screen, from the outside. This is simple as the screen need not be removed from the press.

Wedgewire screens may become smeared from being wiped by the screw or by hard press cake. Wedgewire screens generally work satisfactorily with 30% or even more of their surface smeared over. Usually press liquor will come through a smeared area of a wedgewire screen. If it becomes an issue, it is corrected by running a box cutter blade through the slots.

The screen of a Series KP 6" through 16" press is removed from the cake discharge end of the press. To remove the screen, follow these steps:
• Remove the outboard bushing tailstock assembly.
• Remove the discharge cone and its actuator mechanism. (Be careful not to pinch a finger when removing the lever arm mechanism.)
• Remove the resistor tooth bar.
• It may help to loosen the bolts holding the clamshell closed.
• Remove the bolts holding the cake spout assembly to the C Plate, and remove the spout.
• Pry and slide out the Screen Assembly.
• In the case of a perforated screen, pry the screen sleeve from its assembly. To do this, it is likely that wedges will have to be used to open the clamshell.

When tightening a new perforated screen sleeve into a clamshell fixture, be sure not to tighten beyond the point where the sleeve starts to buckle or bend inward. You want it tight as possible, but not tight enough to beer-can the screen. When changing to different gauge thickness of screens, shims may have to be added or removed from the clamshell joint.

Replacement thin gage screens are provided 1/4" long. This allows 1/8" at either end to be peened over. The resulting 1/8" flange both helps hold the screen in place and prevents the screen from being snagged by the screw.

Excessive rubbing between a screw and the screen can be corrected by grinding the interference off the edge of the screw. Prussian Blue, applied to either the screw or the screen, can be useful.


SHAFT SEAL
The Seal Plate is bolted to the A Plate. This housing may be solid UHMW (ultra high molecular weight polypropylene or polyethylene) or it may contain one or two Johns Manville (JM Clipper) lip shaft seals. There may be a grease fitting on this plate; the grease is used to reduce leakage and to help prevent fiber material from entering and damaging the screw shaft.

Generally, seals are allowed to drip once they start leaking. They are replaced only in conjunction with major maintenance, as when the screw is removed from the press.

In some cases we have found that leakage from a shaft seal can be stopped by simply selectively loosening or tightening the four bolts holding the seal housing to the A plate.

 
SEAL PLATE SPLIT SEAL PLATE


SCREW REMOVAL

The screw is removed through the hole in the C Plate, at the cake discharge end of the press. The operation can be difficult the first time, so we recommend consulting with the factory before getting started.

First of all, remove the four bolts holding the shaft seal housing to the A Plate. This will prevent damage when the screw drops out of the gearbox.

Next, remove the screen of the press, following the instructions in the Screen Removal section.


MODELS KP-6 through KP-16

6" through 16" Series KP screw presses use a key and keyway to couple the screw inside the hollow bore of the gearbox. It may be that a great deal of force will be required to push the screw from the gearbox.

The basic procedure is to push or pull the screw out of the gearbox. If the screw is not tight, the screw is removed easiest by pulling, or by prying with a pry bar.

If it is tight, the screw is removed using a heavy fine-threaded jacking rod to push the screw out of the gearbox. This rod is threaded through a heavy nut Vincent has provided inside the hollow bore of the larger size gearboxes. This nut has a lug that fits in the keyway, to prevent the nut from turning. The nut backs up against an internal snap ring in the hollow bore of the gearbox. The result is that the rod pushes against the end of the screw. Liberal use of Anti Seize or Never-Seez is absolutely required to protect the threads. Loaner tool kits are available in Tampa; see Screw Change Kit Drawing B-94693.

Alternatively, the screw can be removed by constructing a gear puller, allowing the use of a hydraulic jack, combined with a cross bar and two long shafts threaded into the gearbox housing. The holes in the gearbox have metric threads. To avoid the need for metric all-thread rod, a length of all-thread rod with English threads is welded to the head of a metric bolt.

If the press has been badly jammed, the key may have rolled inside its pocket. In these cases it can be impossible to press the screw from the gearbox. It then becomes necessary to cut the screw in half, between the gearbox mounting plate and the A plate. The stub shaft can then be bored out in a large boring mill, and the screw can be repaired at the Vincent factory.


JACKING ROD

 


MODELS KP-24 and KP-30

Model KP-24 and KP-30 presses use a rigid coupling between the gearbox and the screw. The bolts of these coupling have an extremely high torque requirement.

The bars with the resistor teeth must be removed before the screw can be removed.

The flange bearing is mounted on a circular plate which also holds the air cylinders. Do not remove the flange bearing by itself. Instead, leave it on the round plate and remove the air cylinders and bearing as a single unit.

SCREW REPLACEMENT
When reinstalling a screw, the screw must be pulled in until the step in the shaft seats against the thrust bearing of the gearbox. This will position the flights of the screw so as not to hit the resistor teeth. Be careful when guiding this step in the shaft through the shaft seals.

During re-assembly of a keyed shaft, be sure to apply Never-Seez or Anti Seize to the portion of the screw shaft that goes into the gearbox.

When pulling a screw into the press it may become necessary to use a long allthread rod (English threads). This rod is screwed into the threaded hole on the end of the screw. The screw is slid into the press far enough that the allthread rod goes through the hollow bore of the gearbox. A large washer is slid along the allthread rod to form a brace against the gearbox. Running a nut on the rod, against this large washer, will pull the screw into the gearbox.

There are two ways to avoid excessive rubbing between a new screw and the screen: Either a shim can be placed in the clamshell opening at the resistor bar, or the interference can be ground off the edge of the screw. To eliminate the high spots, coat the edge of the screw with Prussian Blue, tighten the assembly, turn the screw, remove the screen, and then grind the screw where interference has occurred.


GEARBOX BASICS
The smaller Series KP presses use Nord hollow bore gearboxes, with a C-face mounted motor. In order to facilitate maintenance, the largest KP presses use foot-mounted gearboxes, with a male output shaft that is coupled to the shaft of the screw.

Gearboxes are rated, and sold, by their torque rating. The manufacturers generally offer their designs in progressively larger sized castings, or boxes. The larger the box, the larger the torque rating. Each box size will be available with different gear ratios. In order to keep the torque fairly constant, larger horsepower motors are used with the high speed boxes. Similarly, reduced horsepower motors must be used when a low output speed is selected.

Screw presses are designed around the size of the gearbox that is selected.


PRESS LUBRICATION
Lubrication is something we generally review with customer personnel during start-up. It is pretty straight forward:

CONE BUSHINGS: Once a shift
BEARINGS: Weekly
BUSHINGS: Weekly
SHAFT SEAL: Weekly
GEARBOX: Annually
AIR REGULATOR: Whenever empty (if a lubricator is being used)
MOTORS: Never

The most critical lubrication item has to do with the cone bushings. Before starting up a new press, the cone should be run in and out a few times to spread the grease around.

Lubrication of the cone bushings depends a lot on what is being pressed. With orange peel, there is enough press liquor juice acting as a lubricant that the bushings are lubricated only at the end of the processing season (to keep them from locking up on dried-out peel juice). The other extreme is at a paper mill where boiler fuel is made out of reject fiber. There is no free water left in the press cake, so everything is very hot and dry. We automatically supply a 900 psi ATS autolube on paper mill jobs. Another tough application is with vapor tight presses where there is apt to be solvent getting into the cone bushings. The solvent can dissolve and wash out the grease, so we like to see frequent lubrication of the cone bushings. (Food grade grease is used in this application since food ingredients are being produced by the press.)

If Vincent supplies an autolube for the cone bushings, it will be either battery powered or require a hard wired power supply. This should be actuated when the press is first placed in service. It should be set to give one small shot of grease every couple hours. We provide autolubes with large grease reservoirs, so they will go at least two weeks at the maximum greasing schedule. Once operations are stabilized, it may be practical to reset the timer dip switches so that it gets one shot every shift or once a day.

In any case, we tell the operators to run the cone open and closed once a shift because this will spread the grease around. This is done with the press in operation.

If Vincent does not provide an autolube for a critical operation, we generally tell the operators to manually grease the cone bushings once a shift. We also tell them to run the cone in and out when they do the greasing, in order to spread the grease.

The bearings and/or bushings holding the screw get greased on the customer's normal schedule for that type of bearing, maybe once a week, or once a day, or once a month. Whatever grease the customer normally uses will be fine.

The shaft seal housing may have a grease fitting. This grease is to prevent fiber from getting into the seal. The seal should be given a shot of grease whenever the screw support bearings or bushings are greased.

The gearbox oil should be changed once a year. Use mineral oil for a normal 1800/1500 rpm input. Use the same grade oil, but synthetic, for input speeds of 2,400 rpm or more.

Some Nord gearboxes have an autolube canister located in the motor adaptor portion of the gearbox. It is located under a cast iron cap. This autolube should be actuated when the press is placed in service. Nord recommends replacing this autolube once a year.

The air regulator used with the discharge cone air cylinder may have a lubricant jar. If so, Vincent includes a can of light oil along with the air regulator which comes with our screw presses. The jar should be filled when placing the press in service and when the jar is empty, about once a year. It takes very light (sewing machine) oil. The oil helps prevent corrosion inside the air cylinder. (Most air cylinder manufacturers no longer recommend the use of lubricators.)

We have never seen nor heard of anyone greasing the motor bearings.


REPLACEMENT PARTS
Most replacement parts are standard OEM components which are purchased from the original manufacturer. The specification of these items (gearbox, seals, air cylinder, etc.) is included in the O&M Manual.

The most common wear parts in the Vincent KP Press are the screw, the screen, the cone, and the outboard bushing at the discharge. Worn screws may be returned for rebuilding at the Vincent factory. Be sure to specify the Serial Number of your press when ordering replacement parts or repairs.


SAFETY
These Operating Hints have left unstated the obvious safety hazard: A screw press, like any screw conveyor, is totally unforgiving. If clothing or a limb gets caught in a rotating screw, the screw will not stop. Vincent Corporation has heard of only one injury of this nature with a screw press; do not let yourself become the second.

With the smaller KP Presses, special care must be taken when removing the lever arm mechanism that actuates the discharge cone. The arms are heavy, clumsy, and they can swing with a scissors-like action. Watch your fingers, wear gloves. The use of common sense is all that is required.


Robert B. Johnston, P.E.

 

 

 


 

AttachmentSize
HINTS-KP.pdf1.08 MB

Operating Hints - CP Presses

(Scroll to bottom for pdf version)

August 16, 2014

RIGGING
Be sure to properly support the press when lifting it from the truck. Usually a sling positioned under the inlet hopper, on the side closest to the gearbox, finds the center of gravity. On the 12" press, do not lift just one end of the press, as it is possible for the frame to deflect, shifting the screw within the press. Interference between the screw and the screen can result.


INSTALLATION
Be careful to not to rack the press when tightening the mounting bolts. In the case of large presses, do not just bolt or weld the press down to a level foundation! Instead, first place the press where it is to be installed. Next, place shims between the press frame and the steelwork (or floor, concrete pedestal or foundation) to fill any gap where the press is to be anchored. Only after shimming (or grouting) should the press be pulled down tight. Doing otherwise can rack the frame of the press, and this can cause screw-to-screen interference.

A large press must be mounted solidly, preferably to a foundation or structural steel. If a press draws its full rated horsepower without the press being anchored to the floor, the frame of the press can twist.

For maintenance, the screw is removed through the cake discharge end of the press. Allow the space required.

To suit individual conditions, a hollow bore gearbox can be rotated 180o, even 90o either way. Consult the owner's manual or factory in these situations as it is likely that the oil level in the gearbox will have to be adjusted.

Material can be fed into the press many ways. You may need to allow for return feed of overflow material in the event that more is fed to the press than it can take.

Spill containment is a consideration which should be taken into account, because it may be possible for un-pressed material to purge from the cake discharge of a press.

We recommend that a manual disconnect, for killing power to the motor, be installed close to the press.

When material is piped to a press in a closed piping system, it is important to have a 2" vent line open to the atmosphere, along with an overflow return line. The vent is necessary to prevent a siphon effect which can induce a vacuum in the inlet hopper and reduce press capacity. The return line should empty above the surface of the return pit. The overflow should fill less than half of the cross section of the return line.

A variable frequency drive (VFD) programmed for either variable speed or auto-reversing may be required, depending on the application. If not, the use of a reversing starter switch is recommended; they cost only a little extra. Such a switch is handy for clearing a jammed press.

It order to provide space for a cake take-away conveyor, the press can be installed tilted with an elevated discharge.

Similarly, it is possible that, when operating intermittently with very wet material left in the press between runs, the liquid may loosen the cake plug at the discharge. This might cause the plug to blow out upon start-up. To minimize this condition, the press can be installed with the discharge tilted upwards.


INVERTER VFD & PLC CONTROL
It is always recommended that an inverter VFD be used to start, protect, and operate the screw press. With a VFD it is possible to establish the optimal combination of screw speed and discharge cone air pressure. The VFD also can be used to reverse the press in case of a jam or to slow it down during upset conditions.

Nine presses out of ten will operate unattended, indefinitely, and just fine at line frequency of 50 or 60 Hertz. If two screw presses are mounted in parallel, they are usually fed with a screw conveyor which drops to fill the first press, with the rest going to the second press; this is followed with a drop-out for overflow.

However, we need to address the exceptions:

Use of level controls is becoming more and more common. These are used to regulate either the flow going into the press or to regulate the screw speed.

In some applications a press is sized for handling upset conditions of large flow, while the normal flow is quite small. In these cases a level control is used, and the PLC can be programmed to turn off the press when a low level is reached in the inlet hopper, and the press re-started when a higher level is signaled.

In some cases the press will tend to jam, overload, and trip out on high amps. In this situation it may be necessary to program the controls so that the cone automatically goes open on high amps, re-closing at a lower set point. This arrangement requires a solenoid operated 4-way air valve, replacing the manual valve which is provided with the press.

In other cases of jamming, a simpler arrangement is to install a Cone Timer. A timer is used to periodically open the cone. The closed period is determined by the amount of time required for press cake to accumulate in the press. The duration of the "cone open" period is long enough to dump most of the press cake that has formed. This type of operation may be used if the press periodically experiences jamming or overload due to fluctuations in the amount of material being fed into the press. Alternatively, it may be used with slippery or slimy press cake that cannot be dewatered to sufficient firmness to force the cone open. Cone Timer panels are available from Vincent at no charge.

Some applications require the use of a specially programmed variable frequency drive. In this case the VFD is not used to change the speed of the press, but, rather to set it for auto-reversing operation. By having the screw run backwards for three or four turns every few minutes, some difficult-to-dewater materials can be pressed much more effectively. This operation can help a great deal with material which tends to blind (cover over) the openings in the screen. Vincent has loaner VFD's if you want to give it a try. The technique works well on bar screens; care must be taken with perf screens so that the screw does not snag the screen during the reverse cycle.

Once through start-up, the cone is almost always permanently left in the closed position at whatever air pressure has been found to be effective. A plug of cake will be left around the cone whenever the press is turned off; this will clear on its own accord on restarting the press.

However, some materials may set up and become hard, or freeze, within the press when the press in turned off. This is especially true in the case of pressing wet coffee grounds or outdoor installations. For these applications it is advisable to open the cone for a period of one minute before turning off the press. This allows the press to partially empty itself, fluffing the material left in the press. Vincent can provide information for automating this procedure.

 

INSTRUMENTATION
The most useful instrument to have when testing a press is an ammeter. The load drawn by the drive motor of the press is indicative of how much work the press is doing. The higher the amps, the better the dewatering. Also, the higher the amps, the closer the press is to jamming, and the greater is the abrasive wear. Very low amps indicate little dewatering is being done; the screen is blinded; low compression is taking place; or the flow into the press has stopped.

A moisture balance is valuable for measuring the moisture content of the inbound material and of the press cake. If an oven is used to dry samples, be sure it is set at 160o F or less if there are sugars in the sample. Samples should be left in the oven overnight. The tare weight of the pan should be much less than the weight of the sample which is being dried.

As mentioned previously, level controls can be useful in operating a press. With a signal providing the depth of material in the inlet hopper, the speed of the press can be varied to match the flow going into the press. With egg shells, a simpler level control is used to only signal when a high level is reached; its signal will increase the screw speed. In special cases the press can be turned off when a low level is reached and re-started when a higher level is reached.

In the case of pressing liquids that contain dissolved sugars or salts, a refractometer is valuable for assessing press performance. The Brix of the inbound flow, the press cake, and the press liquor will all be the same figure. The higher the Brix, the higher will be the solids content of the press cake.

If dissolved (soluble) solids are present, the suspended (insoluble) solids (fiber) in the press liquor are generally measured by filtering and washing a sample and drying the filter paper in an oven. Dissolved solids will be washed from the sample during the washing process.


START-UP
Before putting power to the screw press, it is advisable to bump the motor or even rotate the screw by hand. This will prevent damage to the press in case tramp material has been left in the press. Also, the screw may have shifted so as to hit the screen. (Minor rubbing is normal; it will go away once there is material in the press.) To turn the screw by hand, remove the fan guard on the motor and turn the fan blades.

The screw of the press turns in a counterclockwise direction, when viewed from the drive end of the press.

If problems are encountered, they are apt to be blinding, jamming, channeling or purging. There is a section for each of these ahead in this manual.


FEEDING
Material can be fed into the press many ways. Commonly, screw conveyors, pumps, transition chutes, pre-thickener screens or cyclone separators are used. Consider making provision for return of overflow material, in the event that more is fed to the press than it can take.

Sometimes either a static (sidehill or parabolic) or a rotary drum screen (RDS) must be mounted over the inlet hopper to prethicken the flow ahead of the press; the tailings (solids) from the screen are funneled into the press. This arrangement is desirable when the feed to the press is dilute.

Also, material can be dropped from a shredder or cyclone separator into the press. A shredder is used to increase capacity and dewatering in the case of low bulk density materials like lettuce leaves, alfalfa, onion peel, and cornhusk, or to prevent blockage.

Most commonly, the best screw press performance is achieved if the material in the inlet hopper stays just over the top edge of the screw. Usually presses work the best with only atmospheric pressure in the inlet hopper. In order to minimize static head, press headboxes are kept short, and level controls are used to minimize the depth.

When a pump is used to feed a press, the system can be either open or closed. We recommend the open system where little or no pressure exists in the inlet hopper, thus preventing the press from being force-fed. In this arrangement either there is an open return line allowing flow back to the source feeding the press, or level is controlled in the inlet hopper. It is best to have a line that allows material to recirculate past the press inlet. This will prevent pressurizing the inlet of the press, which can cause both blinding of the screen and purging from the cake discharge.

A port on the side of the inlet hopper is frequently provided on larger Vincent presses. It is used to view the level of material over the screw. It has a bolted cover because it is rarely used.

If a fluid flow is piped through a sealed cover which is bolted to the inlet hopper, force-feeding is possible. A by-pass tee should be provided so that the pressure in the inlet hopper is minimized. In addition, a 2" vent line, open to the atmosphere, must be provided to prevent siphoning material in the inlet hopper out through the recirculation line.

Inlet hopper pressure over one to four psi can force fibrous material against the screen so as to blind off the screen, resulting in unsatisfactory performance.

At pressures above 10 to 15 psi in the inlet hopper, it is possible to blow the "plug" of press cake that forms at the discharge of the press. Unscreened liquid will purge from the cake discharge. Exercise caution if either hot or hazardous material is being pumped into a press.

At inlet hopper pressures of 40 psi and above, the shaft seals will be blown out of their housing. At pressures around 60 psi the screen will start to separate from its support plates, resulting in bypassing of feed material directly into the press liquor flow.


BUILDING A PLUG

In order for the press to work, a plug of cake must form between the cake discharge opening and the pressure cone. The press will almost always do this on its own accord as material is fed into the press.

In the case of sloppy materials like manure and DAF sludge, it may be advisable to start off by first packing the discharge of the press with any available fibrous material.

Alternatively, the press can be turned on and the feed pump allowed to run just long enough to fill the feed line and the press. Then shut off the pump, leaving the press running, and wait until no more liquid drains from the screen of the press. Repeat this process until a plug of cake starts to open the cone.


PRE-THICKENING

Almost always, the thicker a flow going into a press, the better it will work.

If the flow into a screw press is too dilute, the high volume of liquid going through the press screen can cause either of two problems. The flow may either flush most of the solids through the screen, or it may plaster solids against the screen, thus blinding (covering over) the screen.

To prevent these things from occurring, it may be necessary to pre-thicken the flow ahead of the screw press. This is commonly done with a static screen (sidehill or parabolic) or a rotary drum screen (RDS). In the case of very dilute feed to the press, a Vincent Fiber Filter can be used.


AIR CYLINDER REGULATOR
To regulate the air pressure of the discharge air cylinder, presses are supplied with an air pressure regulator along with a Parker four-way reversing valve. These should be installed near the cone end of the press. (Until recently FRL (Filter, Regulator, Lubricator) sets were provided to regulate air pressure. Most air cylinder manufacturers now recommend against the use of lubricators.)

The Parker valve allows manual selection of the shut, open, or "neutral" position. This valve connects air supply from the regulator to one end of the air cylinder, while simultaneously opening the other end to atmosphere. The vent line on the 4-way valve allows air to escape when pressure is switched from one end of the air cylinder to the other.

Continuous air flow from the Parker vent line indicates a leak inside the air cylinder, or possibly a faulty 4-way valve.

Once material is going through the press, set the 4-way valve so that the discharge cone goes shut in the "in" or closed position. Start with a low air pressure, working your way up until the desired performance is obtained.

The neutral position of the Parker valve is used only in testing. If left in the neutral position, the cone will not move unless it is pushed open by press cake. If, later, the flow of press cake is diminished, the cone will remain in the position to which it was pushed, and purging can occur.

FRL AIR REGULATOR WITH 4-WAY REVERSING VALVE

DISCHARGE CONE

The principal adjustment of the press is made with the discharge cone. The cone is the component at the cake discharge end of the press that acts as a door or stopper plug to restrict material from leaving the press. The more pressure exerted by the discharge cone, the drier the cake material will be leaving the press. Also, the motor amps can be expected to increase with added pressure, and throughput may decrease.

The discharge cone is moved in (actuated) by the air cylinders. Typical air cylinder pressures to actuate the discharge cone are in the range of 30 to 60 psi. Some materials will press only in a low range, say 10 to 20 psi. Other materials may press best with a pressure of 60 to 100 psi. Air consumption is minimal in all models, 1 to 2 cfm.

During initial, first-time, start up, presses are generally started up with the discharge cone in the withdrawn position. This will avoid an unnecessary jam.

Note that with many materials it is necessary to start the press with the discharge cone in the closed position at low air pressure. Thin or soupy materials, like pumped manure or clarifier underflow, can tend to purge right through the press if the press is operated with the discharge cone open (in the withdrawn ("out") position).

However, with materials that are dry to begin with, such as sawdust or plastic wash tank sludge, it becomes more important to start with the discharge cone in the open position. This is because these materials may tend to jam or overload the press. Similarly, high freeness materials, from which the water falls away freely, will have a tendency to jam in a press. Be sure to start the press with the cone open, and gradually close it with low air pressure, when running such materials for the first time.

Once you are through the initial start up, it will be unlikely that your press should have the cone opened before starting. Most operators rarely open or shut the cone once it is set.

As the pressure on the discharge cone is increased, not only will the cake become drier, but the flow through the press may also be reduced. With very slippery or slimy feed material it may be possible to apply enough discharge cone pressure to stop the flow altogether.

High discharge cone pressures can result in increased quantities of suspended solids in the press liquor.

Care must be taken if a press is to be left running at a very low pressure like 10 psi. If some fiber enters between the cone bushing and the screw shaft, it will take more than that much air pressure to close a cone which has been pushed open by a heavy flow of cake. The result will be either high moisture content in the cake or, worse, purging.

With some feed materials, the press can be operated with the discharge cone in the withdrawn position. The screw alone may do enough compressing and dewatering to produce a cake at the discharge.

It is acceptable to open the discharge cone, in most cases, during normal operating conditions. This allows inspection, while in operation, of the discharge end of the screw and screen. This will give the operator a chance to observe operation with minimum dewatering and maximum throughput. It is also a good technique for purging bad material i.e., either jammed or spoiled material, from the press. (Do not try this trick if you are pressing hot or chemically aggressive materials.)

Where very low air pressure is required for proper operation, it may be practical to put the 4-way valve in the neutral position, half way between open and closed. A press can not be left permanently in this condition: keep in mind that a slug of cake will push the cone open, and it will not re-close on its own afterwards.

An unusual technique is to set the air pressure so that the cone normally stays completely shut. A timer is used to periodically open the cone. The closed period is determined by the amount to time required for press cake to accumulate in the press. This type of operation is used with slippery or slimy press cake that cannot be dewatered to sufficient firmness to force the cone to open. The duration of the "cone open" period is long enough to dump the press cake that has been formed. Cone Timer panels are available from Vincent at no charge.

Once through start-up, the cone is almost always left in the closed position at whatever air pressure had been found to be effective. A plug of cake will be left around the cone whenever the press is turned off; this will clear on its own accord on restarting the press.

There are a few applications where the air cylinders are removed and replaced with a jacking bolt. This is used if the cone pushes completely closed even with the lowest air pressure. It results in operating the press with a fixed discharge annulus.
Air cylinders with linear actuators are available.


ROTATING CONE OPTION
Some CP screw presses are offered with an optional feature which makes the cone rotate and strip away the press cake. The rotation is driven by two large pins mounted on the back of the cone. These pins engage with a collar which is clamped onto the screw shaft. Press cake is stripped away by studs on the face of the cone. These studs are in an axial position parallel to the screw shaft.

The rotating cone can serve multiple functions. By stripping the cake away it can prevent either jamming or purging. Its use generally results in wetter press cake and lower motor amps. Most commonly it is used when all the press cake tends to channel out past one side of the cone. It is invaluable in situations where press cake props open the cone and allows un-pressed material to purge.

The cake will tend to co-rotate with the screw when the cone is rotating. A spin-stop feature is included to prevent this.

Positioning the drive collar limits the maximum opening of the cone. If a large amount of cake comes form the press, the cone can run into the drive collar and for it to slip along the screw shaft.

If the cake comes out too wet, shorten the length of the studs on the face of the cone.

Disconnect the rotating cone by removing the drive pins and/or drive collar.


INTERMITTENT OPERATION

In the case of intermittent operation, it is recommended that the control panel for the feed pump or conveyor which feeds the press should have a timer. This timer should be set to have the press run for three minutes after the feed pump (or conveyor) shuts off. This will partially clear the press so that it will not trip out on overload when it is re-started. (This applies in high torque applications or in installations where the material in the press dries out or freezes.)

An extreme case occurs when pressing spent coffee grounds and some paper mill fibers. Each time the press is turned off, the cone must first be opened for a minute. If this precaution is not taken, nasty damage to the press screw or screen can occur when the press is re-started.

(See the previous section, INVERTER VFD & PLC CONTROL.)

Minimize the time that the screw press is run with no material being fed into it. The last material admitted to the press will dry to powder, and it can cause severe accelerated abrasive wear.

Initially the press will likely be run empty in order to check rotation. Even though some rubbing may be heard, negligible wear will occur so long that this period is kept to a minimum. Also, since the screw is supported to some extent by the material inside the press, running dry may allow the screw to rub the screen.


DOUBLE PRESSING
Some processes benefit from what is called double pressing. This means that the cake coming from the press is run through the press a second time (or through a second press). If little moisture is removed in the second (double) pressing, then it is known that the liquid removed in the first pressing is all of the free liquid that there is to be pressed out.

Sometimes water is added to the cake in between the first pressing and second pressing. This is done to enhance the recovery of dissolved sugars in the original press cake.

Molasses can be added to press cake between the first and second pressing. This is used to infuse dissolved sugar into the cake, increasing the solids content of the final press cake.

Capital-effective double pressing can be achieved by using an inexpensive Soft Squeeze Series KP screw press for the first pressing, following with a tighter-pressing Series CP/VP in the second position.


MOISTURE CONTENT

A screw press separates free water. This will leave organic water in the press cake. The organic water is either bound to, or part of, the animal or vegetable molecules. Mechanical pressure alone will not remove organic water; it takes heat or chemistry. Frictional heat from the press can remove organic water, but this obviously should be avoided. For chemistry, see the Hydrated Lime, Gypsum and Alum section. For heat, see the Fluid Injection section.

To determine the moisture content of a material (feed to the press, press cake, or press liquor), a sample should be weighed and dried overnight at a temperature slightly less than 100o C. (If sugars are present use less than 70º C to prevent caramelizing.) The sample should weigh six or more times the tare weight of the sample tray or cup.

The moisture content of press cake varies considerably. Tomato press cake will be 90% moisture. Orange peel will be 80%, unless it is reacted with hydrated lime, in which case it will go down to 72% moisture; add molasses and it will go to 65%. Dairy and hog manure will come out at 70% moisture, unless there is sand or sawdust in the sample, which will reduce the moisture content. Cellulose fiber from a paper mill (knots, screen rejects, primary clarifier underflow) will come out about 50%. However, if secondary (biological) sludge is added, then the moisture content of the cake will go up considerably. With high ash content in paper mill samples, moisture may go down to 40%. Moisture contents of only 25% can be achieved pressing things like sand, eggshell, glass, and plastic chips.

The heat from steam injection can change the chemistry of the material being pressed so that cake with lower moisture content is produced. This blanching or parboiling effect works with fish and orange peel, for example.

A quick approximation of what to expect from a screw press is to squeeze as much water out with your fist, and figure that the press will do a little bit better. A better way is to twist a ball of the material in a cotton cloth.


COMPRESSION
A screw press achieves compression using several methods: (1) The discharge cone of the press causes back-pressure on the material being dewatered. The higher the cone pressure, the greater the liquid removal. (2) The pitch of the flights of the screw tightens as the material is conveyed through the press. This forces liquid to go through the screen. (3) The diameter of the shaft of the screw may be increased progressively, forcing material outward, against the screen. This is a tapered shaft design.

 

SCREW WITH TAPERED SHAFT

Force-feeding (supercharging) the press and applying a vacuum to the outside of the screen are two additional methods which may achieve compression. These two are used infrequently because the performance results are uncertain.


PRESS SPEED (RPM)
In general, the slower the screw speed, the greater the dewatering. Longer residence time in the screened area results from lower screw speed, which allows time for more thorough dewatering. Unfortunately, it also goes with reduced throughput capacity.

Screw press speed (rpm) can be changed by using a Variable Frequency Drive (VFD). Alternatively, the drive motor can be switched to a different pole motor (900, 1200, or 3600 versus the standard 1800 rpm). Most modern motors are good for permanent 120 Hertz operation; they are always good for a test at this high speed.

The smaller Nord gearboxes are all rated for 4,000 rpm input, which makes it easy to switch to a 3000/3600 rpm motor. It is best to switch to synthetic lubricant if this change is permanent. With other gearboxes, the higher speed can result in premature gearbox failure. Consult the factory for assistance.

A small change in screw speed, like 20%, will generally not result in a measurable change in performance of the press.

It has become normal for a variable speed drive (frequency inverter VFD) to be used with Vincent presses.

Low screw speeds are used for cooker crumb, potato peel, many sludges, and low freeness materials in general.


CAPACITY MEASUREMENT

The best way to measure capacity of a press is to collect timed samples of press cake and of press liquor. This should be done during a period of sustained, stable operation, rather than by timing a batch through the press.

Press cake is generally captured in a tarpaulin, and press liquor in a 5-gallon pail or 55-gallon drum. When the drain is at floor level, a 3-mil plastic bag can be used to catch press liquor. If the press liquor goes to a pit or tank, the change in depth can be timed.

Sometimes it is possible to collect only one flow, either press cake or press liquor. In these cases it is possible to estimate the press throughput if the solids content of the inbound material and press cake are measured. It is assumed that there are zero suspended solids in the press liquor, although this is never really the case.


A-B-C-D PLATES
There are five vertical plates making up the frame of the press. Starting from the drive end of the press, the first one is the Adapter Plate. The gearbox is bolted to the adapter plate. Through four spacers, the adapter plate is welded to the A Plate. This A Plate forms one wall of the inlet hopper. The shaft seal housing is bolted to the A Plate.

The next plate is the B Plate. It forms the downstream wall of the inlet hopper. The screen starts at the B Plate. There is a notch, called a Cord Cutter, in the B plate. Also, there may be a bar called Brian's Stripper welded to the B Plate, inside the inlet hopper; it is in a position to kiss the edge of the screw flight as it passes. These two features prevent long fiber pieces from balling up at the exit of the inlet hopper. See the section ahead on Cord Cutters.

The final plate, the C Plate, supports the discharge end of the screen. The discharge cone touches this plate when the cone is in the closed position.

There is a fifth plate, called the D Plate, on which air cylinders and thrust bearing are mounted. These plates are called out in the Nomenclature schematic at the end of this manual.


SCREW LIFE

If a press loses its previous throughput capacity, or if cake moisture content increases, it is can be sign of a worn screw.

A screw can last anywhere from six months to twenty years. It depends on the material being pressed and how hard it is being pressed.

Premature screw failure can arise from several causes. The two main ones are:
(a) If a press is allowed to run continuously even when no material is being fed into it, the screw can wear out in one or two months. The same can happen if a very low flow is consistently fed into the press.
(b) If abrasive material is dewatered with high cone air pressure, rapid wear will occur. This condition is avoided if a few drops of water can always be squeezed from a fistful of press cake.

Two effective ways to extend screw life are:
(a) Various grades of hardsurfacing rod can be used to protect the flights of a screw. The best hardsurfacing will have Tungsten Carbide impregnated in it.
(B) Using a VFD or lower speed motor to reduce the screw rpm will extend screw life

If a worn screw is suspected, the thing to do is to shut down the press, open the cone, and dig out the cake until the tips of the last two flights can be seen or felt. Check how badly the tips are worn. If the there is 3/8” between the tips and the screen, wear is evident. It is also an indication that the sharp edges of the flights throughout the press may have worn, becoming rounded. This can cause the flights to act like a putty knife, plastering solids against the screen, preventing water from coming through.

Worn screws are either restored locally or returned to Vincent for rebuilding. The maximum cost of a screw rebuild is around one third the cost of a new screw.


SCREW CONFIGURATION
Almost all Vincent screw presses use the Interrupted Screw Flight design. The interruptions leave room for stationary resistor teeth that are mounted outside of the screen. These teeth go through the screen and reach almost to the shaft of the screw. This design of screw press stands in contrast to a Continuous Screw design. The main advantage of the interrupted design is that solid material must accumulate in the interruptions until sufficient consistency is reached for the solids to be pushed toward the cake discharge. There is a reduced tendency for the material being pressed to co-rotate with the screw. Also, there is more agitation within the press and, consequently, quicker and more thorough dewatering.

The screw starts with a feeder section of continuous flight. This picks up material in the inlet hopper and pushes it into the screen section. The feeder section ends at the first resistor tooth. This feeder section of the screw is followed by compression stages where the flights have reduced pitch. The reduction in pitch of the flights results in compression of the material going through the press.

INTERRUPTED FLIGHTS, RESISTOR BARS, and RESISTOR TEETH
 

PIE CUTTING
Sometimes the compression of a screw is reduced, in the field, in an operation called "pie cutting". This involves cutting pie-shaped segments from certain flights of the screw, leaving a butterfly (end view) configuration. The modification is done to avoid excessive compression and jamming. The "sterile cut" is more dramatic. Consult the factory for assistance before making this modification.

 

4" PIE CUT                                                 PIE CUT


 
NOT PIE CUT                                                      BUTTERFLY CUT

JAMMING
Should a press trip out on overload because it has become jammed, a series of steps can be taken to un-jam the press. Generally, the easiest thing to do is to reverse the leads on the electric motor drive. This will cause the screw to feed material backward into the inlet hopper.

Generally jamming is caused by over-pressing excessively dry material. Running the press backwards will break up this material. If the jamming was caused by tramp material, hopefully this can be found and retrieved from the inlet hopper following operation in the reverse direction.

Having a reversing starter greatly facilitates this operation. These cost little more than a standard starter; they come with a forward-reverse switch. (Having a VFD with a reverse button can be even handier.)

If a press has had extensive use in an abrasive application, the outer diameter of the flights will be worn away at the discharge of the press. Radial wear of 1" to 4" in large presses will lead to serious jamming and, possibly, a burst screen.

When a press is operated in the reverse direction it is possible that solid in the press will be forced against the A Plate. This can damage the shaft seal. For this reason, care should be taken when running the press backwards.

Usually three or four revolutions of the screw are sufficient to clear a press. If running the press backwards several cycles does not clear the jam, the screen should be removed so that the cause of the jam can be determined. Look for a bent flight. Before going to the trouble of removing the screen, shut down the press and try clearing the end of the press with a long screwdriver.

Sometimes when a press is jammed, a flight on the shaft of the screw will fold. This can happen if the press overloads on dry cake or if tramp metal is caught between a flight and a resistor tooth. The weld at the shaft may tear. When this happens flow through the press is greatly impeded.

   

FOLDED FLIGHTS


SCREEN BLINDING
A common problem is for the screen of the press to become blinded (covered over). When this occurs, the flow of press liquor coming through the screen diminishes. The level in the inlet hopper will fill up to where it overflows.

In some cases, the screen can be cleared by periodically reversing the direction of rotation of the screw. This can be programmed with many VFD’s, so that the press runs forward for a given period and then reverses direction briefly for three or four turns when the screen starts to blind. This is one of the easiest possible solutions to test. Sometimes it is the only one that is effective.

Many other methods are used to address blinding: (1) Adding notches to the screw, (2) Reducing or eliminating the pressure in the inlet hopper, (3) Adding press aid to the flow, (4) Changing to a different screen selection, (5) Reducing the screw-to-screen clearance, and/or (6) Employing a screen flush with caustic solution, acid, or high pressure spray.

If blinding occurs after an extended period of satisfactory operation, it is usually due to wear of the screw. Rounded edges of the flights will contribute to blinding.

CHANNELING
A condition somewhat similar to purging can occur with slimy materials, like concord grapes, pineapple pulp, or spent brewer’s grain. These may tend to channel or squirt out from one side of the cone. Two ways to eliminate channeling are to lower the air pressure on the discharge cone and to slow down the speed of the press. Channeling can also be reduced by adding press aid to the material being dewatered, or by reducing the inbound flow to the press.

The rotating cone option is very useful in breaking up channeling. The cone is caused to rotate so that the relative motion between it and the stationary screens breaks the channeling. A pin on the face of the cone will strip away the press cake, breaking up the channeling. If the cake comes out too wet, shorten the length of the pin.

To break up channeling, pieces called wing feeders can be welded to the end tips of the last two flights of the screw. See the Wing Feeder section of this manual.


PURGING
An undesirable condition can occur when the material being admitted to the press purges, without liquid-solid separation, from the cake discharge. This can occur especially if pressure exists in the inlet hopper.

Mechanically, purging occurs when a dry lump of press cake holds open the discharge cone. Un-pressed material will flow around this partial plug.

Purging may occur when there is a much reduced, small flow of cake coming from the press. Usually this is a sign of blinded (covered over) screens. This can be caused by a worn screw. Liquid from the inlet hopper will wick into the press cake, making it soft enough to blow out. Sometimes this condition is avoided by mounting the press inclined at about 5o above horizontal; the simplest way to do this is to place a block under the cone end of the press.

A drop in operating amps can be an indicator that a purging condition has begun. An ammeter circuit can be installed to alarm or trip the system when a reduction in motor amps occurs. This is rarely done.

Purging is prevented with the rotating cone option. To use it, it is necessary for the cone drive to be engaged so that the cone spins with the screw. Pins on the face of the cone will strip away the press cake, preventing it from holding the cone open. If the cake comes out too wet, shorten the length of the pins.


BRIDGING

Sometimes bridging will occur at the inlet hopper, preventing material from flowing into the press. If an independent surge hopper is mounted over the inlet of the press, it should have at least one, preferably two or three, vertical walls. This will minimize bridging.

Bonding Teflon sheets to the inlet hopper of the press is a remedy that has been used to reduce bridging of bulky materials which allow free-draining of water.

A vibrator, mounted on the side of a feed hopper, may also alleviate bridging.

One way to overcome this is to direct a stream of water into the inlet hopper to break the bridge. The nature of the screw press is that essentially all of this added liquid will be removed in the pressing operation. (It may be convenient to pump a jet of the press liquor into the inlet hopper to break the bridging.) This is rarely done.


RESISTOR TEETH

The interrupted screw design press has stationary teeth that protrude into the flow of material as it passes through the press. These fit into the gaps of the screw where there is no flighting. They stop just short of the shaft of the screw.

The resistor teeth are an integral part of the Resistor Bar. In 6" through 12" presses, these resistor bars are positioned axially, parallel to the screw, with one bar above and one bar below the screw. The resistor bars are bolted between the B and C plates; they form a part of the structural frame of the screw press. The screen frames bolt onto the resistor bars.

Rarely the resistor teeth are shortened, usually by half, to increase the capacity of the press. Removing the teeth altogether will result in co-rotation and jamming.

Not infrequently the resistor teeth are drilled so that fluid can be injected into the press during operation. See the next section.


FLUID INJECTION
Resistor teeth can be drilled so to permit injection of steam, solvent or water while the press is in operation. Also, these modified resistor teeth can be used for CIP cleaning, without the need of removing the screen from the press.

Commonly alcohol injection is used to achieve in-line washing to remove sugars. Hot water injection is used to recover dissolved solids in juice production. Steam injection is used in dewatering raw organic materials.

The moisture reduction that results from steam injection is related to a chemical change that comes with blanching, or parboiling, a material. Steam injection works well on pineapple skin, citrus waste, and raw fish. Tests run with steam injection in a Vincent press at Anheuser-Busch showed little benefit. The material being pressed, spent grain, had already been "cooked" before steam was added.

Injection is achieved by drilling holes through the resistor teeth and piping these holes to a manifold outside of the screen. Photos and drawings are available from the factory. Vincent does not charge for providing a drilled resistor bar.

 

 

STEAM INJECTION                                                         LAB PRESS
IN TWIN SCREW PRESS                                               STEAM INJECTION

 

CORD CUTTER AND STRIPPER
Sometimes long stringy material will be pinched where the feeder portion of the screw goes through the hole in the B Plate. This material will co-rotate with the screw and build into a bundle which reduces the flow through the press.

A groove, like a keyway that is 3/8" deep, is cut half way through the hole in the B plate. We call this a Cord Cutter. Material trapped between the flight and the hole in the B plate will pop up slightly as it passes the Cord Cutter. The result is that the material is sheared loose.

A part called Brian's Stripper may be welded to the B Plate, inside the inlet hopper. It goes in a position so that the flight lightly kisses the stripper as it goes past. This strips the material away. Strippers are made of square bar.

 


STRIPPER AND CORD CUTTERS

 

POLYMER
In rare applications, the addition of polymer is indispensable in achieving adequate screw press performance. Polymers are added to dilute waste streams, especially to those containing very small size suspended solids. The long chain molecules of the polymer will flocculate the solids, agglomerating them to the point where they can be pressed. Under the right conditions, drastic improvement can be observed in press throughput, press cake moisture, and press liquor clarity.

Nalco and GE are the leading polymer suppliers. Their sales engineers are anxious to recommend the product best suited for your application.

Low speed operation of the press is usually required in order to achieve good performance.


NOTCHES
Sometimes it is necessary, during press operation, to have the screw wipe the screens clear of blinding material. This is best achieved by having notches in the outer edge of the screw. Fibrous material accumulates in the notches and brushes away slimy material which may be blinding the screens. Shallow notches (1/8" wide by 1/8" deep, 1-1/2" apart) in the outer edge of the screw flights work well. Typically, notching is done from the B plate to the second resistor tooth. Most Vincent presses are supplied with notches.


GRINDING NOTCHES IN THE FLIGHTS

 

WIPERS
Before the advent of notches, wipers, made of UHMW strips or nylon brushes, were (rarely) mounted to the outer perimeter of the screw flights. If at all possible we advise against the use of such wipers because (a) they tend to cause material to co-rotate and jam in the press, (b) they are difficult to replace, and (c) they wear rapidly, resulting in unacceptably frequent shutdowns for replacement. Wipers tend to improve dewatering performance for the first week or so. After that the wiper material wears and press performance reverts to being just a little better than if no wipers were used. Wipers are made either of 1/4" thick high durometer polyurethane or of nylon bristle brush. Wipers are preferably mounted on the downstream edge of the flight.


PRESS AID

Some materials press best if a press aid is mixed into the material to be pressed. Typical press aids are rice hulls, cottonseed hulls, cellulose fiber from a paper mill, and ground newspaper. Ground wood is the best, but most expensive, press aid.

Press aids are most commonly used in producing juice from deciduous fruit. The press aid gives the press something to get a bite on. Press aids also tend to hold back fines (short fibers) and prevent them from going through the screen with the press liquor. If apples are fed into a press, apple sauce will come through the screen. However, if a press aid is added to the apples, then apple juice will come through the screen.

Typically, the amount of press aid used is only 1% to 3% by weight of the flow going through the press. This will look like more than such a small percentage because press aids have a much lower bulk density than the wet materials that are pressed.


HYDRATED LIME, GYPSUM, AND ALUM

Lime (calcium hydroxide) must be added to citrus peel before it can be pressed. The lime breaks down the pectins or cell walls so that the press can remove moisture. Less than 1% by weight is used. A reaction time of several minutes must be allowed prior to pressing. Lime has been used successfully in the same manner with potato, onion, tomato, carrot, and pineapple waste. It works well on acidic materials such as strawberries and coffee bean pulp. Vincent offers lime dosing equipment.

Gypsum and alum salts are also effective chemical press aids. They are typically used in dewatering sugar beet pulp, and they have rarely been found effective on other materials.


VACUUM EFFECT

In some applications, increased screw press capacity can be obtained if the area outside of the screen is under a vacuum. This can be achieved by mounting the press at a high elevation, with the press liquor drain line dropping below the surface of a drain tank or pit.

That is, the drain line from the press should go below the surface of the pit or pond into which it drains. If this line is relatively small in diameter and has a steady downward slope, a vacuum will be induced around the screen of the screw press. The mass and velocity of press liquor flowing through the drain line create this vacuum. To draw air bubbles downwards with the press liquor, the velocity of the fluid must be greater than five feet per second.

The cover over the screen of the press will have to be sealed, usually with Silicone.

The amount of vacuum is a function of the elevation between the press and the drain pond. For good results, the press should be mounted on a stand that is 20' tall or higher.


PRESS LIQUOR
A screw press produces relatively "dirty" press liquor as compared to a Filter Press or Belt Press. Suspended solids will pass through the screen of the screw press along with the liquid being expressed from the inbound material.

If suspended solids need to be removed from the press liquor, the most common method is to pump the press liquor either over a static (sidehill) screen or through a rotary drum screen. Generally, the screen tailings (sludge solids) are fed back into the screw press along with the flow of inbound material. Most of these fines will be captured with the solids of the inbound material and end up in the press cake. Although some of these tailings will once again go through the press screen with the press liquor, equilibrium or recirculating solids is reached and satisfactory pressing operation is realized.

If the press liquor is to be concentrated in an evaporator, better screening than can be achieved with a static screen may be required. Another Vincent machine, the Fiber Filter, provides premium performance. Decanters or centrifuges may be required.


SCREEN SELECTION
The screen of the Series CP presses is made either of wedgewire (slotted screen) or, infrequently, perforated stainless sheet (round holes). Wedgewire screens are expensive one-piece weldments that must be replaced when excessive damage or wear has occurred.

Wedgewire screens in Vincent presses can be reversed in order to achieve double life. That is, wear starts at the cake discharge end of the press. When this occurs, the screen can be turned 180o so that the fresh inlet section is then located in the discharge area.

Screens made of wedgewire come standard with 0.015" to 0.020" slot width; they are also available with slots that are 0.008" to 0.060" wide. With slot widths less than 0.012" there is a tendency for the screen to blind (be covered over) with the material being pressed. However they work well in alcohol and oil separation. Changing the slot width generally has small impact on the clarity of the press liquor or the dewatering capacity of the press.

The most common damage to a wedgewire screen is for part of the surface to be smeared over by rubbing the screw. This rarely is bad enough to affect press performance. Profile bar screens generally work satisfactorily with 30% or even more of their surface smeared over.

Smeared screens can be remedied by running a box cutter blade through the slots.

Perforated metal screens are simple sleeves which are held in the screen assembly. These are less durable but inexpensive to replace. Standard perforated screens have a hole size of 3/32" diameter, although material with 0.060", 0.040", down to 0.023" holes can be supplied.

Frequently, increased press capacity can be achieved by changing a perforated screen to one with smaller holes. This unexpected result arises from a combination of factors: (1) smaller hole screens are made of thinner sheetmetal, so that the press liquor has a shorter distance to travel before it falls free from the screen, reducing the chance of sponging backwards through the screen and (2) particles which fall into and plug a larger hole will roll over a smaller hole. Minor rubbing between the screw and screen is normal, although, obviously, hard rubbing will cause wear and premature failure of the screen. With a clearance greater than 3/16", the dewatering performance of the press can start to deteriorate; this depends a lot on the nature of the material being dewatered.

TIG welding is used to close the gaps in the case where either a few wedgewire slots get spread apart by tramp material or a perforated screen gets torn.

In cases of severe wear or damage, it is common to patch a screen. Stainless sheetmetal is used for this. The reduction in drainage surface is of little consequence as the screens have ample open area.

The most common cause of screen failure ties to failure of the outboard support bearing. If the bearing holding the end of the screw wears out, it can let the screw move enough to rub against the screen.


 
PROFILE BAR (WEDGEWIRE) SCREEN


 
PERFORATED SCREEN WITH REINFORING SHEET


SCREW-TO-SCREEN CLEARANCE
Generally the clearance between the screw and the screen is 1/16", plus or minus 1/16". The screw should not rub the screen hard, as it can cause wear and premature failure of the screen. Tight clearance is used with materials that blind the screen, such as onion skins. Greater clearance, 1/16", is used with eggshells and corn husk. With a clearance greater than 3/16", the dewatering performance of the press can start to deteriorate; this depends a lot on the type of material being dewatered.

To measure the s2s clearance in wedgewire screens, a feeler gauge can be slipped through and along a slot until it hits the edge of the screw. Measure from the outside of the screen to the edge of the flight; then subtract the thickness of the wedgewire (generally either 0.250" or 0.375") from the measured depth in order to calculate the clearance.

Alternatively, on 6" and larger presses, the screw-to-screen clearance can be checked by removing one half of the screen and bolting the other half tight to the resistor bars. Inspection is made from the side from which the screen half has been removed.

In the case of perforated screen installations, a depth gauge can be used to measure the screw-to-screen clearance. This is done by first finding an area where the screw flight is next to the screen; poking a straightened paper clip through the screen is handy for this purpose. The depth from the outside of the screen to the edge of the flight is measured, and then the thickness of the screen is subtracted from that measurement. [3/32" perf is 0.075" thick; 0.050" perf is 0.050 thick; 1/32" perf is 0.024" thick; 0.023" perf is 0.015" thick; 3/8" perf back up screen is 0.120" thick.]

If a screw rubs against the screen in a given area, it may be best to grind some off the OD of the screw. Prussian Blue can be useful in finding the spot that is rubbing.

 

WEDGEWIRE DEPTH GAGE                                    PERFORATED DEPTH GAGE


CONE BUSHING
The cone rides on the shaft of the screw. "Cone Sleeve" is the name given to the portion of the screw on which the cone rides. There will be a bushing in the cone to support and guide it, and to protect the Cone Sleeve journal surface of the screw shaft. Sometimes the bushing is lubricated with liquid from the material being pressed, such as the juice from apples or water from pectin peel.

A grease fitting is provided for lubricating the bushing and to minimize leakage of press liquor through the cone. (Models with two cone bushings may have a single grease line going to a pocket, which serves as a grease reservoir, located between the two bushings.)

Bushing lubrication is extremely important when materials that are dry (like paper mill screen rejects) are being pressed. By the time such materials reach the discharge of the press, they do not have enough free moisture left in them to adequately lubricate the cone bushings. In these applications the operator should, at the start of each shift, pump grease in until it comes out between the cone bushing and the screw shaft. Then he should open and shut the cone three times in order to spread the grease.

Rarely, presses are supplied with additional lubrication fittings so that water, in addition to grease, can be metered to the bushings as a lubricant.

Automatic grease systems are available. These should be the high pressure (900 psi) electric or battery variety. Vincent provides these for critical applications, especially pulp & paper.

Liquid leaking past the cone bushings drains out the back of the cone (at the air cylinder end of the press). Almost always it is minimal compared to the flow of press cake. However, a pan can be provided to collect this liquid and drain it into the main flow of press liquor.


WING FEEDERS
Sometimes there are blades welded to the outside tips of the last two flights of the screw. Called "Wing Feeders", these are mounted parallel to the discharge screen surface. Care must be taken that wing feeders are not made so long that they hit the face of the cone when the cone is in the closed position.

Wing feeders can serve two purposes. (1) In the case of materials that want to channel out the discharge of the press, like pineapple and spent brewer's grains, long wing feeders break up the channeling flow. (2) For abrasive applications, short knobby wing feeders are provided as sacrificial wear elements.

When certain materials are fed through a screw press, clumps of dry material may pack in front of the wing feeders. This buildup can cause wear of the screen. Should the problem occur, grind off the wing feeders.


 
LONG WING FEEDER                                                          KNOBBY WING FEEDER


CLEANING
Commonly, material is cleared from a press by stopping the inbound flow, setting the discharge cone in the withdrawn position, and running the press for a few minutes until no further material is discharged. This will leave some material inside the press, which can be handy for forming a plug at the cake discharge when the press is restarted.
Material will leave a Vincent interrupted flight press only if there is additional inbound material forcing it out. This makes it difficult to clear all material from inside a press without removing the screen. One technique used successfully is to feed crushed ice into the press. Water must be fed along with the ice to prevent jamming. When the ice melts, the press will be relatively clean inside.

There are applications in which the press must be cleaned frequently, such as once a shift, in order to meet sanitary regulations. The screen halves can be hinged to facilitate this. In other cases, the screens are removed from the press. A spare screen assembly may be kept, submerged in cleaning solution, in order to minimize the downtime required.

Cleaning the inside of the screen can be achieved, at least to some extent, by injecting water through the resistor teeth. Holes must be drilled in the resistor teeth to make this possible.

In laboratory applications, the press may be disassembled for cleaning. The various components can be dipped in an appropriate solution or placed in an autoclave.

It is unusual that the outside of the screen needs to be cleaned. Spray systems for this can be built into the press at the Vincent factory. Alternatively, a pressure washer or swabbing with acid solution can be used.


SHAFT SEAL
The Seal Plate is bolted to the A Plate. This may be solid UHMW (ultra high molecular weight polypropylene or polyethylene) or it may contain one or two Johns Manville (JM Clipper) lip shaft seals. There may be a grease fitting on this plate; the grease is used to reduce leakage and to help prevent fiber material from entering and damaging the screw shaft.

Generally, seals are allowed to drip once they start leaking. They are replaced only in conjunction with major maintenance, as when the screw is removed from the press.

In some cases we have found that leakage from a shaft seal can be stopped by simply selectively loosening or tightening the four bolts holding the seal housing to the A plate.


 
SEAL PLATE                                 SPLIT SEAL PLATE


SCREW REMOVAL
The screw is removed through the hole in the C Plate, at the cake discharge end of the press. The operation can be difficult the first time, so we recommend consulting with the factory before getting started.

First of all, remove the four bolts holding the shaft seal plate to the A Plate. This will prevent damage when the screw drops out of the gearbox.

The bars with the resistor teeth must be removed before the screw can be removed.

If a flange bearing is mounted on a plate which also holds the air cylinders, do not remove the flange bearing by itself. Instead, leave it on the plate, and remove the air cylinders and bearing as a single unit.

Series CP screw presses use a key and keyway to couple the screw inside the hollow bore of the gearbox. It may be that a great deal of force will be required to push the screw from the gearbox.

The basic procedure is to push or pull the screw out of the gearbox. If the screw is not tight, the screw is removed easiest by pulling, or by prying with a pry bar.

If it is tight, the screw can be removed using a heavy fine-threaded rod to push the screw out of the gearbox. This rod is threaded through a heavy nut Vincent has provided inside the hollow bore of the larger size gearboxes. This nut has a lug that fits in the keyway, to prevent the nut from turning. The nut backs up against an internal snap ring in the hollow bore of the gearbox. The result is that the rod pushes against the end of the screw. Liberal use of Anti Seize or Never-Seez is absolutely required to protect the threads. Loaner tool kits are available in Tampa; see Screw Change Kit Drawing B-94693.

Alternatively, the screw can be removed by constructing a gear puller, allowing the use of a hydraulic jack, combined with a cross bar and two long shafts threaded into the gearbox housing. The holes in the gearbox have metric threads. To avoid the need for metric all-thread rod, a length of all-thread rod with English threads is welded to the head of a metric bolt.

If the press has been badly jammed, the key may have rolled inside its pocket. In these cases it can be impossible to press the screw from the gearbox. It then becomes necessary to cut the screw in half, between the gearbox mounting plate and the A plate. The stub shaft can then be bored out in a large boring mill, and the screw can be repaired at the Vincent factory.


JACKING ROD

 

SCREW REPLACEMENT
When reinstalling a screw, the screw must be pulled in until the step in the shaft seats against the thrust bearing of the gearbox. This will position the flights of the screw so as not to hit the resistor teeth. Be careful when guiding this step in the shaft through the shaft seals.

During re-assembly of a keyed shaft, be sure to apply Never-Seez or Anti Seize to the portion of the screw shaft that goes into the gearbox.

When pulling a screw into the press, it may become necessary to use a long allthread rod (English threads). This rod can be screwed into the threaded hole on the end of the screw. The screw is slid into the press far enough that the allthread rod goes through the hollow bore of the gearbox. A large washer is slid along the allthread rod to form a brace against the gearbox. Running a nut on the rod, against this large washer, will pull the screw into the gearbox.

There are two ways to avoid excessive rubbing between a new screw and the screen: Either shims can be placed between the screen frames and the resistor bars, or the interference can be ground off the edge of the screw. To eliminate the high spots, coat the edge of the screw with Prussian Blue, bolt the screens in place, turn the screw, remove the screens, and then grind the screw where interference has occurred.


SCREEN REPLACEMENT
For ease of maintenance, the screen halves are held in frames that are split vertically, being bolted to the resistor bars. This allows the screen halves to be removed from the sides of the press.

In the case of screen failure, frequently a solid patch can be welded onto the screen, from the outside. This is simple as the screen need not be removed from the press.

Wedgewire screens may become smeared from being wiped by the screw or by hard press cake. Wedgewire screens generally work satisfactorily with 30% or even more of their surface smeared over. Usually press liquor will come through a smeared area of a wedgewire screen. If it becomes an issue, it is corrected by running a box cutter blade through the slots.

To replace a perforated screen, the screen frame must be removed from the press. Damaged screens should be removed and discarded. The replacement perforated screen first must be clamped tightly against the supporting frame. This can be done by forcing the new screen against the frame, using C-clamps and wood or steel pieces. Be sure that extra screen material sticks out beyond the edges of the frame, to allow trimming with a hand grinder.

Usually perforated screens are tack welded in place. However, if the perforated screens bolt in place, once the new screen is tight against the frame, look through the screen and locate the holes in the frame where the attachment bolts go. Use a center punch to open holes in the screen. These holes must be large enough to allow the attachment bolts to go through the screen and thread into nuts on the far side of the frame.

Once the new screen is tack welded or bolted tightly in place, beat over and grind off the excess screen material.

There are two ways to avoid excessive rubbing between a new screen and the screw: Either shims can be placed between the screen frames and the resistor bars, or the interference can be ground off the edge of the screw. To eliminate the high spots, coat the edge of the screw with Prussian Blue, bolt the screens in place, turn the screw, remove the screens, and grind the screw where interference has occurred.


GEARBOX BASICS

The Series CP presses use Nord hollow bore gearboxes, with a C-face mounted motor.

Gearboxes are rated, and sold, by their torque rating. The manufacturers generally offer their designs in progressively larger sized castings, or boxes. The larger the box, the larger the torque rating. Each box size will be available with different gear ratios. In order to keep the torque fairly constant, larger horsepower motors are used with the high speed boxes. Similarly, reduced horsepower motors must be used when a low output speed is selected.

Screw presses are designed around the size of the gearbox that is selected.


PRESS LUBRICATION
Lubrication is something we generally review with customer personnel during start-up. It is pretty straight forward:

CONE BUSHINGS: Once a shift
BEARINGS: Weekly
BUSHINGS: Weekly
SHAFT SEAL: Weekly
GEARBOX: Annually
AIR REGULATOR: Whenever empty (if a lubricator is being used)
MOTORS: Never

The most critical lubrication item has to do with the cone bushings. Before starting up a new press, the cone should be run in and out a few times to spread the grease around.

Lubrication of the cone bushings depends a lot on what is being pressed. With orange peel, there is enough press liquor juice acting as a lubricant that the bushings are lubricated only at the end of the processing season (to keep them from locking up on dried-out peel juice). The other extreme is at a paper mill where boiler fuel is made out of reject fiber. There is no free water left in the press cake, so everything is very hot and dry. We automatically supply a 900 psi ATS autolube on paper mill jobs. Another tough application is with vapor tight presses where there is apt to be solvent getting into the cone bushings. The solvent can dissolve and wash out the grease, so we like to see frequent lubrication of the cone bushings. (Food grade grease is used in this application since food ingredients are being produced by the press.)

If Vincent supplies an autolube for the cone bushings, it will be either battery powered or require a hard wired power supply. This should be actuated when the press is first placed in service. It should be set to give one small shot of grease every couple hours. We provide autolubes with large grease reservoirs, so they will go at least two weeks at the maximum greasing schedule. Once operations are stabilized, it may be practical to reset the timer dip switches so that it gets one shot every shift or once a day.

In any case, we tell the operators to run the cone open and closed once a shift because this will spread the grease around. This is done with the press in operation.

If Vincent does not provide an autolube for a critical operation, we generally tell the operators to manually grease the cone bushings once a shift. We also tell them to run the cone in and out when they do the greasing, in order to spread the grease.

The bearings and/or bushings holding the screw get greased on the customer's normal schedule for that type of bearing, maybe once a week, or once a day, or once a month. Whatever grease the customer normally uses will be fine.

The shaft seal housing may have a grease fitting. This grease is to prevent fiber from getting into the seal. The seal should be given a shot of grease whenever the screw support bearings or bushings are greased.

The gearbox oil should be changed once a year. Use mineral oil for a normal 1800/1500 rpm input. Use the same grade oil, but synthetic, for input speeds of 2,400 rpm or more.

Some Nord gearboxes have an autolube canister located in the motor adaptor portion of the gearbox. It is located under a cast iron cap. This autolube should be actuated when the press is placed in service. Nord recommends replacing this autolube once a year.

The air regulator used with the discharge cone air cylinders may have a lubricant jar. If so, Vincent includes a can of light oil along with the air regulator which comes with our screw presses. The jar should be filled when placing the press in service and when the jar is empty, about once a year. It takes very light (sewing machine) oil. The oil helps prevent corrosion inside the air cylinders. (Most air cylinder manufacturers no longer recommend the use of lubricators.)

We have never seen nor heard of anyone greasing the motor bearings.


REPLACEMENT PARTS

Most replacement parts are standard OEM components which are purchased from their original manufacturer. The specification of these items (gearbox, seals, air cylinder, etc.) is included in the O&M Manual.

The most common wear parts in the Vincent CP Press are the screw, the screen and the cone bushings. Worn screws may be returned for rebuilding at the Vincent factory. Be sure to specify the Serial Number of your press when ordering replacement parts or repairs.


SAFETY

These Operating Hints have left unstated the obvious safety hazard: A screw press, like any screw conveyor, is totally unforgiving. If clothing or a limb gets caught in a rotating screw, the screw will not stop. Vincent Corporation has heard of only one injury of this nature with a screw press; do not let yourself become the second.

Robert B. Johnston, P.E.



 

AttachmentSize
HINTS-CP.pdf1.31 MB

Operating Hints - VP Presses

(Scroll to bottom for pdf version)

August 16, 2014

RIGGING
Be sure to properly support the press when lifting it from the truck. Do not lift and drag just one end, as it is possible for the frame to deflect, which in turn can shift the screw positioning within the press. Interference between the screw and the screens can result. Use a spreader bar when lifting long presses.


INSTALLATION

Do not just bolt or weld the press down to a level foundation! Instead, first place the press where it is to be installed. Next, place shims between the press frame and the steelwork (or concrete pedestal or foundation) to fill any gap where the press is to be anchored. Only after shimming or grouting should the press be pulled down tight. Doing otherwise may rack the frame of the press, and this can cause misalignment.

The press must be mounted solidly to a concrete pedestal foundation or structural steel. If a press draws more than two thirds of its rated horsepower without the press being anchored to the floor, the frame of the press can twist.

For maintenance, the screw is removed from the cake discharge end of the press. Allow the space required.

It order to provide space for a cake take-away conveyor, the press can be installed tilted with an elevated discharge. Consult the factory, as the oil level in the gearbox may have to be adjusted.

Material can be fed into the press many ways. You may need to allow for bypass or overflow return material in the event that more is fed to the press than it can take.

Spill containment is a consideration which should be taken into account, because it may be possible for un-pressed material to purge from the cake discharge of a press.

We recommend that a manual disconnect, for killing power to the motor, be installed close to the press.

INVERTER VFD & PLC CONTROL
It is always recommended that an inverter VFD be used to start, protect, and operate the screw press. With a VFD it is possible to establish the optimal combination of screw speed and discharge cone air pressure. The VFD also can be used to reverse the press in case of a jam or to slow it down during upset conditions.

Nine presses out of ten will operate unattended, indefinitely, and just fine at line frequency of 50 or 60 Hertz. If two screw presses are mounted in parallel, they are usually fed with a screw conveyor which drops to fill the first press, with the rest going to the second press; this is followed with a drop-out for overflow.

However, we need to address the exceptions:

Use of level controls is becoming more and more common. These are used to regulate either the flow going into the press or to regulate the screw speed.

In some applications a press is sized for handling upset conditions of large flow, while the normal flow is quite small. In these cases a level control is used, and the PLC can be programmed to turn off the press when a low level is reached in the inlet hopper, and the press re-started when a higher level is signaled.

In some cases the press will tend to jam, overload, and trip out on high amps. In this situation it may be necessary to program the controls so that the cone automatically goes open on high amps, re-closing at a lower set point. This arrangement requires a solenoid operated 4-way air valve, replacing the manual valve which is provided with the press.

In other cases of jamming, a simpler arrangement is to install a Cone Timer. A timer is used to periodically open the cone. The closed period is determined by the amount of time required for press cake to accumulate in the press. The duration of the "cone open" period is long enough to dump most of the press cake that has formed. This type of operation may be used if the press periodically experiences jamming or overload due to fluctuations in the amount of material being fed into the press. Alternatively, it may be used with slippery or slimy press cake that cannot be dewatered to sufficient firmness to force the cone open. Cone Timer panels are available from Vincent at no charge.

Some applications require the use of a specially programmed variable frequency drive. In this case the VFD is not used to change the speed of the press, but, rather to set it for auto-reversing operation. By having the screw run backwards for three or four turns every few minutes, some difficult-to-dewater materials can be pressed much more effectively. This operation can help a great deal with material which tends to blind (cover over) the openings in the screen. Vincent has loaner VFD's if you want to give it a try. The technique works well on bar screens; care must be taken with perf screens so that the screw does not snag the screen during the reverse cycle.

Once through start-up, the cone is almost always permanently left in the closed position at whatever air pressure has been found to be effective. A plug of cake will be left around the cone whenever the press is turned off; this will clear on its own accord on restarting the press.

However, some materials may set up and become hard, or freeze, within the press when the press in turned off. This is especially true in the case of pressing wet coffee grounds or outdoor installations. For these applications it is advisable to open the cone for a period of one minute before turning off the press. This allows the press to partially empty itself, fluffing the material left in the press. Vincent can provide information for automating this procedure.


INSTRUMENTATION
The most useful instrument to have when testing a press is an ammeter. The load drawn by the drive motor of the press is indicative of how much work the press is doing. The higher the amps, the better the dewatering. Also, the higher the amps, the closer the press is to jamming, and the greater is the abrasive wear. Very low amps indicate little dewatering is being done; the screen is blinded; low compression is taking place; or the flow into the press has stopped.

A moisture balance is valuable for measuring the moisture content of the inbound material and of the press cake. If an oven is used to dry samples, be sure it is set at 160o F or less if there are sugars in the sample. Samples should be left in the oven overnight. The tare weight of the pan should be much less than the weight of the sample which is being dried.

As mentioned previously, level controls can be useful in operating a press. With a signal providing the depth of material in the inlet hopper, the speed of the press can be varied to match the flow going into the press. With egg shells, a simpler level control is used to only signal when a high level is reached; its signal will increase the screw speed. In special cases the press can be turned off when a low level is reached and re-started when a higher level is reached.

In the case of pressing liquids that contain dissolved sugars or salts, a refractometer is valuable for assessing press performance. The Brix of the inbound flow, the press cake, and the press liquor will all be the same figure. The higher the Brix, the higher the solids will be in the press cake.

If dissolved (soluble) solids are present, the suspended (insoluble) solids (fiber) in the press liquor are generally measured by filtering and washing a sample and drying the filter paper in an oven. Dissolved solids will be washed from the sample during the washing process.

 

START-UP
Before putting power to the screw press, it is advisable to bump the motor or even rotate the screw by hand. This will prevent damage to the press in case tramp material has been left in the press. Also, the screw may have shifted so as to hit the screen. (Minor rubbing is normal; it will go away once there is material in the press.) To turn the screw by hand, remove the fan guard on the motor and turn the fan blades. Alternatively, on belt-drive presses, remove the belt guard and pull on the sheaves or belts; be careful.

The screw of the press turns in a counterclockwise direction, when viewed from the drive end of the press.

If problems are encountered, they are apt to be blinding, jamming, channeling, or purging. There is a section for each of these ahead in this manual.


FEEDING
Material can be fed into the press many ways. Commonly, screw conveyors, pumps, transition chutes, pre-thickener screens or cyclone separators are used. Consider making provision for return of overflow material, in the event that more is fed to the press than it can take.

Sometimes either a static (sidehill or parabolic) or a rotary drum screen (RDS) must be mounted over the inlet hopper to prethicken the flow ahead of the press; the tailings (solids) from the screen are funneled into the press. This arrangement is desirable when the feed to the press is dilute.

Also, material can be dropped from a shredder or cyclone separator into the press. A shredder is used to increase capacity and dewatering in the case of low bulk density materials like lettuce leaves, alfalfa, onion peel, and cornhusk, or to prevent blockage.

Most commonly, the best screw press performance is achieved if the material in the inlet hopper stays just over the top edge of the screw. Usually presses work the best with only atmospheric pressure in the inlet hopper. In order to minimize static head, press headboxes are kept short, and level controls are used to minimize the depth.

When a pump is used to feed a press, the system can be either open or closed. We recommend the open system where little or no pressure exists in the inlet hopper, thus preventing the press from being force-fed. In this arrangement either there is an open return line allowing flow back to the source feeding the press, or level is controlled in the inlet hopper. It is best to have a line that allows material to recirculate past the press inlet. This will prevent pressurizing the inlet of the press, which can cause both blinding of the screen and purging from the cake discharge.

A port on the side of the inlet hopper is frequently provided on larger Vincent presses. It is used to view the level of material over the screw. It has a bolted cover because it is rarely used.

If a fluid flow is piped through a sealed cover which is bolted to the inlet hopper, force-feeding is possible. A by-pass tee should be provided so that the pressure in the inlet hopper is minimized. In addition, a 2" vent line, open to the atmosphere, must be provided to prevent siphoning material in the inlet hopper out through the recirculation line.

Inlet hopper pressure over one to four psi can force fibrous material against the screen so as to blind off the screen, resulting in unsatisfactory performance.

At pressures above 10 to 15 psi in the inlet hopper, it is possible to blow the "plug" of press cake that forms at the discharge of the press. Unscreened liquid will purge from the cake discharge. Exercise caution if either hot or hazardous material is being pumped into a press.

At inlet hopper pressures of 40 psi and above, the shaft seals will be blown out of their housing. At pressures around 60 psi the screen will start to separate from its support plates, resulting in bypassing of feed material directly into the press liquor flow.


BUILDING A PLUG
In order for the press to work, a plug of cake must form between the cake discharge opening and the pressure cone. The press will almost always do this on its own accord as material is fed into the press.

In the case of sloppy materials like manure and DAF sludge, it may be advisable to start off by first packing the discharge of the press with any available fibrous material.

Alternatively, the press can be turned on and the feed pump allowed to run just long enough to fill the feed line and the press. Then shut off the pump, leaving the press running, and wait until no more liquid drains from the screen of the press. Repeat this process until a plug of cake starts to open the cone.


PRE-THICKENING
Almost always, the thicker a flow going into a press, the better it will work.

If the flow into a screw press is too dilute, the high volume of liquid going through the press screen can cause either of two problems. The flow may either flush most of the solids through the screen, or it may plaster solids against the screen, thus blinding (covering over) the screen.

To prevent these things from occurring, it may be necessary to pre-thicken the flow ahead of the screw press. This is commonly done with a static screen (sidehill) or a rotary drum screen (RDS). In the case of very dilute feed to the press, a Vincent Fiber Filter can be used.


AIR CYLINDER REGULATOR
To regulate the air pressure of the discharge air cylinder, presses are supplied with an air pressure regulator along with a Parker four-way reversing valve. These should be installed near the cone end of the press. (Until recently FRL (Filter, Regulator, Lubricator) sets were provided to regulate air pressure. Most air cylinder manufacturers now recommend against the use of lubricators.)

The Parker valve allows manual selection of the shut, open, or "neutral" position. This valve connects air supply from the regulator to one end of the air cylinders, while simultaneously opening the other ends to atmosphere. The vent line on the 4-way valve allows air to escape when pressure is switched from one end of the air cylinders to the other.

Continuous air flow from the Parker vent line indicates a leak inside an air cylinder, or possibly a faulty 4-way valve.

Once material is going through the press, set the 4-way valve so that the discharge cone goes shut in the "in" or closed position. Start with a low air pressure, working your way up until the desired performance is obtained.

The neutral position of the Parker valve is used only in testing. If left in the neutral position, the cone will not move unless it is pushed open by press cake. If, later, the flow of press cake is diminished, the cone will remain in the position to which it was pushed, and purging can occur.



FRL AIR REGULATOR WITH REVERSING 4-WAY VALVE

DISCHARGE CONE
The principal adjustment of the press is made with the discharge cone. The cone is the component at the cake discharge end of the press that acts as a door or stopper plug to restrict material from leaving the press. The more pressure exerted by the discharge cone, the drier the cake material will be leaving the press. Also, the motor amps can be expected to increase with added pressure, and throughput may decrease.

The discharge cone is moved in (actuated) by the air cylinders. Typical air cylinder pressures to actuate the discharge cone are in the range of 30 to 60 psi. Some materials will press only in a low range, say 10 to 20 psi. Other materials may press best with a pressure of 60 to 100 psi. Air consumption is minimal in all models, 1 to 2 cfm.

During initial, first-time, start up, presses are generally started up with the discharge cone in the withdrawn position. This will avoid an unnecessary jam.

Note that with many materials it is necessary to start the press with the discharge cone in the closed position at low air pressure. Thin or soupy materials, like pumped manure or clarifier underflow, can tend to purge right through the press if the press is operated with the discharge cone open (in the withdrawn ("out") position).

However, with materials that are dry to begin with, such as sawdust or plastic wash tank sludge, it becomes more important to start with the discharge cone in the open position. This is because these materials may tend to jam or overload the press. Similarly, high freeness materials, from which the water falls away freely, will have a tendency to jam in a press. Be sure to start the press with the cone open, and gradually close it with low air pressure, when running such materials for the first time.

Once you are through the initial start up, it will be unlikely that your press should have the cone opened before starting. Most operators never open or shut the cone once it is set.

As the pressure on the discharge cone is increased, not only will the cake become drier, but the flow through the press may also be reduced. With very slippery or slimy feed material it is sometimes possible to apply enough discharge cone pressure to stop the flow altogether.

High discharge cone pressures will result in increased quantities of suspended solids in the press liquor.

Care must be taken if a press is to be left running at a very low pressure like 10 psi. If some fiber enters between the cone bushings and the screw shaft, it will take more than that much air pressure to close a cone which has been pushed open by a heavy flow of cake. The result will be either high moisture content in the cake or, worse, purging.

With some feed materials, the press can be operated with the discharge cone in the withdrawn position. The screw alone may do enough compressing and dewatering to produce a cake at the discharge.

It is acceptable to open the discharge cone, in most cases, during normal operating conditions. This allows inspection, while in operation, of the discharge end of the screw and screen. This will give the operator a chance to observe operation with minimum dewatering and maximum throughput. It is also a good technique for purging bad material (i.e., either jammed or spoiled material) from the press. (Do not try this trick if you are pressing hot or chemically aggressive materials.)

Where very low air pressure is required for proper operation, it may be practical to put the 4-way valve in the neutral position, half way between open and closed. A press should not be left permanently in this condition: keep in mind that a slug of cake will push the cone open, and it will not re-close on its own afterwards.

There are a few applications where the air cylinders are removed and replaced with a jacking bolt. This is used if the cone pushes completely closed even with the lowest air pressure. It results in operating the press with a fixed discharge annulus.
Air cylinders with linear actuators are available.


ROTATING CONE OPTION
Some VP screw presses are offered with an optional feature which makes the cone rotate and strip away the press cake. The rotation is driven by two large pins mounted on the back of the cone. These pins engage with a collar which is clamped onto the screw shaft. Press cake is stripped away by studs on the face of the cone. These studs are in an axial position parallel to the screw shaft.

The rotating cone can serve multiple functions. By stripping the cake away it can prevent either jamming or purging. Its use generally results in wetter press cake and lower motor amps. Most commonly it is used when all the press cake tends to channel out past one side of the cone. It is invaluable in situations where press cake props open the cone and allows un-pressed material to purge.

The cake will tend to co-rotate with the screw when the cone is rotating. A spin-stop feature is included to prevent this.

Positioning the drive collar limits the maximum opening of the cone. If a large amount of cake comes form the press, the cone can run into the drive collar and for it to slip along the screw shaft.

If the cake comes out too wet, shorten the length of the studs on the face of the cone.

Disconnect the rotating cone by removing the drive pins and/or drive collar.


INTERMITTENT OPERATION
In the case of intermittent operation, it is recommended that the control panel for the feed pump or conveyor which feeds the press should have a timer. This timer should be set to have the press run for three minutes after the feed pump (or conveyor) shuts off. This will partially clear the press so that it will not trip out on overload when it is re-started. (This applies in high torque applications in installations where the material in the press dries out or freezes.)

An extreme case occurs when pressing spent coffee grounds and some paper mill fibers. Each time the press is turned off, the cone must first be opened for a minute. If this precaution is not taken, nasty damage to the press screw or screen can occur when the press is re-started.

(See the previous section, INVERTER VFD & PLC CONTROL.)

Minimize the time that the screw press is run with no material being fed into it. The last material admitted to the press will dry to powder, and it can cause severe accelerated abrasive wear.

Initially the press will likely be run empty in order to check rotation. Even though some rubbing may be heard, negligible wear will occur so long that this period is kept to a minimum. Also since the screw is supported to some extent by the material inside the press, running dry may allow the screw to rub the screen.


DOUBLE PRESSING
Some processes benefit from what is called double pressing. This means that the cake coming from the press is run through a second press (or through the same press a second time). If little moisture is removed in the second (double) pressing, then it is known that the liquid removed in the first pressing is all of the free liquid that there is to be pressed out.

Sometimes water is added to the cake in between the first pressing and second pressing. This is done to enhance the recovery of dissolved sugars in the original press cake.

Molasses can be added to press cake between the first and second pressing. This is used to infuse dissolved sugar into the cake, increasing the solids content of the final press cake.

Capital-effective double pressing can be achieved by using an inexpensive Soft Squeeze Series KP screw press for the first pressing, following with a tighter-pressing Series VP/CP in the second position.


MOISTURE CONTENT

A screw press separates free water. This will leave organic water in the press cake. The organic water is either bound to, or part of, the animal or vegetable molecules. Mechanical pressure alone will not remove organic water; it takes heat or chemistry. Frictional heat from the press can remove organic water, but this obviously should be avoided. For chemistry, skip ahead to the Hydrated Lime, Gypsum and Alum section. For heat, see the Fluid Injection section.

To determine the moisture content of a material (feed to the press, press cake, or press liquor), a sample should be weighed and dried overnight at a temperature slightly less than 100o C. (If sugars are present use less than 70º C to prevent caramelizing.) The sample should weigh six or more times the tare weight of the sample tray or cup.

The moisture content of press cake varies considerably. Tomato press cake will be 90% moisture. Orange peel will be 80%, unless it is reacted with hydrated lime, in which case it will go down to 74% moisture; add molasses and it will go to 65%. Dairy and hog manure will come out at 70% moisture, unless there is sand or sawdust in the sample, which will reduce the moisture content. Cellulose fiber from a paper mill (knots, screen rejects, primary clarifier underflow) will come out about 50%. However, if secondary (biological) sludge is added, then the moisture content of the cake will go up considerably. With high ash content in paper mill samples, moisture may go down to 40%. Moisture contents of only 25% can be achieved pressing things like sand, eggshells, glass, and plastic chips.

The heat from steam injection can change the chemistry of the material being pressed so that cake with lower moisture content is produced. This blanching or parboiling effect works with fish and orange peel, for example.

A quick approximation of what to expect from a screw press is to squeeze as much water out with your fist, and figure that the press will do a little bit better. A better way is to twist a ball of the material in a cotton cloth.


COMPRESSION
A screw press achieves compression using several methods: (1) The discharge cone of the press causes back-pressure on the material being dewatered. The higher the cone pressure, the greater the liquid removal. (2) The pitch of the flights of the screw tightens as the material is conveyed through the press. This forces liquid to go through the screen. (3) The diameter of the shaft of the screw may be progressively increased, forcing material outward, against the screen. This is a tapered shaft design.


SCREW WITH TAPERED SHAFT


Force-feeding (supercharging) the press and applying a vacuum to the outside of the screen are two additional methods which may achieve compression. These two are used infrequently because the performance results are uncertain.


PRESS SPEED (RPM)
In general, the slower the screw speed, the greater the dewatering. Longer residence time in the screened area results from lower screw speed, which allows time for more thorough dewatering. Unfortunately, it also goes with reduced throughput capacity.

Screw press speed (rpm) can be changed by using a Variable Frequency Drive (VFD). Alternatively, the drive motor can be switched to a different pole motor (900, 1200, or 3600 versus the standard 1800 rpm). Most modern motors are good for permanent 120 Hertz operation; they are always good for a test at this high speed.

Higher speed can result in premature gearbox failure. Switching to synthetic oil, replacing the normal mineral oil, is recommended. Consult the factory for assistance.

A small change in screw speed, like 15%, will generally not result in a measurable change in performance of the press.

Low screw speeds are used for cooker crumb, potato peel, many sludges, and low freeness materials in general.

It has become normal for a variable speed drive (frequency inverter VFD) to be used with Vincent presses.


CAPACITY MEASUREMENT
The best way to measure capacity of a press is to collect timed samples of press cake and of press liquor. This should be done during a period of sustained, stable operation, rather than by timing a batch through the press.

Press cake is generally captured in a tarpaulin, and press liquor in a 5-gallon pail or 55-gallon drum. When the drain is at floor level, a 3-mil plastic bag can be used to catch press liquor. If the press liquor goes to a pit or tank, the change in depth can be timed.

Sometimes it is possible to collect only one flow, either press cake or press liquor. In these cases it is possible to estimate the press throughput if the solids content of the inbound material and press cake are measured. It is assumed that there are zero suspended solids in the press liquor, although this is never really the case.


A-B-C-D PLATES

There are four vertical plates making up the frame of the press. These are called out in the Nomenclature schematic as the end of this manual.

Starting from the drive end of the press, the first one is the A Plate. This A Plate forms the wall of the inlet hopper closest to the gearbox. The shaft seal plate is bolted to the A Plate.

The next plate is the B Plate. It forms the downstream wall of the inlet hopper. The screen starts at the B Plate. There is a notch, called a Cord Cutter, in the B plate. Also, there may be a bar called Brian's Stripper welded to the B Plate, inside the inlet hopper; it is in a position to kiss the edge of the screw flight as it passes. These two features prevent long fiber pieces from balling up at the exit of the inlet hopper. See the section ahead on Cord Cutters.

The final plate, the C Plate, supports the discharge end of the screen. The discharge cone touches this plate when the cone is in the closed position.

There is a fourth plate, called the D Plate, on which air cylinders and thrust bearing are mounted.


SCREW LIFE
If a press loses its previous throughput capacity, or if cake moisture content increases, it can be a sign of a worn screw.

A screw can last anywhere from six months to twenty years. It depends on the material being pressed and how hard it is being pressed.

Premature screw failure can arise from several causes. The two main ones are:
(a) If a press is allowed to run when no material is being fed into it, the screw can wear out in one or two months. The same can happen if a very low flow is consistently fed into the press.

(b) If abrasive material is dewatered with high cone air pressure, rapid wear will occur. This condition is avoided if a few drops of water can always be squeezed from a fistful of press cake.

Two effective ways to extend screw life are:
(a) Various grades of hardsurfacing rod can be used to protect the flights of a screw. The best hardsurfacing will have Tungsten Carbide impregnated in it.
(b) Using a VFD or lower speed motor to reduce the screw rpm will extend screw life

If a worn screw is suspected, the thing to do is to shut down the press, open the cone, and dig out the cake until the tips of the last two flights can be seen or felt. Check how badly the tips are worn. If the there is 1/2” between the tips and the screen, serious wear is evident. It is also an indication that the sharp edges of the flights throughout the press may have worn, becoming rounded. This can cause the flights to act like a putty knife, plastering solids against the screen, preventing water from coming through.

Worn screws are either restored locally or returned to Vincent for rebuilding. The maximum cost of a screw rebuild is around one third the cost of a new screw.


SCREW CONFIGURATION
Almost all Vincent screw presses use the Interrupted Screw Flight design. The interruptions leave room for stationary resistor teeth that are mounted outside of the screen. These teeth go through the screen and reach almost to the shaft of the screw. This design of screw press stands in contrast to a Continuous Screw design. The main advantage of the interrupted design is that solid material must accumulate in the interruptions until sufficient consistency is reached for the solids to be pushed toward the cake discharge. There is a reduced tendency for the material being pressed to co-rotate with the screw. Also, there is more agitation within the press and, consequently, quicker and more thorough dewatering.

The screw starts with a feeder section of continuous flight. This picks up material in the inlet hopper and pushes it into the screen section. The feeder section ends at the first resistor tooth. This feeder section of the screw is followed by compression stages where the flights have reduced pitch. The reduction in pitch of the flights results in compression of the material going through the press.



INTERUPTED FLIGHTS, RESISTOR BARS, and RESISTOR TEETH


PIE CUTTING

Sometimes the compression of a screw is reduced, in the field, in an operation called "pie cutting". This involves cutting pie-shaped segments from certain flights of the screw, leaving a butterfly (end view) configuration. The modification is done to avoid excessive compression and jamming. The "sterile cut" is more dramatic. Consult the factory for assistance before making this modification.

 
4" PIE CUT PIE CUT


 
NOT PIE CUT BUTTERFLY CUT

 


JAMMING

Should a press trip out on overload because it has become jammed, a series of steps can be taken to un-jam the press. Generally, the easiest thing to do is to reverse the rotation with a VFD or to reverse the leads on the electric motor drive. This will cause the screw to feed material backward into the inlet hopper.

Generally jamming is caused by over-pressing excessively dry material. Running the press backwards will break up this material. If the jamming was caused by tramp material, hopefully this can be found and retrieved from the inlet hopper following operation in the reverse direction.

Having a reversing starter greatly facilitates this operation. These cost little more than a standard starter; they come with a forward-reverse switch. (Having a VFD with a reverse button can be even handier.)

If a press has had extensive use in an abrasive application, the outer diameter of the flights will be worn away at the discharge of the press. Radial wear of 1" to 4" in large presses will lead to serious jamming and, possibly, a burst screen.

When a press is operated in the reverse direction it is possible that solid in the press will be forced against the A Plate. This can damage the shaft seal. For this reason, care should be taken when running the press backwards. Three or four turns of the screw are usually all that is required to un-jam a press.

Usually three or four revolutions of the screw are sufficient to clear a press. If running the press backwards several cycles does not clear the jam, a screen should be removed so that the cause of the jam can be determined. Look for a bent flight. Before going to the trouble of removing the screen, shut down the press and try clearing the end of the press with a bar or long screwdriver.

Sometimes when a press is jammed, a flight on the shaft of the screw will fold. This can happen if the press overloads on dry cake or if tramp metal is caught between a flight and a resistor tooth. The weld at the shaft may tear. When this happens flow through the press is greatly impeded.

 

FOLDED FLIGHTS


SCREEN BLINDING

A common problem is for the screens of the press to become blinded (covered over). When this occurs, the flow of press liquor coming through the screens diminishes. The level in the inlet hopper will fill up to where it overflows.

In some cases, the screens can be cleared by periodically reversing the direction of rotation of the screw. This can be programmed with many VFD’s, so that the press runs forward for a given period and then reverses direction briefly for three or four turns when the screens start to blind. This is one of the easiest possible solutions to test. Sometimes it is the only one that is effective.

Many other methods are used to address blinding: (1) Adding notches to the screw, (2) Reducing or eliminating the pressure in the inlet hopper, (3) Adding press aid to the flow, (4) Changing to a different screen selection, (5) Reducing the screw-to-screen clearance, and/or (6) Employing a screen flush with caustic solution, acid, or high pressure spray.

If blinding occurs after an extended period of satisfactory operation, it is usually due to wear of the screw. Rounded edges of the flights will contribute to blinding.

CHANNELING
A condition somewhat similar to purging can occur with slimy materials, like concord grapes, pineapple pulp, or spent brewer’s grain. These may tend to channel or squirt out from one side of the cone. Two ways to eliminate channeling are to lower the air pressure on the discharge cone and to slow down the speed of the press. Channeling can also be reduced by adding press aid to the material being dewatered, or by reducing the inbound flow to the press.

The rotating cone option is very useful in breaking up channeling. The cone is caused to rotate so that the relative motion between it and the stationary screens breaks the channeling. A pin on the face of the cone will strip away the press cake, breaking up the channeling. If the cake comes out too wet, shorten the length of the pin.

To break up channeling, pieces called wing feeders can be welded to the end tips of the last two flights of the screw. See the Wing Feeder section of this manual.


PURGING
An undesirable condition can occur when the material being admitted to the press purges, without liquid-solid separation, from the cake discharge. This can occur especially if pressure exists in the inlet hopper.

Mechanically, purging occurs when a dry lump of press cake holds open the discharge cone. Un-pressed material will flow around this partial plug.

Purging may occur when there is a much reduced, small flow of cake coming from the press. Usually this is a sign of blinded (covered over) screens. This can be caused by a worn screw. Liquid from the inlet hopper will wick into the press cake, making it soft enough to blow out. Sometimes this condition is avoided by mounting the press inclined at about 5o above horizontal; the simplest way to do this is to place a block under the cone end of the press.

A drop in operating amps can be an indicator that a purging condition has begun. An ammeter circuit can be installed to alarm or trip the system when a reduction in motor amps occurs. This is rarely done.

Purging is prevented with the rotating cone option. To use it, it is necessary for the cone drive to be engaged so that the cone spins with the screw. Pins on the face of the cone will strip away the press cake, preventing it from holding the cone open. If the cake comes out too wet, shorten the length of the pins.


BRIDGING

Sometimes bridging will occur at the inlet hopper, preventing material from flowing into the press. If an independent surge hopper is mounted over the inlet of the press, it should have at least one, preferably two or three, vertical walls. This will minimize bridging.

Bonding Teflon sheets to the inlet hopper of the press is a remedy that has been used to reduce bridging of bulky materials which allow free-draining of water.

A vibrator, mounted on the side of a feed hopper, may also alleviate bridging.

One way to overcome this is to direct a stream of water into the inlet hopper to break the bridge. The nature of the screw press is that essentially all of this added liquid will be removed in the pressing operation. (It may be convenient to pump a jet of the press liquor into the inlet hopper to break the bridging.) This is rarely done.


RESISTOR TEETH
The interrupted screw design press has stationary teeth that protrude into the flow of material as it passes through the press. These fit into the gaps of the screw where there is no flighting. They stop just short of the shaft of the screw.

The resistor teeth are an integral part of the Resistor Bar. These resistor bars are positioned axially, parallel to the screw, with one bar above and one bar below the screw. The resistor bars are bolted between the B and C plates; they form a part of the structural frame of the screw press. The screen frames bolt onto the resistor bars.

Rarely the resistor teeth are shortened, usually by half, to increase the capacity of the press. Removing the teeth altogether will result in co-rotation and jamming.

Not infrequently the resistor teeth are drilled so that fluid can be injected into the press during operation. See the next section.


FLUID INJECTION
Resistor teeth can be drilled so to permit injection of steam, solvent or water while the press is in operation. Also, these modified resistor teeth can be used for CIP cleaning, without the need of removing the screen from the press.

Commonly alcohol injection is used to achieve in-line washing to remove sugars. Hot water injection is used to recover dissolved solids in juice production. Steam injection is used in dewatering raw organic materials.

The moisture reduction that results from steam injection is related to a chemical change that comes with blanching, or parboiling, a material. Steam injection works well on pineapple skin, citrus waste, and raw fish. Tests run with steam injection in a Vincent press at Anheuser-Busch showed little benefit. The material being pressed, spent grain, had already been "cooked" before steam was added.

Injection is achieved by drilling holes through the resistor teeth and piping these holes to a manifold outside of the screen. Photos and drawings are available from the factory. Vincent does not charge for providing a drilled resistor bar.


 
STEAM INJECTION                                                            LAB PRESS
IN TWIN SCREW PRESS                                                   STEAM INJECTION

CORD CUTTER AND STRIPPER
Sometimes long stringy material will be pinched where the feeder portion of the screw goes through the hole in the B Plate. This material will co-rotate with the screw and build into a bundle which reduces the flow through the press.

A groove, like a keyway that is 3/8" deep, is cut half way through the hole in the B plate. We call this a Cord Cutter. Material trapped between the flight and the hole in the B plate will pop up slightly as it passes the Cord Cutter. The result is that the material is sheared loose.

Sometimes a part called Brian's Stripper is welded to the B Plate, inside the inlet hopper. It goes in a position so that the flight lightly kisses the stripper as it goes past. This strips the material away. Strippers are made of square bar stock.



STRIPPER AND CORD CUTTERS

POLYMER
Infrequently the addition of polymer may be required in order to achieve adequate screw press performance. Polymers are added to dilute waste streams, especially to those containing very small size suspended solids. The long chain molecules of the polymer will flocculate the solids, agglomerating them to the point where they can be pressed. Under the right conditions, drastic improvement can be observed in press throughput, press cake moisture, and press liquor clarity.

Nalco and GE are the leading polymer suppliers. Their sales engineers are anxious to recommend the product best suited for your application.

Low speed operation of the press is usually required in order to achieve good performance.


NOTCHES

Sometimes it is necessary, during press operation, to have the screw wipe the screens clear of blinding material. This is best achieved by having notches in the outer edge of the screw. Fibrous material accumulates in the notches and brushes away slimy material which may be blinding the screens. Shallow notches (1/8" wide by 1/8" deep, 1-1/2" apart) in the outer edge of the screw flights work well. Typically, notching is done from the B plate to the second resistor tooth. Most Vincent presses are supplied with notches.



GRINDING NOTCHES IN THE FLIGHTS

 
WIPERS

Before the advent of notches, wipers, made of UHMW strips or nylon brushes, were sometimes mounted to the outer perimeter of the screw flights. If at all possible we advise against the use of such wipers because (a) they tend to cause material to co-rotate and jam in the press, (b) they are difficult to replace, and (c) they wear rapidly, resulting in unacceptably frequent shutdowns for replacement. Wipers tend to improve dewatering performance for the first week or so. After that the wiper material wears and press performance reverts to being just a little better than if no wipers were used. Wipers are made either of 1/4" thick high durometer polyurethane or of nylon bristle brush. Wipers are preferably mounted on the downstream edge of the flight.


PRESS AID

Some materials press best if a press aid is mixed into the material to be pressed. Typical press aids are rice hulls, cottonseed hulls, cellulose fiber from a paper mill, and ground newspaper. Ground wood is the best, but most expensive, press aid.

Press aids are most commonly used in producing juice from deciduous fruit. The press aid gives the press something to get a bite on. Press aids also tend to hold back fines (short fibers) and prevent them from going through the screens with the press liquor. If apples are fed into a press, apple sauce will come through the screens. However, if a press aid is added to the apples, then apple juice will come through the screens.

Typically, the amount of press aid used is only 1% to 3% by weight of the flow going through the press. This will look like more than such a small percentage because press aids have a much lower bulk density than the wet materials that are pressed.


HYDRATED LIME, GYPSUM, AND ALUM
Lime (calcium hydroxide) must be added to citrus peel before it can be pressed. The lime breaks down the pectins or cell walls so that the press can remove moisture. Less than 1% by weight is used. A reaction time of several minutes must be allowed prior to pressing. Lime has been used successfully in the same manner with potato, onion, tomato, carrot, and pineapple waste. It works well on acidic materials such as strawberries and coffee bean pulp. Vincent offers lime dosing equipment.

Gypsum and alum salts are also effective chemical press aids. They are typically used in dewatering sugar beet pulp, and they have rarely been found effective on other materials.


VACUUM EFFECT
In some applications, increased screw press capacity can be obtained if the area outside of the screen is under a vacuum. This can be achieved by mounting the press at a high elevation, with the press liquor drain line dropping below the surface of a drain tank or pit.

That is, the drain line from the press should go below the surface of the pit or pond into which it drains. If this line is relatively small in diameter and has a steady downward slope, a vacuum will be induced around the screen of the screw press. The mass and velocity of press liquor flowing through the drain line create this vacuum. To draw air bubbles downwards with the press liquor, the velocity of the fluid must be greater than five feet per second.

The covers over the screen of the press will have to be sealed, usually with Silicone.

The amount of vacuum is a function of the elevation between the press and the drain pond. For good results, the press should be mounted on a stand that is 20' tall or higher.


PRESS LIQUOR
A screw press produces relatively "dirty" press liquor as compared to a Filter Press or Belt Press. Suspended solids will pass through the screen of the screw press along with the liquid being expressed from the inbound material.

If suspended solids need to be removed from the press liquor, the most common method is to pump the press liquor either over a static (sidehill) screen or through a rotary drum screen. Generally, the screen tailings (sludge solids) are fed back into the screw press along with the flow of inbound material. Most of these fines will be captured with the solids of the inbound material and end up in the press cake. Although some of these tailings will once again go through the press screen with the press liquor, equilibrium of recirculating solids is reached and satisfactory pressing operation is realized.

If the press liquor is to be concentrated in an evaporator, better screening than can be achieved with a static screen may be required. Another Vincent machine, the Fiber Filter, provides premium performance. Decanters or centrifuges may be required.


SCREEN SELECTION

The screen of the press is made either of wedgewire (slotted screen) or perforated stainless sheet (round holes). Wedgewire screens are expensive one-piece weldments that must be replaced when excessive damage or wear has occurred.

Wedgewire screens in Vincent presses can be reversed in order to achieve double life. That is, wear starts at the cake discharge end of the press. When this occurs, the screen can be turned 180o so that the fresh inlet section is then located in the discharge area.

Screens made of wedgewire come standard with 0.015" to 0.020" slot width; they are also available with slots that are 0.008" to 0.060" wide. With slot widths less than 0.012" there is a tendency for the screen to blind (be covered over) with the material being pressed. However they work well in alcohol and oil separation. Changing the slot width generally has little impact on the clarity of the press liquor or the dewatering capacity of the press.

The most common damage to a wedgewire screen is for part of the surface to be smeared over by rubbing the screw. This rarely is bad enough to affect press performance. Wedgewire screens generally work satisfactorily with 30% or even more of their surface smeared over.

Smeared screens can be remedied by running a box cutter blade through the slots.

TIG welding is used to close the gaps in the case where either a few wedgewire slots get spread apart by tramp material or a perforated screen gets torn.

Perforated metal screens are simple sleeves which are held in the screen assembly. These are less durable but inexpensive to replace. Standard perforated screens have a hole size of 3/32" diameter, although material with 0.060", 0.040", down to 0.023" holes can be supplied.

Frequently, increased press capacity can be achieved by changing a perforated screen to one with smaller holes. This unexpected result arises from a combination of factors: (1) smaller hole screens are made of thinner sheetmetal, so that the press liquor has a shorter distance to travel before it falls free from the screen, reducing the chance of sponging backwards through the screen and (2) particles which fall into and plug a larger hole will roll over a smaller hole. Minor rubbing between the screw and screen is normal, although, obviously, hard rubbing will cause wear and premature failure of the screen. With a clearance greater than 3/16", the dewatering performance of the press can start to deteriorate; this depends a lot on the nature of the material being dewatered.

In cases of severe wear or damage, it is common to patch a screen. Stainless sheetmetal is used for this. The reduction in drainage surface is of little consequence as the screens have ample open area.

The most common cause of screen failure ties to failure of the outboard support bearing. If the bearing holding the end of the screw wears out, it can let the screw move enough to rub against the screen.

 
PROFILE BAR (WEDGEWIRE) SCREEN


 
PERFORATED SCREEN WITH REINFORING PLATE

 

SCREW-TO-SCREEN CLEARANCE
Generally the clearance between the screw and the screen of a Series VP press is 1/16", plus or minus 1/16". The screw should not rub the screen hard, as it can cause wear and premature failure of the screen. Tight clearance is used with materials that blind the screen, such as onion skins. Greater clearance, 1/16", is used with eggshell, pectin, xanthan gum, and corn husk. With a clearance greater than 3/16", the dewatering performance of the press can start to deteriorate; this depends a lot on the type of material being dewatered.

To measure the s2s clearance in wedgewire screens, a feeler gauge can be slipped through and along a slot until it hits the edge of the screw. Measure from the outside of the screen to the edge of the flight; then subtract the thickness of the wedgewire (generally either 0.25" or 375") from the measured depth in order to calculate the clearance.

Alternatively the screw-to-screen clearance can be checked by removing one half of the screen and bolting the other half tight to the resistor bars. Inspection is made from the side from which the screen half has been removed.

In the case of perforated screen installations, a depth gauge can be used to measure the screw-to-screen clearance. This is done by first finding an area where the screw flight is next to the screen; poking a straightened paper clip through the screen is handy for this purpose. The depth from the outside of the screen to the edge of the flight is measured, and then the thickness of the screen is subtracted from that measurement. [3/32" perf is 0.075" thick; 0.050" perf is 0.050 thick; 1/32" perf is 0.024" thick; 0.023" perf is 0.015" thick; 3/8" perf back up screen is 0.120" thick.]

If a screw rubs against the screen in a given area, it may be best to grind some off the OD of the screw. Prussian Blue can be useful in finding the spot that is rubbing.

 

WEDGEWIRE DEPTH GAGE PERFORATED DEPTH GAGE

 

CONE BUSHINGS
The cone rides on the shaft of the screw. "Cone Sleeve" is the name given to the portion of the screw on which the cone rides. There will be two bushings in the cone to support and guide it, and to protect the Cone Sleeve journal surface of the screw shaft. Sometimes these bushings are lubricated with liquid from the material being pressed, such as the juice from apples or water from pectin peel.

A grease fitting is provided for lubricating the bushings and to minimize leakage of press liquor through the cone. If a single grease line is used, the grease line goes to a pocket, which serves as a grease reservoir, located between the two bushings.

Bushing lubrication is extremely important when materials that are dry (like paper mill screen rejects) are being pressed. By the time such materials reach the discharge of the press, they do not have enough free moisture left in them to adequately lubricate the cone bushings. In these applications the operator should, at the start of each shift, pump grease in until it comes out between the cone bushing and the screw shaft. Then he should open and shut the cone three times in order to spread the grease.

Rarely, presses are supplied with additional lubrication fittings so that water, in addition to grease, can be metered to the bushings as a lubricant.

Automatic grease systems are available. These should be the high pressure (900 psi) electric or battery variety. Vincent provides these for critical applications, especially pulp & paper.

Liquid leaking past the cone bushings drains out the back of the cone (at the air cylinder end of the press). Almost always it is minimal compared to the flow of press cake. However, a pan can be provided to collect this liquid and drain it into the main flow of press liquor.


WING FEEDERS
Sometimes there are blades welded to the outside tips of the last two flights of the screw. Called "Wing Feeders", these are mounted parallel to the discharge screen surface. Care must be taken that wing feeders are not made so long that they hit the face of the cone when the cone is in the closed position.

Wing feeders can serve two purposes. (1) In the case of materials that want to channel out the discharge of the press, like pineapple and spent brewer's grains, long wing feeders break up the channeling flow. (2) For abrasive applications, short knobby wing feeders are provided as sacrificial wear elements.

When certain materials are fed through a screw press, clumps of dry material may pack in front of the wing feeders. This buildup can cause wear of the screen. Should the problem occur, grind off the wing feeders.

 

 
LONG WING FEEDER KNOBBY WING FEEDER


CLEANING

Commonly, material is cleared from a press by stopping the inbound flow, setting the discharge cone in the withdrawn position, and running the press for a few minutes until no further material is discharged. This will leave some material inside the press, which can be handy for forming a plug at the cake discharge when the press is restarted.

Material will leave a Vincent interrupted flight press only if there is additional inbound material forcing it out. This makes it difficult to clear all material from inside a press without removing the screen. One technique used successfully is to feed crushed ice into the press. Water must be fed along with the ice to prevent jamming. When the ice melts, the press will be relatively clean inside.

There are applications in which the press must be cleaned frequently, such as once a shift, in order to meet sanitary regulations. The screen halves can be hinged to facilitate this. In other cases, the screens are removed from the press. A spare screen assembly may be kept, submerged in cleaning solution, in order to minimize the downtime required.

Cleaning the inside of the screen can be achieved, at least to some extent, by injecting water through the resistor teeth. Holes must be drilled in the resistor teeth to make this possible.

It is unusual that the outside of the screen needs to be cleaned. Spray systems for this can be built into the press at the Vincent factory. Alternatively, a pressure washer or swabbing with acid solution can be used.


SHAFT SEAL
The Seal Plate is bolted to the A Plate. This may be solid UHMW (ultra high molecular weight polypropylene or polyethylene) or it may contain one or two Johns Manville (JM Clipper) lip shaft seals. There may be a grease fitting on this plate; the grease is used to reduce leakage and to help prevent fiber material from entering and damaging the screw shaft.

Generally, seals are allowed to drip once they start leaking. They are replaced only in conjunction with major maintenance, as when the screw is removed from the press.

In some cases we have found that leakage from a shaft seal can be stopped by simply selectively loosening or tightening the four bolts holding the seal housing to the A plate.

 
SEAL PLATE SPLIT SEAL PLATE

SCREW REMOVAL
The screw is removed through the hole in the C Plate, at the cake discharge end of the press. The operation can be difficult the first time, so we recommend consulting with the factory before getting started.

Start by removing the four bolts holding the shaft seal plate to the A Plate. This will prevent damage once the screw is loose.

To remove the screw from a Series VP press, the shaft coupling between the gearbox and the screw must be undone. Then the gearbox must be moved out of the way so that the shaft coupling can be pulled from the end of the screw. Falk gear-type couplings are heated to 350º to 500º F so that they can be pulled off; use a temperature stick to measure this temperature.

Next the pillow block bearing is removed.

A hydraulic jack is used to pull the coupling half and the PB bearing from the shaft.so that they can be pulled off; use a temperature stick to measure this temperature.

Next the pillow block bearing is removed.

A hydraulic jack is used to pull the coupling half and the PB bearing from the shaft.

The bars with the resistor teeth must be removed before the screw can be removed.

If the flange bearing is mounted on a circular plate which also holds the air cylinders, do not remove the flange bearing by itself. Instead, leave it on the round plate and remove the air cylinders and bearing as a single unit.

If the coupling half, pillow block bearing, or flange bearing have become seized to the screw shaft, it will be necessary to cut them loose. It is recommended that spare parts be on hand before replacing a screw in a VP press.


SCREW REPLACEMENT
In the case of the Series VP presses, jacking bolts and shims are used to achieve proper alignment. Only after the screw is aligned within the screen can the gearbox be aligned to the screw (not the other way around). When working with gear-type couplings, be sure to leave the prescribed 5/16" gap between the gearbox shaft and the screw shaft. This will protect the gearbox from thrust loads.

Falk gear couplings must be heated to 350º to 500º F in order to slip onto the shaft.

It is important to hand-pack grease into a Falk coupling. Do not rely on the grease fitting because the grease will only lubricate in the path of least resistance.

There are two ways to avoid excessive rubbing between a new screw and the screen: Either shims can be placed between the screen frames and the resistor bars, or the interference can be ground off the edge of the screw. To eliminate the high spots, coat the edge of the screw with Prussian Blue, bolt the screens in place, turn the screw, remove the screens, and grind the screw where interference has occurred.


SCREEN REPLACEMENT
For ease of maintenance, the screen halves are held in frames that are split vertically, being bolted to the resistor bars. This allows the screen halves to be removed from the sides of the press.

In the case of screen failure, frequently a solid patch can be welded onto the screen, from the outside. This is simple as the screen need not be removed from the press.

Wedgewire screens may become smeared from being wiped by the screw or by hard press cake. Wedgewire screens generally work satisfactorily with 30% or even more of their surface smeared over. Usually press liquor will come through a smeared area of a wedgewire screen. If it becomes an issue, it is corrected by running a box cutter blade through the slots.

To replace a perforated screen, the screen frame must be removed from the press. Damaged screens should be removed and discarded. The replacement perforated screen first must be clamped tightly against the supporting frame. This can be done by forcing the new screen against the frame, using C-clamps and wood or steel pieces. Be sure that extra screen material sticks out beyond the edges of the frame, to allow trimming with a hand grinder.

Usually perforated screens are tack welded in place. However, if the perforated screens bolt in place, once the new screen is tight against the frame, look through the screen and locate the holes in the frame where the attachment bolts go. Use a center punch to open holes in the screen. These holes must be large enough to allow the attachment bolts to go through the screen and thread into nuts on the far side of the frame.

Once the new screen is tack welded or bolted tightly in place, beat over and grind off the excess screen material.

There are two ways to avoid excessive rubbing between a new screen and the screw: Either shims can be placed between the screen frames and the resistor bars, or the interference can be ground off the edge of the screw. To eliminate the high spots grind the screw where interference has occurred.


GEARBOX BASICS
The Series VP presses use foot-mounted gearboxes. The foot-mounted gearboxes are most commonly concentric (the motor shaft is in line with the output shaft), although occasionally a parallel shaft reducer is used. The VP presses can use either a C-face motor coupled directly to the gearbox or a foot-mounted motor with either a coupling or a belt drive.

Gearboxes are rated, and sold, by their torque rating. The manufacturers generally offer their designs in progressively larger sized castings, or boxes. The larger the box, the larger the torque rating. Each box size will be available with different gear ratios. In order to keep the torque fairly constant, larger horsepower motors are used with the high speed boxes. Similarly, reduced horsepower motors must be used when a low output speed is selected.

Screw presses are designed around the size of the gearbox that is selected.


PRESS LUBRICATION

Lubrication is something we generally review with customer personnel during start-up. It is pretty straight forward:

CONE BUSHINGS: Once a shift
BEARINGS: Weekly
BUSHINGS: Weekly
SHAFT SEAL: Weekly
GEARBOX: Annually
AIR REGULATOR: Whenever empty (if a lubricator is being used)
GEAR COUPLING: Whenever opened pack by hand, covering all teeth.
MOTORS: Never

The most critical lubrication item has to do with the cone bushings. Before starting up a new press, the cone should be run in and out a few times to spread the grease around.

Lubrication of the cone bushings depends a lot on what is being pressed. With orange peel, there is enough press liquor juice acting as a lubricant that the bushings are lubricated only at the end of the processing season (to keep them from locking up on dried-out peel juice). The other extreme is at a paper mill where boiler fuel is made out of reject fiber. There is no free water left in the press cake, so everything is very hot and dry. We automatically supply a 900 psi ATS autolube on paper mill jobs. Another tough application is with vapor tight presses where there is apt to be solvent getting into the cone bushings. The solvent can dissolve and wash out the grease, so we like to see frequent lubrication of the cone bushings. (Food grade grease is used in this application since food ingredients are being produced by the press.)

If Vincent supplies an autolube for the cone bushings, it will be either battery powered or require a hard wired power supply. This should be actuated when the press is first placed in service. It should be set to give one small shot of grease every couple hours. We provide autolubes with large grease reservoirs, so they will go at least two weeks at the maximum greasing schedule. Once operations are stabilized, it may be practical to reset the timer dip switches so that it gets one shot every shift or once a day.

In any case, we tell the operators to run the cone open and closed once a shift because this will spread the grease around. This is done with the press in operation.

If Vincent does not provide an autolube for a critical operation, we generally tell the operators to manually grease the cone bushings once a shift. We also tell them to run the cone in and out when they do the greasing, in order to spread the grease.

The bearings and/or bushings holding the screw get greased on the customer's normal schedule for that type of bearing, maybe once a week, or once a day, or once a month. Whatever grease the customer normally uses will be fine.

The shaft seal housing may have a grease fitting. This grease is to prevent fiber from getting into the seal. The seal should be given a shot of grease whenever the screw support bearings or bushings are greased.

The gearbox oil should be changed once a year. Use mineral oil for a normal 1800/1500 rpm input. Use the same grade oil, but synthetic, for input speeds of 2,400 rpm or more. (Sumitomo Cyclo's, parallel shaft gearboxes, and Brevini and Bonfiglioli planetaries, are exceptions to this. The OEM manuals from these suppliers detail lubrication requirements.)

The air regulator used with the discharge cone air cylinders may have a lubricant jar. If so, Vincent includes a can of light oil along with the air regulator which comes with our screw presses. The jar should be filled when placing the press in service and when the jar is empty, about once a year. It takes very light (sewing machine) oil. The oil helps prevent corrosion inside the air cylinders. (Most air cylinder manufacturers no longer recommend the use of lubricators.)

If there is a Falk gear-type shaft coupling, Vincent packs these with lubricant grease prior to shipment from the factory. The grease is re-packed only if the coupling is opened (which is rare). Packing must be done by hand so that all the gear teeth have grease.

We have never seen nor heard of anyone greasing the motor bearings.


REPLACEMENT PARTS

Most replacement parts are standard OEM components that are purchased from their original manufacturer. The specification of these items is included in the O&M Manual. Only shaft seals and bearings are apt to require replacement.

The most common wear parts in the Vincent Press are the screens, the screw, and cone bushings. Vincent stocks these for the more popular models. Screws are generally rebuilt at the Vincent factory. Be sure to specify the Serial Number of your press when ordering replacement parts or repairs.


SAFETY
These Operating Hints have left unstated the obvious safety hazard: A screw press, like any screw conveyor, is totally unforgiving. If clothing or a limb gets caught in a rotating screw, the screw will not stop. Vincent Corporation has heard of only one injury of this nature with a screw press; do no let yourself become the second. The use of common sense is all that is required.

Robert B. Johnston, P.E.



 

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HINTS-VP.pdf881.97 KB

Operating Hints - TSP Presses

(Scroll to bottom for pdf version)

August 16, 2014

RIGGING
Be sure to properly support the press when lifting it from the truck. Do not lift just one end and drag, as it is possible for the frame to deflect, which in turn can shift the screws' positioning within the press. Interference between the screws and the screens can result.


INSTALLATION
Be careful to not to rack the press when tightening the mounting bolts. In the case of large presses, do not just bolt or weld the press down to a level foundation! Instead, first place the press where it is to be installed. Next, place shims between the press frame and the steelwork (or floor, concrete pedestal or foundation) to fill any gap where the press is to be anchored. Only after shimming (or grouting) should the press be pulled down tight. Doing otherwise can rack the frame of the press, and this can cause screw-to-screen interference.

A large press must be mounted solidly, preferably to a foundation or structural steel. If a press draws its full rated horsepower without the press being anchored to the floor, the frame of the press can twist.

For maintenance, the screws are removed through the cake discharge end of the press. Allow the space required.

Material can be fed into the press many ways. You may need to allow for return feed of overflow material in the event that more is fed to the press than it can take.

Spill containment is a consideration which should be taken into account, because it may be possible for un-pressed material to purge from the cake discharge of a press.

We recommend that a manual disconnect, for killing power to the motor, be installed close to the press.

When material is piped to a press in a closed piping system, it is important to have a 2" vent line open to the atmosphere, along with an overflow return line. The vent is necessary to prevent a siphon effect which can induce a vacuum in the inlet hopper and reduce press capacity. The return line should empty above the surface of the return pit. The overflow should fill less than half of the cross section of the return line.

It order to provide space for a cake take-away conveyor, the press can be installed tilted with an elevated discharge.

Similarly, it is possible that, when operating intermittently with very wet material left in the press between runs, the liquid may loosen the cake plug at the discharge. This might cause the plug to blow out upon start-up. To minimize this condition, the press can be installed with the discharge tilted upwards.

It order to provide space for a cake take-away conveyor, the press can be installed tilted with an elevated discharge. Consult the factory, as the oil level in the gearbox may have to be adjusted.


INVERTER VFD & PLC CONTROL

An inverter VFD must be used to start, protect, and operate a twin screw press. With a VFD it is possible to establish the optimal combination of speed and discharge cone air pressure. The VFD also can be used to reverse the press in case of a jam or to slow it down during upset conditions.

Nine presses out of ten will operate unattended, indefinitely, and just fine at line frequency of 50 or 60 Hertz. If two screw presses are mounted in parallel, they are usually fed with a screw conveyor which drops to fill the first press, with the rest going to the second press; this is followed with a drop-out for overflow.

However, we need to address the exceptions:

Use of level controls is becoming more and more common. These are used to regulate either the flow going into the press or to regulate the screw speed.

In some applications a press is sized for handling upset conditions of large flow, while the normal flow is quite small. In these cases a level control is used, and the PLC can be programmed to turn off the press when a low level is reached in the inlet hopper, and the press re-started when a higher level is signaled.

In some cases the press will tend to jam, overload, and trip out on high amps. In this situation it may be necessary to program the controls so that the cone automatically goes open on high amps, re-closing at a lower set point. This arrangement requires a solenoid operated 4-way air valve, replacing the manual valve which is provided with the press.

In other cases of jamming, a simpler arrangement is to install a Cone Timer. A timer is used to periodically open the cone. The closed period is determined by the amount of time required for press cake to accumulate in the press. The duration of the "cone open" period is long enough to dump most of the press cake that has formed. This type of operation may be used if the press periodically experiences jamming or overload due to fluctuations in the amount of material being fed into the press. Alternatively, it may be used with slippery or slimy press cake that cannot be dewatered to sufficient firmness to force the cone open. Cone Timer panels are available from Vincent at no charge.

Some applications require the use of a specially programmed variable frequency drive. In this case the VFD is not used to change the speed of the press, but, rather to set it for auto-reversing operation. By having the screw run backwards for three or four turns every few minutes, some difficult-to-dewater materials can be pressed much more effectively. This operation can help a great deal with material which tends to blind (cover over) the openings in the screen. Vincent has loaner VFD's if you want to give it a try. The technique works well on bar screens; care must be taken with perf screens so that the screw does not snag the screen during the reverse cycle.

Once through start-up, the cone is almost always permanently left in the closed position at whatever air pressure has been found to be effective. A plug of cake will be left around the cone whenever the press is turned off; this will clear on its own accord on restarting the press.

However, some materials may set up and become hard or freeze within the press when the press in turned off. This is especially true in the case of pressing wet coffee grounds or outdoor installations. For these applications it is advisable to open the cone for a period of one minute before turning off the press. This allows the press to partially empty itself, fluffing the material left in the press. Vincent can provide information for automating this procedure.


INSTRUMENTATION

Twin screw presses are very sensitive to changes in screw speed. This is one reason a VFD should be used with the drive motor. A more important reason for using a VFD is that it affords instantaneous protection in case of a surge in motor load. Such a surge could indicate that tramp material has entered the press or that the overlapping screw flights are interfering with each other.

The VFD should be set to read motor load or amperage. This is the most useful reading to have when testing a press is an ammeter. The load drawn by the drive motor of the press is indicative of how much work the press is doing. The higher the amps, the better the dewatering. Also, the higher the amps, the closer the press is to jamming, and the greater is the abrasive wear. Very low amps indicate little dewatering is being done; the screen is blinded; low compression is taking place; or the flow into the press has stopped.

A moisture balance is valuable for measuring the moisture content of the inbound material and of the press cake. If an oven is used to dry samples, be sure it is set at 160o F or less if there are sugars in the sample. Samples should be left in the oven overnight. The tare weight of the pan should be much less than the weight of the sample which is being dried.

As mentioned previously, level controls can be useful in operating a press. With a signal providing the depth of material in the inlet hopper, the speed of the press can be varied to match the flow going into the press. With egg shells, a simpler level control is used to only signal when a high level is reached; its signal will increase the screw speed. In special cases the press can be turned off when a low level is reached and re-started when a higher level is reached.

In the case of pressing liquids that contain dissolved sugars or salts, a refractometer is valuable for assessing press performance. The Brix of the inbound flow, the press cake, and the press liquor will all be the same figure. The higher the Brix, the higher will the solids content of the press cake.

If dissolved (soluble) solids are present, the suspended (insoluble) solids (fiber) in the press liquor are generally measured by filtering and washing a sample and drying the filter paper in an oven. Dissolved solids will be washed from the sample during the washing process.


IMPORTANCE OF SYNCHRONOUS TWIN SCREW DRIVE

Synchronous turning of the two screws of a Twin Screw Press is of paramount importance. Should one screw get out of step with the other, severe damage can result to both the screws and screens.

In some twin screw presses, the screws are kept synchronized by a pair of spur gears, one driving the other. If tooth wear occurs, it can lead to tooth breakage. This would allow one screw to run into the other. For this reason the spur gears should be inspected frequently for any sign of wear.

If for any reason screws are to be removed from a Twin Screw Press, care should be taken so that proper synchronization is established upon re-assembly.


MOTION DETECTOR

Most twin screw presses come with speed indicators mounted on the outboard ends of both screws. These are part of an important safety interlock. Their signals are fed to a control box which sends a signal to the VFD to shut down the press should any difference be noted between the speeds of the two screws.

The screw shaft flights over-lap in the twin screw presses, and the flights on one shaft are 90 degrees off-set from the flights on the other shaft. Should one screw stop rotating, or rotate slower than the other, this 90 degree offset could become zero, at which point the overlapped flights crash into each other and serious damage is inflicted.

There are two reasons the screw shafts may rotate at different speeds, causing the motion detector to shut down the press:
1. One screw shaft breaks, either inside the screen or at the gearbox coupling.
2. The gearbox or drive gears fail.
The motion detector interlock is used because one of our competitor's makes a twin screw press and they experienced a failure of this kind, causing their press to "self destruct".

The following explains how the motion detector works and what you need to do during installation for the system to protect your TSP.

The motion detectors are simply solid shaft encoders coupled to the outboard end of the screw shafts. These encoders send a signal to a "rate meter", which is mounted in an enclosure on the tailstock of the press. The rate meter compares the signals from each encoder and calculates (RPM shaft 1):(RPM shaft 2). This ratio should be 1:1, and if the ratio exceeds 1.100 or 0.900, the relay output of the rate meter de-energizes. The rate meter is powered by 115 VAC and is supplied with a 3-prong plug.

In order for the system to work, the relay outputs of the rate meter need to be connected to the relay inputs of the variable frequency drive (VFD). The VFD should be programmed to stop when either relay is de-energized. In the event of shutdown, the relay output of the rate meter latches shut, so it is necessary to cycle the power to the rate meter in order to restart the press.

The encoders are coupled to the screw shafts with a flexible coupling. Should one of these break, there are two spares shipped with the machine. Vincent Corporation can also supply replacement couplings if needed.


START-UP

Before putting power to the screw press, it is advisable to bump the motor or even rotate the screws by hand. This will prevent damage to the press in case tramp material has been left in the press. Also, the screws may have shifted so as to hit the screen. (Minor rubbing is normal; it will go away once there is material in the press.) To turn the screws by hand, remove the fan guard on the motor and turn the fan blades. Alternatively, on belt-drive presses, remove the belt guard and pull on the sheaves or belts; be careful.

The left hand (port side) screw of the press turns in a counterclockwise direction, when viewed from the drive end of the press.

If problems are encountered, they are apt to be blinding, jamming, channeling or purging. There is a section for each of these ahead in this manual.


FEEDING
Material can be fed into the press many ways. Commonly, screw conveyors, pumps, transition chutes, pre-thickener screens or cyclone separators are used. Always make provision for return of overflow material, in the event that more is fed to the press than it can take. Spill containment should be considered.

Sometimes either a static (sidehill) or a rotary drum screen (RDS) must be mounted over the inlet hopper to prethicken the flow ahead of the press; the tailings (solids) from the screen are funneled into the press. This arrangement is desirable when the feed to the press is dilute.

Also, material can be dropped from a shredder or cyclone separator into the press. A shredder is used to increase capacity and dewatering in the case of low bulk density materials like lettuce leaves, alfalfa, onion peel, and cornhusk, or to prevent blockage.

Most commonly, the best screw press performance is achieved if the material in the inlet hopper stays just over the top edge of the screw. Usually presses work the best with only atmospheric pressure in the inlet hopper. In order to minimize static head, press headboxes are kept short, and level controls are used to minimize the depth.

When a pump is used to feed the press, the system can be either open or closed. We recommend the open system where little or no pressure exists in the inlet hopper, thus preventing the press from being force-fed.

A pump can be used to force feed a press. However the superb feeding characteristics of the Twin Screw Press can be expected to void the need for such force-feeding. Nevertheless, if the press is force fed, there should be a line that allows material to recirculate past the press inlet. This will prevent pressurizing the inlet of the press, which can cause both blinding of the screen and purging from the cake discharge.

A port on the side of the inlet hopper is frequently provided on larger Vincent presses. It is used to view the level of material over the screw. It has a bolted cover because it is rarely used.

If a fluid flow is piped through a sealed cover which is bolted to the inlet hopper, force-feeding is possible. However the superb feeding characteristics of the Twin Screw Press can be expected to void the need for such force-feeding. Nevertheless, if force feeding is used, a by-pass tee should be provided so that the pressure in the inlet hopper is minimized. In addition, a 2" vent line, open to the atmosphere, must be provided to prevent siphoning material in the inlet hopper out through the recirculation line.

Inlet hopper pressure over one to four psi can force fibrous material against the screen so as to blind off the screen, resulting in unsatisfactory performance.

At pressures above 10 to 15 psi in the inlet hopper, it is possible to blow the "plug" of press cake that forms at the discharge of the press. Unscreened liquid will purge from the cake discharge. Exercise caution if either hot or hazardous material is being pumped into a press.

At inlet hopper pressures of 40 psi and above, the shaft seals will be blown out of their housing. At pressures around 60 psi the screen will start to separate from its support plates, resulting in bypassing of feed material directly into the press liquor flow.


BUILDING A PLUG
In order for the press to work, a plug of cake must form between the cake discharge opening and the pressure cone. The press will almost always do this on its own accord as material is fed into the press.

In the case of sloppy materials like manure and DAF sludge, it may be advisable to start off by first packing the discharge of the press with any available fibrous material.

Alternatively, the press can be turned on and the feed pump allowed to run just long enough to fill the feed line and the press. Then shut off the pump, leaving the press running, and wait until no more liquid drains from the screen of the press. Repeat this process until a plug of cake starts to open the cone.


PRE-THICKENING
Almost always, the thicker a flow going into a press, the better it will work.

If the flow into a screw press is too dilute, the high volume of liquid going through the press screen can cause either of two problems. The flow may either flush most of the solids through the screen, or it may plaster solids against the screen, thus blinding (covering over) the screen.

To prevent these things from occurring, it may be necessary to pre-thicken the flow ahead of the screw press. This is commonly done with a static screen (sidehill or parabolic) or a rotary drum screen (RDS). In the case of very dilute feed to the press, a Vincent Fiber Filter can be used.

AIR CYLINDER REGULATOR
To regulate the air pressure of the discharge air cylinder, presses are supplied with an air pressure regulator along with a Parker four-way reversing valve. These should be installed near the cone end of the press. (Until recently FRL (Filter, Regulator, Lubricator) sets were provided to regulate air pressure. Most air cylinder manufacturers now recommend against the use of lubricators.)

The Parker valve allows manual selection of the shut, open, or "neutral" position. This valve connects air supply from the regulator to one end of the air cylinders, while simultaneously opening the other end to atmosphere. The vent line on the 4-way valve allows air to escape when pressure is switched from one end of the air cylinders to the other.

Continuous air flow from the Parker vent line indicates a leak inside an air cylinder, or possibly a faulty 4-way valve.

Once material is going through the press, set the 4-way valve so that the discharge cone goes shut in the "in" or closed position. Start with a low air pressure, working your way up until the desired performance is obtained.

The neutral position of the Parker valve is used only in testing. If left in the neutral position, the cone will not move unless it is pushed open by press cake. If, later, the flow of press cake is diminished, the cone will remain in the position to which it was pushed, and purging can occur.


FRL AIR REGULATOR WITH 4-WAY REVERSING VALVE


DISCHARGE CONE

The principal adjustment of the press is made with the discharge cone. The cone is the component at the cake discharge end of the press that acts as a door or stopper plug to restrict material from leaving the press. The more pressure exerted by the discharge cone, the drier the cake material will be leaving the press. Also, the motor amps can be expected to increase with added pressure, and throughput may decrease.

The discharge cone is moved in (actuated) by the air cylinders. Typical air cylinder pressures to actuate the discharge cone are in the range of 30 to 60 psi. Some materials will press only in a low range, say 10 to 20 psi. Other materials may press best with a pressure of 60 to 100 psi. Air consumption is minimal in all models, 1 to 2 cfm.

During initial, first-time, start up, presses are generally started up with the discharge cone in the withdrawn position. This will avoid an unnecessary jam.

Note that with many materials it is necessary to start the press with the discharge cone in the closed position at low air pressure. Thin or soupy materials, like pumped manure or clarifier underflow, can tend to purge right through the press if the press is operated with the discharge cone open (in the withdrawn ("out") position).

However, with materials that are dry to begin with, such as sawdust or plastic wash tank sludge, it becomes more important to start with the discharge cone in the open position. This is because these materials may tend to jam or overload the press. Similarly, high freeness materials, from which the water falls away freely, will have a tendency to jam in a press. Be sure to start the press with the cone open, and gradually close it with low air pressure, when running such materials for the first time.

Once you are through the initial start up, it will be unlikely that your press should have the cone opened before starting. Most operators rarely open or shut the cone once it is set.

As the pressure on the discharge cone is increased, not only will the cake become drier, but the flow through the press may also be reduced. With very slippery or slimy feed material it may be possible to apply enough discharge cone pressure to stop the flow altogether.

High discharge cone pressures can result in increased quantities of suspended solids in the press liquor.

Care must be taken if a press is to be left running at a very low pressure like 10 psi. If some fiber enters between the cone bushings and the screw shaft, it will take more than that much air pressure to close a cone which has been pushed open by a heavy flow of cake. The result will be either high moisture content in the cake or, worse, purging.

With some feed materials, the press can be operated with the discharge cone in the withdrawn position. The screws alone may do enough compressing and dewatering to produce a cake at the discharge.

It is acceptable to open the discharge cone, in most cases, during normal operating conditions. This allows inspection, while in operation, of the discharge end of the screws and screens. This will give the operator a chance to observe operation with minimum dewatering and maximum throughput. It is also a good technique for purging bad material i.e., either jammed or spoiled material, from the press. (Do not try this trick if you are pressing hot or chemically aggressive materials.)

Where very low air pressure is required for proper operation, it may be practical to put the 4-way valve in the neutral position, half way between open and closed. A press can not be left permanently in this condition: keep in mind that a slug of cake will push the cone open, and it will not re-close on its own afterwards.

An unusual technique is to set the air pressure so that the cone normally stays completely shut. A timer is used to periodically open the cone. The closed period is determined by the amount to time required for press cake to accumulate in the press. This type of operation is used with slippery or slimy press cake that cannot be dewatered to sufficient firmness to force the cone to open. The duration of the "cone open" period is long enough to dump the press cake that has been formed. Cone Timer panels are available from Vincent at no charge.

Once through start-up, the cone is almost always left in the closed position at whatever air pressure had been found to be effective. A plug of cake will be left around the cone whenever the press is turned off; this will clear on its own accord on restarting the press.

There are a few applications where the air cylinders are removed and replaced with a jacking bolt. This is used if the cone pushes completely closed even with the lowest air pressure. It results in operating the press with a fixed discharge annulus.
Air cylinders with linear actuators are available.


DISCHARGE CONE

INTERMITTENT OPERATION
In the case of intermittent operation, it is recommended that the control panel for the feed pump or conveyor which feeds the press should have a timer. This timer should be set to have the press run for three minutes after the feed pump (or conveyor) shuts off. This will partially clear the press so that it will not trip out on overload when it is re-started. (This applies in high torque applications or in installations where the material in the press dries out or freezes.)

Minimize the time that the screw press is run with no material being fed into it. The last material admitted to the press will dry to powder, and it can cause severe accelerated abrasive wear.

Initially the press will likely be run empty in order to check rotation. Even though some rubbing may be heard, negligible wear will occur so long that this period is kept to a minimum. Also, since the screw is supported to some extent by the material inside the press, running dry may allow the screw to rub the screen.

(See the previous section, INVERTER VFD & PLC CONTROL.)

 

DOUBLE PRESSING
Some processes benefit from what is called double pressing. This means that the cake coming from the press is run through the press a second time (or through a second press). If little moisture is removed in the second (double) pressing, then it is known that the liquid removed in the first pressing is all of the free liquid that there is to be pressed out.

Sometimes water is added to the cake in between the first pressing and second pressing. This is done to enhance the recovery of dissolved sugars in the original press cake.

Molasses can be added to press cake between the first and second pressing. This is used to infuse dissolved sugar into the cake, increasing the solids content of the final press cake.

Capital-effective double pressing can be achieved by using an inexpensive Soft Squeeze Series KP screw press for the first pressing, following with a tighter-pressing Series TSP in the second position


MOISTURE CONTENT

A screw press separates free water. This will leave organic water in the press cake. The organic water is either bound to, or part of, the animal or vegetable molecules. Mechanical pressure alone will not remove organic water; it takes heat or chemistry. Frictional heat from the press can remove organic water, but this obviously should be avoided. For chemistry, see the Hydrated Lime, Gypsum and Alum section. For heat, see the Fluid Injection section.

To determine the moisture content of a material (feed to the press, press cake, or press liquor), a sample should be weighed and dried overnight at a temperature slightly less than 100o C. (If sugars are present use less than 70º C to prevent caramelizing.) The sample should weigh six or more times the tare weight of the sample tray or cup.

The moisture content of press cake varies considerably. Tomato press cake will be 90% moisture. Orange peel will be 80%, unless it is reacted with hydrated lime, in which case it will go down to 72% moisture; add molasses and it will go to 65%. Dairy and hog manure will come out at 70% moisture, unless there is sand or sawdust in the sample, which will reduce the moisture content. Cellulose fiber from a paper mill (knots, screen rejects, primary clarifier underflow) will come out about 50%. However, if secondary (biological) sludge is added, then the moisture content of the cake will go up considerably. With high ash content in paper mill samples, moisture may go down to 40%. Moisture contents of only 25% can be achieved pressing things like sand, eggshell, glass, and plastic chips.

The heat from steam injection can change the chemistry of the material being pressed so that cake with lower moisture content is produced. This blanching or parboiling effect works with fish and orange peel, for example.

A quick approximation of what to expect from a screw press is to squeeze as much water out with your fist, and figure that the press will do a little bit better. A better way is to twist a ball of the material in a cotton cloth.


COMPRESSION
A screw press achieves compression using several methods: (1) The discharge cone of the press causes back-pressure on the material being dewatered. The higher the cone pressure, the greater the liquid removal. (2) The pitch of the flights of the screw tightens as the material is conveyed through the press. This forces liquid to go through the screen. (3) The diameter of the shaft of the screw may be increased progressively, forcing material outward, against the screen. This is a tapered shaft design.


SCREW WITH TAPERED SHAFT

Force-feeding (supercharging) the press and applying a vacuum to the outside of the screen are two additional methods which may achieve compression. These two are used infrequently because the performance results are uncertain.


PRESS SPEED (RPM)
In general, the slower the screw speed, the greater the dewatering. Longer residence time in the screened area results from lower screw speed, which allows time for more thorough dewatering. Unfortunately, it also goes with reduced throughput capacity.


Screw press speed (rpm) can be changed by using a Variable Frequency Drive (VFD). Alternatively, the drive motor can be switched to a different pole motor (900, 1200, or 3600 versus the standard 1800 rpm). Most modern motors are good for permanent 120 Hertz operation; they are always good for a test at this high speed.

Higher speed can result in premature gearbox failure. Switching to synthetic oil, replacing the normal mineral oil, is recommended. Consult the factory for assistance.

The smaller Nord gearboxes are all rated for 4,000 rpm input, which makes it easy to switch to a 3000/3600 rpm motor. It is best to switch to synthetic lubricant if this change is permanent. With other gearboxes, the higher speed can result in premature gearbox failure. Consult the factory for assistance.

It has become normal for a variable speed drive (frequency inverter VFD) to be used with Vincent presses.

Low screw speeds are used for cooker crumb, potato peel, many sludges, and low freeness materials in general.

 

CAPACITY MEASUREMENT
The best way to measure capacity of a press is to collect timed samples of press cake and of press liquor. This should be done during a period of sustained, stable operation, rather than by timing a batch through the press.

Press cake is generally captured in a tarpaulin, and press liquor in a 5-gallon pail or 55-gallon drum. When the drain is at floor level, a 3-mil plastic bag can be used to catch press liquor. If the press liquor goes to a pit or tank, the change in depth can be timed.

Sometimes it is possible to collect only one flow, either press cake or press liquor. In these cases it is possible to estimate the press throughput if the solids content of the inbound material and press cake are measured. It is assumed that there are zero suspended solids in the press liquor, although this is never really the case.


A-B-C-D PLATES
There are four vertical plates making up the frame of the press. These are called out in the Nomenclature schematic at the end of this manual.

Starting from the drive end of the press, the first one is the A Plate. This A Plate forms the wall of the inlet hopper closest to the gearbox. The shaft seal plates are bolted to the A Plate.

The next plate is the B Plate. It forms the downstream wall of the inlet hopper. The screen starts at the B Plate.

The third plate, the C Plate, supports the discharge end of the screens. The discharge cone touches this plate when the cone is in the closed position.

There is a fourth plate, called the D Plate, on which air cylinders and thrust bearing are mounted.


SCREW LIFE

If a press loses its previous throughput capacity, or if cake moisture content increases, it is can be sign of a worn screw.

A screw can last anywhere from six months to twenty years. It depends on the material being pressed and how hard it is being pressed.

Premature screw failure can arise from several causes. The two main ones are:
(a) If a press is allowed to run continuously even when no material is being fed into it, the screw can wear out in one or two months. The same can happen if a very low flow is consistently fed into the press.
(b) If abrasive material is dewatered with high cone air pressure, rapid wear will occur. This condition is avoided if a few drops of water can always be squeezed from a fistful of press cake.

Two effective ways to extend screw life are:
(a) Various grades of hardsurfacing rod can be used to protect the flights of a screw. The best hardsurfacing will have Tungsten Carbide impregnated in it.
(b) Using a VFD or lower speed motor to reduce the screw rpm will extend screw life

If a worn screw is suspected, the thing to do is to shut down the press, open the cone, and dig out the cake until the tips of the last two flights can be seen or felt. Check how badly the tips are worn. If the there is 3/8” between the tips and the screen, serious wear is evident. It is also an indication that the sharp edges of the flights throughout the press may have worn, becoming rounded. This can cause the flights to act like a putty knife, plastering solids against the screen, preventing water from coming through.

Worn screws are either restored locally or returned to Vincent for rebuilding. The cost of a screw rebuild is around one third the cost of a new screw.


SCREW CONFIGURATION

All Vincent screw presses use the interrupted screw design. The interruptions leave room for stationary resistor teeth, mounted rigid to the frame of the machine, to reach almost to the shaft of the screw. This design of screw press stands in contrast to a continuous screw design. The advantage of the interrupted design is a reduced tendency for the material being pressed to co-rotate with the screw. Also, there is more agitation within the press and, consequently, quicker and more thorough dewatering.

The screw starts with a feeder section of continuous flight. This picks up material in the inlet hopper and pushes it into the screened part of the press. The feeder section ends at the first set of resistor teeth. The feeder section of the screw is followed by compression stages where the flights have reduced pitch. The reduction in pitch of the flights results in compression of the material going through the press. Most twin screw presses are made with five to seven stages of compression.



INTERUPTED FLIGHTS, RESISTOR BARS, and RESISTOR TEETH.


PIE CUTTING

Sometimes the compression of a screw is reduced, in the field, in an operation called "pie cutting". This involves cutting pie-shaped segments from certain flights of the screw, leaving a butterfly (end view) configuration. The modification is done to avoid excessive compression and jamming. The "sterile cut" is more dramatic. Consult the factory for assistance before making this modification.

 

 

4" PIE CUT                                                                                      PIE CUT


 
NOT PIE CUT                                                                   BUTTERFLY CUT



JAMMING

Should a press trip out on overload because it has become jammed, a series of steps can be taken to un-jam the press. Generally, the easiest thing to do is to reverse the leads on the electric motor drive. This will cause the screw to feed material backward into the inlet hopper.

Generally jamming is caused by over-pressing excessively dry material. Running the press backwards will break up this material. If the jamming was caused by tramp material, hopefully this can be found and retrieved from the inlet hopper following operation in the reverse direction.

Having a VFD with a reversing button greatly facilitates this operation.

If a press has had extensive use in an abrasive application, the outer diameter of the flights will be worn away at the discharge of the press. Radial wear of 1" to 4" in larger presses will lead to serious jamming and, possibly, a burst screen.

When a press is operated in the reverse direction it is possible that solid in the press will be forced against the A Plate. This can damage the shaft seals. For this reason, care should be taken when running the press backwards.

Usually three or four revolutions of the screw are sufficient to clear a press. If running the press backwards several cycles does not clear the jam, a screen should be removed so that the cause of the jam can be determined. Look for a bent flight. Before going to the trouble of removing the screen, shut down the press and try clearing the end of the press with a long screwdriver.

Sometimes when a press is jammed, a flight on the shaft of the screw will fold. This can happen if the press overloads on dry cake or if tramp metal is caught between a flight and a resistor tooth. The weld at the shaft may tear. When this happens flow through the press is greatly impeded.

   

FOLDED FLIGHTS

 

SCREEN BLINDING
A common problem is for the screens of the press to become blinded (covered over). When this occurs, the flow of press liquor coming through the screens diminishes. The level in the inlet hopper will fill up to where it overflows.

In some cases, the screens can be cleared by periodically reversing the direction of rotation of the screw. This can be programmed with many VFD’s, so that the press runs forward for a given period and then reverses direction briefly for three or four turns when the screens start to blind. This is one of the easiest possible solutions to test. Sometimes it is the only one that is effective. The technique works well on bar screens; care must be taken with perf screens so that the screw does not snag the screen during the reverse cycle.

Many other methods are used to address blinding: (1) Adding notches to the screws, (2) Reducing or eliminating the pressure in the inlet hopper, (3) Adding press aid to the flow, (4) Changing to a different screen selection, (5) Reducing the screw-to-screen clearance, and/or (6) Employing a screen flush with caustic solution, acid, or high pressure spray.

If blinding occurs after an extended period of satisfactory operation, it is usually due to wear of the screw. Rounded edges of the flights will contribute to blinding.

CHANNELING
A condition somewhat similar to purging can occur with slimy materials, like concord grapes, pineapple pulp, or spent brewer’s grain. These may tend to channel or squirt out from one side of the cone. Two ways to eliminate channeling are to lower the air pressure on the discharge cone and to slow down the speed of the screw. Channeling can also be reduced by adding press aid to the material being dewatered, or by reducing the inbound flow to the press.

To break up channeling, pieces called wing feeders can be welded to the end tips of the last two flights of the screw. See the Wing Feeder section of this manual.


PURGING
An undesirable condition can occur when the material being admitted to the press purges, without liquid-solid separation, from the cake discharge. This can occur especially if pressure exists in the inlet hopper.

Mechanically, purging occurs when a dry lump of press cake holds open the discharge cone. Un-pressed material will flow around this partial plug.

Purging may occur when there is a much reduced, small flow of cake coming from the press. Usually this is a sign of blinded (covered over) screens. This can be caused by a worn screw. Liquid from the inlet hopper will wick into the press cake, making it soft enough to blow out. Sometimes this condition is avoided by mounting the press inclined at about 5o above horizontal; the simplest way to do this is to place a block under the cone end of the press.

Wing Feeders are welded to the tips of the last two flights of the screw in order to break up dry cake which will hold the cone open. Be sure that they hit the cone when it is closed.

A drop in operating amps can be an indicator that a purging condition has begun. An ammeter circuit can be installed to trip the system when a reduction in motor amps occurs. This is rarely done.


BRIDGING
Sometimes bridging will occur at the inlet hopper, preventing material from flowing into the press. If an independent surge hopper is mounted over the inlet of the press, it should have at least one, preferably two or three, vertical walls. This will minimize bridging.

Bonding Teflon sheets to the inlet hopper of the press is a remedy that has been used to reduce bridging of bulky materials which allow free-draining of water.

A vibrator, mounted on the side of a feed hopper, may also alleviate bridging.

One way to overcome this is to direct a stream of water into the inlet hopper to break the bridge. The nature of the screw press is that essentially all of this added liquid will be removed in the pressing operation. (It may be convenient to pump a jet of the press liquor into the inlet hopper to break the bridging.) This is rarely done.


RESISTOR TEETH
The interrupted screw design press has stationary teeth that protrude into the flow of material as it passes through the press. These fit into the gaps of the screw where there is no flighting. They stop just short of the shaft of the screw.

The resistor teeth are an integral part of the Resistor Bar. These resistor bars are positioned axially, parallel to the screw, with one bar above and one bar below each screw. The resistor bars are bolted between the B and C plates; they form a part of the structural frame of the screw press. The screen frames bolt onto the resistor bars.

Not infrequently the resistor teeth are drilled so that fluid can be injected into the press during operation. See the next section.

Rarely the resistor teeth are shortened, usually by half, to increase the capacity of the press. Removing the teeth altogether will result in co-rotation and jamming.


FLUID INJECTION

Resistor teeth can be drilled so to permit injection of steam, solvent or water while the press is in operation. Also, these modified resistor teeth can be used for CIP cleaning, without the need of removing the screen from the press.

Commonly alcohol injection is used to achieve in-line washing to remove sugars. Hot water injection is used to recover dissolved solids in juice production. Steam injection is used in dewatering raw organic materials.

The moisture reduction that results from steam injection is related to a chemical change that comes with blanching, or parboiling, a material. Steam injection works well on pineapple skin, citrus waste, and raw fish. Tests run with steam injection in a Vincent press at Anheuser-Busch showed little benefit. The material being pressed, spent grain, had already been "cooked" before steam was added.

Injection is achieved by drilling holes through the resistor teeth and piping these holes to a manifold outside of the screen. Photos and drawings are available from the factory. Vincent does not charge for providing a drilled resistor bar.

 

   
STEAM INJECTION                                                                       LAB PRESS
IN TWIN SCREW PRESS                                                          STEAM INJECTION

POLYMER
In rare applications, the addition of polymer is indispensable in achieving adequate screw press performance. Polymers are added to dilute waste streams, especially to those containing very small size suspended solids. The long chain molecules of the polymer will flocculate the solids, agglomerating them to the point where they can be pressed. Under the right conditions, drastic improvement can be observed in press throughput, press cake moisture, and press liquore clarity.

Nalco and GE are the leading polymer suppliers. Their sales engineers are anxious to recommend the product best suited for your application.

Low speed operation of the press is usually required in order to achieve good performance.


NOTCHES

Sometimes it is necessary, during press operation, to have the screw wipe the screens clear of blinding material. This is best achieved by having notches in the outer edge of the screw. Fibrous material accumulates in the notches and brushes away slimy material which may be blinding the screens. Shallow notches (1/8" wide by 1/8" deep, 1-1/2" apart) in the outer edge of the screw flights work well. Typically, notching is done from the B plate to the second resistor tooth. Most Vincent presses are supplied with notches.


GRINDING NOTCHES IN THE FLIGHTS

WIPERS
Before the advent of notches, wipers, made of UHMW strips or nylon brushes, were (rarely) mounted to the outer perimeter of the screw flights. If at all possible we advise against the use of such wipers because (a) they tend to cause material to co-rotate and jam in the press, (b) they are difficult to replace, and (c) they wear rapidly, resulting in unacceptably frequent shutdowns for replacement. Wipers tend to improve dewatering performance for the first week or so. After that the wiper material wears and press performance reverts to being just a little better than if no wipers were used. Wipers are made either of 1/4" thick high durometer polyurethane or of nylon bristle brush. Wipers are preferably mounted on the downstream edge of the flight.


PRESS AID
Some materials press best if a press aid is mixed into the material to be pressed. Typical press aids are rice hulls, cottonseed hulls, cellulose fiber from a paper mill, and ground newspaper. Ground wood is the best, but most expensive, press aid.

Press aids are most commonly used in producing juice from deciduous fruit. The press aid gives the press something to get a bite on. Press aids also tend to hold back fines (short fibers) and prevent them from going through the screens with the press liquor. If apples are fed into a press, apple sauce will come through the screens. However, if a press aid is added to the apples, then apple juice will come through the screens.

Typically, the amount of press aid used is only 1% to 3% by weight of the flow going through the press. This will look like more than such a small percentage because press aids have a much lower bulk density than the wet materials that are pressed.


HYDRATED LIME, GYPSUM, AND ALUM

Lime (calcium hydroxide) must be added to citrus peel before it can be pressed. The lime breaks down the pectin or cell walls so that the press can remove moisture. Less than 1% by weight is used. A reaction time of several minutes must be allowed prior to pressing. Lime has been used successfully in the same manner with potato, onion, tomato, carrot, and pineapple waste. It works well on acidic materials such as strawberries and coffee bean pulp. Vincent offers lime dosing equipment.

Gypsum and alum salts are also effective chemical press aids. They are typically used in dewatering sugar beet pulp, and they have rarely been found effective on other materials.


VACUUM EFFECT

In some applications, increased screw press capacity can be obtained if the area outside of the screens is under a vacuum. This can be achieved by mounting the press at a high elevation, with the press liquor drain line dropping below the surface of a drain tank or pit.

That is, the drain line from the press should go below the surface of the pit or pond into which it drains. If this line is relatively small in diameter and has a steady downward slope, a vacuum will be induced around the screens of the screw press. The mass and velocity of press liquor flowing through the drain line create this vacuum. To draw air bubbles downwards with the press liquor, the velocity of the fluid must be greater than five feet per second.

The covers over the screens of the press will have to be sealed, usually with Silicone.

The amount of vacuum is a function of the elevation between the press and the drain pond. For good results, the press should be mounted on a stand that is 20' tall or higher.


PRESS LIQUOR
A screw press produces relatively "dirty" press liquor as compared to a Filter Press or Belt Press. Suspended solids will pass through the screens of the screw press along with the liquid being expressed from the inbound material.

If suspended solids need to be removed from the press liquor, the most common method is to pump the press liquor either over a static (sidehill) screen or through a rotary drum screen. Generally, the screen tailings (sludge solids) are fed back into the screw press along with the flow of inbound material. Most of these fines will be captured with the solids of the inbound material and end up in the press cake. Although some of these tailings will once again go through the press screen with the press liquor, equilibrium of recirculating solids is reached and satisfactory pressing operation is realized.

If the press liquor is to be concentrated in an evaporator, better screening than can be achieved with a static screen may be required. Another Vincent machine, the Fiber Filter, provides premium performance. Decanters or centrifuges may be required.


SCREEN SELECTION

The screen of the press is made either of wedgewire (slotted screen) or perforated stainless sheet (round holes). Wedgewire screens are expensive one-piece weldments that must be replaced when excessive damage or wear has occurred.

Wedgewire screens in Vincent presses can be reversed in order to achieve double life. That is, wear starts at the cake discharge end of the press. When this occurs, the screen can be turned 180o so that the fresh inlet section is then located in the discharge area.

Screens made of wedgewire come standard with 0.015" to 0.020" slot width; they are also available with slots that are 0.008" to 0.060" wide. With slot widths less than 0.012" there is a tendency for the screen to blind (be covered over) with the material being pressed. However they work well in alcohol and oil separation. Changing the slot width generally has little impact on the clarity of the press liquor or the dewatering capacity of the press.

The most common damage to a wedgewire screen is for part of the surface to be smeared over by rubbing the screw. This rarely is bad enough to affect press performance. Profile bar screens generally work satisfactorily with 30% or even more of their surface smeared over.

Smeared screens can be remedied by running a box cutter blade through the slots.

TIG welding is used to close the gaps in the case where either a few wedgewire slots get spread apart by tramp material or a perforated screen gets torn.

Perforated metal screens are simple sleeves which are held in the screen assembly. These are less durable but inexpensive to replace. Standard perforated screens have a hole size of 3/32" diameter, although material with 0.060", 0.040", down to 0.023" holes can be supplied.

Frequently, increased press capacity can be achieved by changing a perforated screen to one with smaller holes. This unexpected result arises from a combination of factors: (1) smaller hole screens are made of thinner sheetmetal, so that the press liquor has a shorter distance to travel before it falls free from the screen, reducing the chance of sponging backwards through the screen and (2) particles which fall into and plug a larger hole will roll over a smaller hole.

Minor rubbing between the screw and screen is normal, although, obviously, hard rubbing will cause wear and premature failure of the screen. With a clearance greater than 3/16", the dewatering performance of the press can start to deteriorate; this depends a lot on the nature of the material being dewatered.

In cases of severe wear or damage, it is common to patch a screen. Stainless sheetmetal is used for this. The reduction in drainage surface is of little consequence as the screens have ample open area.

 
PROFILE BAR (WEDGEWIRE) SCREEN


 
PERFORATED SCREEN WITH REINFORING SHEET


SCREW-TO-SCREEN CLEARANCE

Generally the clearance between the screw and the screen is 1/16", plus or minus 1/16". The screw should not rub the screen hard, as it can cause wear and premature failure of the screen. Tight clearance is used with materials that blind the screen, such as onion skins. Greater clearance, 1/16", is used with eggshells, pectin, xanthan gum, and corn husk, for example. With a clearance greater than 3/16", the dewatering performance of the press can start to deteriorate; this depends a lot on the type of material being dewatered.

To measure the s2s clearance in wedgewire screens, a feeler gauge can be slipped through and along a slot until it hits the edge of the screw. Measure from the outside of the screen to the edge of the flight; then subtract the thickness of the wedgewire (generally either 0.375" or 0.500") from the measured depth in order to calculate the clearance.

Alternatively the screw-to-screen clearance can be checked by removing one half of the screen. Inspection is made from the side from which the screen half has been removed.

In the case of perforated screen installations, a depth gauge can be used to measure the screw-to-screen clearance. This is done by first finding an area where the screw flight is next to the screen; poking a straightened paper clip through the screen is handy for this purpose. The depth from the outside of the screen to the edge of the flight is measured, and then the thickness of the screen is subtracted from that measurement. [3/32" perf is 0.075" thick; 0.050" perf is 0.050 thick; 1/32" perf is 0.024" thick; 0.023" perf is 0.015" thick; 3/8" perf back up screen is 0.120" thick.]

If a screw rubs against the screen in a given area, it may be best to grind some off the OD of the screw. Prussian Blue can be useful in finding the spot that is rubbing.

  

WEDGEWIRE DEPTH GAGE PERFORATED DEPTH GAGE


CONE BUSHINGS

The cone rides on the shaft of the screw. "Cone Sleeve" is the name given to the portion of the screw on which the cone rides. There will be either one or two bushings in the cone to support and guide it, and to protect the Cone Sleeve journal surface of the screw shaft. Sometimes these bushings are lubricated with liquid from the material being pressed, such as the juice from apples or water from pectin peel.

A grease fitting is provided for lubricating the bushings or to minimize leakage of press liquor through the cone. If there is only one grease line per screw, the grease line goes to a pocket, which serves as a grease reservoir, located between the two bushings. Bushing lubrication is extremely important when materials that are dry (like paper mill screen rejects) are being pressed. By the time such materials reach the discharge of the press, they do not have enough free moisture left in them to adequately lubricate the cone bushings.

Rarely, presses are supplied with additional lubrication fittings so that water, in addition to grease, can be metered to the bushings as a lubricant.

Automatic grease systems are available. These should be the high pressure (900 psi) electric or battery variety. Vincent provides these for critical applications, especially pulp & paper.

Liquid leaking past the cone bushings drains out the back of the cone (at the air cylinder end of the press). Almost always it is minimal compared to the flow of press cake. However, a pan can be provided to collect this liquid and drain it into the main flow of press liquor.


WING FEEDERS
There are blades welded to the outside tips of the last two flights of the screw. Called "Wing Feeders", these are mounted parallel to the discharge screen surface. Care must be taken that wing feeders are not made so long that they hit the face of the cone when the cone is in the closed position.

Wing feeders are used in twin screw presses to break up press cake which tends to accumulate between the two cones.

 


LONG WING FEEDER


CLEANING

Commonly, material is cleared from a press by stopping the inbound flow, setting the discharge cone in the withdrawn position, and running the press for a few minutes until no further material is discharged. This will leave some material inside the press, which can be handy for forming a plug at the cake discharge when the press is restarted.

Material will leave a Vincent interrupted flight press only if there is additional inbound material forcing it out. This makes it difficult to clear all material from inside a press without removing the screen. One technique used successfully is to feed crushed ice into the press. Water must be fed along with the ice to prevent jamming. When the ice melts, the press will be relatively clean inside.

There are applications in which the press must be cleaned frequently, such as once a shift, in order to meet sanitary regulations. The screen halves can be hinged to facilitate this. In other cases, the screens are removed from the press. A spare screen assembly may be kept, submerged in cleaning solution, in order to minimize the downtime required.

Cleaning the inside of the screen can be achieved, at least to some extent, by injecting water through the resistor teeth. Holes must be drilled in the resistor teeth to make this possible.

It is unusual that the outside of the screen needs to be cleaned. Spray systems for this can be built into the press at the Vincent factory. Alternatively, a pressure washer or swabbing with acid solution can be used.


SHAFT SEAL
The Seal Plate is bolted to the A Plate. This may be solid UHMW (ultra high molecular weight polypropylene or polyethylene) or it may contain one or two Johns Manville (JM Clipper) lip shaft seals. There may be a grease fitting on this plate; the grease is used to reduce leakage and to help prevent fiber material from entering and damaging the screw shaft.

Generally, seals are allowed to drip once they start leaking. They are replaced only in conjunction with major maintenance, as when the screw is removed from the press.

In some cases we have found that leakage from a shaft seal can be stopped by simply selectively loosening or tightening the four bolts holding the seal housing to the A plate.

 
SEAL PLATE SPLIT SEAL PLATE

SCREW REMOVAL
The screws, as a pair, are removed through the hole in the C Plate, at the cake discharge end of the press. The operation can be difficult the first time, so we recommend consulting with the factory before getting started. Vincent has provides a clamping tool to hold the two screws together as they are removed from the press.

Start by removing the bolts holding the shaft seal plates to the A Plate. This will prevent damage once the screws are loose.

The bars with the resistor teeth must be removed before the screws can be removed.

To remove the screws from a Series TSP press, the shaft couplings between the gearbox and the screws must be undone. Then the gearbox must be moved out of the way so that the shaft couplings can be pulled from the ends of the screws. Falk gear-type couplings should be heated to 350º to 500º F so that they can be pulled off; use a temperature stick to measure this temperature.

Next the pillow block bearing is removed.

A hydraulic jack is used to pull the coupling half and the PB bearing from the shaft.

If a flange (thrust) bearing is mounted on a plate which also holds the air cylinders, do not remove the flange bearing by itself. Instead, leave it on the plate and remove the air cylinders and bearing as a single unit.

If the coupling half, pillow block bearing, or flange bearing have become seized to the screw shaft, it will be necessary to cut them loose. It is recommended to have spares on hand before removing screws from a press.


SCREW REPLACEMENT

In the case of the Series TSP presses, jacking bolts and shims are used to achieve proper alignment. Only after the screws are aligned within the screens can the gearbox be aligned to the screws (not the other way around). When working with gear-type couplings, be sure to leave the prescribed 5/16" gap between the gearbox shaft and the screw shaft. This will protect the gearbox from thrust loads.

Falk gear couplings must be heated to 350º to 500º F in order to slip onto the shaft.

It is important to hand-pack grease into a Falk coupling. Do not rely on the grease fitting because the grease will only lubricate in the path of least resistance.

There are two ways to avoid excessive rubbing between a new screw and the screen: Either shims can be placed between the screen frames and the resistor bars, or the interference can be ground off the edge of the screw. To eliminate the high spots, coat the edge of the screw with Prussian Blue, bolt the screens in place, turn the screw, remove the screens, and grind the screw where interference has occurred.


SCREEN REPLACEMENT
This section covers only presses with perforated metal screens. These screens are held in frames that are split vertically, being bolted to the resistor bars.

In the case of screen failure, frequently a solid patch can be welded onto the screen, from the outside. This is simple as the screen need not be removed from the press.

Wedgewire screens may become smeared from being wiped by the screw or by hard press cake. Wedgewire screens generally work satisfactorily with 30% or even more of their surface smeared over. Usually press liquor will come through a smeared area of a wedgewire screen. If it becomes an issue, it is corrected by running a box cutter blade through the slots.

Usually perforated screens are tack welded in place. However, if the perforated screens bolt in place, once the new screen is tight against the frame, look through the screen and locate the holes in the frame where the attachment bolts go. Use a center punch to open holes in the screen. These holes must be large enough to allow the attachment bolts to go through the screen and thread into nuts on the far side of the frame.

Once the new screen is tack welded or bolted tightly in place, beat over and grind off the excess screen material.

There are two ways to avoid excessive rubbing between a new screen and the screw: Either shims can be placed between the screen frames and the resistor bars, or the interference can be ground off the edge of the screw. To eliminate the high spots, coat the edge of the screw with Prussian Blue, bolt the screens in place, turn the screw, remove the screens, and grind the screw where interference has occurred.

The most common cause of screen failure ties to failure of an outboard support bearing. If the bearing holding the end of the screw wears out, it can let the screw move enough to rub against the screen.

 

GEARBOX BASICS
The Series TSP presses use foot-mounted gearboxes. The foot-mounted gearboxes are concentric (the motor shaft is in line with the output shaft) in smaller TSP's. Parallel shaft reducers are used in the larger models.

Gearboxes are rated, and sold, by their torque rating. The manufacturers generally offer their designs in progressively larger sized castings, or boxes. The larger the box, the larger the torque rating. Each box size will be available with different gear ratios. In order to keep the torque fairly constant, larger horsepower motors are used with the high speed boxes. Similarly, reduced horsepower motors must be used when a low output speed is selected.

Screw presses are designed around the size of the gearbox that is selected.


PRESS LUBRICATION
Lubrication is something we generally review with customer personnel during start-up. It is pretty straight forward:

CONE BUSHINGS: Once a shift
BEARINGS: Weekly
BUSHINGS: Weekly
SHAFT SEAL: Weekly
GEARBOX: Annually
AIR REGULATOR: Whenever empty (if a lubricator is being used)
GEAR COUPLING: Whenever opened pack by hand, covering all teeth.
MOTORS: Never

The most critical lubrication item has to do with the cone bushings. Before starting up a new press, the cone should be run in and out a few times to spread the grease around.

Lubrication of the cone bushings depends a lot on what is being pressed. With orange peel, there is enough press liquor juice acting as a lubricant that the bushings are lubricated only at the end of the processing season (to keep them from locking up on dried-out peel juice). The other extreme is at a paper mill where boiler fuel is made out of reject fiber. There is no free water left in the press cake, so everything is very hot and dry. We automatically supply a 900 psi ATS autolube on paper mill jobs. Another tough application is with vapor tight presses where there is apt to be solvent getting into the cone bushings. The solvent can dissolve and wash out the grease, so we like to see frequent lubrication of the cone bushings. (Food grade grease is used in this application since food ingredients are being produced by the press.)

If Vincent supplies an autolube for the cone bushings, it will be either battery powered or require a hard wired power supply. This should be actuated when the press is first placed in service. It should be set to give one small shot of grease every couple hours. We provide autolubes with large grease reservoirs, so they will go at least two weeks at the maximum greasing schedule. Once operations are stabilized, it may be practical to reset the timer dip switches so that it gets one shot every shift or once a day.

In any case, we tell the operators to run the cone open and closed once a shift because this will spread the grease. This is done with the press in operation.

If Vincent does not provide an autolube for a critical operation, we generally tell the operators to manually grease the cone bushings once a shift. We also tell them to run the cone in and out when they do the greasing, in order to spread the grease around.

The bearings and/or bushings holding the screw get greased on the customer's normal schedule for that type of bearing, maybe once a week, or once a day, or once a month. Whatever grease the customer normally uses will be fine.

The shaft seal housing may have a grease fitting. This grease is to prevent fiber from getting into the seal. The seal should be given a shot of grease whenever the screw support bearings or bushings are greased.

The gearbox oil should be changed once a year. Use mineral oil for a normal 1800/1500 rpm input. Use the same grade oil, but synthetic, for input speeds of 2,400 rpm or more. (Sumitomo Cyclo's, parallel shaft gearboxes, and Brevini and Bonfiglioli planetaries, are exceptions to this. The OEM manuals from these suppliers detail lubrication requirements.)

Some Nord gearboxes have an autolube canister located in the motor adaptor portion of the gearbox. It is located under a cast iron cap. This autolube should be actuated when the press is placed in service. Nord recommends replacing this autolube once a year.

The air regulator used with the discharge cone air cylinders may have a lubricant jar. If so, Vincent includes a can of light oil along with the air regulator which comes with our screw presses. The jar should be filled when placing the press in service and when the jar is empty, about once a year. It takes very light (sewing machine) oil. The oil helps prevent corrosion inside the air cylinders. (Most air cylinder manufacturers no longer recommend the use of lubricators.)

If there is a Falk gear-type shaft coupling, Vincent packs these with lubricant grease prior to shipment from the factory. The grease is re-packed only if the coupling is opened (which is rare). Packing must be done by hand so that all the gear teeth have grease.

We have never seen nor heard of anyone greasing the motor bearings.


REPLACEMENT PARTS

Most replacement parts are standard OEM components that can be purchased from their original manufacturer. The specification of these items is included in the O&M Manual. Only shaft seals and possibly bearings are apt to require replacement.

The most common wear parts in the Vincent Press are the screens, the screws, and discharge cone bushings. Vincent stocks some screens. Screws and cones are generally rebuilt at the Vincent factory. Be sure to specify the serial number of your press when ordering replacement parts or repairs.
A spare set of spur drive gears is recommended for TSP presses which use them.


SAFETY

These Operating Hints have left unstated the obvious safety hazard: A screw press, like any screw conveyor, is totally unforgiving. If clothing or limb gets caught in a rotating screw, the screw will not stop. Vincent Corporation has not heard of one single injury of this nature with a screw press, so do no let yourself become the first. The use of common sense is all that is required.


Robert B. Johnston, P.E.


 

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HINTS-TSP.pdf892.02 KB

Press Lubrication

October 7, 2011
Revised July 2012
, November 2014
 

Lubrication is something we generally review with customer personnel during start-up.  It is pretty straight forward:

CONE BUSHINGS:    Once a shift
BEARINGS:  Weekly
BUSHINGS:  Weekly
SHAFT SEAL:  Weekly
GEARBOX:      Annually
AIR REGULATOR:    Whenever empty
GEAR COUPLING: Whenever opened
MOTORS: 

Never

The most critical lubrication item has to do with the cone bushings. Before starting up a new press, the cone should be run in and out a few times to spread the grease around.

Lubrication of the cone bushings depends a lot on what is being pressed. With orange peel, there is enough press liquor juice acting as a lubricant that the bushings are lubricated only at the end of the processing season, to keep them from locking up on dried-out peel juice. The other extreme is at a paper mill where boiler fuel is made out of reject fiber. There is no free water left in the press cake, so everything is very hot and dry. We automatically supply a 900 psi ATS autolube on paper mill jobs. Another tough application is with vapor tight presses where there is apt to be solvent getting into the cone bushings. The solvent can dissolve and wash out the grease, so we like to see frequent lubrication of the cone bushings. (Food grade grease is used in this application since food ingredients are being produced by the press.)

If Vincent supplies an autolube for the cone bushings, it will be either battery powered or require a hard wired power supply. This should be actuated when the press is first placed in service. It should be set to give one small shot of grease every couple hours. We provide autolubes with large grease reservoirs, so they will go at least two weeks at the maximum greasing schedule. Once operations are stabilized, it may be practical to reset the timer dip switches so that it gets one shot every shift or once a day. In any case, we tell the operators to run the cone open and closed once a shift because this will spread the grease around.

If Vincent does not provide an autolube for a critical operation, we generally tell the operators to manually grease the cone bushings once a shift. We also tell them to run the cone in and out when they do the greasing, in order to spread the grease around.

The bearings and/or bushings holding the screw get greased on the customer's normal schedule for that type of bearing, maybe once a week, or once a day, or once a month. Whatever grease the customer normally uses should be fine.

The shaft seal housing may have a grease fitting. This grease is to prevent fiber from getting into the seal. The seal should be given a shot of grease whenever the screw support bearings or bushings are greased.

The gearbox oil should be changed once a year. Use mineral oil for a normal 1800/1500 rpm input. Use the same grade oil, but synthetic, for input speeds of 3,000 rpm or more. (Sumitomo Cyclo's, parallel shaft gearboxes, and Brevini and Bonfiglioli planetaries, are exceptions to this. The OEM manuals from these suppliers detail lubrication requirements.)

Some Nord gearboxes have an autolube canister located in the motor adaptor portion of the gearbox. It is located under a cast iron cap. This autolube should be actuated when the press is placed in service. Nord recommends replacing this autolube once a year. IT IS VERY IMPORTANT TO DO THIS.

The air regulator set used with the discharge cone air cylinders may have a lubricant jar. Vincent includes a can of light oil along with the air regulator which comes with our screw presses. The jar should be filled when placing the press in service and when the jar is empty, about once a year. It takes very light (sewing machine) oil. The oil helps prevent corrosion inside the air cylinders.

If there is a Falk gear-type shaft coupling, Vincent packs these with lubricant grease prior to shipment from the factory. The grease is re-packed only if the coupling is opened (which is rare). The grease should be hand-packed around the gears; do not count on using the zerk fitting on the coupling.

We have never seen nor heard of anyone greasing the motor bearings.
 

 

Press Start Up

January 19, 2011

When customers request a suggestions for starting up a new screw press, we refer them to the Owners Manual. The key elements are as follows:

Check that there is compressed air for the discharge cone and that there is oil in the gearbox.

Find out where the manual disconnect, stop button, or breaker is located. If there is an emergency of any sort it is important to know where to shut off the press.

Give quick safety instructions: keep hands out of the inlet hopper, and keep hands away from where they might get pinched by the cone. The rest is obvious and normal.

Bump the press to make sure the direction of rotation is correct and that there is no severe rubbing. The direction of rotation is this: if you are sitting on the gearbox looking towards the discharge cone, the screw should be turning counter-clockwise.

Make sure the press liquor drain is hooked up and that there is a way for the press cake to be removed from the area of the press.

Make sure that the lubrication equipment is in place. The most critical item is having lubrication for the cone bushing(s).

Make sure the cone runs in and out. If it is jerky without the press in operation, get the screw turning and make sure that the in-out motion smoothes out.

When ready to get going, set the cone air pressure very low, say one bar. Turn on the press. If practical, start feeding material into the press with the cone in the open (withdrawn) position. Assuming it does not jam and no funny noises or vibration are evident, ease the cone shut once some material is seen coming out of the cake discharge.

Monitor the motor amps while doing this. If the amps spike, the press is jamming. When this happens, stop feeding material into the press and leave the press running with the cone open. If the jamming is bad enough to trip out the motor, run the press in reverse to try and loosen the jammed material. Then switch back to forward in an effort to clear the press.

Assuming that there are no problems, expect the cone (shortly after it is shut) to gradually come open. Some cake should start coming out. At that point, increase the air pressure on the cone. Monitor the motor amps while doing this, backing off the feed or air pressure if the motor seems to be overloading.

Likely it is best to start out at full line frequency, either 60 or 50 Hertz. Once relatively stable operation is achieved, try changing the drive frequency. Generally think of lowering the Hertz in order to get dryer cake.

Check the level in the inlet hopper. Set the feed to the press to be such that the incoming material barely or partly covers the screw in the inlet hopper.

Look out for vibration or shuddering in the press. That would indicate that it may be getting close to jamming or tripping out.

Issue 229

 

 

 

 

Vent Line

February 26, 2011

Even the guys who got A's in Fluids have a hard time explaining this one. In fact, most do not believe it even after they have seen it several times.

In manure applications, a screw press may not operate at its rated throughput unless there is a vent line, open to the atmosphere, mounted at the inlet hopper of the press.

At most dairy farms manure is pumped from the reception pit with a centrifugal pump. Usually 4" or 6" PVC piping is used. This piping is large enough in diameter to avoid plugging with tramp material. It is also large enough to carry a high gpm flow for speedy emptying of the pit.

Similarly, the centrifugal pump is usually at least 3". Otherwise it will plug on rags, cord, and other trash.

This typical piping system can generally pump something from 200 to 800 gpm. At the same time, a typical 10" manure screw press will handle only 20 to 60 gpm. To address this conflict, the piping system for a manure press includes an overflow return line that goes back to the reception pit. Thus there are two lines: pipe from the pump to the inlet of the press, and another pipeline going from the inlet hopper of the press back to the pit.

The phenomena which can easily occur is that the velocity of the flow through the overflow return line is so high that it draws a suction in the inlet hopper of the press. We have seen one installation where, as the pump was shut off, this suction was enough that air could be seen being drawn backwards through the screen of the press and into the inlet hopper.

It can be even more baffling in normal operation: you are pumping 200 gpm into the press, and 10 gpm is coming through the screen of the press and 190 gpm are going back to the pit. But, if you open a vent line at the inlet hopper, two things happen: the flow of press liquor goes up to 30 gpm, and air can be heard and felt sucking through the vent line into the press. You have tripled the capacity of the press by letting air into the system.

Usually the vent line is installed on the cover over the inlet of the press. A better position is in the manure return line where it starts, leaving the press. Do not put the vent line in the pipe that feeds manure into the press.

Vent lines tend to get plugged with manure that splashes or is drawn into the vent. This plugging occurs when manure in the vent dries out and builds up during periods when the press is not running. Therefore it is good to have a vent line made of 1-1/2" pipe or larger, with some provision for rodding it out.

Conditions can occur where manure is pumped out through the vent line. For this reason the vent line is frequently piped through the wall or roof of the building where the press is installed. A union, to allow removal for cleaning, comes in handy.

Issue 230