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December 8, 2012
If there is a problem in the start-up of new press, frequently the first report we get is that the press is jamming.
There are two different situations where people say that a press has jammed:
#1 Jam: After a short period of operation the motor amps will go up and the press trips out on electrical overload.
#2 Jam: After a longer period of operation the level in the inlet hopper will start to go up even though the screw rpm and cone air pressure have not been changed. Material may be observed swirling backwards in the inlet hopper.
With a #2 Jam we know that material is slowly accumulating at some point along the screw, and gradually this material starts to co-rotate with the screw. When this happens, less material can get through the press and gradually the level in the inlet hopper starts to go up. The motor amps do not always go up when this problem is encountered.
The trick in both cases is to find out where the jam is starting. The easiest way to do this is to let the press operate until there is a jam. Then we stop the press and remove the screen. We tell everyone to remove the screens slowly and to be careful not to disturb the material inside the press. Then we look to see where the jam has started. There are four main places we look for:
- A jam can start at the transition from the Inlet Hopper into the start of the screen. That point is called the B Plate. In Operating Hints there are described Cord Cutters and Brian’s Stripper. These prevent jams at the B plate. If they do not cure the problem, it may be that the pitch of the flights in the inlet hopper needs to be increased. [In 2015 the design of KP presses was changed to eliminate a jamming point at the B Plate. It involved eliminating a step in the screw diameter.]
- A jam can start just before the first Resistor Tooth. This can be a difficult jam to correct. It may require that the screen be blanked off so that no press liquor can come out until the flow is past that first tooth. But, it usually requires a modification to the configuration of the screw.
- A jam can occur at the flights between any two of the resistor teeth. When this kind of jam occurs, we usually Pie Cut the flights at that point. (Pie Cutting is described in Hints.)
- Sometimes we see that the jam is at the cake discharge, after the flights. Welding Wing Feeders onto the tips of the last two flights, if the press does not already have Wing Feeders, may solve the problem. These break up the jammed material. Long Wing Feeders are used, rather than the short knobby type that are used as sacrificial wear elements.
Another thing that can be done to break up a jam at the end of the screw is to use the Rotating Cone feature. If the cone is turning with the screw and there is a pin on the face of the cone, the pin will break up cake that wants to get stuck at the discharge. Most Series KP presses come with the Rotating Cone option.
With a #1 Jam we frequently can work a solution that takes advantage of the increase in amps. Sometimes with a #2 Jam there is also an increase in amps. The solutions are easiest if we can work with amps.
By monitoring the motor amps, the press can be set so that the cone swings to the open position on high amps. Alternatively, without regards to motor amps, the press can be set so that the cone automatically goes to the open position every few minutes.
To set the press so that the cone goes open on high amps, the manual 4-way air valve supplied with the press must be replaced with a solenoid operated valve. Then the VFD or PLC used to control the press is set to open the cone when a high amps mark is reached, and then re-close the cone when a lower amps set point is reached. Many paper mills operate their presses in this manner.
To automatically open the cone after a fixed period of time, Vincent offers, at no charge, an electrical panel called a Cone Timer. This 110/220 volt single phase panel has a timer. The timer is set to keep the cone closed for “x” minutes, and then open the cone for “y” seconds. The “x” and “y” periods are determined by trial and error. Note that the “y” seconds open period may be so short that the cone never reaches a fully open position. Also inside the Cone Timer is a 4-way solenoid valve. The plant air supply goes into this valve, with air outlets to either open or close the cone’s air cylinder(s). Presses used to dewater wash tank sludge at plastics recyclers frequently need a Cone Timer.
The capitalized names used in this Pressing News are fully described in the Operating Hints section of the Operating Manual.
Issue 251