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.