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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.
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