23 JULY 2003
The light curtain should also be wired for "fail safe" operation to ensure that beam
misalignment or failed wiring causes the system to de-energize the safety relay and
stop the filter press. If the light curtain causes the plate closing or shifting mechanism to
trip, the system should only be reactivated by use of a lanyard, a local push-button
control, or from a centrally located control panel. However, the reactivation device
should be located far enough away from the filter to ensure the area is clear before it is
restarted. Once the filter is pressurized, the light curtain interlocking is bypassed to
allow automatic maintenance of the hydraulic closure pressure.
b. Interlock Systems. Several types of interlocking systems are typically
used in filter presses. These range from devices that prevent the start of the filtration
cycle if adequate hydraulic pressure does not exist to hold the filter closed to those
preventing discharge of the sludge cake if the cake receptacle is not in place. The most
commonly used interlocking devices include those for starting the filtration cycle and
filter opening, and those for drip trays and bunker covers, conveyors, and sludge cake
(1) The start cycle interlock should prevent the sludge feed from being
pumped to the filter system if adequate hydraulic pressure does not exist to keep the
plates tightly in place. This type of interlock is not only desirable for housecleaning but
also for safety.
(2) The open filter interlock should ensure that the hydraulic pressure holding
the filter closed cannot be released if the pressure within the sludge feed system is
greater than zero. This type of system is desirable to avoid excessive housecleaning
and to provide safe operation. This interlock is essential for feed systems that possess
pressure vessel surge tanks or equalizer tanks.
(3) The drip tray and bunker cover interlock should be used to provide proof
that the drip tray or bunker covers are open before the filter cake is discharged. This
system is normally designed using limit switches that monitor both the hydraulic
pressure applied to the press closing mechanism and the position of the trays and
covers. This interlock will not allow the press to be opened unless the trays and covers
are opened. This avoids inadvertent discharge of sludge cake that may cause problems
and damage to the trays and covers.
(4) The conveying interlock is only used when a conveyor is used in the
sludge cake disposal process. Typically, this type of system will be interlocked with
another sequential treatment process step (i.e., incineration) or to a bunker or
receptacle used for storage prior to ultimate disposal. In general, the following controls
should apply to all conveyor installations:
All conveyors should be equipped with motion switches.
Startup of the conveyor system should be in sequential order, based on
proven startup of upstream conveyors or sequential treatment processes.
Failure of a downstream conveyors or sequential treatment processes should
cause immediate shutdown of upstream conveyors.