UFC 3-280-03
23 JULY 2003
As a minimum, the system control panel should include "start" buttons for
operating the subsystems; selector switches to shift from primary to a redundant oper-
ating unit (i.e., pumps); a feed pressure recorder; level indicators for all tanks and bins
used throughout the system; and alarm indicators for cycle completion, tank/bin low or
high levels, and equipment malfunction. In addition to these components, auxiliary
control and record-keeping instruments at large presses could include noting and
recording sludge concentration, noting cycle start time and project cycle time,
instantaneous cycle feed rate and totalizer, cake and filtrate concentration status, and
total operating hours for major operating components to aid maintenance.
2-4.7.1.3 Fully Automatic Systems. Fully automatic systems are the same as semiau-
tomatic systems, except the complete operation, from the prefill through the end of the
filtration cycle, is begun through a single "start cycle" push button. Although this system
is automatically controlled during the filtration cycle, it is normally equipped with selector
switches for control of subcycles within the filtration cycle.
2-4.7.2 Common Control Elements. Three important elements that should be
incorporated into any control system are feed pressure recording, filtrate flow meas-
urement, and safety and interlocking systems.
2-4.7.2.1 Feed Pressure Recording Systems. Feed pressure monitoring throughout
the filtration cycle is one of the best ways of seeing how well the system is operating. As
described in Paragraph 2-2, the pressure trace during the filtration cycle normally
generates an "S" curve. Deviations noted when comparing the current cycle with
previous cycles may indicate a problem. This can be monitored by a strip recorder
placed within or adjacent to the system control panel.
2-4.7.2.2 Filtrate Measurement Systems. In typical presses, a combination of feed
terminal pressure and reduction in filtrate flow to a predetermined level is used to
indicate a complete filtration cycle. The filtrate flow is normally measured in a weir tank,
equipped with an alarm switch, which is further described in Paragraph 2-4.10.1, that
signals when the flow drops to a predetermined level. The feed terminal pressure is
normally measured by a pressure switch. Once either the specified minimum flow rate
or terminal pressure is achieved, the sludge feed system is shut off.
2-4.7.2.3 Safety and Interlocking Systems. Safety and interlocking systems are
essential for all filter press installations. For this discussion, safety systems will include
only those that, when engaged, interrupt service, such as a safety light curtain. Inter-
locking systems prevent or start a sequential step of the process unless a specific
condition is met, such as the filter press not opening if the feed pressure is greater than
zero.
a. Safety Systems. The primary safety system used in filter presses is a
safety light curtain. Safety light curtains consist of a bank of photo cells on alternative
ends of the press that, when activated, form a continuous light curtain during closing or
plate shifting. If the light curtain is interrupted, the work is stopped immediately. The
light curtain, therefore, acts as an interlock when the plate is closed, opened, or shifted.
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