UFC 3-280-03
23 JULY 2003
cake begins to form and resistance increases, the feed system should be capable of
providing a constant flow with increasing pressures. When the maximum design
pressure for the filter press is reached, the feed system must be capable of reducing the
flow rate, while maintaining the constant press design pressure.
2-4.3.1.2 Two types of systems are typically used for feeding sludge to the filter press.
The first system uses a single pump or several pumps in combination with variable
speed drives to achieve the required changes in both flow and pressure. The second
system uses the combination of a pump and pressure tank.
2-4.3.1.3 The first pumping system, often called stepping, uses one variable-speed
pump or several pumps. These pumps are typically equipped with automatic controls
that are used to vary their speed to achieve desired flow rates until the maximum
pressure is reached, and then to reduce the flow while maintaining the constant
maximum pressure. A two-pump or multiple pump system is used when the initial
pressure requirements are too high or the available flow rate turndown is too limited to
be achieved by the initial pump. The second pump or multiple pumps are used in
conjunction with the initial pump to achieve the higher flow rates and will operate until
the flow rate drops within the range of the initial pump. For an example of the use of a
step-pumping system, assume that a filter press system's terminal pressure is 1550 kPa
(100 psig), a maximum requirement of 2.5 L/s (40 gpm), and an filtration operating time
of approximately 90 minutes. Applying a stepping pump system to this example may
consist of the following sequence:
Period
Pressure
Fill Rate
Initial Fill (15 min)
170 kPa (25 psi)
2.5 L/s (40 gpm)
Filtration (30 min)
345 kPa (50 psi)
2.5 L/s (40 gpm)
Filtration (30 min)
520 kPa (75 psi)
2.5 L/s (40 gpm)
Terminate Filtration
690 kPa (100 psi)
2.5 L/s (40 gpm)
(15 min)
to terminal flow
(5 to 7%
of initial flow).
The pump and pressure tank system uses one pump and a pressure tank. The pressure
tank is initially filled with sludge and pressurized with air. The filter press fill cycle is then
started by allowing the sludge in the pressurized tank to be discharged into the filter
press at a high rate. As the level of sludge in the pressure tank starts to decrease, the
sludge feed pump is engaged to maintain a constant pressure in the tank. The pressure
in the tank is also controlled by the addition or release of air. At the end of the filter
cycle, the pressure tank is closed to stop the sludge feed pumping. The pump and
pressure tank method does offer the advantage of a more rapid and positive fill;
however, it is typically not used because it requires more equipment room and is less
flexible than the integral or multiple pumping system.
2-4.3.2 Pump Characteristics. Sludge feed pumps should be positive displacement
pumps capable of delivering sludge to the filter press over a wide range of pressures
and flows. As the filter cycle begins, the pumps must deliver a maximum flow at a very
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