23 JULY 2003
low back-pressure. As the filtration cycle continues, the back-pressure increases
because the solids accumulate, and the flow rate drops to a very low rate at the
maximum pressure. The pumping system should be equipped with flow control devices
that automatically adjust (lower) the flow rate with increasing pressure. Although several
types of pumps can be used for sludge feed systems, the most commonly used are
progressive cavity, piston, or piston-membrane pumps. Following is a brief discussion of
each of these types of pumps and their application.
2-220.127.116.11 The progressive cavity pumps are variable-speed drive pumps that operate
based on the geometrical fit between the rotating element of the pump (rotor) and sta-
tionary element (stator). The pumping action is achieved by the rotor turning eccentri-
cally within the stator, which causes fluid to enter cavities formed between the rotor and
stator at the pump inlet and to progress within that cavity to the pump outlet. These
pumps are often constructed with multiple stages to achieve the high discharge pres-
sure required for the filter press operation. These multistage pumps typically have
increased pressures and decreased flow rates with additional stages, with a maximum
pump speed of 200 to 250 rpm to minimize wear on the pump rotor and stator.
2-18.104.22.168 Piston pumps are generally driven by a hydraulic power pack with a
compensator that varies the pump discharge while maintaining the required filter press
pressure. These types of pumps typically have infinite turndown capability from maxi-
mum to zero discharge. Because the piston and cylinder are in direct contact with the
sludge, these pumps should be equipped with wear-resistant pistons and ceramic-lined
cylinders. Other design considerations include the proper sizing of surge arrestor
vessels on the suction and discharge sides of the pump. Problems from incorrect sizing
include pulsating discharge and excessive wear on the suction and discharge ball check
valves. Piston pumps are normally available for pressures up to 1720 kPa (250 psig)
and flow capacities ranging from 1 to 30 L/s (15 to 450 gpm).
2-22.214.171.124 Piston-membrane pumps are similar to piston pumps, except that the moving
parts of the pump (i.e., piston and cylinder) have no direct contact. These moving parts
are separated by a flexible membrane. This type of pump operates primarily by regu-
lating the amount of hydraulic fluid displaced against the membrane. Piston-membrane
pumps are typically available for terminal pressures of 690 and 1550 kPa (100 and 225
psig) and for flows ranging from 0.3 to 30 L/s (5 to 500 gpm).
2-126.96.36.199 Air operated diaphragm pumps can also be used for pumping sludge.
Typically, the use of these pumps has been somewhat restricted because they were
limited to a 1:1 ratio of air pressure to discharge of slurry pressure and most plant air
systems are limited to about 690 kPa (100 psig). However, designs with 2:1 ratios (i.e.,
delivering slurry at 1380 kPa [200 psig] with 690 kPa [100 psig] motive air) have been
recently developed that allow broader use of this type of pump.
2-188.8.131.52 Centrifugal pumps have been used with only limited success for filter press
sludge pumping. Although these types of pumps have suitable flow and discharge
pressure characteristics required for initially filling the filter press, the shearing force of
the impeller destroys floc generated from sludge conditioning, thus reversing favorable