MIL-HDBK-1005/16
recommended for primary solids pumping applications where the
sludge is relatively thin (2 percent solids or less).
The screw feed centrifugal pump incorporates some of
the features of the torque-flow design but also has an
auger-shaped portion of the impeller that extends into the
volute. This feature results in higher pumping efficiency and a
steeper head curve than the conventional torque-flow design,
which helps in flow control. However, this type of centrifugal
pump is more likely to accumulate stringy material if the solids
stream is not ground properly beforehand.
Centrifugal cutter pumps, or grinder pumps, overcome
clogging problems by incorporating a sharp cutting edge on the
impeller, which serves to grind the solids passing through.
The main disadvantage of the cutter pump and the other types of
centrifugal pumps is that they cannot generate the high heads
that a positive displacement pump can.
10.4.2
Progressing Cavity Pumps. Progressing cavity pumps
produce a relatively even flow and can achieve high pressures,
which permit longer pumping distances than centrifugal pumps and
aids in flushing of plugged lines. Two-stage progressing cavity
pumps can generate from 120 to 150 psi (827 to 1,034 kPa) of
pressure; higher pressures can be achieved, if necessary, by
using more stages. Up to nine stages are available, although
using one to four is most common.
Rapid wear of the rotor and stator from abrasive grit
is one of the primary drawbacks to using progressing cavity pumps
for primary solids. However, rapid wear is less of a problem
with thickened activated sludge pumping or scum pumping. For
concentrated primary scum, progressing cavity pumps are the only
type of pumps recommended.
Progressing cavity pumps require a dry pit
installation. The pumps are self-priming at suction lifts up to
28 feet (8.5 m). Run-dry protection, such as a high-temperature
sensor in the stator, is recommended. A grinder ahead of the
pump is also recommended.
10.4.2.1 Capacity and Power Considerations. Whenever
progressing cavity pumps are used for pumping primary solids (raw
or digested), a generous allowance should be made in capacity.
The pump should be designed to pump at least 50 percent more than
its required peak hydraulic capacity, and speed control should be
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