TM 5-814-2/AFM 88-11, Vol. 2
pumped under low head through pressure sewers as
thereby prevented from entering the stuffing box, thus
small as 1 inches in diameter. Grinder pumps are for
reducing wear on the shaft and packing. The advantage
submersible installation, with a recommended operating
of less frequent repairs to the shaft and less frequent
range of 10 to 100 gpm. These pumps are available in
repacking should be considered in relationship to the
discharge heads of 10 to 150 feet TDH. The peak
cost of providing the water line and other necessary
design efficiency is generally very low. Grinder pumps
facilities for the water seal. Where freezing of seal
will be used only to handle domestic type wastes from
water is likely to occur, protective measures will be
one or more individual buildings, and only in remote
taken. There must not be, under any circumstances, a
areas or areas where gravity sewers and centralized
direct connection between wastewater pumps and the
pumping facilities are not feasible (see paragraph 1-3b).
potable water system, nor any possibility of backflow of
wastes into the potable water system.
Air Force
facilities will comply with AFM 85-21.
4-2.
Pump drives
b. Screw pumps. The screw pump is classified as
a Electric motors. As a general rule, electric
a positive displacement pump, and as such, maintains
motors will be provided as the primary drive unit in
two distinct advantages over centrifugal pumps. It can
sanitary and industrial wastewater pumping stations.
pass large solids without clogging, and can operate over
Small pump stations serving remote areas where
a wide range of flows with relatively good efficiencies.
electric power is not available, will usually require
Screw pumps are normally available in capacities
engine drives. The three types of electric motors most
ranging from 150 to 50,000 gpm with a maximum lift of
commonly used in wastewater pumping are (1) squirrel-
30 feet. Because of its nonclog capabilities and wide
cage induction, (2) wound-rotor induction, and (3)
pumping range, the screw pump is best suited for lifting
synchronous.
Squirrel-cage induction motors will
raw untreated wastewater into the treatment facility, and
normally be selected for constant speed pump
for the pumping of treated effluent. Its use in sludge
applications because of their simplicity, reliability and
pumping is discussed in TM 5-814-3/AFM 88-11, Vol. 3.
economy. They can also be used for variable speed
Also, when treatment plants are upgraded, screw pumps
operation when provided with the proper speed control.
may be installed to overcome the additional head losses
Synchronous motors may be more economical for large
created by new treatment units, so that existing
capacity, low rpm, constant speed pumps. Wound-rotor
discharge facilities can be retained. Screw pumps are
induction motors are most commonly used for pumps
usually driven by a constant speed motor with gear
requiring variable speed operation. For a 60 cycle,
reducer, and are inclined at angles of 30 to 38 degrees
alternating current power supply, the maximum
from the horizontal. In most instances, screw pumps will
synchronous motor speed allowed for wastewater pumps
be installed outdoors with only the drive unit enclosed.
will be 1800 rpm (approximately 1770 rpm induction
c. Pneumatic ejectors. Pneumatic ejector stations
speed). The normal range of speeds is from 600 to
will generally be used only in remote areas where
1200 rpm, with speeds below 450 rpm unusual at
quantities of wastes are small, and where future
military installations. Lower speed pumps and motors
increases in waste flows are projected to be minimal. A
are larger and more expensive, but generally are more
pneumatic ejector consists of a receiving tank, inlet and
reliable. The selection of electric motors will depend
outlet check valves, air supply, and liquid level sensors.
upon the type, size and location of the pumps, type of
When the wastewater reaches a preset level in the
speed control used, and the power available at the site.
receiver, air is forced in ejecting the wastewater. When
Pump location will determine the type of motor
the discharge cycle is complete, the air is shut off and
enclosure.
For dry pit pump installations, motor
wastewater flows through the inlet into the receiver.
enclosures will normally be the open, drip proof type.
Generally, duplex ejectors operate on a 1-minute cycle,
Pumps installed outdoors, or in dirty or corrosive
filling for 30 seconds and discharging for 30 seconds.
environments, will require totally enclosed motors.
Thus, each receiver tank will be equal in volume to 30
Submersible pumps will have motor enclosures which
seconds of the extreme peak flowrate. Pneumatic
are watertight. Motors installed outdoors will have
ejector stations are available in capacities ranging from
temperature ratings adjusted to suit ambient operating
30 to 200 gpm with recommended operating heads up to
conditions. For pumps designed to operate on an
60 feet TDH. A typical ejector installation will include
intermittent basis, space heaters will be provided in
duplex units with two compressors, receivers, level
motor housings to prevent condensation.
Motors
sensors, etc.
installed in wet wells will be explosion proof. Motor
d Grinder pumps. Grinder pumps shred solids
starting equipment will be selected in accordance with
normally found in domestic wastewater, including rags,
paper and plastic, into a slurry. The slurry can be
4-2
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