CEMP-E
EI 11C201
1 March 1997
within a reasonable distance. The water seal system will be capable of supplying 0.2 L/s (3 gpm)
per pump minimum. The principal advantage of a water seal over a grease seal is the positive
pressure maintained on the stuffing box by the seal pump, and small amount of water which
flows from the stuffing box into the pump casing. Grit and other abrasive materials that may be
in the wastewater are thereby prevented from entering the stuffing box, thus reducing wear on
the shaft and packing. The advantage of less frequent repairs to the shaft and less frequent
repacking should be considered in relationship to the cost of providing the water line and other
necessary facilities for the water seal. Where freezing of seal water is likely to occur, protective
measures will be taken. There must not be, under any circumstances, a direct connection
between wastewater pumps and the potable water system, nor any possibility of backflow of
wastes into the potable water system. Air Force facilities will comply with AFM 85-21.
b. Screw pumps. The screw pump is classified as a positive displacement pump, and as
such, maintains two distinct advantages over centrifugal pumps. It can pass large solids without
clogging, and can operate over a wide range of flows with relatively good efficiencies. Screw
pumps are normally available in capacities ranging from 10 to 3150 L/s (150 to 50,000 gpm) with
a maximum lift of 9 meters (30 feet). Because of its nonclog capabilities and wide pumping
range, the screw pump is best suited for lifting raw untreated wastewater into the treatment
facility, and for the pumping of treated effluent. Its use in sludge pumping is discussed in TM
5-814-3/AFM 88-11, Vol. 3. Also, when treatment plants are upgraded, screw pumps may be
installed to overcome the additional head losses created by new treatment units, so that existing
discharge facilities can be retained. Screw pumps are usually driven by a constant speed motor
with gear reducer, and are inclined at angles of 30 to 38 degrees from the horizontal. In most
instances, screw pumps will be installed outdoors with only the drive unit enclosed.
c. Pneumatic ejectors. Pneumatic ejector stations will generally be used only in situations
where quantities of wastes are too small for more conventional pumps, and where future
increases in waste flows are projected to be minimal. A pneumatic ejector consists of a
receiving tank, inlet and outlet check valves, air supply, and liquid level sensors. When the
wastewater reaches a preset level in the receiver, air is forced in ejecting the wastewater. When
the discharge cycle is complete, the air is shut off and wastewater flows through the inlet into the
receiver. Generally, duplex ejectors operate on a 1-minute cycle, filling for 30 seconds and
discharging for 30 seconds. Thus, each receiver tank will be equal in volume to 30 seconds of
the extreme peak flowrate. Pneumatic ejector stations are available in capacities ranging from 2
to 13 L/s (30 to 200 gpm) with recommended operating heads up to 18 meters (60 feet) TDH. A
typical ejector installation will include duplex units with two compressors, receivers, level
sensors, etc.
d. Grinder pumps. Grinder pumps shred solids normally found in domestic wastewater,
including rags, paper and plastic, into a slurry. The slurry can be pumped under low head
through pressure sewers as small as 32 millimeter (1-1/4 inches) in diameter. Grinder pumps
are for submersible installation, with a recommended operating range of 0.6 to 6 L/s (10 to 100
gpm). These pumps are available in discharge heads of 3 to 45 meters (10 to 150) feet TDH.
The peak design efficiency is generally very low. Grinder pumps will be used only to handle
domestic type wastes from one or more individual buildings, and only in remote areas or areas
where gravity sewers and centralized pumping facilities are not feasible (see paragraph 1-3b).
6-3. PUMP DRIVES.
a. Electric motors. As a general rule, electric motors will be provided as the primary drive
unit in sanitary and industrial wastewater pumping stations. Small pump stations serving remote
6-3
Previous Page
