EI 11C201
CEMP-E
1 March 1997
suction passage, thereby limiting its use to handling only clear water. Radial flow centrifugal
pumps are available in discharge sizes of 50 to 500 millimeters (2 to 20 inches). However,
pumps with a capacity to pass 75-millimeter (3-inch) minimum solids will be required. The
recommended capacity range for these pumps is 6 to 1,260 L/s (100 to 20,000 gpm). Pumps are
available in discharge heads of 8 to 60 meters (25 to 200 feet) total dynamic head (TDH). Peak
design efficiency ranges from 60 percent for smaller pumps to 85 percent for larger pumps.
Radial flow pumps are suitable for either wet well or dry well applications. They can be installed
with horizontal or vertical shafting, however, vertical shaft pumps require considerably less
space.
(2) Mixed flow pumps. The mixed flow centrifugal pump is an intermediate design
between the radial flow type and the axial flow type, and has operating characteristics of both.
The mixed flow pump is designed with wide unobstructed passages, and is therefore suitable for
handling wastewater or clear water. Mixed flow centrifugal pumps are available in 200-millimeter
through 2100-millimeter (8-inch through 84-inch) discharge sizes. The recommended capacity
range for these pumps is 60 to 5,000 L/s (1,000 to 80,000 gpm). Pumps are available to operate
at 3 to 18 meters (10 to 60 feet) TDH. Peak design efficiency depends on the size and
characteristics of the individual pump, but generally ranges from 80 to 90 percent. Mixed flow
pumps are used only in dry well applications, with either horizontal or vertical shafting
configuration.
(3) Axial flow pumps. Axial flow centrifugal pumps will not be used to pump raw untreated
wastewater. This pump is designed primarily for clear water service and for wet well
installations. The pump is furnished with vertical shaft having a bottom suction, with the
propeller mounted near the bottom of the shaft and enclosed in a bowl. The propeller is totally
submerged and can be clogged by large solids, rags or trash. Therefore, this pump will only be
used for clear well applications. Axial flow centrifugal pumps are available in 200-millimeter
through 1800-millimeter (8-inch through 72-inch) discharge sizes. The recommended capacity
range for these pumps is 30 to 6,300 L\s (500 to 100,000) gpm. Pumps are available to operate
from 0.3 to 12 meters (1 to 40 feet) TDH.
(4) Pump construction. Centrifugal wastewater pumps will normally be constructed of cast
iron with bronze or stainless steel trim, and with either cast iron or bronze impellers. When
operating in wastewater containing substantial quantities of grit, impellers made of bronze, cast
steel or stainless steel will be required. Enclosed impellers will be specified for waste water
pumps required to pass solids. Pump casings of the volute type will be used for pumping raw
untreated wastes and wastewaters containing solids. Diffusion or turbine type casings may be
utilized for effluent or clear water service at waste treatment facilities. Pump shafts will be high
grade forged steel, and will be protected by renewable bronze or stainless steel sleeves where
the shaft passes through the stuffing box. Stuffing boxes will utilize either packing glands or
mechanical type seals.
(5) Stuffing box seals. The stuffing box will be lubricated and sealed against leakage of
wastewater (into the box) by grease, potable water, or another clear fluid. The lubricating and
sealing medium will be supplied to the stuffing box at a pressure of 35 to 70 kPa (5 to 10 psi)
greater than the pump shutoff head. Grease seals are usually provided by cartridges which are
either spring loaded or pressurized by connections off the pump discharge. These arrangements
generally do not maintain sufficient seal pressure on the stuffing box. However, they will be
acceptable for low head pumps and where the wastewater contains little grit, as when pumping
treated effluent. When pumping raw untreated wastes containing the usual quantities of grit, a
potable water seal system with seal pump will be required if a potable water line is accessible
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