EI 11C201
CEMP-E
1 March 1997
areas where electric power is not available, will usually require engine drives. The three types of
electric motors most commonly used in wastewater pumping are (1) squirrel-cage induction, (2)
wound-rotor induction, and (3) synchronous. Squirrel-cage induction motors will normally be
selected for constant speed pump applications because of their simplicity, reliability and
economy. They can also be used for variable speed operation when provided with the proper
speed control. Synchronous motors may be more economical for large capacity, low rpm,
constant speed pumps. Wound-rotor induction motors are most commonly used for pumps
requiring variable speed operation. For a 60 cycle, alternating current power supply, the
maximum synchronous motor speed allowed for wastewater pumps will be 1800 rpm
(approximately 1770 rpm induction speed). The normal range of speeds is from 600 to 1200
rpm, with speeds below 450 rpm unusual at military installations. Lower speed pumps and
motors are larger and more expensive, but generally are more reliable. The selection of electric
motors will depend upon the type, size and location of the pumps, type of speed control used,
and the power available at the site. Pump location will determine the type of motor enclosure.
For dry pit pump installations, motor enclosures will normally be the open, drip proof type.
Pumps installed outdoors, or in dirty or corrosive environments, will require totally enclosed
motors. Submersible pumps will have motor enclosures which are watertight. Motors installed
outdoors will have temperature ratings adjusted to suit ambient operating conditions. For pumps
designed to operate on an intermittent basis, space heaters will be provided in motor housings to
prevent condensation. Motors installed in wet wells will be explosion proof. Motor starting
equipment will be selected in accordance with paragraph 7-3, and will be suitable for the type of
motor and the required voltage. Motor starters will be designed for limiting the inrush current
where shocks or disruptions to the electrical supply are likely to occur as a result of pump
start-up. Where low starting inrush current is required for constant speed pumps, such as when
using engine driven generator sets, wound-rotor motors will be considered as an alternative to
squirrel-cage motors. The voltage required for operation of motors and other equipment will be
determined in accordance with paragraph 9-6.
b. Internal combustion engines. Internal combustion engines (I.C.) engines will be used
primarily at large pumping stations where electric motors are the primary drive units, and where
emergency standby facilities are required. Conditions which dictate the use of fixed, standby
power at wastewater pumping stations are outlined in paragraph 9-7. I.C. engines will be
required for small pump stations in remote locations where no electric power source exists. At
large wastewater treatment plants where abundant digester gas is produced, it will generally be
more feasible to use I.C. engines which are fueled by the waste gas. I.C. engines may be
arranged to drive horizontal pumps by direct or belt connections, or they may drive vertical
pumps through a right angle gear drive with an electric motor as the primary drive unit (dual
drive). It is more common however, and will be the general rule at large pump stations, to
provide fixed emergency generator sets powered by I.C. engines. Generators produce electric
power not only for pumps, but also for auxiliary equipment such as heaters, lights, alarms, etc.,
and for critical pump control systems. The types of internal combustion engines normally used
include (1) diesel, (2) gasoline, (3) natural gas, primarily digester gas, and (4) dual-fuel diesel.
The use of gasoline engines for anything except small, remotely located pumping stations is not
recommended due to the hazards associated with fuel handling and storage. Dual-fuel diesel
engines fire a mixture of diesel oil and natural gas, with a minimum of 10 percent diesel fuel
required to ignite the mixture. Propane is usually provided as a backup fuel for gas and dual-fuel
diesel units. The selection of I.C. engines will be coordinated with the installation's Facility
Engineer to insure that adequate operation and maintenance can be made available.
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