CEMP-E
EI 11C201
1 March 1997
Larger, conventional type pump stations will normally be constructed with wet wells divided into
two or more sections, or compartments, so that a portion of the station can be taken out of
service for inspection or maintenance. Each compartment will have individual suction pipes, and
will be interconnected with slide or sluice gates. The floor of the wet well will be level from the
wall to a point 300 to 450 millimeters (12 to 18 inches) beyond the outer edge of the suction bell,
and then will be sloped upward at a minimum 1:1 slope.
7-6. CAVITATION.
a. Cavitation is a term used to describe a phenomenon that may occur in a pumping
installation and may occur in piping systems because of liquid velocity changes. Cavitation in
centrifugal pumps is explained in TM 5-813-5/AFM 88-10, Vol. 7. Cavitation in pipe lines may
take place at sudden enlargements of the pipe cross section, at sharp bends, or similar
situations. The designer should avoid the following conditions for centrifugal pumps:
(1) Operating heads much lower than rated head at peak efficiency of the pump.
(2) Operating capacities much higher than rated capacity at peak efficiency of the pump.
(3) Suction lift higher or positive suction head lower than recommended by the
manufacturer.
(4) Liquid temperatures higher than that for which the system was originally designed.
(5) Pump speeds higher than manufacturer's recommendations.
b. For propeller pumps, the designer should avoid these conditions except conditions (1) and
(2) will be stated as follows:
(1) Operating heads much higher than the peak efficiency of the pump.
(2) Operating capacities much lower than capacity at peak efficiency of the pump.
Cavitation will not be a problem in a pump installation if the system is designed, selected,
installed, and operated in accordance with the recommendations of the designer and the
manufacturer.
7-7. SURGE ANALYSIS AND CONTROL.
a. Water hammer. Sudden changes in flow and velocity in force mains can occur as a result
of pump startup, pump shutdown, power failure, or rapid closing of a valve. These velocity
changes can produce large pressure increases or surge phenomena known as water hammer.
The most severe water hammer conditions are usually caused by a pump shutdown or power
failure. An analysis of water hammer will include calculating the critical time, determining the
maximum pressure increase, and selecting a method of control.
b. Critical time. When flow is suddenly changed in a force main, a pressure wave is
generated which rapidly travels the entire length of conduit, and back to the point of change. The
time required for this round trip is given by:
Tc
=
2L/a
7-9
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