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CEMPE
EI 11C201
1 March 1997
constructed by plotting the TDH over a range of flowrates from zero to the maximum expected
value. Friction losses can be expected to increase with time, thus affecting the capacity for the
pumping units and their operation. Therefore, system curves well reflect the maximum and
minimum friction losses to be expected during the lifetime of pumping units, as well as high and
low wet levels. The typical set of system curves will generally consist of two curves using a
HazenWilliams coefficient of C = 100 (one for maximum and one for minimum static head), and
two curves using a HazenWilliams coefficient of C = 140 (for maximum and minimum static
head). These coefficients represent the extremes normally found in wastewater applications.
c. Pump headcapacity curve. The head that a particular pump can produce at various
flowrates is established in pump tests conducted by the pump manufacturer. The results of
these tests are plotted on a graph to form the pump characteristic curve. Along with the
discharge head developed, the pumps operating efficiency, required power input, and net
positive suction head are generally included on the same diagram. Additional information on
pump curves and pump design is available in TM 58139. Software is also commercially
available for personal computers to aid in the design of pumping systems and the selection of
pumps.
(1) Efficiency and power input. Pump efficiency is the ratio of the useful power output to
the power input, and is given by:
E
=
wQ TDH
)))))))
PK
where:
E
=
pump efficiency (100 E = percent)
w
=
specific weight of fluid in kilonewtons per cubic meter (pounds per cubic foot)
Q
=
pump capacity in cubic meters per second (cubic feet per second)
TDH
=
Total dynamic head in meters (feet)
P
=
power in kilowatts (brake horsepower)
K
=
constant, 1 for SI units (550 for IP units)
Pump efficiencies usually range from 60 to 85 percent. Most characteristic curves will indicate a
best efficiency point (BEP) at which pump operation is most efficient. Where possible, pumps
will be selected to operate within a range of 60 to 120 percent of the BEP.
(2) Net positive suction head. When pumps operate at high speeds and at capacities
greater than the BEP, the potential exists for pump cavitation. Cavitation can reduce pumping
capacity and may in time damage the pump impeller. Cavitation occurs when the absolute
pressure at the pump inlet drops below the vapor pressure of the fluid being pumped. To
determine if cavitation will be a problem, the net positive suction head (NPSH) available will be
computed, and compared with the NPSH required by the pump. The NPSH is not normally a
problem when discharge heads are less than 18 meters (60 feet). However, when heads are
greater than 18 meters (60 feet), or when the pump operates under a suction lift, or far out on its
curve, the NPSH will be checked. The NPSH available at the eye of the impeller in meters (feet)
will be calculated with the following formula:
NPSHA = Hs + Pa/w  Pv/w
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