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272 Chapter 8 Pumping, Storage, and Dual Water Systems
the specific weight of water and g the gravity constant. The energy grade line is at a dis-
2
tance h P P v 2g from the eye of the impeller to the head delivered by the
s
s
s
a
pump, where p is the atmospheric pressure. The ratio H h , where h P h , is
s
sv
a
s
a
called the cavitation parameter
8.2.3 Performance Characteristics
Common performance characteristics of a centrifugal pump operating at constant
speed are illustrated in Fig. 8.4. Note that the shut-off head is a fixed limit and that
power consumption is minimum at shut-off. For this reason, centrifugal pumps, after
being primed or filled with water, are often started with the pump discharge valve
Shut-off head System head
at Q = 0
2
Head for two
equal pumps
Operating head, H Q at H a 1 Q b = 2Q at H a

a
a

Head for
Static head at
Q = 0 single pump
1 Efficiency of
two equal pumps
Efficiency of 2
single pump
E at Q a E = E at (Q = 2Q )
a
Efficiency, E
b
a
a
b
Power input for
two equal pumps
2

P = 2P a at (Q b = 2Q a )
b
Break horsepower, P Power input for P at P d 1 Operating characteristics

single pump
1
Single pump
2
Two pumps
a

Rate of discharge, Q

Figure 8.4 Performance Characteristics of Single and Twin Centrifugal Pumps Operating at
Constant Speed