Page 100 - Know and Understand Centrifugal Pumps

Understanding Pump Curves

B

Figure 7-7
At point ‘C’, the flow is high and the head (pressure) is low on the
H-Q curve. This pump is operating with reduced efficiency, this
time to the right of the optimum efficiency point. The BHP is rising
and may overload the installed motor. The NPSHr has risen to the
point that the pump is being strangled; the liquid is leaving the
pump faster than it can come into the pump. The pumped liquid is
prone to vaporize or boil. This is the zone where classic
vaporization cavitation occurs. And, the shaft is under a deflection
load, stressing the seal and bearings.
Let’s see these four elements, as they appear on the same graph (Figure
7-8).
You can see in Figure 7-9 that the pump should run at or near zone
‘A’, its best efficiency point, the BEP. This is the preferred sweet or
happy zone. The pump should be specified and operated in this zone.

H
Feet

-

0
0 Q
GPM
Figure 7-8 ~ ~ ~~~ -.
83 n