Page 99 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

The pump’s minimum requirements (NPSH)

The last component of the pump performance curve is the curve of the
minimum requirements, or NPSH. Actually, the reading on the pump
curve is the NPSHr, the Net Positive Suction Head required by the
pump. There is a complete discussion on NPSHr and NPSHa, and the
result of not respecting or understanding them in Chapters 3 and 4.
Basically, the NPSHr curve, beginning at 0 flow, is mostly flat or
modestly rising until it crosses through the BEP zone. As the NPSHr
curve crosses through the BEP of the pump, the curve and values begin
rising exponentially. Normally it is seen this way (Figure 7-6).

HI
Feet

The NPSHr curve is a flat to modestly
rising curve. It begins rising
sharply as the pump crosses thru
its BEP.

Y

0 Q
GPM

Review
See Figure 7-7 for the pump performance curve components.
As you can see in the four components of the pump curve:
w At point ‘A’ on the H-Q curve, the pump is pumping Q gpm
(gallons per minute), at H feet of head. This point on the curve
corresponds to the best efficiency, and it is also seen at
approximately the middle of the energy curve, and also on the
NPSHr curve where it begins its sharp rise.
At point ‘B’, the flow is reduced and the head is elevated on the H-
Q curve. The pump is being operated to the left of its best efficiency
zone. Note that the pump has lost efficiency at this point. The
minimum requirements of the pump, the NPSHr, and the
horsepower consumption, BHP, have also been reduced, but with
the efficiency drop and reduced flow, the pump is vibrating and
heating the pumped liquid. The shaft is under deflection, causing
stress to the bearings and mechanical seal (or shaft packing rings).

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