Page 104 - Centrifugal Pumps Design and Application

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NPSH

The expressions NPSHR and NPSHA are accepted abbreviations for
net positive suction head required and net positive suction head available.
Probably more has been written on NPSH than any other subject involv-
ing pumps. With so much literature available, one might assume that
NPSH and its relationship to cavitation is well understood. Nothing could
be further from the truth. To this day, NPSH is still misunderstood, mis-
used, and misapplied, resulting in either costly over-design of new sys-
tems or unreliable operation of existing pump installations. Avoiding
these problems requires accurate prediction of NPSHR, supply of suffi-
cient NPSHA, and attention to suction piping approach. NPSHR can be
considered the suction pressure required by the pump for safe, reliable
operation,

Establishing NPSHA
Establishing NPSHA, which is the head available characteristic of the
system that provides flow of liquid to the pump, is the responsibility of
the system designer. As NPSHR increases with pump capacity, normal
practice is to establish NPSHA at the operating condition, then add a rea-
sonable margin to accommodate any anticipated increase in pumping ca-
pacity. All too often, future operating problems begin here. It is not un-
usual for the ultimate user to add some anticipated increase in capacity,
then for insurance the contractor designing the system adds even more.
When the pump designer finally gets the data sheet, he designs the impel-
ler inlet and suction nozzle geometry for operating capacity, plus user
margin, plus contractor margin. If these margins are not carefully con-

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