October 2004

Pumps/Reliability

NPSHA —how much is enough?

Follow these guidelines to avoid pump damage

Henshaw, T., Consulting Engineer

I know of no more important, and yet more misunderstood, pump characteristic than net positive suction head (NPSH). Many pump installations are plagued by cavitation problems. With centrifugal pumps, excessive cavitation causes a reduction in head and efficiency, but less cavitation can cause increased mechanical problems that range from an eroded impeller to short life of seals and bearings. Industry standards1,2 call for NPSH requirements to be based on a cool-water (one-stage) pump head drop of 3%. So, by definition, if the net positive suction head available (NPSHA) is equal to the net positive suction head required (NPSHR), pump head will be reduced (3% if pumping cool water—less

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