Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

NPSH 87
Predicting NPSHR
The other side of the coin necessary for reliable operation is of course
accurate prediction of NPSHR by the pump designer.
In considering NPSHR, it is necessary to understand that a centrifugal
pump is designed as a hydraulic machine to move liquids. Any amount of
entrained air or gas present will cause a deterioration in pump performance.
Various tests substantiate the claim that a volume of only one percent
air or gas will cause a loss of head and efficiency. As liquid travels
from suction nozzle to impeller eye, it will experience pressure losses
caused by friction, acceleration, and shock at blade entry. If the summation
of these losses permits vaporization of the liquid, vapor bubbles will
form in the impeller eye, travel through the impeller, and upon reaching a
high pressure region, collapse. This collapse or implosion of the vapor
bubbles is classic cavitation, which can lead to impairment of performance
and impeller damage (Figures 8-2 and 8-3). Thus, predicting
Figure 8-2. Cavitation damage—looking into impeller eye.