Defined as the net head developed at the suction port of the pump, in excess of the head due to the vapor pressure of the liquid at the temperature in the pump. NPSH must be positive for preventing the liquid from boiling. Boiling or cavitations may damage the pump.
where, is the vapor–pressure of the liquid. If the pump is placed at a height above the free surface of a liquid where the atmosphere pressure is the NPSH may be evaluated by writing the Bernoulli’s equation between the free surface and the suction port of the pump as
where frictional loss in the suction pipe between the liquid–surface and the pump.
Therefore, it is obvious that for NPSH to be positive or maximum, Z and should be minimum.
is the vapor–pressure of the liquid. If the pump is placed at a height 
above the free surface of a liquid where the atmosphere pressure is 
the NPSH may be evaluated by writing the Bernoulli’s equation between the free surface and the suction port of the pump as 
frictional loss in the suction pipe between the liquid–surface and the pump.
should be minimum. 
and 
