NPSH

NPSH

A minimum amount of suction pressure (head) is needed for a pump to operate without cavitating. The term used to describe this suction pressure is Net Positive Suction Head (NPSH). The amount of NPSH the pump requires to avoid cavitation is called NPSHr. The amount of NPSH available to the pump from the suction line is termed NPSHa. When selecting a pump it is necessary to see how much NPSH it requires at the duty point and make sure the NPSH available exceeds that amount. It is normal practice to have at least 2 feet of extra NPSH available at the suction flange to avoid any problems at the duty point. Also, if the pump were inadvertently operated at a flow higher than the rating point then a higher NPSH would be required to avoid cavitation\

NET POSITIVE SUCTION HEAD:

Flooded suction:
NPSH = ha - hv + hs - hf

Suction lift:
NPSH = ha – hv - hs - hf

ha = the absolute pressure in feet of liquid on the surface of the supply liquid.
hv = the vapor pressure of the liquid being pumped expressed in feet of head.
Hs = the height in feet of the supply liquid surface with respect to the pump inlet.
Hf = suction line friction losses expressed in feet of head.

These calculations yield the available net positive suction head for a given system. This must be compared to the required net positive suction head NPSHr calculated by the manufacturer. NPSHa must exceed NPSHr.