Net Positive Suction Head Required, NPSHR, Net Positive Suction Head Available, NPSHA

Net Positive Suction Head Required, NPSHR:-

As the liquid passes from the pump suction to the eye of the impeller, the velocity increases and the pressure decreases. There are also pressure losses due to shock and turbulence as the liquid strikes the impeller. The centrifugal force of the impeller vanes further increases the velocity and decreases the pressure of the liquid. The NPSH required is the positive head (absolute pressure) required at the pump suction to overcome these pressure drops in the pump and maintain the liquid above its vapor pressure.

Net Positive Suction Head Available, NPSHA :-
Net Positive Suction Head Available is a function of the system in which the pump operates. It is the excess pressure of the liquid in feet absolute over its vapor pressure as it arrives at the pump suction,
to be sure that the pump selected does not cavitate.