Net positive suction head

NPSH
NPSH is the difference between pressure at the suction of the pump and vapour pressure of the liquid. It is very important to avoid cavitation in centrifugal pumps.
NPSH=PRESSURE AT SUCTION INLET-                         VAPOUR PRESSURE OF LIQUID

Available NPSH - NPSHa or NPSHA



The Net Positive Suction Head available from the application to the suction side of a pump is often named NPSHa. The NPSHa can be estimated during the design and the construction of the system, or determined experimentally by testing the actual physical system.

Required NPSH - NPSHr or NPSHR

The NPSHr, called as the Net Suction Head as required by the pump in order to prevent cavitation for safe and reliable operation of the pump.
The required NPSHr for a particular pump is in general determined experimentally by the pump manufacturer and a part of the documentation of the pump.
                    
The available NPSHa of the system should always exceeded the required NPSHr of the pump to avoid vaporization and cavitation of the impellers eye. The available NPSHa should in general be significant higher than the required NPSHr to avoid that head loss in the suction pipe and in the pump casing, local velocity accelerations and pressure decreases, start boiling the fluid on the impellers surface.

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