NPSH

NPSH:

Net positive suction head measures the difference in head (differential head) & not the difference in pressure.

NPSHA	NPSHR
Absolute Pressure required at the pump suction above the vapor pressure of the liquid at that temperature. (Psuction > Pvap)	It is the minimum absolute pressure required at the pump suction to avoid vaporization. (Psuction = Pvap)
It is the function / requirement of your actual process / system	It is pump specific.
Hence it is calculated with the help of process parameters & conditions.	It is provided by pump manufacturer. It is calculated using water at room temperature by the manufacturer.

It is worth noting that while NPSHa must be greater than NPSHr, NPSHr test values stem from a procedure defined by Hydraulic Institute.
When the NPSHr value is determined by the pump manufacturer using water, the pump performance has already decreased by 3% in order to measure the change & so, NPSHa must actually be atleast a few feet/meter greater than NPSHr, not just equal.

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