Net Positive Suction Head (NPSH) - Total suction head in meters (feet) of liquid absolute determined at suction nozzle and referred to datum elevation, minus the vapor pressure of liquid in meters (feet) absolute. The datum elevation is the shaft centerline for horizontal pumps, the suction nozzle centerline for vertical in-line pumps, and the top of the foundation for other vertical pumps.
Net Positive Suction Head Available (NPSHa)is the NPSH measured in feet or meters of liquid determined by the characteristics of the pumping installation with the liquid at rated flow and normal pumping temperature.
Net Positive Suction Head Required (NPSHr) - NPSH in feet or meters and is determined by characteristics within the pump. This is evaluated usually with water by the pump manufacturer. NPSHr is measured at the suction flange and corrected to the datum elevation.
NPSHr is not the pressure at which there is no cavitation in the pump. NPSHr is the minimum NPSH at rated capacity required to prevent a head drop of more than 3% (first stage head in multistage pumps) due to cavitation within pump.