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.
In the photo above, the NPSH that is available at the pump suction, P2 is characterized by pressure at the pressure source, P1, minus pressure drop due to change of elevation, ∆ Height, and friction loss ∆Pf. It can be calculated using a modified form of the Bernoulli Equation:
\(NPSH_A = P_2 = P_1 - P_v - ∆P_f - ∆Height\)
NPSHA = Available Net Positive Suction Head
P1 = Initial Pressure
Pv = Vapor Pressure
∆Pf = pressure Loss due to friction
∆ Height = Pressure loss due to elevation change
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.