NPSHr

NPSHR stands for Net Positive Suction Head Required. It is a pump-specific parameter that represents the minimum suction head (or pressure) needed at the pump inlet to avoid cavitation at a given flow rate. NPSHR is the “required” head determined by the pump design and operating condition, and it is distinct from NPSHA, the Net Positive Suction Head Available in the system.

NPSHR depends on factors such as flow rate, impeller design, pump speed, and fluid properties. For centrifugal pumps, NPSHR typically increases with higher flow rates and varies with the geometry of the suction path. It is not a fixed property of the pump alone but a curve that must be consulted for the expected operating point.

Cavitation occurs when the local pressure at the suction reduces below the liquid’s vapor pressure, causing vapor bubbles to form and then collapse as they move to higher pressure regions. This leads to noise, vibration, reduced performance, and potential damage. If NPSHA is less than NPSHR, cavitation is likely; if NPSHA exceeds NPSHR, the pump can operate without cavitation at that flow rate.

In practice, engineers compare NPSHR (from manufacturer curves) with NPSHA (calculated from the system’s suction head, elevation, vapor pressure, and losses). A safe design maintains NPSHA above NPSHR across the intended operating range, often with a margin. NPSHR is commonly expressed in meters or feet of liquid.