NPSH

NPSH stands for Net Positive Suction Head, a key concept in pump performance that describes the pressure head available at the pump suction to keep the liquid from boiling or cavitating as it enters the impeller. In practice, NPSH is discussed in terms of two quantities: NPSH_A (available) and NPSH_R (required).

NPSH_A is the head of liquid at the pump suction that is available from the system. It is determined by the system pressure at the suction, adjusted for elevation difference to the pump, the liquid’s vapor pressure, the velocity head in the suction line, and any suction-line losses. NPSH_R is the minimum suction head required by the pump to operate at a given flow without cavitation; it is specified by the pump manufacturer and shown on pump curves.

Cavitation occurs when the suction pressure falls to or below the liquid’s vapor pressure, causing vapor bubbles that can implode and damage the impeller, reduce flow, and increase noise. Safe operation requires NPSHA to exceed NPSHR (NPSH_R).

NPSHA depends on liquid properties (density, vapor pressure), temperature, system elevation, and suction losses; units are usually meters or feet of liquid. NPSHR depends on pump design and operating point and varies with flow. To improve NPSHA, strategies include raising the liquid level at the source, reducing suction-line losses, lowering liquid temperature, or selecting a pump with a lower NPSHR or operating at a more favorable point. Manufacturers’ data and standards provide guidance for design and verification.