Glossary

Net Positive Suction Head (NPSH)

Net Positive Suction Head (NPSH) is the margin between the absolute pressure at a pump’s suction and the vapor pressure of the liquid, expressed as head. NPSH available (NPSHa) is a property of the system; NPSH required (NPSHr) is a property of the pump. If NPSHa does not exceed NPSHr with adequate margin, the liquid vaporises at the impeller eye and the pump cavitates.

Definition

NPSHa = (absolute pressure on the free liquid surface at the source — atmospheric for an open tank, or the vapor-space pressure for a closed or pressurised vessel) + (elevation difference between the source liquid surface and the pump centreline, positive for a flooded suction and negative for a suction lift) − (friction head loss in the suction line at the design flow) − (vapor pressure of the liquid at the pumping temperature), all expressed as head of the pumped liquid. NPSHr is determined by the pump manufacturer from testing and is conventionally defined as the suction condition at which total head has already fallen by 3%.

Engineering context

NPSHa is a system calculation, which is why it belongs in the network model rather than a standalone hand calculation: the suction line pressure drop, elevation, and fluid temperature all interact. FluidFlow reports NPSHa at every pump from the solved network and compares it against the NPSHr curve entered from manufacturer data. Typical practice is a margin of at least 0.5–1 m NPSHr depending on service criticality and the consequences of failure.

Related definitions

Cavitation · Pump curve · System curve · Best Efficiency Point (BEP) · Vapor pressure

See it in FluidFlow

Check NPSHa at every pump in your system. FluidFlow reports the available suction margin against pump NPSHr as part of the solution.

Go deeper

Knowledge Base Energy & Hydraulic Grade Lines (EGL/HGL) in FluidFlow How suction-side pressure margin is visualised along a network using EGL/HGL plots. Knowledge Base Pump & Pipe Sizing Best Practices Sizing choices that keep available suction head safely above the pump's NPSHr. Article FluidFlow Liquid module Model incompressible liquid systems and read NPSHa at every pump from the solution.

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