Mechanical & Fluids Environmental Troubleshooting Diagnose a problem

Why a pump is cavitating and what NPSH margin it needs

Working out the net positive suction head available at the pump eye, and why a few centimetres of margin matter.

The engineering question

The pump is noisy, the flow is short of the curve, and it gets worse when hot. What is happening at the suction, and how much margin is enough?

Worked example

The figures below were produced by running the NPSH Calculator with the same inputs — during the build, from its authoritative engine.

Results
NPSH available	11.1739 m
NPSH required	3 m
Margin	8.1739 m
Vapour pressure at temperature	7.38 kPa
Assessment	NPSH(A) 11.2 m exceeds NPSH(R) 3.0 m with 8.2 m margin.

Atmospheric pressure gives metres of head above the liquid.
Subtract the vapour pressure at the actual liquid temperature — this is what rises sharply when the fluid is hot.
Add the static suction head and subtract suction friction to get NPSH available.

Sanity check

Warmer liquid reduces NPSH available. If a pump cavitates only in summer, temperature is the variable to check first.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

NPSH Calculator Net positive suction head available at the pump from pressure, vapour pressure, static head and suction friction.

Engineering review required

This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.

References & standards

Hydraulic Institute — NPSH margin practice
Crane TP-410 — suction piping design
ASME B31.3 — cavitation precautions