NPSH and Cavitation

A centrifugal pump lowers the pressure at the impeller eye in order to draw liquid in. If that pressure falls to the vapour pressure of the liquid, the liquid boils, forms bubbles, and those bubbles collapse violently as they move into the higher-pressure part of the impeller.

The collapse removes metal. That is cavitation, and it is a property of the suction system, not of the pump.

NPSH Available and NPSH Required

NPSH required is a pump characteristic, published by the manufacturer against flow. It is the margin the pump needs at the impeller eye to avoid vaporising the liquid.

NPSH available is a system characteristic. It is set by the liquid level, the atmospheric pressure, the suction pipe losses and the liquid temperature.

The system must supply more than the pump demands, with a margin. When NPSH available falls below NPSH required, the pump cavitates.

What Reduces NPSH Available

A long, narrow or convoluted suction pipe. Every metre of pipe, every bend and every fitting takes head out of the suction.

A liquid level that drops — a sump emptying, a tank running down, a river falling in a dry season.

Higher liquid temperature, which raises vapour pressure. A duty that is fine at 20 °C can cavitate at 80 °C.

Altitude, which lowers atmospheric pressure and therefore the head pushing liquid into the pump.

Recognising It

A distinctive noise, usually described as gravel passing through the pump.

Unstable discharge pressure and fluctuating flow.

Vibration that rises with flow rather than falling.

On strip-down, pitting on the impeller vane on the low-pressure side near the eye — not general erosion across the whole vane.

Fixing It

Shorten and straighten the suction line. This is usually the cheapest and most effective change available.

Increase the suction pipe diameter to cut friction losses.

Raise the liquid level relative to the pump, or lower the pump relative to the liquid.

Reduce the flow rate. NPSH required rises with flow, so throttling the discharge can move the pump back into a safe region — at an efficiency cost.

Where the sump level is the constraint and cannot be changed, a vertical wet-installed pump removes the suction lift entirely. That is what the VNP series is for.

Designing It Out

Ask for the NPSH required curve at selection, not after commissioning.

Calculate NPSH available at the worst case: lowest liquid level, highest temperature, dirtiest strainer.

Keep a margin. A design that just meets NPSH required at ideal conditions will cavitate in service.

On pit and sump duties, consider a vertical pump from the start rather than fighting a suction lift with a horizontal one.