NPSH and Cavitation: A Practical Troubleshooting Guide for Maintenance Teams
That gravel-rattling sound is your pump eating itself. How to recognise cavitation, confirm it, and fix it without replacing the pump.
Cavitation is the most destructive common pump fault, and the most misdiagnosed. It sounds like gravel or marbles rattling inside the casing, and left running it erodes impellers to lace and destroys seals and bearings. The good news: most cavitation problems are installation problems, fixable without touching the pump.
What is actually happening
Every liquid vaporises when its pressure drops below vapour pressure. Inside a pump, pressure dips at the impeller eye; if it dips far enough, vapour bubbles form, then collapse violently as they move into higher pressure — thousands of micro-implosions hammering the metal. That is the noise, and the damage.
NPSH is simply the margin between the pressure at the suction and the liquid's vapour pressure. NPSH-required is what the pump demands (from our test curve). NPSH-available is what your installation delivers. Available must exceed required, with margin — we recommend at least 1 metre, more for hot liquids.
Recognising it
Gravel noise that changes with discharge valve position. Discharge pressure gauge fluttering. Flow wandering below expectation. Impeller with a sponge-textured, pitted surface on inspection — concentrated near the eye on the low-pressure side.
The usual causes, in order of how often we find them
Clogged suction strainer or choked foot valve — check first, costs nothing. Suction lift too high or suction line too long, too small, or with too many elbows. Liquid hotter than the selection assumed — vapour pressure climbs steeply with temperature, and a pump fine at 30 °C cavitates at 70 °C. Tank level fallen below the level the piping was designed around. And the classic: the pump is running far right of its curve because someone opened a bypass, and NPSH-required has risen past what the system gives.
The fixes
Clean the strainer. Raise the tank level or lower the pump. Shorten or enlarge the suction line, remove unnecessary fittings, and keep the suction run rising steadily to the pump with no air pockets. Throttle the discharge valve to pull the duty point back left — counter-intuitive, but effective as a temporary measure. For persistent cases, we can trim the impeller or re-select the pump against the true system conditions.
Prevention at specification stage
Tell us the real suction condition — flooded or lift, height, line size, liquid temperature at its worst, not its average. Half of the cavitation cases we repair were born in an enquiry form that said 'suction: OK'.
Have a duty point to discuss?
Send it over — an applications engineer responds within 24 working hours.