Concept reference

Cavitation

C.1Plain-language definition

Cavitation begins where local absolute pressure falls to the liquid’s vapour pressure. Vapour cavities travel into higher-pressure regions and collapse, creating noise, vibration and repeated surface impact. It is a phase-change problem; air pulled through a vortex or leaking joint can sound similar but follows different physics.

C.2Governing relation or decision logic

The available suction margin is framed by NPSHa − NPSHr, in m (ft) of pumped liquid, but the required margin depends on pump, service, test criterion and consequence. Vapour pressure rises strongly with liquid temperature, reducing NPSHa even when the level and piping do not change.

C.3The failure to name

Replacing a damaged impeller without correcting low source level, excess suction loss or high temperature guarantees recurrence. Diagnosing every gravel-like sound as cavitation can miss vortex air, recirculation, debris or mechanical looseness.

C.4Field review checklist

• Correlate noise, pressure, vibration and capacity with flow and source level.
• Calculate NPSHa with worst credible temperature, atmosphere and suction condition.
• Inspect the location and pattern of pitting and compare with hydraulic symptoms.
• Test competing causes—entrained air, recirculation, solids and mechanical faults—before prescribing a fix.

C.5Where it fits in the course

§6.5 links symptoms to margin; §8.3 diagnoses vortex air.