Cavitation is the formation and collapse of vapour bubbles in a liquid as it flows through a pump. When uncontrolled, it damages impellers within hours, destroys seals, and reduces pump performance. Understanding why it occurs is essential for anyone specifying, operating, or maintaining centrifugal pumps in industrial and marine applications.
What causes cavitation?
Every liquid has a vapour pressure — the pressure at which it begins to boil at a given temperature. Water at 20 °C boils at about 0.023 bar absolute; at 80 °C, it boils at 0.474 bar absolute. Inside a pump, the absolute pressure at the suction can drop below the local vapour pressure, especially at the impeller eye where velocity is highest. When this happens, the liquid flashes into vapour, forming bubbles.
As the vapour bubbles travel along the impeller into higher-pressure regions, the surrounding pressure recompresses them. They collapse violently — a process that generates shockwaves, noise, vibration, and microscopic jets capable of eroding metal. Continuous cavitation leaves the impeller eye looking pitted and porous within weeks.
NPSH: the prevention measure
The technical concept used to prevent cavitation is Net Positive Suction Head (NPSH). Two values matter:
- NPSH Required (NPSHr) — specified by the pump manufacturer at each flow rate. It is the minimum suction pressure (expressed as head above vapour pressure) the pump needs to operate without cavitation.
- NPSH Available (NPSHa) — calculated for the actual installation. It depends on atmospheric pressure (or tank pressure), static suction height, friction losses, and fluid temperature.
The design rule is straightforward: NPSHa must exceed NPSHr by at least 0.5–1 metre across the full operating range. A small margin is risky because manufacturer NPSHr values are typically defined at 3% head drop — by the time you reach NPSHr, partial cavitation is already happening.
How to recognise cavitation
Cavitation produces several characteristic symptoms:
- Noise like gravel passing through the pump — the most recognisable sign, especially at the suction.
- Erratic discharge pressure — sudden swings of several percent.
- Reduced flow and head — performance falls below the pump curve.
- Pump vibration — especially at the impeller frequency.
- Pitting on the impeller eye and inlet vanes — visible on inspection, often with a sponge-like surface texture.
- Premature seal failure — vibration shortens seal life dramatically.
Causes of insufficient NPSH
- Excessive suction lift — pump installed too high above the fluid source.
- Long or undersized suction piping — friction losses eat into NPSHa.
- Partially closed suction valve or clogged strainer — should never be throttled.
- High fluid temperature — raises vapour pressure and reduces NPSHa.
- Operation at high flow rates — NPSHr rises with flow, so running far right of design can outpace NPSHa.
- Reduced atmospheric pressure — altitude or unusual barometric conditions.
- Vapour-locked or air-bound suction line — common after maintenance work.
Fixing a cavitating pump
Solutions depend on the cause. In order of effort:
- Open the suction valve fully and clean the strainer.
- Increase suction line diameter or shorten the run.
- Lower the pump physically, increasing suction head (or flooded suction).
- Lower fluid temperature where process-permissible.
- Reduce flow to bring NPSHr below NPSHa.
- Pressurise the supply tank (where chemistry permits).
- Select a pump with lower NPSHr at the required flow.
- For repeated NPSH problems, install an inducer or a vertical multi-stage pump with low NPSHr.
Some cavitation problems are unsolvable with the original pump and require re-selection. This is more economical than continuing to replace impellers and seals.