How Cavitation Occurs
In a centrifugal pump, the impeller accelerates the fluid from eye to tip, creating a high-velocity, low-pressure zone at the suction eye. If the pressure at this point drops below the liquid's vapour pressure, the liquid vaporises locally, forming bubbles of vapour.
As these bubbles travel from the low-pressure zone at the impeller eye to the higher-pressure zone at the impeller tip, they collapse. The collapse is not gentle — it is an implosion that creates local pressure spikes of up to 1,000 bar and temperatures exceeding 5,000°C in the microscopic collapse zone. Repeated over billions of cycles, this micro-implosion erodes even hardened stainless steel impellers.
Suction Cavitation vs Discharge Cavitation
| Type | Cause | Damage location | Fix |
|---|---|---|---|
| Suction cavitation | NPSHa < NPSHr — insufficient suction pressure | Impeller vane leading edges (suction side), inlet | Improve NPSHa: lower pump, reduce suction friction, pressurise suction vessel, reduce fluid temperature |
| Discharge cavitation | Pump operating far below BEP (very high head, low flow) | Impeller vane trailing edges, volute tongue | Open discharge valve; select correctly sized pump; trim impeller |
| Recirculation cavitation | Operating below minimum continuous stable flow (MCSF) | Impeller eye, suction recirculation zone | Maintain flow above MCSF; install recirculation bypass valve |
Recognising Cavitation
Cavitation produces characteristic symptoms:
- Sound: Crackling, rattling, or grinding noise — described as "gravel in the casing." Severity can range from faint crackling (mild) to loud continuous rattle (severe).
- Vibration: Elevated vibration at pump casing and bearing housing, typically broadband random vibration rather than specific harmonics
- Performance: Fluctuating flow rate; pump output drops below curve; pressure gauge oscillates
- Visual on disassembly: Pitting on impeller vane leading edges (suction cavitation) or trailing edges (discharge recirculation); rough, cratered surface like a rough casting
Prevention Strategies
For suction cavitation:
- Ensure NPSHa ≥ NPSHr + 1.0 m margin (see our NPSH guide)
- Use large-bore suction pipework (fluid velocity <1.5 m/s suction side)
- Minimise suction lift — locate pump close to and below fluid source where possible
- Avoid partially closed suction valves — always fully open before starting
- For hot liquids: pressurise the suction vessel or cool the fluid before the pump
For discharge/recirculation cavitation:
- Select the pump to operate within ±20% of BEP during normal operation
- Install a recirculation bypass line to maintain minimum flow during low-demand periods
- If flow requirements have changed, re-rate the pump with a smaller impeller diameter or lower speed
Assessing Impeller Damage
On disassembly, inspect the impeller with good lighting. Mild pitting (isolated rough patches <1 mm deep) — pump can continue running with monitoring. Moderate pitting (interconnected craters, material loss visible) — inspect bearing condition, balance impeller if reassembling. Severe pitting (vane cross-section reduced >30%) — replace impeller; running risks catastrophic failure and seal damage.