Pump Cavitation Causes: Diagnosis & Prevention

What is Cavitation?

Cavitation occurs when the local pressure at the pump impeller eye drops below the vapour pressure of the liquid being pumped. The liquid literally boils — vapour bubbles form. As these bubbles travel through the impeller to the higher-pressure discharge zone, they collapse violently, creating micro-jets that impact the impeller surface at up to 500 m/s, eroding the metal.

The sound of cavitation is unmistakable — like gravel passing through the pump. If left uncorrected, cavitation will destroy an impeller in weeks or even days.

The 7 Causes of Cavitation

• 1. Insufficient NPSHa: The most common cause. The available NPSH (from the suction installation) is less than the required NPSH (from the pump curve). This happens when the suction lift is too high, the liquid is too hot, or the suction piping friction losses are excessive. See our Pump NPSH guide for calculation details.
• 2. High liquid temperature: As temperature increases, vapour pressure increases exponentially. Water at 80°C has a vapour pressure of 47 kPa — compared to 2.3 kPa at 20°C. This dramatically reduces NPSHa. A pump that works fine on cold water may cavitate severely on hot water.
• 3. Blocked suction strainer: A clogged strainer creates a large pressure drop on the suction side, reducing the pressure at the impeller eye. Fix: clean the strainer regularly.
• 4. Undersized suction piping: Suction piping that is too small (or too long) creates excessive friction losses, reducing NPSHa. Rule: suction pipe should be at least one size larger than the pump suction nozzle.
• 5. Operating too far right on the curve: At high flow rates (beyond the best efficiency point), NPSHr increases sharply. The pump requires more NPSH than the system provides. Fix: throttle the discharge valve or trim the impeller.
• 6. Air entrainment: If air enters the suction line (through leaks in fittings, vortexing in the suction tank, or dissolved gases coming out of solution), it forms bubbles that behave like cavitation. This is technically 'false cavitation' but causes similar damage.
• 7. Suction recirculation: At very low flow rates (below 30% of BEP), the flow pattern inside the impeller breaks down, creating recirculation vortices that cause localised cavitation. This damages the impeller inlet vanes.

Diagnosing the Cause

Prevention

1. Calculate NPSHa and compare to NPSHr: Ensure NPSHa exceeds NPSHr by at least 0.5–1.0 m at the operating flow rate.
2. Size suction piping correctly: One size larger than the pump suction nozzle, as short as possible, with minimum fittings.
3. Maintain suction strainers: Clean regularly. Install a differential pressure gauge across the strainer to monitor fouling.
4. Control liquid temperature: If the liquid is hot, ensure the suction source is above the pump (flooded suction) or use a booster pump.
5. Operate within the pump's range: Keep flow between 30% and 110% of BEP. Avoid both high-flow and low-flow cavitation.
6. Install an inducer: For low-NPSH applications, an inducer (axial-flow pre-impeller) can reduce NPSHr by 30–50%.
7. Eliminate air leaks: Check all suction fittings, gaskets, and valve packings. Use threaded connections with sealant, or better, welded/flanged joints.