Cavitation is a damaging pump condition that happens when local pressure in a liquid falls below the liquid vapor pressure. Vapor bubbles form in low-pressure regions, especially near the pump suction and impeller eye, then collapse violently when they move into higher pressure regions. This collapse can pit metal surfaces, create noise and vibration, reduce flow, and shorten pump life.
Engineers use NPSH to predict whether a pump has enough suction pressure margin to avoid cavitation.
Key Facts
- Cavitation begins when local absolute pressure drops below vapor pressure: Plocal < Pv.
- NPSHa = (Ps,abs / rho g) + (Vs^2 / 2g) - (Pv / rho g) at the pump suction reference point, often adjusted from reservoir conditions.
- For an open suction tank: NPSHa = (Patm / rho g) + zstatic - hloss - (Pv / rho g).
- Safe operation requires NPSHa > NPSHr, usually with an added margin such as 0.5 m to 1 m or 10 percent to 20 percent.
- Suction pipe losses increase strongly with flow rate, so hloss is often proportional to Q^2.
- Raising suction liquid level, lowering liquid temperature, reducing suction losses, or selecting a lower NPSHr pump helps prevent cavitation.
Vocabulary
- Cavitation
- Cavitation is the formation and collapse of vapor bubbles in a liquid when pressure falls below and then rises above the liquid vapor pressure.
- Vapor pressure
- Vapor pressure is the absolute pressure at which a liquid can boil at a given temperature.
- NPSHa
- Net positive suction head available is the suction pressure head margin above vapor pressure provided by the piping system.
- NPSHr
- Net positive suction head required is the minimum suction head margin a pump needs at a given flow rate to limit cavitation.
- Impeller eye
- The impeller eye is the inlet region of a centrifugal pump impeller where fluid enters and pressure is often lowest.
Common Mistakes to Avoid
- Using gauge pressure instead of absolute pressure in NPSH calculations is wrong because vapor pressure and atmospheric pressure must be compared on the same absolute scale.
- Ignoring suction pipe friction losses is wrong because these losses reduce NPSHa and become much larger at high flow rates.
- Assuming cavitation only occurs when the whole liquid boils is wrong because cavitation can start in small local low-pressure zones inside the pump.
- Treating NPSHr as a constant is wrong because pump NPSHr depends on flow rate, impeller design, speed, and manufacturer test conditions.
Practice Questions
- 1 An open tank feeds a pump with water at 20°C. Patm / rho g = 10.3 m, vapor pressure head Pv / rho g = 0.24 m, the liquid level is 2.0 m above the pump centerline, and suction losses are 1.1 m. Calculate NPSHa.
- 2 A pump requires NPSHr = 4.0 m at the operating flow rate. The system has Patm / rho g = 10.1 m, Pv / rho g = 1.25 m, static suction head = 0.5 m, and suction losses = 3.0 m. Calculate NPSHa and decide whether the pump has at least a 1.0 m NPSH margin.
- 3 A pump begins making a gravel-like noise after the flow rate is increased by opening a discharge valve. Explain why increasing flow can cause cavitation even if the tank level and water temperature do not change.