Cavitation Risk Calculator (Cavitation Number and NPSH Ratio)

What is cavitation?

If the pressure of a liquid drops to its vapour pressure at some point, the liquid boils there and bubbles form. When the bubbles collapse where pressure recovers, they pit metal surfaces, cause noise and vibration and cut the head of a pump. It shows up at pump inlets, in throttled valves, at orifices and on fast propellers.

Which numbers are used?

For a flow the cavitation number applies: σ = (p − pv) / (ρ × v² / 2). It divides the margin above vapour pressure by the dynamic pressure of the flow. The smaller σ, the higher the risk, and every geometry has a value of σ at which cavitation starts.

For a pump the measure is the NPSH ratio: the NPSHa the installation provides divided by the NPSHr the manufacturer asks for. Below 1 the pump cavitates. The Thoma number makes the same comparison relative to the pump head: σT = NPSH / H.

For valves the cavitation index σ = (p1 − pv) / (p1 − p2) is compared with the limit given by the manufacturer.

Example

Water at 20 °C flows at 12 m/s under 2 bar absolute. The dynamic pressure is 71.9 kPa, the margin above vapour pressure 197.7 kPa and the cavitation number 2.75. If inception for the geometry is at 1.5, no cavitation is expected. The limit is reached once the velocity climbs to 16.2 m/s.

Limits

This calculator does not produce the limits: NPSHr, the inception number and the valve index come from the manufacturer or from tests. Dissolved gas, part load and transients can bring cavitation on earlier. This is a preliminary assessment.