IntermediateTopic 65 of 105Thermodynamics & Fluids
Cavitation and NPSH
Local pressure below vapour pressure causes bubbles that damage pumps.
In depth
Cavitation happens when local pressure falls to vapour pressure, forming bubbles that collapse violently, causing noise, vibration and pitting. NPSH available must exceed NPSH required: NPSH_a=(P_s-P_v)/ρg+V²/2g-losses. Avoid by lowering suction lift, enlarging suction pipe, cooler liquid.
Key points
- Pitting damage on impellers
- Margin of 0.5–1 m typical
- Occurs at propellers and valves too
ExampleA pump drawing hot water from a low tank may cavitate.
What is Cavitation and NPSH?
Local pressure below vapour pressure causes bubbles that damage pumps.
Why is it important to understand cavitation and NPSH?
Cavitation damages pumps and reduces performance, and NPSH is the check that keeps the local pressure above vapour pressure.
What is NPSH?
Net positive suction head, the head available at the pump inlet above the fluid's vapour pressure, which must exceed the pump's required NPSH.
What happens when cavitation occurs?
Bubbles form at low pressure and collapse at higher pressure, which produces noise, vibration and surface damage.
How is Cavitation and NPSH used in practice?
Typical applications include checking the suction lift of a pump on a hot water system, predicting damage to an impeller from bubbles formed at the inlet and specifying a minimum suction pressure for a pump drawing from a tank.
What should I learn before Cavitation and NPSH?
Study "Pump affinity laws" first (topic 64 of 105 in Thermodynamics & Fluids). It covers the groundwork this topic builds on.
What are the key points of Cavitation and NPSH?
Pitting damage on impellers. Margin of 0.5–1 m typical. Occurs at propellers and valves too.
Can you give an example of Cavitation and NPSH?
A pump drawing hot water from a low tank may cavitate.
What comes after Cavitation and NPSH?
The next topic is "Turbines", which builds directly on this one.
How difficult is Cavitation and NPSH?
It is classed as Intermediate in Thermodynamics & Fluids (topic 65 of 105). It is an intermediate topic. Work through the beginner topics in Thermodynamics & Fluids first.