NPSH and cavitation: what they are and how to avoid them
Cavitation is the forming and collapsing of vapour bubbles inside the pump. It causes noise (like gravel being sucked in), vibration, loss of performance and, over time, impeller erosion. The key to avoiding it is NPSH.
What NPSH is
NPSH (Net Positive Suction Head) is the suction pressure available above the liquid's vapour pressure. It splits into NPSH available (the system's) and NPSH required (the pump's, on the curve).
The golden rule
To run safely: NPSH available ≥ NPSH required + margin (typically 0.5–1 m). If available drops below required, the pump cavitates.
How to raise NPSH available
- Lower the pump relative to the liquid level (or give it a flooded suction).
- Make the suction pipe shorter and wider, with fewer bends and valves.
- Lower the liquid temperature (less vapour pressure).
Choose a pump with low NPSH required
Well-designed pumps have a low required NPSH. Compare models in our Grundfos catalogue or start from pumps for industry and process, where suction is often critical.
Häufige Fragen
How do I know if a pump is cavitating?
Typical symptoms are a gravel-like sucking noise, vibration, a drop in flow and head and, over time, visible erosion on the impeller.
How much NPSH margin is needed?
A typical margin is 0.5–1 m between available and required NPSH; for hot or volatile liquids it should be larger.