What is NPSH?
When a centrifugal pump draws fluid, the pressure at the suction eye of the impeller drops below the inlet pressure. If this local pressure drops below the vapour pressure of the liquid, the liquid flashes to vapour — creating bubbles. These bubbles collapse violently as pressure rises through the impeller, causing cavitation: noise, vibration, impeller erosion, and catastrophic damage.
NPSH quantifies the margin between available suction pressure and the vapour pressure. Both NPSHa and NPSHr are expressed in metres of fluid head (not pressure).
Calculating NPSHa
Habs = absolute pressure on suction fluid surface (m head) = atmospheric pressure + gauge pressure on vessel
Hz = suction static head = vertical height from fluid surface to pump centreline (positive if fluid above pump, negative if pump above fluid)
Hf = friction losses in suction pipework (m head)
Hvp = vapour pressure of fluid at pumping temperature (m head)
Worked example — water at 80°C
Pump draws water at 80°C from an open tank 2 m above pump centreline. Suction pipe friction losses = 0.4 m. Atmospheric pressure = 10.3 m head. Vapour pressure of water at 80°C = 4.82 m head.
NPSHa = 10.3 + 2.0 − 0.4 − 4.82 = 7.08 m
If the pump's NPSHr at operating flow = 3.5 m, NPSH margin = 7.08 − 3.5 = 3.58 m — adequate.
Factors That Reduce NPSHa
- High fluid temperature — vapour pressure increases with temperature; at 100°C, water vapour pressure equals atmospheric pressure (NPSHa effectively zero without pressurisation)
- Suction lift — pump above fluid surface reduces Hz (becomes negative)
- High suction pipe friction — undersized suction pipe, long suction runs, partially open valve
- High altitude — lower atmospheric pressure reduces Habs
- Operating above rated flow — NPSHr increases steeply above BEP; system resistance decreases; flow increases beyond pump curve data
How to Improve NPSHa
| Problem | Solution |
|---|---|
| Pump above fluid level (suction lift) | Lower pump or raise tank; use submersible pump |
| Hot fluid (high vapour pressure) | Increase suction tank pressure; cool fluid before pump; use pressurised suction header |
| High suction pipe friction | Increase suction pipe diameter; shorten suction run; eliminate unnecessary fittings |
| Pump NPSHr too high | Select pump with lower NPSHr at duty point; reduce pump speed; use double suction impeller |
Cavitation Symptoms
If your pump is cavitating, you'll typically observe:
- Crackling, rattling, or gravel-in-the-pipe noise from the pump casing
- Vibration increase — measurable with a vibration pen
- Flow rate pulsation and reduced output
- On disassembly: pitting erosion on the impeller vane leading edges (suction side)
Do not ignore cavitation. Even 30 minutes of severe cavitation can pit an impeller to the point of replacement. See our pump cavitation guide for prevention strategies.