NPSH Available Calculator
Check a pump suction for cavitation: calculate the net positive suction head available (NPSHa) and compare it with the NPSH required (NPSHr) from the pump curve.
- NPSHa = Ps/(ρg) + z − hf − Pv/(ρg)
- = 10.36 + 2.5 − 0.6 − 0.3242 = 11.94 m
NPSH Available Formula
Ps is the absolute pressure on the liquid surface in the suction vessel, z the height of the liquid surface above the pump centreline (negative for suction lift), hf the friction loss in the suction line at the flow considered, Pv the liquid vapour pressure at pumping temperature and ρ the density. Use the lowest liquid level and the highest temperature for the worst case.
Worked example
Water at 25 °C (Pv 0.0317 bar a) from an open tank 2.5 m above the pump, with 0.6 m friction: NPSHa ≈ 10.36 + 2.5 − 0.6 − 0.32 = 11.9 m, a comfortable 8.7 m margin over 3.2 m NPSHr.
Frequently Asked Questions
What is NPSH?
Net positive suction head: how far the suction pressure is above the vapour pressure, expressed as head of liquid.
What happens if NPSHa is less than NPSHr?
The liquid boils in the impeller eye, causing cavitation: noise, vibration, loss of head and impeller damage.
Where do I find NPSHr?
On the pump performance curve from the manufacturer; it rises with flow.
Learn more on the blog
What factors affect the stability of flow meter readings?
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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