What NPSH actually measures
Net positive suction head is the pressure margin, expressed in metres of liquid, between the pressure at the pump suction and the pressure at which that liquid would boil. Run out of margin and the liquid flashes to vapour at the impeller eye — cavitation.
Four terms, each of which can quietly erode the margin. The pressure term is what the atmosphere and any tank pressurisation contribute. The vapour term is what the liquid's temperature takes away. Static head helps when the source is above the pump and hurts when it is below. Friction always subtracts.
Worked example
Water at 30 °C from an open tank whose level sits 2 m above the pump, with 1.5 m of friction loss in the suction line:
- Pressure head = 101,325 ÷ (995.7 × 9.807) = 10.38 m
- Vapour head = 4,246 ÷ (995.7 × 9.807) = 0.43 m
- NPSHa = 10.38 − 0.43 + 2.0 − 1.5 = 10.44 m
- Against an NPSHr of 3 m, the margin is 7.44 m — comfortable
Now heat the same water to 80 °C. Vapour pressure climbs to 47.4 kPa, the vapour head becomes 5.02 m, and NPSHa collapses to 5.85 m. Nothing about the piping changed.
Field notes
- Everything is absolute, and everything is in metres of the pumped liquid. A head expressed in metres is density-independent only within one fluid — converting kPa to metres needs that fluid's density. Our gauge to absolute converter handles the first step.
- Calculate at the worst case, not the normal case: lowest tank level, highest temperature, maximum flow (friction rises with the square of flow), and lowest tank pressure.
- NPSHr is defined at 3% head drop — the point where performance has already measurably degraded. That is why margin above NPSHr matters rather than merely exceeding it.
- Suction line friction is the term you can most easily fix: larger bore, fewer fittings, a shorter run. Check velocity with the pipe velocity calculator and the true bore from the pipe schedule lookup — suction lines commonly run one or two sizes above the discharge.
- A partially blocked suction strainer is a classic cause of cavitation appearing months after successful commissioning.
Frequently asked questions
What is the formula for NPSH available?
NPSHa = (P_absolute − P_vapour) ÷ (ρ·g) + static head − friction loss, with all terms expressed in metres of the pumped liquid. Static head is positive when the liquid level sits above the pump centreline and negative for a suction lift.
How much NPSH margin should I have?
A common rule is NPSHa exceeding NPSHr by at least 0.5 to 1 metre, or by 10 percent, whichever is greater. Large or high-energy pumps and hydrocarbon service often call for more; HI 9.6.1 gives margin ratios by application.
Why does hot liquid cause cavitation?
Vapour pressure rises steeply with temperature, and it is subtracted directly in the NPSHa equation. Water at 20 °C has a vapour pressure of about 2.3 kPa, but at 80 °C it is 47.4 kPa — roughly 4.6 metres of head removed from the available margin.
What happens if NPSHa is less than NPSHr?
Liquid vaporises at the impeller eye and the bubbles collapse violently as pressure recovers. The result is the characteristic gravelly noise, vibration, loss of head and flow, and pitting erosion of the impeller.
Provided for reference and education. Pump selection and NPSH margin must follow the pump manufacturer's data and the applicable Hydraulic Institute standards. See our disclaimer.