NPSHa Calculator
Net Positive Suction Head available from an energy balance at the pump suction, with the margin against the pump's NPSHR.
When to use this calculator
Use when checking suction conditions on a new pump selection or diagnosing cavitation on an installed one. It builds NPSHA from the four terms that set it — surface pressure over the liquid, static suction level, suction line loss, and vapour pressure at the pumping temperature — then subtracts an optional design margin and compares the result against the NPSHR the vendor curve gives at the duty point. The margin, not the raw NPSHA, is the number that tells you whether the pump is safe. Works in SI or US units.
Required inputs
- Liquid density ρ at the pumping temperature
- Surface pressure P_surf over the liquid in the suction vessel
- Vapour pressure P_v at the pumping temperature
- Static suction head z_s — positive for a flooded suction, negative for a lift
- Suction line friction and fitting loss h_f at design flow
- Optional safety margin SM, and the pump NPSHR from the vendor curve
Expected outputs
- Surface pressure head H_surf and vapour pressure head H_v
- NPSHA
- NPSH margin against the pump NPSHR
- Warning when the surface pressure is at or below vapour pressure
Formula overview
SI: pressures in kPa, density in kg/m³, all head terms in metres of the pumped liquid, g = 9.80665 m/s². US: psi, lb/ft³, and feet. Enter h_f and SM as positive numbers — both are subtracted.
Energy balance at the pump suction:
H_surf = P_surf · 1000 / (ρ · g) SI, P in kPa
H_v = P_v · 1000 / (ρ · g)
NPSHA = H_surf + z_s − h_f − H_v − SM
Margin = NPSHA − NPSHR
US units use H = P · 144 / ρ with P in psi and ρ in lb/ft³.Worked example
Water at 20 °C from an atmospheric tank, flooded suction:
ρ = 998.2 kg/m³, P_surf = 101.325 kPa, P_v = 2.339 kPa,
z_s = 2.0 m, h_f = 1.5 m, SM = 0.5 m, pump NPSHR = 2.0 m
H_surf = 101.325 × 1000 / (998.2 × 9.80665) = 10.35 m
H_v = 2.339 × 1000 / (998.2 × 9.80665) = 0.24 m
NPSHA = 10.35 + 2.0 − 1.5 − 0.24 − 0.5 = 10.11 m
Margin = 10.11 − 2.0 = 8.11 m → ample
Pump the same water at 80 °C and P_v rises to 47.4 kPa, so H_v becomes
4.84 m and NPSHA drops to 5.51 m. Temperature dominates this calculation.Common mistakes
- Taking vapour pressure at ambient instead of at the pumping temperature. P_v climbs steeply with temperature and is the term that most often destroys NPSHA — hot service and boiling liquids need the value at the real temperature.
- Using atmospheric pressure for a closed vessel. On a vessel at its bubble point, P_surf equals P_v and the two terms cancel, leaving NPSHA as static head minus friction loss and nothing more.
- Comparing NPSHA to NPSHR with no margin. NPSHR is defined at 3% head drop, which means the pump is already cavitating slightly at that point — treat it as a floor to clear by a margin, not a target to meet.
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