What this calculator does
Cavitation happens when the pressure at the pump eye drops below the vapour pressure of the liquid. This calculator gives you the number that matters — the NPSH available at the pump — and compares it with the NPSH the manufacturer requires, so you know whether the installation is safe or whether the water will boil.
How to use it
- Enter the ambient atmospheric pressure at your site altitude — 101.3 kPa at sea level, falling about 1 kPa per 85 m of altitude.
- Enter the liquid temperature; the vapour pressure is interpolated from the saturation table.
- Enter the static suction head as a positive number for a flooded suction, or a negative number for a suction lift.
- Add the friction loss in the suction line, then compare the available margin with your pump curve.
Governing formula
NPSH(A) = (P_a − P_v)/(ρg) + H_s − H_f Where:
-
P_a= Absolute pressure above the liquid [kPa] -
P_v= Vapour pressure of the liquid [kPa] -
H_s= Static suction head, positive or negative [m] -
H_f= Friction loss in the suction line [m]
Derived Equations:
barometric head = P_a / (ρ g) margin = NPSH(A) − NPSH(R) Worked example — A flooded suction on warm water at sea level
The figures below were produced by running this page's own engine with the example inputs (Atmospheric pressure at site (kPa), Liquid temperature (°C), Static suction head (+ flooded / − lift) (m), Suction line friction loss (m), NPSH required (pump curve) (m), Fluid density (kg/m³)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| NPSH available | 11.17 m |
|---|---|
| NPSH required | 3 m |
| Margin | 8.17 m |
| Vapour pressure at temperature | 7.38 kPa |
| Assessment | NPSH(A) 11.2 m exceeds NPSH(R) 3.0 m with 8.2 m margin. |
- At 40 °C the vapour pressure of water is about 7.4 kPa, equivalent to 0.75 m of water.
- Atmospheric pressure of 101.3 kPa gives 10.3 m of head.
- NPSH(A) = 10.3 − 0.75 + 2 − 0.4 = 11.2 m, comfortably above the 3 m required.
Assumptions & limitations
Engineering calculation. Computes NPSH available at the pump from absolute pressure, vapour pressure, static suction head and suction-line friction. Vapour pressure is interpolated for water from a standard saturation table.
- Interpolated vapour pressure is for water; use the correct saturation data for glycol mixtures, solvents or other fluids.
- Static suction head must be measured at the liquid surface relative to the pump centreline, not taken from the drawing nominally.
- NPSH required must be taken from the pump curve at the actual flow point, not the shut-off value.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- Hydraulic Institute — NPSH margin practice.
- Crane TP-410 — process fluid mechanics, suction piping design.
- ASME B31.3 — process piping, cavitation precautions.