Net positive suction head available (NPSHa)
Net positive suction head available (NPSHa) for a pump taking liquid from a tank, separator or vessel, and its margin over the NPSH required (NPSHr) from the pump curve.
NPSHa = (p_a − p_v) / (ρ · g) + z − h_fΔ = NPSHa − NPSHrk = NPSHa / NPSHrThe pump runs reliably if NPSHa exceeds NPSHr with a margin: ANSI/HI 9.6.1 recommends an NPSHa / NPSHr ratio of 1.1 to 2.5 depending on the application; in practice use at least 1.1–1.3 and at least 0.5–1 m. At NPSHa ≈ NPSHr the head already falls by 3 % and cavitation erodes the impeller. For a liquid at its bubble point (p_v = p_a) only the static height z provides the margin, so such pumps are installed below the vessel.
Source: ANSI/HI 9.6.1 — NPSH margin; ISO 9906 and API 610 — NPSH3
Inputs
You can change a field's unit: the value is converted to the formula's units automatically.
Open tank: atmospheric pressure (101.3 kPa at sea level); separator: its gauge pressure plus atmospheric.
Water: 2.34 kPa at 20 °C, 19.9 kPa at 60 °C, 47.4 kPa at 80 °C. Oil or condensate separated from gas in this vessel is at its bubble point: p_v = p_a.
Fresh water 1 000, crude oil 800–900, formation water 1 050–1 180 kg/m³.
Use the lowest level; negative if it is below the pump centreline (suction lift).
Pipe friction, strainer, valves and bends at the highest flow rate.
From the pump curve at the highest flow rate; catalogues give NPSH3, the NPSH at which the head drops by 3 %.
Unit converter for this formulaPressure · Density and °API · Length
- MPa1
- kPa1,000
- bar10
- atm9.86923
- kgf/cm²10.1972
- mmHg7,500.62
- MPa(g)0.898675
- barg8.98675
- psi145.038
- psia145.038
- psig130.342
- ksi0.145038
psi and psia are absolute pressure; gauge units (barg, MPa(g), psig) are above atmospheric (101.325 kPa = 14.696 psi). Pressure differences in the formulas do not depend on this.
- Δ — Margin over NPSHr–
- k — Ratio NPSHa / NPSHr–
More in Pumps
Hydraulic power from the flow rate and head, shaft power from the pump efficiency, power drawn from the supply, and the smallest motor rating with a margin.
The actual total head of a running pump from the suction and discharge pressures, the gauge height difference and the pipe velocities — to compare with the rated curve.
Flow rate, head and power of a centrifugal pump at another speed — including an ESP or a pump on a variable speed drive — and with a trimmed impeller.
Results are engineering estimates from standard formulas; for design decisions check them against the codes, project documents and specialists' calculations. The formulas carried over from the original set are unchanged, and their errors are described in the notes.