NPSH Calculator (NPSHa)
Compute the Net Positive Suction Head available at a pump from the tank pressure, liquid level, fluid temperature and suction-line losses. Water properties come from IAPWS-IF97; the margin verdict applies the same 0.5 m rule SimuPipe's solver uses when it checks every pump in a simulated network.
ρ = 998.2 kg/m³, Pv = 2.339 kPa abs (IAPWS-IF97)
P_atm = 101.33 kPa abs (ISA standard atmosphere)
Total loss of the suction pipe, fittings and strainer at the operating flow — estimate it with the friction loss calculator.
From the pump curve at the operating flow rate.
NPSH available
11.31 m
= 37.1 ft of liquid
Enter the pump's NPSHr to get a cavitation-margin verdict.
| T (°C) | Pv (kPa abs) | NPSHa (m) |
|---|---|---|
| 10 | 1.23 | 11.41 |
| 20 | 2.34 | 11.31 |
| 40 | 7.38 | 10.85 |
| 60 | 19.95 | 9.64 |
| 80 | 47.41 | 6.86 |
| 95 | 84.61 | 2.97 |
Same surface pressure, level and friction loss as entered above — only the water temperature (and with it ρ and Pv) changes. This is why hot condensate service needs generous submergence.
- NPSH available (NPSHa) — the absolute head at the pump suction above the liquid's vapour pressure. A property of your installation, not of the pump.
- NPSH required (NPSHr) — the head the pump needs at a given flow to keep cavitation at the 3% head-drop criterion (NPSH3). Read it from the manufacturer's curve; it rises steeply with flow.
- Vapour pressure Pv — the absolute pressure at which the liquid boils at its operating temperature. It is subtracted in full, which is why hot liquids are hard to pump.
- Static head z — the vertical distance from the liquid surface to the pump shaft centreline: positive when flooded (level above the pump), negative for a suction lift.
- Friction loss h_f — every loss between the tank and the pump flange at the operating flow: pipe friction, fittings, strainer, foot valve. Always subtracts.
- Margin — NPSHa minus NPSHr. This calculator flags margins under 0.5 m (the rule SimuPipe's solver applies); Hydraulic Institute guidance recommends application-specific margins that can be larger.
- Datum — all heads are referenced to the pump shaft centreline. For vertical pumps use the impeller-eye elevation.
Cold-water transfer: an open tank at sea level, water at 20 °C with the surface 2 m above the pump centreline, and 0.8 m of suction-line losses. The steps below are computed live by the same function the calculator runs.
- Water at 20 °C (IAPWS-IF97): ρ = 998.2 kg/m³, Pv = 2.339 kPa abs. Open tank at sea level: P_surface = 101.325 kPa abs.
- Surface-pressure head: P/(ρg) = 101,325 / (998.2 × 9.80665) = 10.351 m.
- Vapour-pressure head: Pv/(ρg) = 2339 / (998.2 × 9.80665) = 0.239 m.
- NPSHa = 10.351 − 0.239 + 2.0 − 0.8 = 11.31 m.
- Against a pump needing NPSHr = 4.0 m at duty flow, the margin is 7.31 m — comfortably above the 0.5 m warning threshold.
These numbers are evaluated at page load by lib/npsh.ts — the same code path as the calculator above — so the example can never drift from the tool.
- Tank-surface energy balance: NPSHa = (P_surface − Pv)/(ρg) + z − h_f. No velocity-head term appears — for a large tank the surface velocity is essentially zero, and the suction-line velocity head cancels against its own share of the pump-inlet pressure. (SimuPipe's solver computes the same quantity from the solved pump-nozzle pressure, where the local V²/2g term does appear — the two formulations give identical numbers for the same physical state.)
- Single liquid phase with no entrained gas. Dissolved air coming out of solution near the vapour pressure reduces effective NPSHa before flashing does.
- Vapour pressure is taken at the bulk liquid temperature. Local heating (hot wells, recirculation) raises Pv above the bulk value.
- Friction loss is your input, at the actual operating flow — remember the strainer and foot valve, and re-check it for a fouled strainer.
- The altitude option uses the ISA standard atmosphere; day-to-day weather moves the true barometric pressure by roughly ±3 kPa (±0.3 m of water).
- The verdict mirrors SimuPipe's solver: insufficient below NPSHr, thin margin below NPSHr + 0.5 m. ANSI/HI 9.6.1 recommends margins by application that can be substantially larger for high-energy pumps.
SimuPipe computes NPSHa automatically at every pump in a simulated network — from solved pressures, with temperature-correct fluid properties — and applies this same margin check. See the NPSHa vs NPSHr guide for the full theory, or the validation library for how the solver is benchmarked.
The formulation and margin guidance follow standard pump-engineering references:
- ANSI/HI 9.6.1. Rotodynamic Pumps — Guideline for NPSH Margin, Hydraulic Institute — NPSH margin guidance by application; the basis for treating the 0.5 m rule as a floor, not a target.
- Karassik, I. J., Messina, J. P., Cooper, P. and Heald, C. C. Pump Handbook, McGraw-Hill — the NPSHa formulation and suction-design practice used here.
- Gülich, J. F. Centrifugal Pumps, Springer — cavitation physics and the NPSH3 (3% head drop) definition behind quoted NPSHr curves.
- Crane Co. Flow of Fluids Through Valves, Fittings, and Pipe (TP-410) — suction-line friction and fitting losses for the h_f term.
- IAPWS R7-97(2012). IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam. iapws.org — water density and vapour pressure used by the water option.
How NPSH available is calculated
Net Positive Suction Head available is the absolute head at the pump suction in excess of the liquid's vapour pressure. If it falls below the pump's required NPSH, vapour bubbles form at the impeller eye and collapse violently further in — cavitation — eroding the impeller, dropping the head, and eventually destroying the pump. The calculator evaluates the tank-surface form:
- — absolute pressure on the liquid surface (Pa); for an open tank, atmospheric pressure at your altitude
- — vapour pressure of the liquid at operating temperature (Pa abs)
- — liquid density (kg/m³); g = 9.80665 m/s²
- — static head from liquid surface to pump centreline (m); negative for a suction lift
- — friction loss of the suction line, fittings and strainer at operating flow (m)
Hot liquids and altitude
The two effects engineers most often underestimate are temperature and altitude. Water's vapour pressure grows from 2.3 kPa at 20 °C to 47.4 kPa at 80 °C, cutting NPSHa by about 4.5 m for the same tank; near-boiling condensate has almost no pressure-head contribution left, which is why condensate-return and boiler-feed pumps need flooded suctions. Altitude works on the other term: atmospheric pressure falls to 84.6 kPa at 1,500 m, costing an open-tank installation about 1.7 m of NPSHa compared to sea level.
Vapour pressures for water at any temperature are tabulated in the water properties table, and the full theory is in the NPSHa vs NPSHr guide. To check a complete system — suction and discharge, with the solver computing suction pressure and NPSHa at every pump — model it in SimuPipe.
Frequently Asked Questions
What is the difference between NPSHa and NPSHr?
How much NPSH margin should I keep?
Why does hot water reduce NPSHa?
Does altitude affect NPSH?
Why is there no velocity-head term in the formula?
What happens when a pump cavitates?
Check NPSH across a whole network
SimuPipe solves your suction and discharge piping together — computing pressure, flow and NPSHa at every pump, with cavitation warnings built into the solver.