NPSH Calculator - Net Positive Suction Head Available

Your details

Operating temperature of the liquid. Used to look up water vapor pressure and density automatically.
°C
Leave at 0 to use the built-in water density for the entered temperature, or enter a custom value for other liquids.
kg/m³
Leave at 0 to use the built-in water vapor pressure. For other fluids, enter the absolute vapor pressure at the operating temperature.
bar
Flooded suction (tank above) gives positive static head. Suction lift (tank below) subtracts static head and must not exceed about 5-6 m for water.
Absolute vertical distance between the liquid surface in the tank and the pump centerline. Sign is applied automatically based on tank position.
m
Absolute pressure at the liquid surface. Use 1.01325 bar (14.696 psi) at sea level, or a lower value at altitude, or the vessel gauge pressure + atm for a pressurised tank.
bar
Select the nominal bore of the suction pipe. The internal diameter is used for velocity and friction calculations.
Total straight length of suction pipe from tank to pump.
m
Add extra equivalent pipe length to account for valves, elbows, tees, etc. Rule of thumb: gate valve ~0.5D, 90° elbow ~30D, tee (through) ~60D.
m
Volumetric flow through the suction pipe.
m³/h
Enter the NPSHr from the pump curve at the design flow point. Leave at 0 if unknown. The HI 9.6.1 standard recommends NPSHa exceed NPSHr by at least 0.6 m (2 ft).
m
NPSHa AvailableSafe margin
11.72m

Net Positive Suction Head Available at the pump inlet

Cavitation margin (NPSHa - NPSHr)8.72m
Atmospheric / surface head10.36m
Static head (z)2m
Friction head loss (h_f)0.4m
Vapor pressure head (h_vp)0.24m
Pipe velocity1.69m/s
Reynolds number180,674
Darcy friction factor0.0188
Fluid density used997.1kg/m³
Vapor pressure used2,330Pa
Cavitation assessmentSafe - NPSHa exceeds NPSHr by adequate margin
Atm / surface head10.36
Static head (z)2
Friction loss (-)0.4
Vapor pressure head (-)0.24
NPSHa (result)11.72

NPSHa is 11.72 m with a +8.72 m margin over NPSHr.

  • The system meets the HI 9.6.1 minimum 0.6 m cavitation margin. For critical service (high-energy pumps, boiler feed, hydrocarbons) a margin of 1.0-2.0 m is advisable.
  • Pipe velocity is 1.69 m/s, which exceeds the recommended 0.6-1.5 m/s for suction lines. Consider increasing the suction pipe diameter to reduce friction losses and improve NPSHa.

Next stepVerify the pump curve NPSHr at your operating flow point and confirm the margin holds across the flow range, including at start-up.

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