MachineCalcs

Pump NPSH Calculator

Calculate pump NPSH available from absolute suction pressure, static head, vapor pressure, suction-line loss and fluid density, then compare it with pump NPSHr.

Calculator

Inputs

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Absolute pressure at the suction vessel/free surface or pump suction datum. Atmospheric pressure is about 1.013 bar / 14.7 psi absolute.

bar

Positive when liquid level is above the pump centerline; negative for suction lift.

m

Use vapor pressure at the pumping temperature. Water near 20 C is about 0.023 bar absolute.

bar

Friction and minor losses in the suction line, expressed as head.

m

Density at the operating temperature.

kg/m³

Required NPSH from the pump curve at the design flow rate.

m

Results

Default result
Edit inputs
NPSH available(NPSHa)
10.11m
Pass

Absolute pressure head plus static head, minus vapor pressure head and suction losses.

Also computed

NPSH margin(NPSHa - NPSHr)Pass7.114m

NPSH ratio(NPSHa/NPSHr)Pass3.371

Absolute pressure head(P_abs/(rho g))10.35m

Vapor pressure head(P_v/(rho g))0.2391m

NPSHa pressure equivalent(rho g NPSHa)0.9898bar

Method notes 3 notes
  • NPSHa = P_abs/(rho*g) + static suction head - P_v/(rho*g) - suction line head loss.
  • Compare against NPSHr from the pump curve at the same flow. Many installations need additional margin beyond simply exceeding NPSHr.
  • Final pump selection still needs the full system curve, inlet geometry, acceleration head, strainers, valves, temperature, altitude and manufacturer guidance.

Pump NPSH available is a suction-side head balance: NPSHa = P_abs/(rho*g) + static suction head - P_v/(rho*g) - suction line loss. This calculator converts absolute suction pressure and vapor pressure into head, adds flooded head or suction lift, subtracts suction losses, then compares NPSHa with pump NPSHr from the pump curve as a margin and ratio.

Continue workflow

All Hydraulics
Next 1 Check pipe pressure drop Estimate suction-line head loss from actual pipe ID, length, fittings and fluid properties. Next 2 Size pipe ID Back-solve suction pipe ID when velocity is driving the NPSH margin. Next 3 Check pump flow and power Carry the selected flow and pressure back into pump power and drive sizing. Chart Use reference data Steel pipe schedule chart

How to use this calculator

  1. Enter absolute suction pressure. Use absolute pressure at the suction source, not gauge pressure.
  2. Add static suction head. Use positive flooded head or negative suction lift relative to the pump centerline.
  3. Subtract vapor and line loss. Enter vapor pressure at temperature and suction-line head loss.
  4. Compare with NPSHr. Use the pump curve NPSHr at the same design flow rate and review margin.

How it works

NPSH available is a head balance at the pump suction:

NPSHa = P_abs/(rho x g) + H_s - P_v/(rho x g) - h_f

The available head is then compared with the pump manufacturer's required NPSH at the same flow: margin = NPSHa - NPSHr

For the pressure-loss part of the suction line, use the pipe flow pressure drop calculator or pipe size by flow velocity calculator.

Worked example

Verified against the live calculator

At atmospheric pressure, room-temperature water, 1 m flooded suction head, 1 m suction-line loss and a pump requiring 3 m NPSHr, the calculator reports about 10.1 m NPSHa and roughly 7.1 m margin.

Frequently asked questions

What is NPSH available?

NPSH available is the absolute suction pressure head at the pump, corrected for static head, vapor pressure and suction-line losses.

What is the difference between NPSHa and NPSHr?

NPSHa is what the installation provides. NPSHr is what the pump requires at a given flow rate, read from the pump curve.

Should suction lift be positive or negative?

Enter static suction head as positive when the liquid level is above the pump centerline and negative when the pump is lifting from below.

Does this replace a pump curve?

No. It only calculates the NPSH side of the suction check. Final pump selection still needs the pump curve and full system curve.

Method & assumptions

  • Absolute suction pressure is required. Do not enter gauge pressure unless it has been converted to absolute.
  • Vapor pressure must match the fluid temperature at the pump suction.
  • Suction line loss should include pipe, fittings, strainers, valves and entrance losses.
  • Does not model acceleration head, transients, vortexing, entrained gas, poor inlet geometry, pump-specific suction energy or manufacturer margin requirements.
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