HesapHanem

NPSH Available Calculator

Enter pressures, heads and fluid density to compute the available NPSH.

How it is calculated

The tool applies NPSHa = (Patm − Pvap) × 1000 / (ρ × g) + hs − hf. Pressures patm and pvap are in kPa (the factor 1000 converts kPa to Pa), ρ is the fluid density in kg/m³ and g is fixed at 9.80665 m/s². The static head hstat and friction loss hfric are in metres; enter a negative static head when the pump sits above the liquid surface (suction lift). Defaults are 101.325 kPa for atmospheric pressure and 1000 kg/m³ for density. Vapour pressure, static head and friction loss are required. A density of zero or less gives no result. The answer is in metres, to three decimals, and must exceed the pump's NPSHr.

Worked example

Water at about 20 °C: Patm = 101.325 kPa, Pvap = 2.34 kPa, ρ = 1000 kg/m³, static head 3 m (flooded suction), friction loss 1.5 m. Pressure head = (101.325 − 2.34) × 1000 / (1000 × 9.80665) = 98,985 / 9,806.65 = 10.094 m. Then NPSHa = 10.094 + 3 − 1.5 = 11.594 m. If the pump datasheet asks for an NPSHr of 4 m, this system has a comfortable margin.

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Common mistakes

  • Entering pressures in bar, psi or Pa. The fields expect kPa, so 1.013 bar must be typed as 101.3.
  • Using gauge instead of absolute pressure for the tank or atmosphere. NPSH works from absolute pressure minus vapour pressure.
  • Forgetting the sign of the static head: when the liquid level is below the pump axis the head is negative and it reduces NPSHa.
  • Using the vapour pressure of water at 20 °C for a hot or different liquid. Vapour pressure rises quickly with temperature, so look up the value at the real operating temperature.
  • Comparing NPSHa with the pump's NPSHr without a margin. Available head should be clearly above required head, not just equal to it.

About this calculator

NPSHa (Net Positive Suction Head available) tells you how much pressure margin exists at a pump’s suction inlet above the fluid’s vapor pressure. It’s derived from atmospheric pressure and vapor pressure, converted to head, plus the static head of the fluid at the source, minus friction losses in the suction piping: roughly NPSHa = (Patm − Pvapor)/(ρg) + hs − hf.

Pump engineers compare NPSHa against the pump manufacturer’s required value, NPSHr: if available head falls below required head, the fluid flashes to vapor inside the pump and causes cavitation — a condition that erodes impellers, causes noise and vibration, and can wreck a pump over time. This calculator is a standard part of the workflow for mechanical and process engineers sizing pumps and laying out suction piping in industrial and municipal water systems.

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Comments (1)

  • Editorial team

    Treat NPSHa as one operating point, not a final verdict. Vapour pressure climbs steeply with liquid temperature, so rerun the numbers for the hottest duty the pump will see and with the friction loss of a partly clogged strainer. A bare pass over the datasheet NPSHr is thin; most designers want a margin. What margin rule does your plant or standard apply for this kind of service?

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