NPSH Calculator

What Is Net Positive Suction Head?

Net Positive Suction Head (NPSH) measures how much energy is available at the pump suction above the fluid's vapor pressure. If NPSH available drops below the pump's required NPSH, cavitation can damage impellers and reduce flow.

NPSH Available Formula

Available NPSH in meters of fluid is:

$$\text{NPSH}_a = \frac{P_{atm}}{\rho g} + h_s - \frac{P_{vapor}}{\rho g} - h_{loss}$$

Here $h_s$ is static suction head (positive when the liquid level is above the pump), $h_{loss}$ is friction and fitting loss in the suction line, and pressure terms are converted to head using fluid density $\rho$ and gravity $g$.

Frequently Asked Questions

What is a good NPSH margin?

Compare NPSH available to the pump manufacturer's NPSH required value. A safety margin of 0.5 to 1.0 m (or more for hot or volatile fluids) is common in practice.

Why does vapor pressure matter?

As pressure drops toward vapor pressure, the liquid boils locally and forms vapor bubbles. Subtracting vapor head ensures suction energy stays above that threshold.

Can static suction head be negative?

Yes. A suction lift (pump above the liquid level) counts as negative $h_s$, which reduces NPSH available and increases cavitation risk.

What fluid density should I use for water?

Use about 1000 kg/m³ for water near room temperature. Hot water has lower density and higher vapor pressure, both of which lower NPSH available.

How do I increase NPSH available?

Lower the pump relative to the supply, shorten suction piping, use larger suction pipe, reduce fittings, or cool the fluid to lower vapor pressure.

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What Is Net Positive Suction Head?

NPSH Available Formula

Frequently Asked Questions

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