NPSH Available Calculator (Suction Head Check)

What is NPSH?

NPSH stands for net positive suction head. If the liquid pressure at the pump suction falls to the vapour pressure, the liquid flashes and the bubbles collapse in the impeller, causing noise, vibration and erosion. This is cavitation. NPSH available (NPSHa) is what the installation provides and NPSH required (NPSHr) is what the pump needs. The rule is simple: NPSHa must exceed NPSHr by a margin.

Formula

NPSHa = (p0 − pv) / (ρ × g) ± z − hL, where p0 is the absolute pressure on the liquid surface, pv the vapour pressure at the pumping temperature, z the vertical distance between the liquid level and the pump centreline and hL the total head loss in the suction line. Add z when the level is above the pump and subtract it when below.

Example

Water at 20 °C in a tank open to atmosphere, level 3 metres below the pump centreline, suction line loss 0.8 metres. With a vapour pressure of 2.339 kPa the pressure head difference is 10.11 metres, so NPSHa = 10.11 − 3 − 0.8 = 6.31 metres. If the pump needs 3 metres there are 3.3 metres to spare. With water at 80 °C in the same installation the vapour pressure would be 47.4 kPa and NPSHa would fall to roughly 1.9 metres.

Limits

This is a preliminary check. Read NPSHr from the manufacturer's curve at the duty flow. The suction line loss has to be worked out separately and entered. Hot water, volatile liquids, altitude and a fouled strainer all make things worse. Reciprocating pumps, dissolved gas and transients are not covered.