NPSH Available (Cavitation Margin) Calculator

How it works

NPSHa = h_atm (+ vessel gauge pressure) − suction lift (or + submergence) − suction friction − h_vapour; h (ft) = psi × 2.31 ÷ SG; P_atm = 14.696 × (1 − 6.88×10⁻⁶ × ft)^5.256; margin = NPSHa − NPSHr (≥ 3–5 ft)

A pump does not suck; atmospheric pressure (or tank pressure) pushes liquid into it, and the pressure at the impeller eye must stay above the liquid’s vapour pressure or bubbles form and collapse — cavitation, which sounds like gravel and eats impellers. NPSH available is the head pushing liquid in, minus everything working against it: the lift from a source below the pump, friction in the suction line, and the vapour pressure of the liquid at its temperature. The manufacturer’s NPSH required, read from the curve at the duty flow, must be comfortably lower — a margin of at least 3–5 feet or 35%. Altitude trims atmospheric head (about 1.2 ft per 1,000 ft), and temperature is brutal: water at 90 °C has 23 feet of vapour head, which is why hot condensate pumps sit below their tanks.

Worked example

At 1,500 ft elevation (13.9 psi, 32.3 ft of water) pumping 20 °C water with a 15 ft lift, 4 ft of suction friction and 0.8 ft of vapour head: NPSHa = 32.3 − 15 − 4 − 0.8 = 12.5 ft. Against an NPSHr of 12 ft that is a 0.5 ft margin — on the edge; the maximum comfortable lift here is about 13.5 ft.

Frequently asked questions