NPSH Available Calculator

Net positive suction head available is the energy at the pump suction above the liquid’s vapour pressure. If it drops below the pump’s required NPSH (NPSHr), the liquid flashes and bubbles collapse inside the impeller — cavitation — which erodes the impeller, damages seals and drops performance. The formula: atmospheric head ± suction static head − friction loss − vapour-pressure head.

Atmospheric head is about 10.33 m at sea level. Suction static is positive for flooded suction and negative for lift. Friction covers the suction pipe, fittings, valves and strainer. Vapour-pressure head comes from the liquid at pumping temperature — the largest, and most forgotten, term for hot liquids.

Water at 30 °C has a vapour pressure of about 0.042 bar, roughly 0.43 m of head. Water at 90 °C has about 0.70 bar — 7.1 m — so NPSHa collapses by nearly 7 m just from heating. The same effect applies to acids and alkalis: pumping HCl or NaOH at 60 °C instead of 30 °C can take several metres off NPSHa. The calculator reads the vapour pressure and SG of the selected liquid at the entered temperature automatically.

This is why hot-liquid pumps need elevated tanks or shorter, larger suction lines — and why "it worked in winter, now it cavitates" is almost always a vapour-pressure story.

The calculator compares NPSHa with the NPSHr from the pump curve and reports the margin. Above 0.5 m is acceptable; 0 to 0.5 m is a warning; negative margin means cavitation is happening. If NPSHr is unknown, the tool still returns NPSHa so you can compare with the curve yourself.

Fixes, in order of preference: raise the tank, lower the pump, shorten or enlarge the suction line, cool the liquid, or reduce flow. A suction strainer that is half clogged can eat the entire margin — it is the first thing Teflow’s engineers check on a cavitation call. Call +91 98251 62709 for a suction-system review.