How to calculate NPSH available
NPSH available is the head margin at the pump suction that keeps the liquid from flashing into vapor. The working equation is NPSHa = surface pressure head - vapor pressure head ± static height - suction friction loss, subtracting the static height for a suction lift and adding it for a flooded suction. If NPSHa falls below the NPSH required printed on the pump curve, bubbles form at the impeller eye and the pump cavitates.
For a tank open to atmosphere at sea level the surface pressure is 14.696 psia, and water at 68 °F has a vapor pressure of about 0.34 psia. Vapor pressure climbs steeply with temperature, reaching roughly 3 psia at 140 °F and 7 psia at 176 °F, so hot water services need a far shorter lift through the same piping. This calculator derives the vapor pressure of water from the temperature you enter, and lets you type in a value directly for any other fluid.
The verdict follows the NPSH margin concept in the Hydraulic Institute guidance: the result is flagged as adequate only when NPSHa exceeds NPSHr by at least 2 ft and the ratio NPSHa ÷ NPSHr is 1.3 or more. High-temperature service, liquids near their boiling point and high suction specific speed pumps all call for a larger margin, so confirm the final selection against the manufacturer curve and have a mechanical engineer review it.
Frequently asked questions
Not really. NPSHr is defined at the point where head has already dropped 3%, meaning cavitation has begun. Keep at least 2 ft of margin and preferably a ratio of 1.3 or better.
Lowering the pump closer to the liquid level, or flooding the suction, has the biggest effect. Shorter and larger suction piping cuts friction loss, and running the liquid cooler lowers vapor pressure.