How this calculator works
Express reservoir absolute pressure and vapour pressure as metres of the pumped liquid. Add surface elevation above the pump datum, or subtract suction lift when the surface is below it, then subtract suction losses. The reservoir surface velocity is assumed negligible. Negative available NPSH remains visible because it is a meaningful indication that the assumed liquid state or suction arrangement cannot be sustained. Required NPSH comes from the pump curve at the actual operating flow.
Formula
NPSHa = p_surface/(rho g) + z - h_loss - p_vapour/(rho g); margin = NPSHa - NPSHr.
Worked example
With 100 kPa absolute reservoir pressure, 2 kPa vapour pressure, density 1000 kg/m³, g=10 m/s², 2 m positive elevation and 1 m loss, NPSHa is 10.8 m. If NPSHr is 3 m, the simple difference is 7.8 m.
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Frequently asked questions
Can I enter gauge reservoir pressure?
Convert it to absolute pressure first by adding local atmospheric pressure.
Does a positive margin guarantee no cavitation?
No. Manufacturers often require extra margin beyond a published NPSHr or NPSH3 curve. Check their operating guidance.
Does water temperature matter?
Yes. Temperature changes vapour pressure and density. Enter properties for the operating liquid temperature.