NPSH = hatm ± hz(s) - hf(s) - hvp
hatm = Atmospheric pressure 14.7 (33.9ft)
hz(s) = Elevation above pump suction (ft), note ithat f pump is located above the water level the hz(s) will be negative
hf(s) = Friction loss in suction pipe (ft)
hvp = Vapor pressure (from steam table per water temp.) – (ft), note that Vapor pressure in steam table should be converted to feet by multiplying by 2.307.
To prevent captivation in pump’s impeller, the NPSH at pump’s suction pipe should be more than the NPSH pump: NPSHA > NPSHR
NPSHA = Net Positive Suction Head Available (system)
NPSHR = Net Positive Suction Head required (pump)