The system NPSHavailable value is defined as:
pe = Pressure available system inlet liquid level
pb = Barometric pressure
pD = Vapour pressure of pumped fluid at the pump suction inlet
p = Density of the pumped fluid at the pump suction inlet
g = Local gravitational acceleration (9.81 m/s2)
ze = Static level difference between system inlet liquid level and a reference level, the negative sign becoming applicable if the reference level is above the system inlet liquid. The reference point is the centre of the impeller.
Hv = Friction loss in suction-side system.
The duty point of a centrifugal pump can only be a permanent duty point if complying with:
NPSHavailable > NPSHrequired + safety margin.