NPSHavailable = NPSH of system
pe = Pressure available system inlet liquid level
pb = Barometric pressure
pD = Vapour pressure of pumped fluid at the pump suction inlet
ρ = 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 reference point for the NPSH value is the centre of the impeller, i.e. the intersection of the pump shaft axis with the plane perpendicular to it through the outer points of the blade leading edge.
The duty point of a centrifugal pump can only be a permanent duty point if complying with:
NPSHavailable > NPSHrequired + safety margin.