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.
That means that the local atmospheric pressure p
b is higher than the sum of net positive suction head H
H and vapour pressure p
v; the inlet pressure is thus no longer required. This interrelation is based on the drastic reduction of the vapour pressure of cold water. In practice, that means:Pumps operating at a negative minimum inlet pressure H
req are capable of creating a suction lift (not self-priming).The suction capacity is approximately equal to the level of the negative minimum inlet pressure minus 1m safety factor.As pumps normally used in conjunction with building services are generally not of selfpriming characteristics, the following conditions for suction lift operation must be met:
- Filling and venting of the suction-side pipework including the pump before commissioning.
- Avoiding air entrainment during pump operation (aeration will result in break-down of suction capability).
- Avoiding drainage of suction-side piping on standdown of pump by providing and installing a footvalve (danger of leakage due to dirt particles).
Reliance on non-return valves in the discharge pipe is not sufficient, as air can be entrained by way of the shaft seal (mechanical or packed gland seals) on pump standdown.The suction capability of non-selfpriming pumps is, on account of their construction features, generally limited to the range of max. 2 to 4 m. Higher suction lifts (max. 8 to 9 m) and selfpriming operation require the use of Special Pumps.
The vapour pressure of the pumped fluid greatly influences Cavitation Behaviour and thus the System NPSHavailable.The vapour pressure is one of the important liquid properties for pump selection and depends on the fluid temperature.
