NPSH:
●NPSH: The net positive suction head is the total suction head in feet of liquid
(or meters), less the vapor pressure (in feet or meters) of the liquid being pumped.
●NPSH is defined as the total head of fluid at the centre line of the impeller less
the fluid's vapour pressure.
●NPSH is normally considered in two forms: NPSH-R (NPSH Required) and
NPSH-A (NPSH Available).
● (NPSH)A is the amount of NPSH that the system has available at the eye of the
pump impeller.
● (NPSH)A =Liquid pressure at pump suction - Vapor pressure of the liquid
(Always Liquid pressure at eye of impeller >Vapor pressure of the same liquid)
●Always (NPSH)A > (NPSH)R is recommended by process engineers during design
and (NPSH)A > 0 to avoid cavitation in pump.
●(NPSH)R is Net Positive Suction Head Required, that is given by OEM(Original
Equipment Manufacturer)
Formula:
● (NPSH)A =P-hfs -Pv ± Z
Where P=Absolute pressure
hfs =Frictional Losses in suction line due to pipe fittings,valves
Pv =Vapour pressure of liquid
Z=Satic Head
CAVITATION IN CENTRIFUGAL PUMP:
●When (NPSH)A < (NPSH)R or pressure at eye of pump impeller become less than vapour pressure of the same liquid then vapour bubbles creates inside liquid and the vapour bubbles goes through the impeller and on impeller vane it collapse. Due to repeated collapse of vapor bubbles on a metal surface of pump parts , This causes mild physical damage to protective films, severe deformation and fracture of surface. So that is called Cavitation.
●It occurs in equipment where high velocity liquid flow and pressure changes are encountered, such as pump impellers, resulting in vapor bubbles.
●Cavitation also results in vibration and noise in the pump, placing greater strain on the drive shaft and other components, and also in downstream pipework.
●This can lead to greater maintenance costs and a higher incidence of pump failures.
