When a liquid boils at a suction line of a pump it is said to be cavitation.
As the liquid boils it forms cavity of gases in the suction line. This happens when liquid pressure is well below the vapour pressure of the liquid.
Boiling starts when pressure of the liquid is reduced to vapor pressure of the liquid at the actual temperature.
Vapour pressure of water:
The pressure at which the water turns into vapor at the given temperature.
for example: water turns into vapor at 40deg at pump suction pressure below 0.07 bar, so pump suction pressure should not go below 0.07 bar at 40 deg to avoid cavitation.
Causes of cavitation
Cavitation can causes serious damage to the pump in the form of vibration and abnormal sound. cavitation leads to bearing and mechanical seal failure, shaft breakage , pitting on the impeller.
The damage occurs when these bubbles pass to the high pressure region of the pump.The bubbles explode at the vanes of the pump.
Problems due to cavitation
- Loud noise
- reduced flow
- pitting formation in the impeller
Net positive suction head
The Minimum pressure required at the suction port of the pump to keep the pump from cavitating
Net positive suction head available
The pressure available at the suction side of the pump and it should be calculated and it should be more than NPSH R
Net positive suction head required
It is the minimum pressure required at the pump suction to keep the pump away from cavitation and to be provided by the manufacturer.
Net positive suction lift
If the pump is positioned above the tank it has some net positive suction head. if the pump is raised above the tank the net positive suction head reduces and at some point the head becomes zero and the fluid starts to evaporate.