A pump is said to cavitate when the pressure at the pump suction has fallen to such an extent that bubbles of vapour are formed in the fluid being pumped.
In a centrifugal pump bubbles can form with great rapidity at the pump suction and may be carried away to regions of higher pressure within the pump. The bubbles then collapse rapidly and in the process of collapsing creates a hammer effect. This effect, though minute, if repeated frequently, may cause damage to the pump.
The residual effects of cavitation, as well as the cavitation itself, can be a problem and include:-
- Erosion of metallic surfaces which, if severe, may create flow disturbances.
- Vibration, which may damage the pump or, more commonly, the instrumentation connected with the pump and installation.
Centrifugal pumps must not be allowed to cavitate.
You may also know about self priming of pump :
In a self-priming system a discharge control valve performs two duties:-
- By restricting the pump throughput, the head is artificially raised to a level above the normal discharge system resistance at that particular throughput.
- Throughput is reduced to a level equal to, or below, the natural flow of liquid to the pump so that cavitation does not occur, i.e. the flow of liquid to the tank suction strum is closely matched to the pump throughput.
The performance of a centrifugal pump declines if gas is entrained in the liquid being pumped.
Manual matching of pump input and output is difficult, and without external aid suction cannot be regained once the strum has been uncovered and air allowed to enter the suction piping. It is for this reason that separate positive displacement pump stripping systems are usually installed. Centrifugal pumps can be made self-priming if air or gas is removed from the liquid being pumped before it enters the pump suction.
The discharge valve on the pump performs as important function during this process by matching the discharge rate of the pump to the natural flow of liquid to the pump suction.
The sequence of events is:
- Discharge commences, pump and separator fill with oil.
- Vacuum pump and valve in gas extraction line are shut-off by float switch in pump separator.
- Discharge valve is fully open, the control system reacting to separator level.
- Tank level falls to point where separator level begins to fall causing the vacuum pump to start to extract the gases filling the top of the separator preventing cavitation. The extracted gases are vented to a slop tank.
- As the separator level falls, the control system partly closes the discharge valve on the pump to reduce the output of the pump.
In this way the discharge and stripping operation can continue, the discharge valve being progressively closed, as the vacuum pumps have to work harder to keep the separator full.