What is cavitation of a pump
Pump cavitation is the formation and subsequent collapse or implosion of vapor bubbles in a pump. It occurs when gas bubbles are formed in the pump due to drop in absolute pressure of the liquid below vapor pressure. These gas bubbles occupy space inside the pump and affect the pump's operating pressure and flow.
With vapor bubbles in the low-pressure zones of the pump, the motor's energy is wasted expanding the bubbles instead of bringing more liquid into the pump. As the bubbles pass into the pump's high-pressure zones, the motor's energy is wasted compressing the bubbles instead of expelling the liquid from the pump.
The bubbles can collapse as they pass from low- to high-pressure zones in the pump. When vapor bubbles collapse inside the pump the liquid strikes the metal parts at the speed of sound. The noise generated from these collisions of gas bubbles into the metal parts of pump sounds like pumping marbles and stones.
Cavitation of a pump can be a serious obstacle to smooth operation of a centrifugal pump. Therefore it is important to understand different pump cavitation causes in order to understand how to prevent pump cavitation.
Pump cavitation causes
Drop in pressure at the suction nozzle due to low NPSHa
If the fluid at pump suction is not available sufficiently above the vapor pressure of liquid at operating conditions, then vaporization of liquid and formation of gas bubbles is very likely, leading to cavitation.
Increase of the temperature of the pumped liquid
Increase in liquid temperature at the pump suction point increases the vapor pressure of the liquid. Thus it becomes more likely for operating pressure to fall below this vapor pressure limit, hence leading to bubbles and cavitation.
Increase in the fluid velocity at pump suction
Increase in fluid velocity at pump suction can typically be caused by higher liquid flowrates than the design case. As per Bernoulli’s principle, higher liquid velocity means higher velocity and lower pressure head. Frictional pressure drop in the pump suction also rises with rise in the flowrate, making low pressure and cavitation at pump suction more likely to occur.
Reduction of the flow at pump suction
Certain minimum flow is required by the centrifugal pumps to keep them from running dry, as indicated by the pump curves. If liquid flow falls below this limit, possibility of developing vapor in pumps and cavitation increases.
Undesirable flow conditions caused by obstructions or sharp elbows in the suction piping
Sharp elbows, valves, other fittings and obstructions cause more frictional pressure loss in the pump suction, thus increasing possibility of low pump suction pressure leading to cavitation.
The pump is not selected correctly.
Every centrifugal pump has a certain requirement of positive suction head (NPSHr). If the pump is not selected properly NPSHa might fall below this NPSHr limit, causing cavitation.
Symptoms and Effects of Pump Caviation
Pump cavitation causes noise and vibration. If the pump operates under cavitating conditions for a long time, the following damage can occur:
- Pitting marks on the impeller blades and on the internal volute casing wall of the pump
- Premature bearing failure
- Premature mechanical seal failure
- Shaft breakage and other fatigue failures in the pump
Preventing pump cavitation
Pump cavitation can result in severe damage to the pump impeller blades and other internal parts. However these damages and the cavitation itself can be prevented by taking certain measures.
These preventive measures depend on what is causing the pump cavitation in the first place, which may be one of the following factors -
- Vaporization at pump suction, NPSHa being too low
- Internal re-circulation resulting from restriction on the pump discharge flow
- Vane passing syndrome
- Air aspiration
- Due to turbulence
Each one of these cases is discussed in detail in this post, along with the preventive measures to be taken in each case.