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Cavitation

Cavitation is the phenomenon where small cavities of partial vacuum form in fluid then rapidly collapse, producing a sharp sound.

Table of contents
1 Pump
2 See also
3 External link

Pump

Cavitation occurs in pumps. Cavitation means that cavities are forming in the liquid that we are pumping. Cavitation's shocks can cause pitting and flaking of parts such as impellers, shortening their life.

Cavitation by the propellers of submarines are the giveaway of the vessels' stealthiness.

"The faster a propeller spins, the lower the water pressure around it becomes. If this pressure falls fast enough, then the water reaches its vapor pressure, at which, like boiling water, it vaporizes and forms small bubbles of gas." [1]

Cavitation may occur in two different forms:

Suction Cavitation

Suction Cavitation occurs when the pump suction is under a low pressure/high vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition has large chunks of material removed from its face causing premature failure of the pump.

Discharge Cavitation

Discharge Cavitation occurs when the pump discharge is extremely high. It normally occurs in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller it must pass through the small clearance between the impeller and the pump cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater similar to what occurs in a venturi and turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump cutwater. In addition due to the high pressure condition premature failure of the pump mechanical seal and bearings can be expected and under extreme conditions will break the impeller shaft.

Cavitation is believed to be the cause of cracking knuckles.

See also

External link