Cavitation is the phenomenon where small cavities of partial vacuum form in fluid, then rapidly collapse, producing a sharp sound.
Pump
Cavitation occurs in pumps, as well as around propellers, or at restrictions in a flowing liquid. Cavitation means that cavities are forming in the liquid being pumped. Cavitation's shocks can cause pitting and flaking of parts such as impellers, shortening their life. Cavitation also reduces efficiency dramatically.
Cavitation by the propellers of submarines destroys the vessels' stealthiness.
As an impeller's (in a pump), or propeller's (as in the case of a ship or submarine) blades move through a fluid, low pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around it can become. If this pressure falls far enough, then the fluid will reach its 'vapor pressure'. As it reaches vapor pressure, the fluid, like boiling water, vaporizes and forms small bubbles of gas [1]. This is cavitation. When the bubbles collapse later, they typically cause very strong local shockwaves in the fluid, which may be audible and may even damage the blades.
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) which 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. Under extreme conditions this can break the impeller shaft.
Discharge cavitation is believed to be the cause of the cracking of joints.
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