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Monday, January 25, 2010

Pump Cavitations

The formation and subsequent collapse of vapor filled cavities in a liquid due to dynamic action are called pump cavitation. The cavities may be bubbles, vapor filled pocket or a combination of both. The local pressure must be at or below the vapor pressure of the liquid for cavitation to begin, and the cavities must encounter a region of pressure higher than the vapor pressure in order to collapse. Dissolved gasses often are liberated shortly before vaporization begins. This may be an indication of impending cavitation, but true cavitation requires vaporization of the liquid. Boiling accomplished by the addition of heat or the reduction of static pressure without dynamic action of the liquid is arbitrarily extruded from the definition of cavitation.

When a liquid flows over a surface having convex curvature, the pressure near the surface is however and the flow tends to separate from the surface. Separation and cavitation are completely different phenomena. Without cavitation, a separated region contain turbulent eddying liquid at pressures higher than the vapor pressure. When the pressure is low enough, the separated region may contain a vapor pocket that fills from the downsteam end, collapses, and forms again many times each second. This causes noise and, if severe enough, vibration. Vapor filled bubbles usually are present and collapse very rapidly in any region where the pressure is above the vapor pressure.

Bubble that collapse on a solid boundary may cause severe mechanical damage. Shutler and Mesler photographed bubbles that distorted into toroidal shaped rings during collapse and produced ring shaped indentation in a soft metal boundary. The bubbles rebounded following the initial collapse and caused pitting of the boundary. Pressure on the order of 104 atm have been estimated during collapse of a bubble. All known material can be damaged by exposure to bubble collapse for a sufficiently long time. This is properly called cavitation erosion, or pitting.

It has been postulated that high temperature and chemical action may be present at bubbles collapse, but any damaging effects due to them appear to be secondary to the mechanical action. It seems possible that erosion by foreign material in the liquid and cavitation pitting may augment each other. Cavitation pitting, as measure by weight of of the boundary material removed per unit time, frequently increase with time. Cast iron and steel boundaries are particularly vulnerable. Controlled experiment have shown that cavitation pitting in metal such as aluminum, steel and stainless steel depends strongly on the velocity of the fluid in the disturbed flow past the surface.

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