Cavitation Explained and Illustrated

The phenomenon of cavitation consists in the disruption of continuity in the liquid where there is considerable local reduction of pressure. The formation of bubbles within liquids (cavitation) begins even in the presence of positive pressures that are equal to or close to the pressure of saturated vapor of the fluid at the given temperature.

Various liquids have different degrees of resistance to cavitation because they depend, to a considerable degree, upon the concentration of gas and foreign particles in the liquid.

Wear Mechanism

The mechanism of cavitation can be described as follows: Any liquid will contain either gaseous or vaporous bubbles, which serve as the cavitation nuclei. When the pressure is reduced to a certain level, bubbles become the repository of vapor or of dissolved gases.

The immediate result of this condition is that the bubbles increase rapidly in size. Subsequently, when the bubbles enter a zone of reduced pressure, they are reduced in size as a result of condensation of the vapors that they contain.

This process of condensation takes place fairly quickly, accompanied by local hydraulic shocks, the emission of sound, the destruction of material bonds and other undesirable phenomena. It is believed that reduction in volumetric stability in most liquids is associated with the contents of various admixtures, such as solid unwetted particles and gas-vapor bubbles, particularly those on a submicroscopic level, which serve as cavitation nuclei.

A critical aspect of the cavitation wear process is surface destruction and material displacement caused by high relative motions between a surface and the exposed fluid. As a result of such motions, the local pressure of the fluid is reduced, which allows the temperature of the fluid to reach the boiling point and small vapor cavities to form.

When the pressure returns to normal (which is higher than the vapor pressure of the fluid), implosions occur causing the cavity or vapor bubbles to collapse. This collapse of bubbles generates shock waves that produce high impact forces on adjacent metal surfaces and cause work hardening, fatigue and cavitation pits.

Thus, cavitation is the name given to a mechanism in which vapor bubbles (or cavities) in a fluid grow and collapse due to local pressure fluctuations. These fluctuations can produce a low pressure, in the form of vapor pressure of the fluid. This vaporous cavitation process occurs at approximately constant temperature conditions.

Cavitation Types

Two principal types of cavitation exist: vaporous and gaseous.

1.Vaporous cavitation: An ebullition process that takes place if the bubble grows explosively in an unbounded manner as liquid rapidly changes into vapor. This situation occurs when the pressure level goes below the vapor pressure of the liquid.

2.Gaseous cavitation: A diffusion process that occurs whenever the pressure falls below the saturation pressure of the noncondensable gas dissolved in the liquid. While vaporous cavitation is extremely rapid, occurring in microseconds, gaseous cavitation is much slower; the time it takes depends upon the degree of convection (fluid circulation) present.

Cavitation wear occurs only under vaporous cavitation conditions – where the shock waves and microjets can erode the surfaces. Gaseous cavitation does not cause surface material to erode.

It only creates noise, generates high (even molecular level cracking) temperatures and degrades the chemical composition of the fluid through oxidation. Cavitation wear is also known as cavitation erosion, vaporous cavitation, cavitation pitting, cavitation fatigue, liquid impact erosion and wire-drawing.

Cavitation wear is a fluid-to-surface type of wear that occurs when a portion of the fluid is first exposed to tensile stresses that cause the fluid to boil, then exposed to compressive stresses that cause the vapor bubbles to collapse (implode).

This collapse produces a mechanical shock and causes microjets to impinge against the surfaces, unifying the fluid. Any system that can repeat this tensile and compressive stress pattern is subject to cavitation wear and all the horrors accompanying such destructive activity.

Cavitation wear is similar to surface fatigue wear; materials that resist surface fatigue (hard but not brittle substances) also resist cavitation damage.