Cavitation

• Vapour pressure of liquid may causes an unplanned OR undesirable effect called cavitation.
• When a liquid flows into a region where its pressure is reduced to vapour pressure, it starts vaporizing (or boiling), and vapour pockets or bubbles are formed in the liquid. These vapour bubbles are carried along with the flowing liquid until a region of higher pressure is reached, where they suddenly collapse as the vapour condenses to liquid again. Moreover, these bubbles may also collapse even at the point where they are formed, because of the momentary local increase in pressure at this point. When a vapour bubble collapses a cavity is formed and the surrounding liquid rushes to fill it. This process of formation of vapour bubbles and their collapsing is called Cavitation.
• Liquids generally contain some dissolved air. This dissolved air is released as the pressure is reduced and thus air pockets or bubbles are formed in the liquid. The air pockets are often termed as air locks due to which air cavitation occurs.
• Any solid surface in the vicinity of the cavities is also subjected to intense pressure, because, even if the cavities are not actually at the solid surface, the pressure is propagated from the cavities by pressure waves. The formation and collapse of the large number of bubbles on the boundary surface subject the surface to intense local stressing.
• The boundary surface is thus alternatively stressed and relieved of the stresses, thereby ultimately damaged by fatigue, resulting in serious erosion of the surface, which is known as Pitting.
• The phenomenon of cavitation is always accompanied by considerable noise and vibrations.
• Vapour pressure increases with temperature, which allows the liquid to break down at lower energy and consequently. Therefore at higher temperature, chances of cavitation is more.
• When cavitation occurs, the flow is disturbed, and the efficiency is lowered.
• Air cavitation is less damaging than vapour cavitation but, both reduces efficiency equally.
• The cavitation number (σ) is used to characterize the susceptibility of the system to cavitate.
  It is defined by, σ = p-p_v/(ρv²/2)
  where, p = absolute pressure at point under consideration; p_v = vapour pressure of liquid ρ = mass density of liquid; V = reference velocity of flow of liquid
• Cavitation number, σ is a dimensionless parameter.
• In two geometrically similar systems, if the value of σ is same, they would be equally likely to cavitate or they would have the same degree of cavitation.
• Theoretically, cavitation starts when σ = 0, i.e., p = pv
• Cavitation may, generally, occur at the exit of reaction turbine, inlet of pump, at the top of siphon and on the surface of spillway.
• Rise in elevation, increase in velocity, decrease in atmospheric pressure, increase in temperature of liquids are some of the causes which favours the chances of cavitation.

Protection against cavitation:

• By adopting such a hydraulic design of the system which would avoid the development of low pressure.
• If it is not possible to develop completely cavitation free regions (e.g. in case of turbines and pumps), we can go for special cavitation resistant materials as a coating such as alloys of aluminium, bronze, stainless steel etc.

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