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Cavitation is a curious phenomenon to strangers where as for engineers; it is a nothing short of menace. It occurs in hydraulic machinery and results in serious material erosion. In a liquid, whenever the pressure at any point reduces to vapour pressure, the liquid begins to boil and vaporisation starts. If the liquid contains dissolved gases, they are also liberated at such low pressures. Large number of tiny bubbles of liquid vapour and liberated gas are thus formed.
These are carried along the flow. Whenever these bubbles reach the high pressure zones, they will condense and collapse suddenly. Then, the surrounding liquid rushes with very high speeds to fill the void or cavity left by the collapsing bubble. The flow begins from all directions to collide at the centre of cavity and generates a high pressure wave which propagates nearly at the velocity of sound. This will give rise to very high local pressures. So far, fine; if the collapse of bubbles takes place at or near a solid boundary, the solid surface will come under these high pressures. These very high local pressures are often beyond the elastic limit of materials used for hydraulic structures and machines. Repeated cavitations in the flowing fluids would eventually result in fatigue failure. Even when these bubble collapses and the consequent formation of cavities, occur away form solid boundary, the extreme pressures will be propagated to the boundary through the wave.
This cavitation, the sequential formation of vapour bubbles, their transporting and collapse in zones of high pressure would normally repeat in quick successions. This frequency depends directly on the velocity of flow and is many times a second. This obviously, generates noise and vibrations. The sound of cavitation in pumps and turbines can be heard as if a few pebbles are flowing through them. These very high pressure waves pouncing repeatedly at high frequency, is analogous to repeated hammer blows on the solid boundaries. So much so that before the material fails by fatigue, it may become severely dented. The presence of bubbles also cause constriction to flow and reduces efficiency of system. The dents may appear as if the material is eroded off. This dent formation is called as pitting and the process of bubble formation in flowing fluid culminating in the pitting is the cavitation.
As the cavitation mechanism makes it clear, the phenomenon of cavitation can be observed in many high speed hydraulic machines and large hydraulic structures. Turbines, pumps. propellers of ships and other hydro craft, spillways and sluice openings of high dams, flow measurement instrumentation and certain portions of the pipelines are a few examples.
So, what did the theory have to say? Well, let us now study the factors responsible for cavitation in a flow. Bernoulli’s equation can be expressed as energy per unit volume as given below:
ρ (V2 / 2) + p+ γZ = A constant (say Ev)
The pressure at a point is
p = Ev - ρ (V2 / 2) - γZ
The total energy per unit volume Ev is a constant along a streamline in a rotational flow and at all points in an irrotational flow. So, from the above equation, it can be seen that the pressure p at a point, depends on the velocity and the datum head and is inversely proportionate to them. In other words, the pressure gets reduced when the velocity or elevation head or both are increased. Certain regions in the flow geometry may reduce the pressure of water to its vapour pressure and exposes the structure or machine to cavi¬tation.
Engineering designs, therefore give due consideration to operate the hydraulic machines and structures without incidence of cavitation. So, it is necessary to ensure that the pressure would never fall to the vapour pressure of the liquid at the flow temperature. To prevent cavitation in flowing water, the absolute pressure head must not be less than 2.5 m.
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