What is Cavitation?
Derived from the Latin word 'Cavus', which means a cavity or a hollow space.
Defined as the rapid formation and collapse of cavities in a flowing liquid in regions of very low pressure.
If the pressure at any point inside pump particularly at the suction line, drops below the vapour pressure corresponding to the temperature of liquid, the liquid will vaporize and appear as cavities of vapour or vapour bubbles.
These cavities are carried up stream along with the liquid until a region of high pressure formed inside the impeller, where the cavities collapse with a tremendous impact on the side walls of impeller vanes. This phenomenon is called cavitation.
How does cavitation occur ?
Cavitation occurs when a pump is under low pressure or high vacuum conditions, it does not receive enough flow. When this happens, bubbles or cavities will form at the eye of the impeller.
This formation and accumulation of bubbles around a pump impeller is called cavitiation.
Steps in Cavitation
- FORMATION OF BUBBLES INSIDE THE LIQUID
- GROWTH OF BUBBLES
- COLLAPSE OF BUBBLES
- CAVITATION
Formation of bubbles inside the liquid
Bubbles are formed due to vaporization of the liquid 'irrespective of its viscosity being pumped when the local static pressure at any point inside the pump becomes equal to or less than the vapour pressure of the liquid at the pumping temperature.
In a nutshell, the bubbles form inside the liquid when it vaporises that is change from liquid state to vapour state.
Growth of bubbles
After the bubbles form inside the liquid, new bubbles are formed continuously and the old bubbles continue to grow in size.
Collapse of bubbles
The bubbles then are carried in the liquid as it flows from impeller eye to the impeller exit along the vane trailing edge. As the bubbles continue to move along the impeller vanes, there is an increase in the presure around the bubbles. The pressure continues to build up until it reaches the point where the pressure inside the bubble is lesser than the pressure on the outside of the bubble. Collapse of bubble begins at this point. The collapse is inward bursting that is implosion and not explosion. On each of the impeller vane, at the approximately the same point, hundreds of bubbles collapse non-symmetrically. Due to this non-symmetrical collapse, the surrounding liquid rushes in to fill the void and liquid microjet is formed which subsequently ruptures the bubble with force creating a hammering effect that can pit the pump impeller.
Cavitation
When cavitating, the pump experiences leakage, impeller failure etc., leading to irreplaceable pump damage.
The bubbles start to occupy space inside the pump and as a result the motor's energy gets wasted on expanding the bubbles instead of bringing in more liquid into the pump.
When bubbles collapse inside the pump, the liquid strikes the metal parts at high speed. It can cause excessive vibration and erosion.
As a result, pumps face problems ranging from a partial loss in capacity and head resulting in total pumping failure.
Adverse effects of cavitation
1.Metallic erosion
Implosion of bubbles destroys metal 's protective cover. There is loss of pump material (pitting of material).
2.Deterioration in pump performance
A dramatic decline in pump performance results due to 'cavitation break-down'. Hydraulic design modification is required.
3.Failure of pump components
Such as Shafts, Wear rings, Seals, Impeller and Bearing occurs.
4.Instability in flow pattern of pumping liquid
Instability in flow pattern of pumping liquid results in oscillating flow and pressure that threatens the structural integrity of pump specially inlet and discharge ducts.
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