CAVITATION

 

 

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CAVITATION

It is process of vapor formation  in an accelerated liquid  flow at a certain temperature. The accelerated flow is found in pumps and turbines etc. Due to acceleration, there is an increase in velocity. Thus, there is fall of static pressure resulting in low pressure below the saturation pressure of the liquid. These vapors move to high pressure region. Hence there is a sudden collapse of these vapors. It causes serious damage. As a result of this, erosion is caused by the bombardment of vapor bubbles on the surface in high pressure region. Powerful shock waves are repeatedly created.

Types of Cavitation

Discharge Cavitation is found  due to very high pressure at discharge. Suction Cavitation is due to very low suction pressure (vacuum) at the suction of the pump.

Different titles for Cavitation

Different names of Cavitation are Cavitation pitting, Cavitation corrosion and Cavitation damage.

After effects of Cavitation

Cavitation causes dynamic shaft imbalance of the pump. Consequently, there is a  serious damage to the to the structure. Thus, it reduces the life significantly.This collapsing causes loud noise, vibrations, loss of efficiency, wear and tear of the equipment. It causes erosion (cavities) in the operating machine.

PRACTICAL APPLICATIONS OF CAVITATION

It is found in hydrodynamic machinery like pump impellers, turbine blades, control valves, sub-marines. Cavitation is found in hearts and knee joints too.

Causes of Cavitation

The liquid enters hydraulic turbines at high pressure. Fast moving blades increase local dynamic pressure. Thus, the static pressure decreases.  Consequently, there is vapor formation. Cavitation is formation of vapors in an accelerated liquid flow  at a certain temperature. These vapors move to high pressure regions and collapse suddenly. It produces a loud noise, vibrations and loss of efficiency. Shock waves created cause damage and reduce life significantly. Size of bubbles greatly affects the damage. Smaller is the size of the bubble, more will be the frequency and more will be the damage. These small bubbles combine to form bigger bubbles. These bigger bubbles disturb the liquid flow and also causes flow separation. Cavitation affects overall performance.  Cavitation decreases pressure rise, flow rate and increases power consumption in a pump. It also causes axial imbalance which shortens seal, bearing and impeller life.

 HARMFUL EFFECTS OF CAVITATION

The formation of vapor bubbles in Cavitation does not cause much harm.  However, the collapse of these bubbles in high pressure zones generates pressure waves of very high frequencies. This in turn,  causes damage to the machinery. The collapsing of these bubbles  near the machine surface  cause erosion of the surface. It is called as Cavitation erosion. . It reduces the machine performance significantly. Cavitation  causes loud noise, vibrations, repeated shock waves, loss of efficiency, structural damage. It reduces the life significantly and dramatically.

HOW TO REDUCE CAVITATION

Extreme care should be taken so that at any point of flow static pressure may not fall below the vapor pressure of the liquid. In order to achieve this, tests should be conducted on hydrodynamic machine models with controlled parameters which will not cause Cavitation. These parameters are  pressure head, flow rate and exit pressure of the liquid. In order to reduce Cavitation,  suction pressure should be increased and liquid temperature should be increased as far as possible.

SUPER CAVITATION

There are situations for bodies moving in water that the Cavitation can be used with an advantage. It will reduce the drag and vehicles will move faster. This type of Cavitation has been named as Super Cavitation. Super Cavitation  phenomenon is found in underwater objects to decrease their drag force.

CONCLUSIONS

Cavitation occurs in accelerated liquid flow and causes lot of harm to the pump impellers, turbine blades, control valves etc. Suitable steps must be taken to reduce the Cavitation to a minimum.