CAVITATION CLASS NOTES FOR MECHANICAL ENGINEERING

 

 

CAVITATION CLASS NOTES

FOR MECHANICAL ENGINEERING

It is process of vapor formation  in an

accelerated liquid  flow at a certain

temperature. Flow accelerates

in pumps and turbines etc. Velocity

increases. Static pressure falls below the

saturation pressure of the liquid. Vapors

form. These vapors move to high pressure

region. Sudden collapse of these bubbles

cause erosion on the surface in high

pressure region. Powerful shock waves are

repeatedly created.

Types of Cavitation

 Two types of cavitation, discharge & suction.

 High pressure causes discharge cavitation.

 Vacuum, at suction of the pump, causes suction

cavitation.

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.  Damages the structure & reduces life. 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

Cavitation exists in pump impellers, turbine blades, control valves, sub-marines.  Cavitation also exists in hearts & knee joints.

Causes of Cavitation

The liquid enters hydraulic turbines at high pressure. Fast moving blades increase local dynamic pressure. Decrease in static pressure causes 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 cause damage and reduce life. 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 cause flow separation. Cavitation affects performance.  Cavitation decreases pressure rise, flow rate and increases power consumption in a pump. Axial imbalance comes. Axial imbalance shortens life of shaft seal, bearing and impeller.

 HARMFUL EFFECTS OF CAVITATION

The formation of vapor bubbles in Cavitation does not cause much harm.  Bubbles collapse  in high pressure zones. It generates pressure waves of high frequencies. This in turn,  causes damage to the machinery. The collapsing of these bubbles  near the machine surface cause erosion of the surface. Machine performance reduces. Cavitation  causes loud noise, vibrations, repeated shock waves, loss of efficiency, structural damage. Life reduces.

HOW TO REDUCE CAVITATION

 Static pressure may not fall below the vapor pressure of the liquid. Hydrodynamic models tested under controlled parameters. Controlled parameters will not cause Cavitation. These parameters are  pressure head, flow rate and exit pressure of the liquid. In order to reduce Cavitation,  suction pressure & liquid temperature should be increased.

 Cavitation help bodies moving in water. It will reduce the drag and vehicles will move faster. This is Super Cavitation. Super Cavitation decreases drag force on underwater objects.

CONCLUSIONS

Cavitation occurs in accelerated liquid flow and causes lot of harm to the pump impellers, turbine blades, control valves etc.  Minimum Cavitation should occur.

 

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