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
Types of Cavitation
Two types of cavitation, discharge & suction.
High pressure causes discharge cavitation.
Vacuum, at suction of the pump, causes suction
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.
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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