Question

The phenomenon of vapour bubble formation, and sudden collapsing of its in a turbine, is called
1. vibration 
2. cavitation
3. bubbling
4. priming

Answer 1

Correct Answer - Option 2 : cavitation

Explanation:

Cavitation:

• Cavitation is defined as the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure and the sudden collapsing of these vapour bubbles in a region of higher pressure. 
• When the vapour bubbles collapse, a very high pressure is created.
• The metallic surfaces, above which these vapour bubbles collapse, is subjected to these high pressures, which cause pitting action on the surface. 
• Thus cavities are formed on the metallic surface and also considerable noise and vibrations are produced.
• Cavitation includes formation of vapour bubbles of the flowing liquid and collapsing of the vapour bubbles. 
• Formation of vapour bubbles of the flowing liquid take place only whenever the pressure in any region falls below vapour pressure.
• When the pressure of the flowing liquid is less than its vapour pressure, the liquid starts boiling and vapour bubbles are formed.
• These vapour bubbles are carried along with the flowing liquid to higher pressure zones where these vapours condense and bubbles collapse.
• Due to sudden collapsing of the bubbles on the metallic surface, high pressure is produced and metallic surfaces are subjected to high local stresses. Thus the surfaces are damaged. 

Effects of Cavitation:

• The metallic surfaces are damaged and cavities are formed on the surfaces.
• Due to sudden collapse of vapour bubble, considerable noise and vibrations are produced. 
• The efficiency of a turbine decreases due to cavitation. Due to pitting action, the surface of the turbine blades becomes rough and the force exerted by water on the turbine blades decreases. Hence, the work done by water or output horse power becomes less and thus efficiency decreases.

Precaution against Cavitation:

• The pressure of the flowing liquid in any part of the hydraulic system should not be allowed to fall below its vapour pressure. If the flowing liquid is water, then the absolute pressure head should not be below 2.5 m of water.
• The special materials or coatings such as aluminium-bronze and stainless steel, which are cavitation resistant materials, should be used.