Cavitation free notes for Diploma / BTech.

1. What is Cavitation?

Cavitation is a phenomenon in hydraulic machines where vapour bubbles form in a liquid due to local pressure falling below vapour pressure, and then suddenly collapse when they move to a region of higher pressure.

This process produces shock waves, leading to noise, vibration, and damage to machine components like turbine blades and pump impellers.

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2. Mechanism of Cavitation

Step-by-step process:

1. Pressure Drop
   • When fluid flows through a turbine or pump, velocity increases → pressure decreases.
   • If pressure falls below vapour pressure, liquid starts vaporizing.
2. Bubble Formation
   • Tiny vapour bubbles (cavities) are formed in low-pressure regions.
3. Bubble Movement
   • These bubbles travel with the flowing liquid to higher pressure zones.
4. Collapse of Bubbles
   • At higher pressure, bubbles implode violently.
   • This creates micro-jets and shock waves.

3. Effects of Cavitation

1. Surface Damage (Pitting)
   • Continuous collapse causes erosion of metal surfaces.
2. Noise and Vibration
   • Produces a characteristic crackling sound.
3. Loss of Efficiency
   • Disturbs smooth flow → reduces performance.
4. Structural Damage
   • Can lead to failure of turbine blades or pump impellers.

4. Types of Cavitation

1. Vapour Cavitation
   • Due to pressure falling below vapour pressure.
2. Gas Cavitation
   • Due to release of dissolved gases.
3. Suction Cavitation (in pumps)
   • Occurs at pump inlet due to insufficient suction head.
4. Discharge Cavitation
   • Occurs at pump outlet due to flow disturbances.

5. Cavitation in Turbines

• Common in reaction turbines like Kaplan and Francis turbines.
• Occurs mainly at:
  • Runner outlet
  • Draft tube entrance

Important Term:

Thoma’s Cavitation Factor (σ)

$$\sigma = \frac{H_a – H_v – H_s}{H}$$Where:

• $H_a$ = Atmospheric pressure head
• $H_v$​ = Vapour pressure head
• $H_s$​ = Suction head
• $H$ = Net head

Condition to avoid cavitation:$$\sigma > \sigma_{critical}$$

6. Cavitation in Pumps

Cavitation in pumps is the phenomenon in which vapor bubbles form in the liquid when the local pressure falls below the liquid’s vapor pressure. As these bubbles move to a higher-pressure region inside the pump, they collapse violently, producing shock waves that can damage the pump.

Net Positive Suction Head (NPSH)

Net Positive Suction Head (NPSH) is the amount of pressure available at the pump suction above the liquid’s vapor pressure. It is one of the most important parameters for preventing cavitation.

Formula:$$\boxed{\text{NPSHA} = H_a + H_s – H_f – H_v}$$

Where:

• $H_a$​ = Atmospheric pressure head
• $H_s$​ = Static suction head (positive if liquid is above the pump, negative if below)
• $H_f$​ = Friction loss in the suction piping
• $H_v$​ = Vapor pressure head of the liquid

NPSH Condition for cavitation:

$$NPSH_{available} > NPSH_{required}$$If this condition is not satisfied → cavitation occurs.

7. Factors Affecting Cavitation

• Low pressure in flow
• High fluid velocity
• High temperature (increases vapour pressure)
• Poor design of blades or passages
• High suction lift in pumps

8. Prevention of Cavitation

1. Maintain Adequate Pressure
   • Ensure pressure does not fall below vapour pressure
2. Proper Design
   • Smooth flow passages
   • Proper blade angles
3. Reduce Suction Lift
   • Place pump below water level
4. Use of Draft Tube (in turbines)
   • Helps maintain pressure at outlet
5. Material Selection
   • Use cavitation-resistant materials (stainless steel, alloys)
6. Temperature Control
   • Lower temperature reduces vapour formation

9. Advantages and Disadvantages of Cavitation

Advantage	Disadvantage
Used in ultrasonic cleaning Helps in fuel atomization Used in water treatment processes	Severe equipment damage Efficiency loss Maintenance cost increases Operational instability

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