Short Answer:

Cavitation is a phenomenon that occurs in liquids when the local pressure drops below the vapor pressure, causing the formation of small vapor bubbles. These bubbles travel with the fluid and collapse violently when they enter a region of higher pressure, producing shock waves. This process can cause serious damage to pump impellers, turbine blades, and pipes.

In simple words, cavitation is the formation and sudden collapse of vapor bubbles in a liquid due to pressure variations. It reduces the efficiency of hydraulic machines and can lead to vibration, noise, and surface erosion in the components.

Detailed Explanation:

Cavitation

Cavitation is a very important and destructive phenomenon that occurs in hydraulic machines and fluid systems. It takes place when the pressure in a flowing liquid falls below its vapor pressure, leading to the formation of vapor bubbles. These vapor bubbles move along with the liquid stream and collapse when they enter regions of higher pressure. The sudden collapse generates high-pressure shock waves that can erode and damage the surfaces of nearby components, such as impellers, blades, or valve seats.

Cavitation is commonly observed in pumps, turbines, propellers, and valves, where rapid pressure changes occur. It not only causes mechanical damage but also leads to reduced efficiency, vibration, and noise in the hydraulic system.

1. Formation Process of Cavitation

The process of cavitation can be explained in three main stages:

1. Formation of vapor bubbles:
   When the pressure in a part of the liquid falls below the vapor pressure, small vapor-filled cavities or bubbles form. This usually happens at points where the velocity is very high and pressure is low, such as at the suction side of pumps or behind turbine blades.
2. Growth and movement of bubbles:
   These vapor bubbles grow in size and move along with the liquid flow to regions of higher pressure within the system.
3. Collapse of bubbles:
   When these bubbles enter a high-pressure region, they collapse suddenly. The collapse creates a small but powerful impact pressure, often as high as several hundred atmospheres, which damages the nearby metal surface.

This entire process happens very quickly and repeatedly, resulting in continuous wear and tear on hydraulic machine components.

2. Causes of Cavitation

Cavitation occurs due to several reasons related to pressure and flow conditions:

• Low pressure at suction side: When the suction pressure in a pump falls below the vapor pressure of the liquid.
• High velocity flow: According to Bernoulli’s principle, an increase in flow velocity decreases pressure, which may cause cavitation.
• Improper design: Sharp bends, sudden contractions, or rough surfaces can create local low-pressure zones.
• High temperature: At high temperatures, vapor pressure increases, making cavitation easier to occur.
• Excessive suction lift: Drawing fluid from a deep level increases suction head loss and may reduce pressure below vapor pressure.

These causes are often related to poor design, improper operation, or high-speed conditions in hydraulic systems.

3. Effects of Cavitation

Cavitation can lead to several negative effects on hydraulic machines and systems:

1. Material erosion: The collapse of vapor bubbles produces micro-jets and shock waves that erode the metal surface.
2. Noise and vibration: The repetitive collapsing of bubbles generates loud noise and mechanical vibrations.
3. Loss of efficiency: Cavitation reduces the flow capacity and performance of pumps and turbines.
4. Reduced lifespan: Continuous erosion weakens components and shortens machine life.
5. Unstable operation: Cavitation can cause irregular flow, affecting system stability and performance.

If not controlled, cavitation can cause severe damage, requiring expensive repairs or replacement of machine parts.

4. Types of Cavitation

Cavitation can be classified into different types based on its cause and location:

• Vaporous Cavitation: Occurs when the pressure drops below vapor pressure, causing vapor bubble formation.
• Gaseous Cavitation: Happens when dissolved gases in the liquid are released as bubbles.
• Hydrodynamic Cavitation: Caused by changes in velocity and pressure in flowing fluids, such as near impeller blades.
• Transient Cavitation: A temporary and unstable form occurring during pressure fluctuations.

Understanding the type of cavitation helps engineers find suitable methods for prevention.

5. Prevention of Cavitation

To prevent or reduce cavitation, the following methods are commonly used:

1. Maintain sufficient suction head: Keep the suction pressure above the vapor pressure by proper pump positioning and design.
2. Reduce pump speed: Lowering the operating speed reduces velocity and prevents pressure from dropping too low.
3. Use suitable impeller design: Streamlined impeller shapes and smoother surfaces reduce flow disturbances.
4. Avoid sharp bends and contractions: Ensure smooth transitions in pipelines to prevent sudden pressure drops.
5. Control operating temperature: Keeping the fluid temperature lower reduces vapor pressure and avoids bubble formation.
6. Use cavitation-resistant materials: Materials such as stainless steel or bronze are more resistant to erosion caused by cavitation.

By applying these preventive measures, the occurrence of cavitation can be minimized, improving machine life and performance.

6. Importance in Hydraulic Machines

Cavitation is particularly critical in centrifugal pumps and hydraulic turbines.

• In pumps, cavitation mainly occurs at the suction side, reducing discharge and damaging the impeller.
• In turbines, it happens at the runner blades or draft tube, lowering output power and efficiency.

Engineers use Net Positive Suction Head (NPSH) in pump systems to evaluate and avoid cavitation. Maintaining NPSH above the required value ensures smooth operation without vapor formation.

Conclusion:

Cavitation is a destructive process caused by the formation and collapse of vapor bubbles in a fluid when pressure falls below vapor pressure. It results in material damage, noise, vibration, and reduced efficiency in hydraulic machines. Proper design, operating conditions, and maintenance can help prevent cavitation and extend the life of pumps and turbines. Controlling cavitation is essential for the safe, efficient, and reliable operation of any hydraulic system.