Short Answer:

Cavitation is the formation and sudden collapse of vapor bubbles in a fluid when the pressure falls below its vapor pressure. It commonly occurs in hydraulic machines like turbines and pumps, especially at points where fluid pressure becomes very low. These collapsing bubbles create strong shock waves that can damage metal surfaces.

Cavitation occurs due to high flow speed or poor design that causes pressure drops in certain areas. It results in pitting, vibrations, noise, reduced efficiency, and sometimes serious mechanical failure in turbines, pumps, and valves if not prevented.

Detailed Explanation:

Cavitation and how it occurs in hydraulic machines

Cavitation is a destructive hydraulic phenomenon that occurs in flowing liquids when the local pressure drops below the vapor pressure, causing the liquid to change into vapor bubbles. These bubbles form in low-pressure zones and collapse when they move into high-pressure regions. This collapse happens violently, producing shock waves and intense localized forces that can damage machine parts.

In civil and mechanical engineering, cavitation is a major concern in hydraulic machines such as pumps, turbines, and valves, where high-speed flow and pressure changes are common.

How Cavitation Occurs

1. Formation of Vapor Bubbles
   When water or any other fluid flows through a region of low pressure—such as the suction side of a pump, the exit of a turbine blade, or a sharp bend in a pipe—its pressure may drop below the vapor pressure. At this point, small vapor bubbles form within the liquid.
2. Collapse of Bubbles
   As the flow continues and enters a high-pressure region, these vapor bubbles suddenly collapse (implode). The collapse happens so quickly that it generates very high pressure and temperature locally, which can exceed thousands of atmospheres.
3. Damage to Surfaces
   The force from the collapsing bubbles attacks the nearby metal surfaces, causing tiny holes or pits. Over time, this effect grows, leading to surface erosion, vibration, noise, and even failure of mechanical components.

Common Conditions That Lead to Cavitation

• High-speed flow or sharp changes in velocity
• Sudden change in pipe diameter or shape
• Low pressure at the suction side of pumps
• Improper design of impellers or blades
• Operating hydraulic machines at off-design conditions
• High temperature environments (which reduce vapor pressure)

Effects of Cavitation in Hydraulic Machines

1. Erosion and Pitting
   The collapsing bubbles create deep pits on metal parts like pump impellers, turbine blades, and valve seats. This weakens the components.
2. Vibrations and Noise
   Cavitation creates loud crackling or knocking sounds and causes vibrations that reduce machine life and affect performance.
3. Reduced Efficiency
   Cavitation disturbs the smooth flow of water, reducing energy transfer and causing loss of efficiency in pumps and turbines.
4. Structural Damage
   Long-term cavitation can break down parts of the machine, leading to expensive repairs or complete failure.

Prevention of Cavitation

• Maintain adequate inlet pressure (high Net Positive Suction Head – NPSH)
• Avoid sharp turns and sudden expansions in flow path
• Design blades and impellers properly to reduce pressure drops
• Use cavitation-resistant materials like stainless steel
• Install air release valves to prevent pressure drops due to trapped air
• Operate within recommended flow and speed conditions

Conclusion:

Cavitation is the formation and collapse of vapor bubbles in a hydraulic machine due to low pressure. It causes serious damage like erosion, noise, and reduced efficiency. Cavitation occurs mostly in pumps and turbines and can be prevented with proper design, pressure control, and regular maintenance. Understanding cavitation is essential for ensuring safe and efficient operation of hydraulic systems.