Short Answer:

Cavitation is defined as the formation and collapse of vapor bubbles in a liquid when the local pressure falls below the vapor pressure of the liquid. It usually occurs in pumps, turbines, and propellers where high-speed flow causes low-pressure zones.

In simple words, cavitation happens when liquid boils at low pressure, forming vapor bubbles that later collapse violently as they move into higher-pressure regions. This collapse creates noise, vibration, and even damage to metal surfaces like pump blades and turbine vanes. Cavitation reduces the efficiency and life of hydraulic machines.

Detailed Explanation :

Cavitation

Cavitation is a physical phenomenon that occurs in fluids when the local static pressure in the liquid becomes lower than the vapor pressure of that liquid. When this happens, tiny vapor-filled cavities or bubbles form within the fluid. These bubbles grow in low-pressure regions and collapse suddenly when they move to high-pressure zones.

This formation and violent collapse of vapor bubbles release high energy, creating shock waves that can cause severe erosion, noise, and vibration in hydraulic machines such as pumps, turbines, and propellers. Cavitation is therefore one of the most undesirable occurrences in fluid machinery.

Mechanism of Cavitation

Cavitation occurs in three main stages:

1. Formation of Vapor Bubbles:
   When a fluid flows through a region of low pressure (such as behind an impeller blade or at the throat of a nozzle), the local pressure may drop below the liquid’s vapor pressure. At this stage, small vapor bubbles form within the liquid.
2. Growth of Bubbles:
   These vapor bubbles grow as they travel through the low-pressure zone. The surrounding liquid continues to vaporize, adding to the bubble’s size.
3. Collapse of Bubbles:
   As the fluid moves to a region of higher pressure, the vapor bubbles cannot exist in this high-pressure environment. They collapse violently, producing strong shock waves that strike the nearby metal surfaces.

The collapse of millions of bubbles every second causes pitting, erosion, and damage to machine components, along with noise and vibration.

Conditions Leading to Cavitation

Cavitation occurs when the local static pressure in a fluid becomes equal to or less than its vapor pressure. The following conditions favor cavitation:

• High flow velocity: Increased velocity causes a pressure drop according to Bernoulli’s principle.
• Sharp changes in flow direction: Flow separation at bends or sharp edges causes local pressure reduction.
• Improper pump operation: Low suction head (NPSH) or excessive speed can lead to cavitation.
• Temperature: Higher fluid temperatures increase vapor pressure, making cavitation more likely.

Thus, maintaining sufficient pressure above the vapor pressure of the liquid is necessary to prevent cavitation.

Types of Cavitation

1. Vaporous Cavitation:
   This occurs when the liquid vaporizes due to local pressure dropping below vapor pressure. It is the most common type seen in pumps and turbines.
2. Gaseous Cavitation:
   This happens when dissolved gases come out of the liquid due to pressure reduction. When pressure rises again, the gas bubbles may not collapse completely, causing less damage but reducing machine efficiency.

Effects of Cavitation

Cavitation has several harmful effects on hydraulic machines and fluid systems:

1. Metal Erosion and Pitting:
   The shock waves produced during bubble collapse cause small pits on the metal surface. Continuous action leads to rough surfaces and material removal.
2. Noise and Vibration:
   The repeated bubble collapses create loud noises similar to gravel striking metal and cause severe vibration, affecting machine stability.
3. Reduced Efficiency:
   Cavitation disrupts the smooth flow of liquid and reduces the energy transfer efficiency in pumps and turbines.
4. Structural Damage:
   Long-term cavitation can lead to cracking and failure of components like impeller blades, valves, or turbine runners.
5. Performance Deterioration:
   Cavitation changes flow patterns, leading to loss of head, reduced discharge, and unstable operation.

Detection of Cavitation

Early detection of cavitation is essential to prevent damage. Common indicators include:

• Unusual noise: A rattling or cracking sound similar to boiling or stones hitting metal.
• Vibration: Excessive vibration in pumps, turbines, or valves.
• Drop in efficiency: Reduced discharge, pressure head, or power output.
• Damage inspection: Pitted or eroded surfaces upon physical examination.

Prevention of Cavitation

To avoid cavitation, the following preventive measures should be adopted:

1. Maintain Proper Pressure:
   Ensure that the pressure in the system never falls below the vapor pressure of the liquid.
2. Increase Suction Head (NPSH):
   Provide sufficient Net Positive Suction Head (NPSH) in pumps by keeping suction pipes short and straight and reducing frictional losses.
3. Control Pump Speed:
   Avoid operating pumps at very high speeds that cause low pressure on the suction side.
4. Reduce Sharp Bends and Restrictions:
   Smooth flow passages prevent sudden pressure drops that initiate cavitation.
5. Lower Fluid Temperature:
   Cooling the liquid reduces its vapor pressure and lowers the chances of cavitation.
6. Use Cavitation-Resistant Materials:
   Impellers and turbine blades can be made from materials like stainless steel or bronze, which resist erosion and pitting.

Cavitation in Hydraulic Machines

1. In Pumps:
   Cavitation occurs mainly at the suction side of the impeller where pressure is low. It causes damage to impeller blades and loss of head.
2. In Turbines:
   Cavitation occurs at the outlet of turbine blades where pressure is lower than vapor pressure. It damages the runner blades and reduces efficiency.
3. In Propellers:
   Cavitation occurs on the rear surface of marine propellers when local pressure falls. It reduces propulsion efficiency and causes severe noise and vibration.

Engineering Importance of Cavitation

Understanding cavitation is crucial in the design of hydraulic systems because it affects performance, safety, and durability. Engineers calculate the NPSH required (Net Positive Suction Head) and ensure it is always greater than the NPSH available to prevent cavitation. Cavitation studies also help in selecting materials, optimizing blade design, and controlling flow velocity in hydraulic machines.

Conclusion

In conclusion, cavitation is the phenomenon of formation and collapse of vapor bubbles in a liquid when local pressure falls below the vapor pressure. It is undesirable in hydraulic machines because it causes erosion, noise, vibration, and efficiency loss. Cavitation can be prevented by maintaining sufficient suction head, reducing flow restrictions, and using proper materials. Understanding cavitation is essential in mechanical and fluid engineering to ensure smooth, safe, and efficient machine operation.