Short Answer:

Cavitation is a phenomenon that occurs in pumps, turbines, or propellers when the pressure of a liquid falls below its vapor pressure, causing the formation of vapor bubbles. These bubbles collapse violently when they move into a high-pressure region, producing shock waves.

This sudden collapse of bubbles can cause serious damage to pump parts such as the impeller or casing. Cavitation also reduces efficiency, causes vibration and noise, and can lead to complete pump failure if not properly controlled.

Detailed Explanation :

Cavitation

Cavitation is one of the most common and destructive problems found in hydraulic machines like pumps, turbines, and propellers. It takes place when the local pressure in a liquid drops below its vapor pressure, resulting in the formation of vapor-filled cavities or bubbles. These bubbles are carried by the flowing liquid to regions of higher pressure, where they collapse suddenly, creating intense shock waves.

These micro-implosions produce high local pressures and temperatures that damage metal surfaces and cause pitting, erosion, vibration, and noise. Cavitation not only shortens the life of equipment but also reduces its performance and efficiency.

Formation of Cavitation

The process of cavitation occurs in three main steps:

1. Reduction in Pressure:
   When the liquid flows through a section where the velocity increases suddenly, such as at the eye of a centrifugal pump impeller, the pressure in that region drops.
2. Formation of Vapor Bubbles:
   If the pressure falls below the liquid’s vapor pressure, vapor bubbles start forming. These bubbles contain vapor or gas.
3. Collapse of Bubbles:
   As these bubbles move into a region of higher pressure (for example, near the impeller blades or discharge section), they collapse violently, releasing energy that damages the surrounding surface.

This collapse generates localized high pressure (up to 1000 atmospheres) and high temperature (up to 1000°C), which leads to severe pitting and erosion of metal surfaces.

Types of Cavitation

1. Suction Cavitation:
   It occurs when the pump suction pressure is too low or the suction line is blocked. This leads to vapor bubble formation at the pump inlet, which collapse as they reach higher pressure zones.
2. Discharge Cavitation:
   It happens when the discharge pressure is too high, restricting fluid flow and causing vapor bubbles to form in the discharge area.
3. Vaporous Cavitation:
   It occurs when the liquid temperature is near its boiling point, making it easier for vapor bubbles to form.
4. Gaseous Cavitation:
   It occurs due to the release of dissolved gases in the liquid, forming bubbles that collapse similarly to vapor bubbles.

Causes of Cavitation

• Low suction head or high suction lift
• High fluid temperature that lowers vapor pressure
• Obstructed suction pipe causing pressure drop
• Improper pump design or speed
• Air leakage into the suction line
• Inadequate Net Positive Suction Head (NPSH) available

Effects of Cavitation

1. Erosion and Pitting:
   The most visible effect is the pitting of metal surfaces, especially on the impeller blades or casing.
2. Loss of Efficiency:
   Cavitation reduces the flow of liquid and increases turbulence, leading to poor pump performance.
3. Vibration and Noise:
   The collapse of bubbles produces a characteristic crackling or rumbling noise, often described as “gravel noise.”
4. Damage to Components:
   Continuous cavitation leads to cracks, corrosion, and weakening of mechanical parts, resulting in costly repairs.
5. Reduced Lifespan:
   The wear caused by cavitation significantly shortens the service life of the pump.

Detection of Cavitation

Cavitation can be detected by observing the following signs during pump operation:

• Sudden drop in discharge pressure or flow rate
• Loud crackling or rattling noise
• Unusual vibrations in the pump
• Decrease in pump efficiency
• Visible damage to the impeller after inspection

Prevention of Cavitation

1. Increase Suction Pressure:
   Maintain sufficient suction head to prevent pressure from dropping below vapor pressure.
2. Reduce Pump Speed:
   Slower speeds reduce velocity and thus avoid sudden pressure drops.
3. Lower Fluid Temperature:
   Cooler liquids have higher vapor pressure, reducing the chances of vapor bubble formation.
4. Proper Pump Selection:
   Choose pumps that match system requirements and have the required Net Positive Suction Head (NPSH) available.
5. Smooth Flow Design:
   Avoid sharp bends, sudden contractions, or restrictions in the suction line.
6. Use Cavitation-Resistant Materials:
   Materials like stainless steel or bronze can withstand the erosion caused by bubble collapse.
7. Regular Maintenance:
   Cleaning filters and checking for suction leaks help prevent conditions that lead to cavitation.

Mathematical Relation

Cavitation is often linked to the concept of Net Positive Suction Head (NPSH), which must be sufficient to prevent vapor formation.

Where,

•  = Atmospheric head
•  = Vapor pressure head
•  = Suction lift
•  = Friction head loss

If the available NPSH is lower than the required NPSH, cavitation will occur.

Importance in Engineering

Understanding and controlling cavitation is essential for ensuring the longevity and efficiency of hydraulic machines. Engineers must carefully design suction systems, maintain proper NPSH, and choose suitable materials to avoid the harmful effects of cavitation. In modern designs, computational fluid dynamics (CFD) is also used to study flow patterns and prevent low-pressure zones.

Conclusion

Cavitation is a destructive hydraulic phenomenon caused by the formation and collapse of vapor bubbles when pressure drops below the vapor pressure of the liquid. It leads to erosion, noise, vibration, and reduced efficiency of pumps and turbines. Preventing cavitation requires maintaining adequate suction pressure, reducing flow restrictions, and ensuring correct pump design. Proper understanding and control of cavitation are essential for reliable, safe, and efficient operation of hydraulic systems.