Short Answer:

Cavitation in a pump is a condition where vapor bubbles form in the liquid inside the pump due to low pressure and then suddenly collapse when they reach a high-pressure area. This causes loud noise, vibration, and damage to pump parts like the impeller.

To prevent cavitation, the suction pressure must be kept high enough, and the pump should be installed and operated correctly. Proper priming, avoiding high suction lift, using the right pump type, and ensuring smooth fluid flow can help avoid cavitation and extend pump life.

Detailed Explanation

Cavitation in a Pump 

Cavitation is a harmful phenomenon that occurs in pumps when the pressure in the liquid falls below its vapor pressure, causing the formation of vapor bubbles. These bubbles are carried along with the flowing liquid and suddenly collapse when they enter a high-pressure zone inside the pump. This collapse generates intense energy, leading to noise, vibration, pitting, and even serious damage to internal parts like the impeller and casing.

Cavitation not only reduces pump efficiency but also shortens the life of the pump. It is one of the most common problems in pump systems and must be understood and controlled carefully, especially in civil engineering applications like water supply, irrigation, and sewage systems.

Causes of Cavitation

1. Low Suction Pressure
   When the pressure at the pump inlet is too low, it may fall below the vapor pressure of the liquid, causing vapor bubbles to form.
2. High Suction Lift
   If the pump is installed too far above the water source, the high vertical lift reduces inlet pressure and encourages cavitation.
3. High Fluid Temperature
   Warmer fluids have lower vapor pressure, which makes it easier for cavitation to occur.
4. Obstruction in Suction Line
   Blocked or narrowed suction pipes restrict flow and reduce pressure, leading to cavitation.
5. Pump Running Too Fast
   A very high pump speed can lower the pressure at the eye of the impeller, causing vaporization of the liquid.

Effects of Cavitation

• Loud noise similar to gravel hitting metal.
• Strong vibrations that damage bearings and seals.
• Erosion or pitting on impeller blades.
• Drop in pump performance and flow.
• Total pump failure if not treated.

Prevention of Cavitation

1. Maintain Proper Suction Head
   Ensure Net Positive Suction Head Available (NPSHA) is always greater than Net Positive Suction Head Required (NPSHR). This keeps pressure above the vapor pressure of the fluid.
2. Lower the Pump Position
   Place the pump closer to the water source to reduce suction lift and maintain higher inlet pressure.
3. Reduce Pump Speed
   Operating the pump at the manufacturer-recommended speed helps avoid pressure drops at the impeller eye.
4. Avoid Sharp Bends or Restrictions in Suction Pipe
   A straight, short, and properly sized suction pipe allows smooth fluid flow and maintains pressure.
5. Use of Inducers or Booster Pumps
   Inducers help increase pressure at the impeller inlet, and booster pumps provide additional suction pressure.
6. Proper Priming and Regular Maintenance
   Keep the pump fully primed and free from air pockets. Clean filters and suction lines to avoid clogs.
7. Monitor Fluid Temperature
   Avoid pumping liquids at very high temperatures unless the system is designed for it.

Conclusion

Cavitation in a pump is the formation and sudden collapse of vapor bubbles due to low pressure, leading to noise, vibration, and severe damage. It can be prevented by maintaining sufficient suction head, avoiding restrictions in the suction line, keeping the pump well-primed, and operating it at the correct speed. Understanding and controlling cavitation is essential for long-lasting and efficient pump performance in civil engineering systems.