Centrifugal Pumps - Cavitation

⚙️ Centrifugal Pumps – Cavitation

Cavitation is one of the most common and damaging phenomena in centrifugal pumps. It occurs when the pressure in the pump falls below the vapor pressure of the liquid at a given temperature, causing the liquid to boil and form vapor bubbles. These bubbles collapse violently when they move into regions of higher pressure, producing shock waves that can erode pump components and reduce efficiency.

🔹 What is Cavitation?

- Definition: Local vaporization of liquid inside the pump due to pressure dropping below vapor pressure.  

- Process:  

  1. Vapor bubbles form in low‑pressure zones (usually at the suction side).  

  2. Bubbles grow as liquid velocity increases and static pressure decreases.  

  3. When transported to higher pressure zones, bubbles implode.  

  4. Collapse generates extremely high localized pressures, causing noise, vibration, and erosion.  

🔹 Effects of Cavitation on Pumps

- Noise and Vibration: Cavitation produces a characteristic “gravel‑like” noise.  

- Surface Erosion: Metal surfaces of the impeller and casing are damaged by bubble collapse.  

- Pressure Fluctuations: Discharge pressure becomes unstable due to varying fluid density.  

- Power Variations: Pump power absorbed fluctuates, visible on ammeter readings.  

- Reduced Efficiency: Cavitation lowers pump performance and shortens equipment life.  

🔹 Where Cavitation Occurs

- Primarily at the pump suction side, where liquid velocity rises rapidly and static pressure falls.  

- Can also occur in areas of high turbulence or restrictions in the suction piping.  

🔹 How to Reduce Cavitation

1. Improve Suction Conditions  

   - Reduce suction pipe friction losses.  

   - Keep suction piping short, straight, and properly sized.  

2. Increase Net Positive Suction Head (NPSH)  

   - Ensure NPSHa > NPSHr (available suction head greater than required).  

   - Raise suction tank level or reduce suction lift.  

3. Control Pump Flow Rate  

   - Avoid operating pumps at excessive flow rates.  

   - Use throttling or variable speed drives to maintain optimal conditions.  

4. Proper Start‑Up Procedure  

   - Start the pump against a closed discharge valve to minimize losses during startup.  

🔹 Key Takeaways

- Cavitation occurs when pressure < vapor pressure, forming bubbles that collapse violently.  

- It causes noise, vibration, erosion, and efficiency loss in centrifugal pumps.  

- Prevention involves optimizing suction conditions, reducing friction losses, and ensuring NPSHa exceeds NPSHr.  

- Proper design, operation, and maintenance are essential to avoid cavitation damage.