Cavitation Damage in Pumps
(a) How Cavitation Damage Occurs:
Cavitation damage within a pump is a destructive process caused by the formation, growth, and collapse of vapor bubbles in the liquid being pumped. Here’s a breakdown of the phenomenon:
- Pressure Drop: As liquid flows through the pump, pressure decreases in specific areas, particularly at the inlet of the impeller or around the leading edges of the impeller vanes.
- Bubble Formation: When the pressure drops below the liquid’s vapor pressure at a specific temperature, dissolved gases and vapor pockets within the liquid begin to form small vapor bubbles.
- Bubble Growth: As the low-pressure zone persists, the vapor bubbles expand in size.
- Bubble Collapse: When the liquid flows to a higher pressure zone within the pump (e.g., discharge side of the impeller), the pressure on the bubbles rapidly increases. This causes the vapor bubbles to implode violently.
- Shockwaves and Erosion: The rapid collapse of the bubbles generates intense shockwaves that travel through the liquid. These shockwaves can damage the pump components, particularly the impeller vanes and housing, by causing erosion, pitting, and material fatigue.
Over time, continuous cavitation can lead to significant wear and tear on the pump, reducing its efficiency, increasing noise and vibration, and potentially causing complete pump failure.
(b) Operational Causes of Increased Cavitation:
(i) Low Suction Pressure:
- Reason: If the pressure at the pump inlet (suction side) drops too low, it creates a larger pressure differential within the pump, promoting more extensive bubble formation and collapse. This can occur due to:
- Clogged inlet filter or piping: Restrictions on the suction side increase resistance to flow, leading to a lower pressure at the pump inlet.
- Pumping from a deep sump: The higher the suction lift (vertical distance between the pump and the liquid source), the lower the pressure at the pump inlet.
- Insufficient available NPSH (Net Positive Suction Head): NPSH is a parameter that considers the available pressure head at the pump inlet and the vapor pressure of the liquid. If the available NPSH is lower than the pump’s required NPSH, cavitation is more likely to occur.
(ii) High Operating Speed:
- Reason: Increasing the pump speed (RPM) can also exacerbate cavitation. At higher speeds, the pressure drop across the impeller vanes becomes more significant, creating conditions more favorable for bubble formation and collapse. This can happen due to:
- Operating the pump above its design speed: Running the pump faster than recommended can lead to cavitation issues.
Increased system demand requiring higher flow rates: If the system demands a higher flow rate than the pump’s design capacity, the pump might need to operate at a higher speed to compensate. This can increase the risk of cavitation.