Q13Basic Mechanical Engineering

Understanding Cavitation: Cavitation is a severe phenomenon in centrifugal pumps. If the absolute pressure of the liquid flowing into the eye of the impeller drops below its vapor pressure at the operating temperature, the liquid will spontaneously boil, forming thousands of tiny vapor bubbles. As these bubbles are swept into the higher-pressure regions of the impeller, they violently collapse (implode). This generates microscopic, supersonic shockwaves that erode the metal impeller like a sandblaster, causing intense noise, severe vibration, and rapid mechanical failure.

Elimination Strategies: To eliminate cavitation, the pressure at the pump inlet must be kept artificially high. This is governed by ensuring $NPS{H}_{available}>NPS{H}_{required}$.

• 1. Reduce Suction Lift: The most effective method is to physically place the pump closer to the water level. Ideally, the pump should be installed below the liquid level (flooded suction) so gravity pushes liquid into the pump.
• 2. Reduce Friction Losses: The suction pipe should be kept as short and straight as possible. Avoid using unnecessary elbows, valves, or strainers on the suction side. Increase the diameter of the suction pipe to lower fluid velocity and friction.
• 3. Decrease Liquid Temperature: Hotter liquids have higher vapor pressures, making them boil easier. Cooling the liquid before pumping prevents bubble formation.
• 4. Operate at Design Flow: Avoid running the pump at excessive speeds or capacities far beyond its Best Efficiency Point (BEP), which heavily increases $NPS{H}_{required}$.