Question

Cavitation can be prevented by

• A. suitably designing the pump
• B. maintaining the suction head sufficiently greater than the vapor pressure
• C. maintaining the suction head equal to developed head
• D. maintaining the suction head lower than the vapor pressure

Hint:

To eliminate cavitation in any pumping installation: - Keep the inlet fluid pressure high. - Lower the suction lift height or reduce suction line friction losses. - Lower the fluid temperature to reduce its vapor pressure (\(P_v\)).

Answer 1

The Correct Option is B

Concept: Cavitation is a destructive phenomenon that occurs inside hydraulic machinery, such as pumps or turbines, when the local static pressure of the liquid drops below its vapor pressure (\(P_v\)) at the operating temperature. When this happens, the liquid boils locally, forming vapor bubbles. As these bubbles travel into higher-pressure zones within the impeller, they collapse rapidly. This violent collapse generates localized high-velocity micro-jets and extreme shock waves (up to thousands of atmospheres), causing severe pitting, structural erosion, loud noise, vibrations, and a drop in pump efficiency.

Step 1: Analyzing the fundamental criterion for cavitation prevention.
To prevent vapor bubbles from forming anywhere inside the pump passages, the absolute static pressure head of the liquid (\(H_s\)) at the inlet must always remain strictly above its saturation vapor pressure head (\(H_v\)): \[ H_{\text{local}} \gt H_v \implies \text{No boiling} \implies \text{No cavitation} \] This margin is measured using the Net Positive Suction Head (NPSH).

Step 2: Evaluating the system requirements (\(\text{NPSH}_A\) vs \(\text{NPSH}_R\)).
To prevent cavitation, the actual net pressure head available at the suction inlet from the external piping system (\(\text{NPSH}_{\text{available}}\)) must be greater than the minimum pressure head required by the pump's internal design (\(\text{NPSH}_{\text{required}}\)), with an added safety margin: \[ \text{NPSH}_{\text{available}} \gt \text{NPSH}_{\text{required}} \] This is achieved by keeping the total suction head sufficiently higher than the fluid's vapor pressure.

Step 3: Evaluating why alternative options fail.
Let's review the other options:

• Option (A) helps optimize internal flow, but cannot prevent cavitation if the external suction piping delivers fluid below its vapor pressure.

• Option (D) explicitly promotes cavitation by allowing the fluid pressure to drop below the vapor pressure, triggering widespread boiling.
Thus, maintaining the suction head sufficiently higher than the vapor pressure is the primary operating requirement.