Cavitation occurs when NPSHa and NPSHr satisfy which condition?

• A. Cavitation occurs when NPSHa > NPSHr.
• B. Cavitation occurs when NPSHa < NPSHr.
• C. Cavitation occurs when NPSHr equals atmospheric pressure.
• D. Cavitation occurs only at zero flow.

Answer: (B)

Cavitation happens when the pressure at the pump suction is low enough to cause the liquid to vaporize. In suction-head terms, that means the available suction head (NPSHa) is less than the head the pump needs to avoid cavitation (NPSHr). When NPSHa < NPSHr, vapor bubbles form and can collapse as they move into higher-pressure regions, leading to noise, vibration, and potential damage. If NPSHa > NPSHr, there’s enough pressure to keep the liquid from boiling, so cavitation does not occur. When NPSHa equals NPSHr you’re at the onset of cavitation. Cavitation can happen at many flow rates, not just zero flow.