Why must NPSHa exceed NPSHr in pumping applications?

• A. To minimize energy use.
• B. To prevent cavitation and maintain reliable flow.
• C. To increase the pump speed.
• D. To ensure the pump head remains unchanged.

Answer: (B)

Preventing cavitation is the core idea here. NPSHa must exceed NPSHr because when there is more suction head available than what the pump requires, the pressure at the suction face stays above the liquid's vapor pressure. That keeps the liquid from forming vapor pockets as it enters the impeller, so cavitation can't start. Cavitation damages the impeller, reduces flow and head, and increases vibration and noise, compromising reliability. Having a safe margin also allows consistent performance across operating conditions. The other options don't address the risk of vapor formation: energy use isn't the primary reason, increasing pump speed doesn't fix the suction limit, and keeping pump head unchanged isn't controlled by the NPSH comparison.