What happens when NPSHa equals NPSHr?

• A. Cavitation just begins to occur (onset threshold).
• B. Cavitation is completely prevented.
• C. Flow stops.
• D. There is no flow.

Answer: (A)

At the point where NPSHa equals NPSHr, you’re at the cavitation inception threshold. NPSH available (NPSHa) is the head present at the suction that can keep the liquid from boiling, while NPSH required (NPSHr) is the minimum head the pump needs to avoid cavitation at the operating flow. When these two are equal, the suction pressure is just enough to reach the liquid’s vapor pressure; any small disturbance—such as a momentary drop in suction pressure, a temperature rise in the liquid, or a transient surge—can push local pressures below the vapor pressure and start forming vapor bubbles. There is no safety margin, so cavitation begins at this point. To prevent cavitation, you want the available head to exceed the required head by a comfortable margin.

The other statements aren’t correct because cavitation is not completely prevented at equality, flow hasn’t inherently stopped, and there isn’t a situation of “no flow” simply because the threshold is reached.