The minimum pressure head required to avoid cavitation is represented by which of the following?

• A. NPSH_available
• B. NPSH_required
• C. both
• D. neither

Answer: (C)

Cavitation risk arises from the suction head at the pump. You have two related quantities: how much head the system can supply at the suction (NPSH available, NPSHa) and how much head the pump needs to avoid cavitation (NPSH required, NPSHr). The safe operating condition is when the available head is at least equal to the required head: NPSHa ≥ NPSHr. The minimum head necessary to prevent cavitation, from the pump’s perspective, is NPSHr, while the head actually available from the system is NPSHa. In practice, both concepts are used together to assess cavitation risk, since you must ensure the system provides at least as much suction head as the pump requires. For example, if NPSHr is 4 m and NPSHa is 5 m, cavitation is avoided; if NPSHa drops to 3 m, cavitation would occur. Therefore, both quantities are relevant in describing the suction-head situation, which is why the answer reflects the involvement of both.