Which statement best describes NPSHa in relation to cavitation risk?

• A. NPSHa is the maximum pressure head available to avoid cavitation.
• B. NPSHa is the actual head available at the pump suction, considering atmospheric, suction static, suction losses, and vapor pressure head.
• C. NPSHa is always greater than NPSHr.
• D. NPSHa depends only on atmospheric pressure.

Answer: (B)

The concept being tested is what Net Positive Suction Head available means and how it relates to cavitation risk. NPSHa represents the actual pressure head available at the pump suction to keep the liquid from flashing into vapor. It isn’t a fixed maximum; it depends on the current conditions in the suction path. Specifically, it combines the atmospheric pressure head, the suction static head, the losses in the suction line, and the vapor pressure head of the liquid. If the suction pressure falls toward the liquid’s vapor pressure, cavitation can begin, so you compare NPSHa to what the pump itself requires, NPSHr. The best description is the one that names all these factors: atmospheric pressure head plus suction static head minus suction losses minus the vapor pressure head. The other statements misstate what NPSHa represents or its relationship to cavitation risk: it is not just atmospheric pressure, it is not a fixed maximum, and it is not guaranteed to be greater than NPSHr in all cases.