In a pump, the difference between the actual pressure, or head, of a liquid available (measured on the suction side) and the vapor pressure of that liquid is called Net Positive Suction Head (NPSH) available. The minimum head required to prevent cavitation with a given liquid at a given flow rate is called Net Positive Suction Head __________. Inadequate NPSH can result in cavitation.

In fluid dynamics and pump system design, cavitation occurs when the local static pressure inside a pump drops below the vapor pressure of the liquid, causing vapor bubbles to form and violently collapse. To prevent cavitation damage, the pressure available at the pump suction nozzle, known as Net Positive Suction Head Available (NPSHa), must exceed the minimum margin required by the pump design. This minimum threshold is defined as Net Positive Suction Head Required (NPSHr). NPSHr is a characteristic determined by pump manufacturers through empirical testing at specific flow rates. If NPSHa falls below NPSHr, vapor bubbles form and collapse against impeller surfaces, causing localized high-impact pressure waves that result in metal loss and severe pitting.

A. Surplus is not an official engineering term for pump NPSH requirements; excess head is described simply as the NPSH margin (NPSHa minus NPSHr).
C. Reserve is an informal term and not the recognized standard pump engineering term defined by hydraulic standards or damage mechanism codes for required suction head.
D. Option B is the correct, standard technical term defined in fluid machinery standards and API 571 cavitation descriptions, making "None of the above" incorrect.

1. API Recommended Practice 571, "Damage Mechanisms Affecting Fixed Equipment in the Refining Industry," Third Edition, March 2020, Section 3.16, Cavitation.

2. Hydraulic Institute Standards (HI 1.3), "Rotodynamic Pumps for Design and Application," 2019, Section 1.3.5.

3. Igor J. Karassik et al., "Pump Handbook," 4th Edition, McGraw-Hill, 2008, Chapter 2.1, Centrifugal Pump Theory.