In pump engineering, the net positive suction head (NPSH) of a centrifugal pump is defined as the available suction-side head (static pressure head plus velocity head) referred to the pump datum, after subtracting the liquid’s vapor pressure head at the suction temperature. Which expression best captures this definition?

Introduction / Context:
NPSH (Net Positive Suction Head) is a critical concept in turbomachinery and fluid systems. It quantifies how far the local absolute pressure at a pump’s suction is above the liquid’s vapor pressure. Adequate NPSH prevents cavitation, performance loss, and impeller damage.

Given Data / Assumptions:

• Centrifugal pump operating with a liquid at a known suction temperature.
• Static pressure head and velocity head are evaluated at the pump suction flange (datum).
• Vapor pressure head corresponds to the same temperature as at suction.

Concept / Approach:

NPSH is defined on the suction side. The available head is the sum of the static pressure head and velocity head (and elevation if used consistently), minus the vapor pressure head. This expresses the margin against vaporization at the inlet. Engineers compare NPSHa (available) to NPSHr (required by pump) to ensure cavitation-free operation (NPSHa > NPSHr).

Step-by-Step Solution:

Verification / Alternative check:

Manufacturer curves specify NPSHr; field calculations of NPSHa use suction pressure readings corrected to absolute, velocity head, and the appropriate vapor pressure of the fluid at temperature. This agrees with standard pump handbooks and industry practice.

Why Other Options Are Wrong:

At discharge: NPSH is a suction-side criterion, not a discharge metric. At suction (without vapor pressure): Missing the essential subtraction of vapor pressure head. At discharge minus vapor pressure at discharge temperature: Location and temperature are incorrect for NPSH definition.

Common Pitfalls:

Forgetting to use absolute pressure; using gauge pressure without atmospheric correction; using vapor pressure at the wrong temperature; ignoring suction-line losses and resulting velocity head changes.

Final Answer:

At suction minus the vapor pressure head of the liquid at the suction temperature.