Centrifugal pump fundamentals — need for priming: A centrifugal pump must be primed before starting when it is installed at which relative level to the water surface in the source reservoir?

Introduction / Context:
Centrifugal pumps cannot handle air effectively and require the casing and suction line to be filled with liquid to create the necessary suction at startup. This initial filling is called priming and is critical when the pump is above the water level (suction lift installation).

Given Data / Assumptions:

• Standard end-suction centrifugal pump.
• Source reservoir open to atmosphere.
• No foot-valve or self-priming feature unless stated.

Concept / Approach:
When the pump is above the water level, the suction line initially contains air. The impeller cannot evacuate air to create vacuum unless primed. Priming fills the line and casing with water, enabling atmospheric pressure to push water up the suction line. If the pump is below water level (positive suction head), the line remains flooded and priming is inherently satisfied.

Step-by-Step Solution:

Verification / Alternative check:
Self-priming pumps or foot-valves reduce the need for manual priming; however, the general rule remains that pumps above source level require priming.

Why Other Options Are Wrong:
Option b: with positive suction head, priming is naturally satisfied; options c/d ignore the physics; “only when discharge valve is open” is unrelated to priming necessity.

Common Pitfalls:
Starting a dry pump; air leaks on suction side preventing prime; ignoring NPSH requirements leading to cavitation.

Final Answer:
At a higher level than the reservoir water surface (suction lift condition)