Centrifugal pumps — effect of throttling discharge valve When the discharge of a centrifugal pump is throttled on the delivery side, what happens to the suction lift (all else unchanged)?

Introduction / Context:
Operators often adjust flow by throttling the discharge valve of a centrifugal pump. Understanding which variables change (flow, head, power) and which remain essentially unaffected (suction lift/NPSH available) is a key field skill.

Given Data / Assumptions:

• Pump speed and suction piping conditions remain constant.
• Discharge valve is partially closed to reduce flow.
• Suction source level and vapor pressure do not change.

Concept / Approach:
Throttling a centrifugal pump reduces flow and moves the operating point along the pump curve to higher head and typically lower power. Suction lift (static suction head/height) depends on liquid level and piping, not on the discharge valve setting. NPSH required usually decreases as flow decreases; NPSH available remains determined by system suction conditions.

Step-by-Step Solution:
Close discharge valve slightly → flow decreases.Operating head increases modestly, pump power often drops.Suction side geometry and levels unchanged → suction lift remains the same.

Verification / Alternative check:
Consult typical pump performance curves: throttling shifts along the same speed curve; suction static head is independent of discharge throttling.

Why Other Options Are Wrong:

• Increases/decreases: would require change in suction level, friction, or vapor conditions—not caused by discharge throttling alone.
• Data insufficient: standard practice shows negligible effect on suction lift.

Common Pitfalls:

• Confusing suction lift with NPSH required; the latter can vary with flow, but the static lift does not.

Final Answer:
remains unchanged