In a centrifugal pump installation, where should the regulating (throttling) valve be located to control discharge safely without inducing cavitation at the inlet?

Introduction:
Flow control in centrifugal pumps is commonly achieved by throttling. The location of the regulating valve is critical to avoid harmful low pressures that can cause cavitation and reduce pump life.

Given Data / Assumptions:

• Standard end-suction centrifugal pump.
• Ambient water temperature; NPSH margin must be preserved.
• Goal: control Q while maintaining safe suction conditions.

Concept / Approach:
Throttling on the delivery side raises back pressure and shifts the operating point along the pump curve, reducing discharge with minimal risk to inlet pressure. Throttling on the suction side would lower inlet pressure, reduce NPSH available, and promote cavitation—therefore it must be avoided.

Step-by-Step Solution:
1) Install valve on delivery line downstream of pump.2) Adjust opening to meet required flow/head.3) Monitor discharge pressure and vibration to stay in stable operating region.4) Keep suction line as short, straight, and unobstructed as possible.

Verification / Alternative check:
NPSH analysis shows suction throttling reduces NPSH available, increasing cavitation risk; delivery throttling does not affect inlet static pressure adversely.

Why Other Options Are Wrong:
Casing/impeller: no such control element is placed there.

Suction pipe: dangerous due to NPSH loss.

Foot valve: non-return at sump, not used for regulation.

Common Pitfalls:
Using suction valves for flow control; neglecting the role of variable-speed drives as a superior alternative when available.

Final Answer:
delivery pipe