Centrifugal pump characteristics: As the flow rate through a centrifugal pump increases from low to high values, how does the hydraulic efficiency typically vary?

Introduction / Context:
Hydraulic efficiency reflects how effectively a pump converts input energy into fluid energy, excluding mechanical and volumetric losses. Its variation with flow rate guides selection of the best efficiency point (BEP) for reliable operation.

Given Data / Assumptions:

• Single-stage centrifugal pump with typical radial-flow characteristics.
• Clear liquid and steady operation.
• Standard impeller and casing geometry.

Concept / Approach:
Hydraulic efficiency is low at very small flows due to recirculation and incidence losses, rises to a maximum near the BEP where flow aligns with blade angles, then falls at high flows due to increased shock, separation, and hydraulic losses at the impeller exit and casing tongue.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer pump curves show efficiency contours forming a ridge at BEP, confirming the increase-then-decrease trend with Q.

Why Other Options Are Wrong:

• Monotonic decrease/increase or constant: inconsistent with standard performance curves.
• Decrease then increase: opposite to observed behavior; low-flow efficiency increases toward BEP before dropping off.

Common Pitfalls:
Operating far from BEP increases vibration, noise, and bearing loads; users may misinterpret overall efficiency changes with motor and drive losses included.

Final Answer:
increases and then decreases