Brief Description of the Impact of Selecting Over-Sized or Under-Sized Pumps

Preface

Engineers often worry about being overly conservative in system design. Many uncertainties exist during the design process, including differences in actual operating conditions, changes in fluid properties, equipment aging over time, pipeline scaling, etc. Engineers use design factors to account for these variables and prevent selecting/purchasing under-sized equipment. These factors also address the impact of aging on the system.

However, some engineers fail to consider that applying excessive safety margins in design can actually increase wear on flow components and shorten system life, particularly when sizing pumps.

The primary goal in selecting pump and pipeline systems is typically to achieve the required pressure (head) and flow rate for the application, whether in cooling water systems, fuel transfer pipelines, chemical production plants, or many other applications. The required system flow rate is used to determine the pump's total dynamic head, which is then compared against the performance curves of all available pump models. Choosing an over-sized or under-sized pump can have severe impacts on the system, highlighting the importance of using an accurate operating point in this process.

Impacts of Over-Sizing or Under-Sizing

· Under-Sized Pumps:
Inadequate pump sizing results in flow rates below requirements. This may necessitate adding extra pumps or adjusting the system (e.g., opening discharge valves to force the pump to operate at higher-than-rated conditions).

· Over-Sized Pumps:
Over-sized pumps deliver more flow than needed. Depending on the application, throttling valves or impeller trimming may be required to reduce flow. While adjusting system valves might seem to correct the operating point, the impact becomes more significant when considering pump efficiency.

Best Efficiency Point (BEP)

The BEP is the ideal operating point where the maximum percentage of energy used to run the pump is transferred to the fluid. When the pump’s operating point deviates from the BEP:

· Efficiency decreases, requiring more drive power. Unused energy is released as heat or vibration.

· Increased vibration and heat generation occur; the further the deviation from BEP, the more severe the effects.

Centrifugal pump standards (e.g., Hydraulic Institute standard HI 9.6.3) typically recommend operating pumps within 80% to 110% of BEP to avoid these issues:

· >110% of BEP: Risk of cavitation due to low Net Positive Suction Head (NPSH) margin, plus vibration/heat damage.

· <80% of BEP: Risks include impeller stalling, low-flow cavitation, internal recirculation, and overheating.

· Long-term effects: Higher maintenance costs, increased energy consumption, and shorter pump lifespan.

Key Considerations to Avoid Mis-Sizing

1. 

Prudent Determination of Design Factors

2. 

· Clarify how much design margin is needed and when to apply it. Stakeholders (system designers, project managers, pump manufacturers) must align on assumptions and boundary conditions to avoid unreasonable safety margins.

3. 

Account for Operating Limits

4. 

· Ensure sizing calculations and design factors consider extreme operating conditions (e.g., peak flow).

· Select pumps that can operate close to BEP under all conditions, not just extreme scenarios. For systems with variable demands, consider Variable Frequency Drives (VFDs) to maintain optimal operation.

Conclusion

Designing a system that meets operational requirements is critical, but "bigger is better" is not always optimal. Effective design reduces material and installation costs while preventing excessive wear on components like pumps. Balancing accuracy with practical safety margins ensures reliable, efficient, and cost-effective system performance.