Optimizing Pump Operation with VFDs: Halving Total Flow in Parallel Pumps

When operating two pumps in parallel with Variable Frequency Drives (VFDs), reducing the total flow by half can be achieved through different strategies. Here’s a detailed breakdown of the two options you mentioned, along with considerations for reliability, maintenance, and energy efficiency.

Reducing the RPM of Both Pumps by Half

If you reduce the RPM of both pumps by half, the flow from each pump will essentially decrease. However, the relationship between pump speed and flow is typically not linear. According to the affinity laws, flow is proportional to the speed (RPM). Reducing the speed of both pumps to 50% will result in approximately 25% of their original flow. Therefore, the total flow from both pumps would be around 50% of the original flow, achieving the desired reduction.

This approach can lead to better energy efficiency as running the pumps at lower speeds generally consumes less energy than running one at full speed. Additionally, this can help in maintaining a more balanced operation, which is beneficial for overall system performance.

Running One Pump at Full Speed

Alternately, running one pump at full speed while turning off the other pump can also achieve the desired total flow reduction. However, this method could lead to higher energy consumption and potentially more wear on the pump that is running at full speed. It is important to consider the pump's capacity and whether it can handle the reduced flow requirement without undue stress.

If the system is designed for two pumps, running one alone may not be optimal unless the pump’s capacity is sufficient to meet the reduced flow demand without significant loss in efficiency or increased risks of mechanical failure.

Conclusion

In summary, running both pumps at reduced RPMs is generally more energy-efficient and can lead to a more balanced operation. However, if one pump can handle the required flow at full speed without issues, this could also be a viable option. The best choice often depends on the specific system characteristics, including pump curves, efficiency considerations, and operational requirements.

Reliability and Maintenance

Connecting centrifugal pumps in parallel is primarily aimed at increasing the discharge flow above each individual pump. Having VFDs in operation allows you to adjust the speed of each pump based on the required discharge flow and optimal pump pressure head, ensuring minimal operating costs. When operating two pumps together, either by altering their speeds or halving each pump's speed, this is not the most economical or maintenance-friendly approach. A single pump is typically required to give the necessary discharge flow, and it is more even in terms of maintenance and operational efficiency to have one pump as a standby.

Key Takeaways

Reduce total flow by halving pump RPM for better energy efficiency and balanced operation. Consider higher energy consumption and pump wear when running one pump at full speed. Opt for a single pump at full speed if capacity allows and ensure reliability and maintenance.

Final Thoughts

By carefully considering the benefits and drawbacks of each approach, you can make an informed decision that best meets your specific needs in terms of energy efficiency, reliability, and maintenance. Always ensure that the chosen method aligns with your operational requirements and the characteristics of your system.