Parallel Pumping: When Two Pumps Are Better Than One

A colleague and I were recently talking about the benefits of parallel pumping and how cost-effective and efficient it can be for a wide variety of applications. Yet, for some reason, the benefits of parallel pumping seem to be known mainly for large flow rate applications such as chilled water distribution systems. This is unfortunate because there are many benefits to be realized for the “smaller” applications as well.

In fact, once you look closely at parallel pumping and how to apply it properly, you will find that you don’t need to use bigger in-line or base-mounted pumps as often because two smaller pumps in parallel will handle the job just as well. Not to mention that if you get stuck in a jam and cannot find the big pump you need to do the job; by applying parallel pumping you will be able to use two smaller stock pumps.

Here are some ‘refresher’ truths about parallel pumping:

  • For any given discharge head, flows for parallel pumps are additive
  • The system flow rate will be determined by the intersection of the system-head curve and the performance curve of the parallel pumps
  • Pumps of different hydraulic characteristics may be operated in parallel to the extent that they share common discharge head characteristics
  • Pumps of different hydraulic characteristics may encounter severe problems when operated in parallel
  • All pumps have different hydraulic characteristics
  • To produce flow, a pump must generate a greater discharge pressure at start-up than the pressure already present in the system

When two pumps are better than one

As you know, parallel pumping involves installing two circulating pumps in a piping system in parallel with each other. When selected properly, each circulator will pump half of the total required flow rate at the design head loss. This means that each pump is capable of pumping half of the gallons per minute needed at the total designed pressure drop for an application.

Why Bother With Parallel Pumps?

  • Using parallel pumps can be beneficial because installation often costs less to buy, install, and maintain compared to a single large pump
  • Takes less space in the equipment room (especially if one utilizes in-line pumps that may be stacked)
  • Uses less energy
  • Provides better than 50 percent flow redundancy during single pump operation

However, small systems or systems that rarely change flow may not be good candidates for parallel pumps. Of course, if the system requires 100 percent redundancy, the designer must provide a single pump system with another 100 percent standby pump.

Parallel pumps for two systems. Curve S1 represents a friction-only system, while Curve S2 represents a system that has fixed and variable head.

System Curves and Pump Curves

To really appreciate parallel pumping and all its benefits, you have to consider system curves and pump curves. Pump curves as we know, are designed by the manufacturer and are based on the horsepower, the diameter of the impeller and the shape of its volute (the wet end of the pump that contains the impeller). No matter what the conditions of a system, the pump has to operate somewhere on this curve.

System curves, on the other hand, represent the flow-head relationships that exist for particular installations. For any given system, once a design condition is calculated, you can establish other flow and head conditions. You can use a software program such as System Syzer® Calculator to determine the flow rate.

One major benefit of parallel pumping is the high degree of standby capacity provided by single pump operation. When one pump is out of operation, the other pump continues to pump water through the system. But the flow rate isn’t cut in half just because only one pump is operating. Remember, the pump has to operate at the intersection of its pump curve with the already determined system curve.

In summary, operating pumps in parallel is viable providing that it is done with a full understanding of the individual characteristics of the pumps involved and the ability to monitor or ensure minimum flow thresholds are met for each pump.

Parallel Pumping can be a great way to save energy and money.

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