In fact, once you understand parallel pumping and how to apply it properly, you will find that you don't need to use bigger inline or base-mounted pumps as often because two smaller pumps in parallel will handle the job just as well. Also, if you get stuck in a jam and your supply house doesn't have that one big pump you need to do the job, by applying parallel pumping you will be able to use two smaller stock pumps.

When two pumps are better than one

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.

For example, if you had a system with a heat loss of 200,000 Btu/h and you calculated the flow rate based upon a design temperature drop of 20_F, you would need to pump 20 8pm. And for argument's sake, let's say that at this flow rate, the system has a total pressure drop of 8'. At this point you have several choices:

1.you can select one pump that is capable of meeting the design conditions,
2.you can also pick one additional pump as 100% stand-by, or
3.you can pick two smaller pumps in parallel to meet the condition.

If you decide to use two smaller pumps in parallel, you could choose two B&G NRF22 pumps. Each NRF-22 is capable of pumping 10 8pm at 8'. When they are both piped into the system and turned on, they will provide a total flow rate of 20 8pm at 8' of head.

System Curves and Pump Curves

To really appreciate parallel pumping and all its benefits, we have to consider system curves and pump curves. A pump curve is the path that a pump has to operate on. In graph form, it tells us the performance of a pump in gallons per minute flow rate versus the head in feet. The curve is designed by the manufacturer and is 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 by using scale #5 on B&G's System Syzer. This scale states that "head will change as the square of the change in flow." For example, if you had a system pumping 5 8pm at 4' head loss and wanted to increase the flow rate to 10 8pm, the head loss would increase to 16' head. Using scale #5 from the System Syzer, you can plot a system curve that is specific to a given system. We need to know this information so we can determine the flow rate when only one of the pumps is operating.

What happens when one of the pumps shuts off?

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 our design example of 20 8pm at 8', we can build a system curve based on these points: (7 GPM-1'), (10 GPM-2'), (15 GPM-4.5'), (22GPM-9.5').