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Centrifugal Pumps – When and How to Use Parallel Pumping

Parallel Pump Operation

One of the advantages of parallel pumping is the ability to shut one of the pumps off and operate with one pump most of the season. When a pump stages off, the operation of the remaining pump or pumps will move out on their curves to a higher flow. This flow is determined by the system curve. When a single pump is operating, the system curve shows the pump will flow much more than 50% of the design flow rate.

Why Use Parallel Pumping?

A single variable speed pump selected for the total design flow rate is simple and saves energy. There are situations where a single pump operating may not be the best choice. Here are five examples:

Standby Capacity Without a Standby Pump

When a parallel pump is turned off or destaged, the operating pumps will run out on their curves. They will supply a higher flow rate than if they were selected for at design. It will not be 100% but it will be significant. If your client does not require 100% standby and the project budget is important, parallel pumping may be a solution. If a pump is offline, the remaining two pumps will produce 84% of the design flow rate. Remember that in hydronic systems, an 84% flow rate provides more heat transfer than 84%.

Project First Cost Savings

When you select parallel pumps, they are smaller. The flow rate is a fraction of the total system design flow rate. Each pump will, normally, end up being smaller with a smaller horsepower. Smaller pumps mean less cost. In addition, the piping components of each pump are sized for a lower flow rate and a smaller pipe size. Smaller pipes mean less cost.

Operation Near Design Flow Rate May Never Happen

The perfect example of this is plumbing pressure boosters. Every plumbing engineer knows that the standards used in determining a cold-water flow rate are very safe. Rarely will there be a standby pump on a multiple pump pressure booster. The two, three or four pumps are staged in parallel with all the pumps designed to operate. If your system design is too safe, parallel pumping may be an answer.

Future Additions to the System

The owner expects future additions but is not clear when and even if the addition will happen. Design the system for parallel pumping in the future. Leave the room and place the housekeeping pad for a future pump. Use a single pump now with a standby. Add another pump in the future.

System Minimum Flow is Very Low

Most centrifugal pumps have a required minimum flow rate. The minimum flow rate gets larger as the pump gets larger. A single 3000 GPM pump selection would have a minimum flow rate required of about 400 GPM at the minimum speed. The minimum flow rate at minimum speed of the three-pump parallel design would drop to under 100 GPM.

Staging of Parallel Pumps and EOC Protection

Centrifugal pump end of curve (EOC) operating points is an important thing to note when using parallel pumps. Assume the chilled water or heating system follows the system curve. The single pump operation has the operating point off the end of the curve. The staging controls must recognize this and increase the speed of the pump to allow operation on the pump curve. This minimum speed point must then disable during the two-pump operation and establish a new EOC point. Systems do not always follow a theoretical system curve. The controls must protect the pump in all system conditions.

The Bell & Gossett PPS Parallel Pump Controller is a perfect solution to the proper and safe operation of parallel pumps.

Next week, we will introduce series pumping. This is not as common as parallel pumping but has its advantages in applications.