Mailbag: Parallel Pumps

Hello Bill,

The simple and theoretical answer to your question is: Two identical pumps with best efficiency rated at 100 feet of head @ 840 GPM each, installed in a parallel system, will deliver the same head (100 ft.) at double the flow (1,680 GPM). However, the real answer is more involved.

It depends on the profile of the system curve. These pumps will perform on their parallel curves where the system makes the pumps perform. The system controls the pumps. It is most important that the system – meaning the suction and discharge pipe arrangement with fittings and accessories – is designed to accommodate double the flow (1,680 GPM) at the same head (100 ft.). What does this mean?

It means the suction pipe arrangement must supply adequate energy to the suction nozzles of both (or all) pumps. Consult your pump performance curves, and look for the NPSHr at the duty coordinates. The energy arriving into the impeller eye must be greater than the pump’s energy requirement (the NPSHr).

Be aware, the pump curve will read in feet of head. If you use instrumentation on the pump, the gauges will record pressure in PSI. For cold water, there should be 1-2 PSI safety margin, or 2-5 feet of head safety margin.
If the suction energy is inadequate, your pumps may not perform on the curve. This means the information on your gauges will not coincide with the information on the curve.

The discharge pipe arrangement with fittings and accessories must provide 100 ft. of resistance at a flow of 1,680 GPM. If the resistance is too high, the pumps will back-up on their curves. This is similar to four lanes of traffic trying to squeeze onto a two-lane roadway. The cars will back up and form a traffic jam.

With high resistance in the discharge pipe arrangement, the pumps will store energy. This stored energy expresses itself as vibrations, heat and maintenance. It is really important to design the system for two pumps operating together.
With inadequate resistance in the discharge pipe arrangement, the pumps will deliver high flow with insufficient head. It can lead to cavitation and may overload the motors. System design (and the profile of the system curve) is more important than most people think.

It is always nice when the pump’s duty coordinates are also the best efficiency coordinates. If you do not have your pump curve, then contact your pump distributor and demand the performance curve. If you can’t get good service from your local pump distributor, maybe you can download the curve from the pump manufacturer’s website. You need the performance curve.

In your communication to me, you already said your liquid is cold water. Your pump curves are generated on cold water, so you are already one step ahead of the game. The information on the gauges should coincide with the information on the curve.

To answer the second and third parts of your email: Yes, the head, flow and horsepower will change if the system changes. You will see how they will change on the pump curve.

These are many words to describe a relatively simple concept. We cover pump curves and system curves in detail in my public lectures.

Flow Control magazine will bring the Pump Guy Seminar Series to Florida later this year. New Orleans is on the schedule for next year. Come if you can!

Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a retired member of ASME and lectures in both English and Spanish. He can be reached at [email protected].