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What is a Duplex Pump?

Lori Kilchermann
Lori Kilchermann

A duplex pump is a pump typically found in waste water treatment plants and oil fields. It consists of two pumps that alternate the pumping process, thereby allowing it to be more efficient than a single version. This pump has a higher flow rate due to its not having a dead spot in the pump stroke. As one pump is completing its stroke cycle, the other is beginning its stroke — maintaining maximum pumping action without a break in the cycle.

The first duplex pump was steam operated. It consisted of two steam chambers as well as two pump housings. The pumps were capable of pumping twice as much liquid as a single-stage pump. There are no dead spots in this type of pump. The pumping cylinders are constantly under pressure and operate just enough out of time with each other to ensure that one or the other is always pumping.

Man with a drill
Man with a drill

The steam-operated pump was replaced with a duplex pump, which operates on compressed air. The compressed air acts in the same manner as the steam and maintains constant pressure on the cylinders of the pump. The original steam duplex pump was created in the 1800s and steam-operated pumps are still in use, but compressed air has taken over as the primary operating system.

The duplex pump is used where a high volume of sustained pumping is required. Waste water treatment plants utilize the pumps because of the need to move large quantities of water without the threat of backwash. Any backwash could potentially contaminate large freshwater storage tanks and render the system unfit for use.

In oil fields, the need to pump large quantities of water to cool drilling heads as well as to flush out the boring holes requires the use of a duplex pump. The ability to move the great quantity of water allows the drilling rigs to operate at a much faster pace than those equipped with a single-stage pump.

Duplex pumps are also used to pump diesel fuel and heating oils that do not require heating. The thin viscosity of these oils allow the pumps to transfer them to holding tanks as well as to pass the oils through a strainer to clean them of impurities. Thicker oils require heating to allow them to be pumped. The thin make-up of the heating oil allows the oil to flow freely through the pumps at a high rate.

Discussion Comments


The duplex pump terminology is also used for two completely independent pumps that do not operate concurrently. Their controls can be configured for primary and backup or alternating.


@Charred - I think that’s a fair analogy, although I am no mechanic myself.

The thing that I wonder about is the use of compressed air instead of steam to provide the locomotion. I would think that steam boilers would deliver greater power and the steam would act as a better lubricant to the pump’s metal parts.

I believe it would take a lot more compressed air to deliver comparable force. I bought compressed air in a can once to dust off my computer monitor, and while it can be very forceful, it tends to run out in a hurry.


The two stroke action of the duplex pump reminds me of the operation of the internal combustion engine, although I realize that in principle they are not the same and have different purposes.

The combustion engine uses pistons that move back and forth, propelled by chamber explosions, to generate motion. The duplex pump is powered by compressed air which drives the pumps back and forth, to generate motion need to pump fluids.

Personally I think that the concepts are very similar, although admittedly I am making a very loose analogy.

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      Man with a drill