Self-Priming Positive Displacement Pumps

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In this Pump Report, Chad demonstrates how PD pumps are able to self-prime - as well as a few things to keep in mind. To learn more or to view other Pump Reports, please visit our website at vikingpump.com.

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Priming is the process of removing air from the pump and the suction line. For a centrifugal pump, this means filling the pump and the suction line with liquid before turning the pump on. The pump can't pump until fully filled with liquid.
Positive displacement pumps on the other hand are self-priming. This means that the pump is able to develop enough vacuum to allow atmospheric pressure to push the liquid up and into the pump. You can observe the same principle whenever you drink water through a straw. By sucking on the straw to create a vacuum, the liquid is pushed up the straw by the relatively higher atmospheric pressure.

Now, PD pumps feature tight running clearances. This means that not only are they able to move liquids, but they're also capable of pumping some air. We've set our pump 12 feet over the liquid below. Which means that this positive displacement pump is going to have to evacuate that air to suck the liquid up 12 feet to prime.

Once the pump is switched on, the air is evacuated from the suction pipe, and the atmospheric pressure quickly pushes the liquid up the pipe towards the pump. A vacuum gauge at the pump inlet illustrates the vacuum necessary to do this. The taller the lift, the higher the vacuum required.

There are some considerations and limitations to keep in mind. And the first limitation is atmospheric pressure - that force that's pushing the liquid into the pump. Here at our lab in Cedar Falls, Iowa we're at an elevation of roughly 900 feet. At that elevation our atmospheric pressure is about 29 inches of mercury. This means that even at a perfect vacuum the maximum lift is only 32 feet of water. And it would be even less for heavier liquids or at higher altitudes.

And remember, that's only theoretical. The second consideration is NPSH. Once the pump is primed and the liquid is moving we still need to make sure that the pressure at the pump's inlet port is adequate to prevent cavitation.

We also have to consider that amount of lift possible is a function of the pump. PD pumps running at high speeds with tight clearances tend to work the best. And wetting the pump with a small amount of liquid helps to seal the running clearances to improve priming and has the additional benefit of lubricating the pump internals.

Self-priming is critical for applications such as pulling liquid from drums, totes, and railcars or for lifting liquids from underground tanks or sumps. This inherent ability to self-prime is just one advantage to using positive displacement pumps in your application.

To learn more about PD pumps or to view other Pump Reports, please visit our website at VikingPump.com.