Self-priming pumps are liquid pumps designed to have liquid already inside the pump body to start the priming process. They’re used to increase operational efficiencies because the start-up liquid is already inside the pump, making it primed and ready to pump.
Self-priming pumps are found most often in process plants and are used for various repeated, intermittent functions.
However, they are also the pump of choice for many industrial applications, including:
Self-priming centrifugal pumps are different from standard centrifugal pumps because they have a liquid reservoir built into the unit. The built-in reservoir, sometimes called the separation chamber, enables the pump to discharge air from the casing and the suction line by recirculating liquid during the priming cycle.
The reservoir may be above the impeller or in front of the impeller.Self-priming centrifugal pumps:
Some disadvantages include:
There are essentially nine steps for a self-priming pump to work, which we detail below. You can also see them in action in the Self-Priming Basics video from Gorman-Rupp.
Centrifugal self-priming pumps have two phases or cycles to operate: the priming cycle and the pumping cycle.
PRIMING PUMPING AT REST
rotating impeller creates a vacuum at the impeller’s eye that draws compressed air from the suction line into the pump.
around the impeller, creating an air-tight seal. The seal prevents air in the discharge line from leaking back into the suction line.
where the liquid and air bubbles separate.
the heavy liquid flows back down through a recirculation port
to the pump reservoir, and the air evacuates through the open-ended discharge line or an air-release device.
we create a low-pressure system at the impeller’s eye. Due to a differential in pressure, higher atmospheric pressure outside the pump forces liquid up the suction line.
at this point, stopping the recirculation process so the pump can discharge the liquid.
A failure to prime is one of the most common issues technicians and maintenance workers have to troubleshoot. Although there are plenty of reasons why a pump loses its prime, it’s most commonly caused by air leaks, air binding, face clearance issues, or clogging.
As you know, liquid must recirculate inside the pump casing to create the low-pressure vacuum that draws liquid up the suction line into the pump. If there’s an air leak, the pressure differential inside the pump can’t drop below the atmospheric pressure outside, making it impossible for the suction line to pull a vacuum.
During priming, air is handled through the recirculation process and evacuated off the discharge side. Air binding occurs when air is not vented to the atmosphere and collects on the discharge side. The now pressurized discharge side stalls the priming process until the air is vented.
The higher the clearance (or the more space) between the wear plate and the impeller, the weaker the vacuum created by the pump. If the vacuum isn’t strong enough to lower the internal pressure of the pump, atmospheric pressure can’t force liquid up the suction line. This clearance issue is often caused by general wear on the plate or impeller.
If debris clogs up the impeller’s eye, you’ll lose all the impeller’s hydraulic capability and priming will never complete.
If debris plugs up the recirculation port, the pump won’t properly recirculate liquid within the casing, resulting in a failure to prime.
You can see these five issues in action in Gorman-Rupp’s Failure to Prime video.
Choosing the right pump isn’t always easy, which it’s important to lean on the experts. In the Selection & Application brochure from Gorman-Rupp, you’ll learn some of the things you’ll need to know before settling on the right pump for the job.
Some things you should know include:
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