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Self-Priming [wet] or Priming Assisted [dry]? Which is better?
 Mar 20, 2022|View:501

Self-priming pumps are divided into two categories. True [or wet] self-priming pumps and those that use an external priming system. These are called “priming assisted”, “auto-prime”, “vacuum prime” or “dry prime” pumps. Choosing which is better is very dependent on the application.

Dry Pumps

Standard centrifugal pumps are used here, which cannot create their own vacuum. These pumps rely on several “external” components to create the required vacuum in the suction line to prime the pump. They need:-

  1. A vacuum source. This can be a dedicated vacuum pump, a diaphragm pump, or a compressor/venturi system. These components are usually situated near the pump shaft so that this shaft can be used to run drive belts to the vacuum pump or compressor.

  2. An air separation chamber [or “priming hopper”]. This is mounted on the suction side of the pump immediately prior to the pump inlet. The vacuum created is “pulled” through this device to access the suction line. It comprises a cannister with a float inside. As the water rises in the suction line, it enters the priming hopper, raising the float, and through a series of linkages, closes [seals] the chamber to prevent water entering the priming devices.

  3. A non-return valve. This is positioned immediately after the pump.

Advantages of a Dry Prime pump

  • Because these pumps use an external priming source, a very wide range of centrifugal pumps can be used. Pumps with high flows or high heads or a combination of both can be utilised. Flows to in excess of 1000 litres per second can be achieved, as can pressures in excess of 200 metres.

  • Also, because they use an external vacuum source, suction lines can be long and/or large.

  • There is also a very wide range of materials available because priming systems can be retrofitted to any centrifugal pump.

Cons of the Dry Prime pump

  • The priming ability relies on the vacuum pump [or equivalent] to be functioning, including the belt drive system to be in good condition. A broken belt is a pump out of action.

  • The priming hoppers have a screen to try to prevent foreign matter from the suction line entering the hopper. If the screen is too fine, sludge-like fluids can block it, preventing the vacuum accessing the suction line. If the screen is too coarse, it can allow access to fibres and other solids [like cigarette butts and rags] which can build up inside the chamber, fouling the float.

  • The non-return valve in the system needs to seal perfectly for the pump to prime. If it does not seal perfectly, the vacuum system not only “pulls” air from the suction line, but also from the discharge line. This can cause priming times to be much greater than they should be, or can prevent priming all together.

  • Because of the many components in the system, they are more expensive than self-priming pumps.

  • They are more complicated and require higher levels of intervention.

Dry Prime pump Applications

These pumps are suited to a wide range of applications hydraulically. This includes high head and high flow mining and quarry applications, also for construction site dewatering and temporary sewer station bypass. But they are best when they are attended. Too many components need to be relied upon for the pumps to be applied to prime automatically at an unattended site.

Wet Prime Pumps

Wet prime, or “true” self-priming pumps do not rely on external priming systems. Instead, they are designed with an integral external priming tank [pump casing]. After the casing is filled with liquid, the pump simply uses the rotating impeller [which is immersed in the liquid] to create a low-pressure area [vacuum] at the impeller eye, which continually removes air from the suction line until the pump and suction line is completely primed.

All self-priming pumps are not created equal though. Wet prime pumps can also be split into two categories. “Standard” self-priming pumps and those that are “guaranteed” to re-prime after each and every priming cycle is completed.

All wet prime pumps can produce a high vacuum when their casings are completely full, but in an unattended “pump-down” application where solids are present in the pumped fluid, solids can lodge in the pump’s suction check valve at the completion of a pumping cycle, causing the suction leg to drain, which in turn causes a siphoning of the pump casing. “Standard” self-priming pumps will not retain enough liquid to “guarantee” re-priming – particularly on a high suction lift.

“Guaranteed re-primers”, like Gorman-Rupp’s Super T and Ultra V series pumps, are guaranteed to reprime to levels published on their performance curves. Because of this capability, these pumps can be relied upon to operate automatically every single time at an unattended site.

“Wet-primers” do have their limitations. They initially need to be filled with liquid [generally water] before they can start to prime. They also have flow limitations of approximately 300 litres per second, and pressure limitations to approximately 100 metres.

Advantages of Wet Prime pumps

  • They have a simple priming system that does not require a combination of systems to be operational to ensure priming.

  • The “re-primers” can operate reliably and automatically at unattended sites.

  • Small solids like cigarette butts don’t cause a problem with the priming system.

  • They are less expensive than prime assist pumps.

Cons of the Wet Prime pump

  • Wet prime pumps do not have the hydraulic range of dry primers.

  • They are not capable of priming suction lines as long as the dry prime pumps can manage.

  • Because they are purpose built to be self-primers, they do not have the full range materials available to dry primers.

Wet Prime Applications

Wet prime pumps can be applied to numerous fixed installations, and are also suitable for mobile plant [such as diesel drives]. Re-priming solids handling pumps can be applied to all manner of fixed installations including raw sewage pumping and pumping out of industrial wastewater pits including those found in food process, paper process, steelmaking and those requiring automatic liquid level control.

So which is better?

  • If it is an unattended fixed installation application, a wet prime re-primer is the choice.

  • If it is an application with easy access to first-fill the pump casing, and within the hydraulic limits of a wet primer, a wet primer is the choice.

  • If suction lines are long at an attended site, a dry prime would be the pick.

  • If hydraulic limitations exceed that of a wet primer, then a dry primer is the only choice.