Self-Priming Wet or Priming Assisted Dry. Which Is better ?

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.

This diagram shows the flow of air [and water] through the priming system of a “Dry Prime” pump.


  • 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
  • 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.


  • 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.


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.

Diagram below shows water being separated from air, as air is primed and water is recirculated.

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.

This video demonstrates the re-priming capability of Gorman-Rupp’s Super T series Pump.

“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.

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Self-Priming Pumps

Ask Our Team

The Gorman-Rupp 80 Series range of self priming centrifugal grey water are heavy duty 24/7, quick priming “work horses” for industry. With multi-vane impellers and a “straight-in” suction design, the 80 Series pumps deliver high efficiencies and quick positive self priming.

Advantages include:

  • Ability to mount high and dry above the wet well [not in it]
  • Capital cost savings over wet well/ dry well constructions
  • Low cost of ownership
  • Greatly reduced OH&S issues
  • Easy access for monitoring or service

Ask Our Team

The EDUR range of multi-stage self priming centrifugal pumps can self prime on suction lifts up to 7.5 metres, and deliver heads to 160m. These self priming pumps are also available in a variety of materials (Cast Iron, All Bronze, All Stainless Steel) enabling them to pump corrosive fluids including seawater.

These Grey Water Pumps can be close or long-coupled.

Advantages include:

  • Self Priming to 7.5 metres
  • All Iron, All Bronze and All Stainless Steel construction
  • Flows to 80 l/s
  • Heads to 160m
  • Handle temperatures from -40oC to 90oC depending on model
  • Single and double mechanical seals available

Ask Our Team

The Gorman-Rupp ES Auto-Start sewage pump stations are the same as any other self priming pump station, except they have an auxiliary combustion engine to drive one or more of the pumps in times of power outage. They are pre-engineered and factory tested packaged units with one, two or more sewage pumps. The motors, auxiliary engine/s, all the valves, interconnecting piping etc are all supplied as a package, inclusive of state-of-the-art purpose built controls and liquid level control systems.

The sewage pumps in the station can either be Gorman-Rupp’s Super T or Ultra V Series sewage pumps. Flows to 250 l/s for a single pump can be achieved, and this can be expanded when using self priming pumps, because only the suction line needs to be in the wet well [not whole pumps], so that 3 or more pumps can be utilised to deliver peak efficiencies when pumping ADWF [average dry weather flow] right through to peak wet weather flows. A variety of enclosures are available to house the pumping equipment.

Advantages include:

  • Lower civil costs/ engineering
  • Sewage pump stations are fully equipped and ready to “plug and play”
  • Lower operational costs
  • Greatly reduced OH&S issues
  • Easy access for monitoring or service
  • Intuitive control system
  • New advanced level control system keeps electrical equipment out of the wet well
  • Able to solve many application problems, including those with steep system head curves
  • All components within Gorman-Rupp packaged sewage pump stations are warranted for 5 years
  • Can work with the revolutionary Self Cleaning Sump (SCS) for a complete sustainable system

Ask Our Team

The Gorman-Rupp O Series pumps were originally designed for handling petroleum products. Later, O Series pumps were introduced into the industrial market for effective handling of solvents and similar liquids, as well as excellent clean liquids handling. The straight-in-suction design without a check valve eliminated entrance restrictions provides quick, reliable self-priming with few moving parts offering saving through minimal maintenance and increased efficiency. The O Series has a variety of seal configurations to choose from allowing it to handle a number of different liquids.

Advantages include:

  • Capital cost savings over wet well/ dry well constructions
  • Low cost of ownership
  • Easy access for monitoring or service

Ask Our Team

The Gorman-Rupp Roto-Prime range of self priming centrifugal pumps is specifically designed for pumping petroleum products, petrochemicals and solvents. The Roto-Prime fuel pump incorporates a variable capacity vane [priming] pump which quickly and automatically evacuates air and vapour from suction lines, making them ideal for loading, off-loading transports and stripping hoses.

Advantages include:

  • Ability to mount high and dry above the liquid source
  • Eliminates vapour lock or air lock in suction lines
  • Easy access for monitoring or service

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We are leaders in Self Priming Centrifugal Wastewater Pump Technology. Hydro Innovations is the exclusive distributor of Gorman-Rupp pumps for all the municipal and industrial markets in Australia. Gorman-Rupp is one of the world’s leading manufacturers of self-priming sewage pumps and wastewater pumping equipment and has been for more than 75 years.

Holding a very large inventory of pumps and spare parts allows us to deliver industry-leading service. With over 88 years combined experience in applying, selecting and troubleshooting pumps, we are also in a good position to help design the best and most cost-effective fluid transfer systems for our customers.

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