Sealless gear pumps make a comeback with new design
16 May 2013
In June 1990 a USA consultancy firm predicted that sealless gear pumps would take an increased percentage of the market; it didn’t happen.
The technology at that time was not reliable enough, writes AxFlow managing director Tony Peters, with too many instances of failures that were brought about by excessive bearing loads and design deficiencies that led to breakdown and leakage issues. These failures created a stigma that many manufacturers of sealless pumps are still trying to overcome today.
Adopting a fresh, clean-sheet approach to confronting the conundrums inherent in traditional sealless pump design has led to the creation of a new line of sealless gear pumps. In short, the new Envirogear pump from Axflow increases reliability while eliminating the unacceptably high ownership, maintenance and environmental costs that have dogged previous sealless gear pump designs.
With its simple design comprising seven primary parts, the Envirogear pump is designed to eliminate all the operational concerns found in old-style sealless gear pumps. The leading causes of high maintenance in conventionally sealed pumps include those associated with mechanical seal replacement and the premature wear of the bushings and close-fitting metal parts due to insufficient support of the rotating pumping elements.
There are also the environmental costs of leakage include cleanup, possible forced shut-down and environment hazard fines and penalties, plus the resulting bad press and community mistrust if a leak or accident occurs.
Times have changed (and so have sealless pumps)
Traditional sealless gear pumps are designed with a cantilevered load where a large rotor gear is attached to the end of the pump shaft. As hydraulic force is applied to the rotor during pump operation, extra pressure is put on the shaft and bearings. This pressure can lead to shaft deflection and increased bearing wear, which results in more rotor-to-casing or rotor-to-head contact wear. The result is rapid loss of pressure and flow performance.
Secondly, traditional sealless gear pumps feature a magnetic drive chamber and a fluid pumping chamber which are joined together. These elements result in a long, complex pump with elongated, narrow flow-paths and the need for more components, which make the pump more expensive and difficult to maintain. This also limits the viscosity of the liquids that can be pumped.
What to change?
The first step is to identify the areas in which sealless pumps fall short then look for improvements. Traditional sealless pump bearings are severely affected by the pump’s cantilever load. It was important to find a superior replacement for the classic magdrive two chamber design that complicates pump operation and limits fluid handling capabilities.
The Envirogear pump features two design enhancements to overcome these challenges. These enhancements are:
Between-the-bearing support systems – As opposed to the performance-robbing, one-sided support found on the cantilevered load design that exists in traditional sealless pumps, the new gear pump supports the rotor and idler gears at three locations. This is achieved through the creation and incorporation of:
- A patented eccentric spindle that is supported in the head, the crescent location and the back of the containment canister, eliminating much of the cantilever load.
- Large diameter journals that provide more rigid support for less shaft deflection and bearing.
- Large, long radial bushings that support the entire length of the rotating element, which spreads out the hydraulic forces and gives extended operational life.
The improved design has only one fluid chamber with the pump’s magnets piggy-backed on the rotor gear element. This design gives the pump a much shorter, simpler flow-path. It also allows the pump to easily handle solvents or liquid viscosities up to 30,000cP.
Envirogear pumps have dimensional interchange-ability with 95 percent of the other internal gear pumps that are currently available. A plant can run a traditional sealed pump in the morning and have it pulled out in the afternoon and drop the single fluid chamber Envirogear pump into the footprint while reusing same piping, gearbox, motor and base plate.
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