A report entitled “Meeting Emission Regulations with Mechanical Seals” released in April 1990 by the Seals Technical Committee of the Society of Tribologists and Lubrication Engineers (STLE) stated that “eliminating seals in pumps is not the solution to emission controls.” The standards committee included seven leading seal manufacturing companies working in conjunction with chemical company clients. The report also stated “sealless pumps seem like the perfect solution but rely on bearings being lubricated by the product being pumped. Therefore, bearing problems result from converting to sealless pumps.” The seal manufacturers effectively removed themselves as the weak link and focused on the perceived and sometimes real bearing issues.
The report listed a number of perceived problems that were present when relying on the product being pumped for lubrication, including: the sometimes poor lubricity of the pumped product; high instances of costly downtime for in-shop repairs; and the elevated chance that leaks will still occur, which exposes plant personnel and the environment to the pumpage. As pump manufacturers rushed their sealless offerings to market, an overzealous sales force misapplied or over-applied their products. Initial failures, most common among high-speed centrifugal manufacturers lent credibility to the seal manufacturer's warnings. End-users became cautious. Those burned would hesitate to consider sealless technology again.
Then, most damningly, the report concluded: “Obviously, there is questionable, if any, benefit (of using sealless pumps) to the end-user who is genuinely concerned with the environment and his personnel."
Times Have Changed (So Have Sealless Pumps)
Traditionally, 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 reduced pressure and flow rate.
Secondly, traditional sealless gear pumps feature two fluid chambers—a hydraulic chamber in which the gears work and a second chamber for the magdrive coupling unit—that are joined together by a bracket, which also serves as a barrier between the two chambers. This complicated design requires that a portion of the material being pumped through the hydraulic chamber must be used to cool the magnets in the other chamber. These requirements result in a long, complicated pump with elongated, narrow flow paths and the need for more parts which makes the pump more expensive and difficult to maintain. This also limits the viscosity of the liquids that can be pumped, as well as the types of solids that can be handled.
The approach to finding an ultimate solution to the sealless pump quandary removed the word sealless from the development process. When looking to create a gear pump that is affordable, controls leaks and reduces maintenance costs and environmental concerns, the first step is to identify the areas in which sealless pumps fall short and look for improvements. As mentioned, the No. 1 area in which traditional sealless pump operation is compromised is the bearings and how they interact with, and are affected by, the pump's cantilever load. The second step is to find a superior replacement for the two-fluid-chamber design that complicated the pump's operation and limited its fluid-handling range.
Taking these main concerns into account, and approaching the design process with an open mind, the result is a gear pump line that is sealless, not because the designers and engineers felt that it needed to be, but because its design enhancements led them to the conclusion that it would operate most effectively as a sealless pump.
This pump also features two design enhancements to overcome the long-time challenges of excessive bearing wear and a fluid chamber design that complicates operation and limits product range. These enhancements are:
Between-the-bearing support system—As opposed to the performance-robbing, one-sided support found in a cantilevered-load design that exists in traditional sealless pumps, the new gear pump supports the rotor and idler gears at three locations 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 effects of cantilever load. In tests in which 200 psi of pressure was applied to the rotor, only 0.005 inch of shaft deflection occurred in this pump, compared to 0.056 inch of shaft deflection in a traditional sealless pump, giving the new design 11 times less shaft deflection.
Larger diameter materials that provide more rigid support for less shaft deflection and bearing wear. For example, a traditional 3-inch sealless pump will have a shaft that is 17⁄16 inch in diameter. The diameter of the new design's eccentric spindle is 2 inches.
Large, long radial bushings that support the entire length of the rotating element, which spreads out the hydraulic forces and allows the bushings to last longer. The new pump's bushings are also made of premium-grade carbon graphite that will last up to eight times longer than more common bushing materials.