Rollings says the mechanical seals of conventional pumps are delicate pieces of machinery. Seal manufacturers have developed numerous innovations to address seal-life and leakage problems: labyrinth seals, double or dual seals and gas-barrier seals. The gas-barrier designs, for example, pair a utility gas (typically nitrogen) and a seal with etched faces to create a flow pattern such that the gas becomes a barrier between the seal faces.
An overbearing issue
The enemy of sealless pumps (both mag-drive and canned-motor) is bearing wear or failure. Most conventional sealless pumps rely on the process fluid to provide lubrication to the bearings. If the process fluid has a high solids content, flow around the bearings becomes constricted, leading to rising temperature in the bearings, potential solidification of the fluid from a thermal reaction and, ultimately, bearing failure. Similar problems can arise if the pump runs dry. Although it is common knowledge that sealless pumps must have fluid to operate properly, many typical plant operations, such as emptying and filling tanks, can result in dry running. One manufacturer’;s literature calls a customer’;s fuel-transfer station a “pump nightmare.” High-solids process fluids also can erode the bearings or their sleeves.
To address the solids issue, pump designers have looked carefully at the pathways by which process fluid encounters the bearings. They also are investing in more durable materials for bearing components, especially silicon carbide faces on bearing housings. Goulds, which introduced its EZMag line of mag-drive pumps last year, offers the Safeglide bearing cartridge (Figure 1), which is made of silicon carbide and has a synthetic-diamond coating for better resistance to dry running. “This bearing can tolerate up to 20 minutes of dry operation, while other silicon-carbide bearings break down after one to two minutes,” Biver says, adding that there is no cumulative damage (i.e., the next time dry running is experienced, durability is not affected by the previous episode).
Figure 1. Vanes move fluid around the back of the containment shell where grooves on the bearing cartridge transport it through the cartridge.
Source: Goulds Pumps
Goulds also has developed an external-flushing system that pushes bearing fluid against the flow of process fluid. Normally, process fluid flowing into the containment shell contacts the sealless pump bearings. Instead, Gould relies on a pressurized flow of lubricating fluid from outside the shell, so that process fluid is kept away from the bearings. With this, Biver says, the EZMag pumps can handle as much as 10% solids (depending upon the type of solid).
Magnatex is experimenting with silicon carbide seals on its smaller mag-drive units; they are also a feature of the Ansimag and HMD/Kontro lines of Sundyne Corp., Arvada, Colo., Flowserve Corp. Pump Division, Vandalia, Ohio, and others. However, Teikoku USA, Houston, a leading supplier of canned-motor pumps, uses carbon-graphite bearings and provides a bearing monitor with its products to measure bearing wear. At Sundyne, Bill Mabe, director of technology development, says conventional carbon bearings offer a benefit: They “have tolerant wear — if a silicon carbide bearing fails, it can destroy the pump shaft.” On the other hand, carbon bearings cannot tolerate process fluids with abrasive particles. “We sell both types of bearings,” he says.
Another worry from dry running is heat buildup. Mag-drive pumps generate heat from eddy-current losses in the containment shell covering the rotor and from the rotor and stator being out of synchronization as pump loads vary. Making the gap between the rotor and stator as small as possible can minimize eddy currents, but the downside to this is the rising potential of fluid blockage from dirty process fluid.
Canned or mag-drive?
Pump experts agree that, in most cases, a careful analysis of process conditions, fluids and operating practices must be performed to choose between canned-motor and mag-drive pumps. Canned pumps provide double containment, both around the rotating shaft and around the entire unit, which is added insurance against emissions from catastrophic failure. Mag-drive pumps, on the other hand, generally offer a smaller footprint on the plant floor. Because the pump’;s liquid end and motor are separate, access to either is somewhat easier. Pump literature indicates that the chemical reactivity and volatility of the process fluid can be the deciding factor between the two.
For both types of pumps, manufacturers have developed protective plastic linings for the containment shell. In the case of mag-drive units, the lining is on the inner containment shell or the pump casing. Several companies, such as Met-Pro, Hauppauge, N.Y., and Vanton Pump, Hillside, N.J., offer medium-capacity units with the containment shell made from plastic, such as PTFE, PVDF or PFA. Goulds has developed mag-drive containment shells lined with PFA, says Biver, but uses a metallic shell to maintain an acceptable pressure rating and dimensional stability. “At our pressure ranges, you need to have some kind of stiff backing for safety.”
Sundyne has given the mag-drive/canned-motor question a novel twist: A hybrid design that features a motor-type (windings) stator — similar to what a canned-motor pump would have — and a magnet-based inner rotor like a conventional mag-drive unit. The stator, however, is removable, so the pump can be serviced in place, which is an advantage when compared to conventional canned-motor units. Because of some design modifications, the unit also has fewer bearings — and potentially fewer bearing-related problems — than typical mag-drive units.
“What we’;ve done is remove some of the objections of both canned-motor and mag-drive pumps,” says Sundyne’;s Mabe. “The unit is easier to service but runs a little cooler and can use a somewhat smaller motor.” The pumps, called MagMax, have been on the market since 2001.
Sealless pumps are not limited to centrifugal designs. Viking Pump, for example, offers a mag-drive-based line of internal, external or spur gear pumps. Jim Murphy, marketing manager, says that the units especially suit applications involving low flow (less than about 50 gal/min) or viscous or heat-sensitive process fluids. They also can run at 1,200 rpm (or lower, in some cases), which minimizes eddy current problems. “At low flow rates, centrifugal pumps tend to âhunt’; for the right speed because the pump curve is relatively flat. These positive-displacement pumps avoid that,” he says. The gear-pump lines recently have been upgraded with modularized designs for interchangeability of parts and improved bearings.
Nick Basta is controbuting editor for Chemical Processing magazine. E-mail him at email@example.com.