Process pumps take center stage at many plants when it comes to addressing regulatory requirements and safety issues about fugitive emissions. After all, leaks through seals on conventional pumps usually play a significant role in such emissions. This is pumping up interest in designs that avoid seals altogether by relying on magnetic drives or motors contained within the pump shell. These designs have been around since the late 1940s, yet new magnetic materials, better bearings and improved versions of the pumps roll out regularly. Suppliers also have been raising the horsepower, percent-solids and dry-running capabilities of their units.
Meanwhile, the cost premium over conventional pumps has been dropping. According to sealless pump makers, pricing has come down, mostly due to the greater availability of lower-cost cobalt-samarium permanent magnets (which are used on the pump rotor). At the same time, enhanced mechanical seals or other emission control measures have added to the cost of conventional pumps. “By my estimation, the difference between the pumps’; prices has shrunk by 30% over the past 15 years, depending on a great variety of design and mechanical details of both sealless and sealed pumps,” says Jack Parker, an applications specialist at Magnatek, Houston, which sells only sealless mag-drive designs.
“Business isn’;t growing by leaps and bounds, but every year we sell more sealless units than the previous year,” says Paul Biver, a product manager at ITT Goulds Pumps, Seneca Falls, N.Y. Part of the driver for this might be changing customer demand: The latest market study from Freedonia Group, Cleveland, predicts U.S. pump demand (of all types) will rise 5% per year from 2002 to 2007. Process manufacturing accounts for about a third of the market, and should show the highest growth rate, 5.7% per year, the study reports. With sealless pumps the design of choice for handling the hazardous fluids common in the process industries, these styles of pumps may be benefiting from the evolving marketplace.
Sealless pump manufacturers expected to get a boost from a recently written standard from the American Petroleum Institute, Washington, D.C. API 685 (2000) defines pump specifications for petroleum refining and hydrocarbon processing. The standard took a decade to develop; even so, U.S. response so far has been lackluster. In mid-2003, two participants in the effort presented a paperNT> at the Pump Symposium run by Texas A&M University, College Station, Texas, noting that thousands of copies of other API pump standards, such as 610 and 611, have been bought, but only 125 copies of the new standard had been sold at that point. There continues to be strong interest from European concerns, while the U.S. market has lagged behind, the presenters concluded.
The Hydraulics Institute, Parsippany, N.J., which has had a sealless pump committee for many years, updated its sealless standards (which are more for testing and definition rather than specifying) in 2000. Greg Romanyshyn, technical director, says the next revisions will be completed in 2005 and will reflect new pump features.
The basic mag-drive pump employs a motor to drive the external portion of a magnetic coupling that surrounds the rotating shaft. The other part of the coupling consists of a set of permanent magnets arranged around the rotor shaft inside a thin containment shell. These magnets are synchronized to move with the exterior portion of the coupling. Process fluid passes throughout this shell, providing lubrication to bearings and cooling to the rotor. Notably absent is a seal between the motor and coupling that is in contact with the process fluid.
In contrast, canned-motor pumps place the motor fully inside the pump to avoid the need for a seal. The stator and rotor have windings like a conventional motor, but both are contained within metallic shells with process fluid surrounding both. The inner containment shell can be thinner than the corresponding shell of a mag-drive motor because the stator supports it.
The role of reliability
The big news, to hear sealless pump marketers tell it, is that the absence of seals in these units offers an inherently more reliable design — so much so that some pump buyers are specifying them for general-purpose applications. “Pure and simple, every study of pump reliability finds that seal problems cause the majority of pump failures,” says Jim Murphy, a marketing director at Viking Pump Inc., Cedar Falls, Iowa. “Companies looking at the cost of maintaining or repairing their pumps can be more comfortable with sealless units in many applications.”
While cautioning that comparative failure data are highly dependent on the application, ITT Gould’;s Biver says mag-drive pumps demonstrate a mean time between failures (MTBF) of two to three years versus one-and-a half to two years for sealed pumps.
Those claims don’;t hold much water with at least one pump specifier, Bob Rollings, pump consultant in the engineering department at DuPont, Wilmington, Del. “[If they’;re] properly operated — and I emphasize that — few pumps wear out from old age,” he says. “Any pump, sealless or sealed, is going to need routine maintenance, and if the pump is properly maintained, there’;s little or no cost difference between the two. Sealless pumps make sense for certain applications, especially with low-solids streams where fugitive emissions could be an issue.”
Mike D’;Ambrosia, pump hydraulics engineer at the King County Wastewater Treatment facility, Seattle, says, “Mechanical seals do have their problems, but they are able to reduce our maintenance costs sufficiently relative to conventional seals that we use them for most of our pumps. That’;s one of the reasons why our mag-drive pumps are used only in our laboratories, where fugitive emissions are more of a concern.”
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 firstname.lastname@example.org.