Many processing units, including pumps, compressors and blowers, depend on rotating shafts to operate safely and productively. These shafts must be equipped with reliable seals to prevent releases of potentially hazardous materials.
The choices of seal technology for the given application are many. Mechanical seals are one option. “In my experience, double-seal cartridge types are most common,” says Dan Towse, senior consultant at T.A. Cook’s consulting office in Raleigh, North Carolina.
“We deal primarily with process machinery, with the notable exception of pumps, so mechanical packing is the most common seal we find in service,” says Starkey Steuernagle, general manager at Meco Seal, a subsidiary of Woodex Bearing.
“Specific seal types and models vary significantly from industry to industry, because the pump type, operating conditions and fluid characteristics might require the design features that are unique to a specific seal type,” says Michael Huebner principal engineer at Flowserve. Pusher style mechanical seals are the most common type, says Huebner, but the choice depends on the specific industry. “Most industries have moved toward cartridge style seals because they simplify installations and generally improve reliability,” he says.
Like nearly every other device, sooner or later, regardless of technology chosen, the shaft seal will exhibit signs of impending failure. The concept of mean time between failures (MTBF) might be elusive. “Any discussion about MTBF must begin with a caveat that not all users define MTBF the same way,” warns Huebner. He clarifies this by saying that, while the concept is simple, the actual definitions and recording methods used in industry vary widely. For example, if a seal fails because of a bearing failure, does that count as a seal failure? How does a user account for a seal that was changed out before failure? Different methods might be used even within the same company, which makes it difficult to compare MTBF data from different sources accurately.
Figure 1. This air-purged, double-faced sanitary mechanical is designed for service in an ATEX environment on a pharmaceutical product mixer. The carbon brushes prevent triboelectric discharge. (Meco-Woodex)
Much depends on what went into the seal selection and its installation and ongoing care. “If the shaft run out, motor/pump alignment, coupling condition, cooling, and PM and PdM of pump and motor are within design specifications,” says Towse, “I’d expect 18-month to 2-year MTBF in a centrifugal pump application.” Other factors include minimizing shock loading by means of soft starts and barrier fluid tank operation.
Attention to such details should extend the average MTBF. “I’ve witnessed dramatic improvement in MTBF after the implementation of best practices methodology, timely PM/PdM practices and an increase in operator awareness of asset reliability best practice,” reports Towse.
But such improvements aren’t automatic. “MTBF has a direct correlation to an end user’s focus on reliability,” says Huebner. “End users who continuously identify causes of seal failures and address underlying problems see improvements in MTBF. Customers who don’t try to improve reliability systematically generally don’t see any improvements.”
Monitor and predict
Failures come in many flavors. “Seals often fail as a result of other equipment damage,” says Steuernagle. “Constant high shaft runout causes packing to wear and fail prematurely; the presence of abrasive process material between shaft and packing often damages shafts. Bearing failures frequently result in failure of packing and mechanical shaft seals.”
Towse cuts to the chase. The primary causes of failure are “poor PM and PdM practices, process upsets and operator error,” he says.
“There have been numerous studies on seal failure that show the wide range of failure modes seen in industry,” added Huebner. “The predominant seal failures are caused by poor equipment condition and operation of the seal outside of the seal’s rated operating window. These manifest themselves in failure modes such as dry running, fretting, broken faces, hang-up and heavy face wear. Most seal failures can be addressed by ensuring pumps are in good condition, the seal OEM understands the application and the user operates the equipment correctly.”
Maintenance teams have been focused on predictive technologies for many years now. It’s a source of competitive advantage. The most useful predictive technologies for seals are those that are actually performed, says Paul Wehrle, chief engineer at Meco. “In purged, double-face seals, monitoring the quality of purge retention often can predict the need for seal maintenance,” he says.
“Monitoring purge/barrier fluid pressures and retention is useful, but performance monitoring should always include regular visual inspections of the seals,” adds Steuernagle. “This informs, not only regarding the condition of the seal, but also of other aspects of machine operation that might affect sealing.”
Most of the current techniques involve monitoring different aspects of the seal and support system and providing early detection of a degradation in performance, says Huebner. “Some of these are as simple as upgrading from switches to transmitters, which allows the user to trend performance over time,” he says. “Other methods might be more complex and involve monitoring specific aspects of the seal or system. This is still an area of development for the sealing industry.”
Towse argues that maintenance teams should focus on vibration analysis, leak detection — visual or VOC sampling — and repair. While there might be several approaches to predictive maintenance, not all practices are equally valuable, and “predictive maintenance doesn’t work if frequencies aren’t aligned with process conditions,” he warns.
“The biggest confusion in industry is that most users consider an early failure detection as a predictive technique,” adds Huebner. “In practice, predictive maintenance should allow the user to know the condition of the equipment and detect a degradation before it affects performance. Relying on failure detection is too late and prompts a reactive response instead of a proactive plan.”
Get to the root
Whatever maintenance approach one uses, seals, being mechanical devices subject to wear and aging, ultimately show signs of failure. The prudent maintenance team should ask a lot of probing questions to identify the root cause of the trouble.
“Save old parts and photograph each step of seal disassembly,” advises Steuernagle. “When examining failed packing seals, note how well the shaft is centered in the stuffing box before removing a failed packing, after the box is emptied of packing and lantern ring, and after the packing is replaced. If the shaft is off-center in the unpacked box, replacement packing will be functioning as a bearing and wear will accelerate.”
In mechanical seals, look for any sign of the shaft having come into contact with the stationary parts of the seal or of the machinery, suggests Wehrle. “This could indicate shaft centering problems,” he says. “Burned packing or seal faces can indicate failure of quench/cooling water. Torn or broken O-rings in mechanical seals can indicate installation errors. Broken or corroded springs or chemically attacked rubber parts in mechanical seals can indicate chemical compatibility issues.”
Because many maintenance professionals learned failure analysis techniques through on-the-job training, this often has encouraged them to sacrifice accuracy for speed, warns Huebner. “Failure analysis techniques such as root cause analysis are formalized structured methods that require a thorough examination of all aspects of a problem,” he explains. “The biggest tip for doing this correctly is to provide personnel with proper, specific training in failure analysis techniques and to ensure they apply them correctly.”
Towse recommends cataloging as many system conditions prior to the failure as possible before making any repairs. “Communicate with operations to ensure it fully understands system conditions leading up to the failure,” he recommends. “Build an historical database of equipment failures to ensure that each RCA case is searchable and the seal OEM is actively engaged in RCA process.”
Run, failure, run
Predictive maintenance and root cause analysis consumes resources, time and money. There exists some economic balance between the cost of idealized maintenance and the potential cost of seal failure.[pullquote]
Run-to-failure is not a good practice because of the additional damage that could occur to the sealing surfaces and associated equipment, explains Towse.
Steuernagle agrees. “Run-to-failure requires a willingness to accept not only uncontrolled leakage and product loss, but also the ancillary consequences of seal failure: damaged shafts and bearings or gearboxes, and unpredicted, lost production time, usually when it's least affordable,” he says. “Nonetheless, many facilities use run-to-failure as standard practice, accepting the costs of unplanned downtime and machinery damage in the interest of continuing production and filling orders as long as possible.”
Run-to-failure is a concept that has historically been used across most industries, so it has some level of practicality, argues Huebner. “Is it the most effective method to operate equipment?” he asks. “Clearly, there’s supporting evidence that run-to-failure is a more expensive method of operating equipment and will lead to more unscheduled down time than proactive techniques. Environmental and safety considerations also are important justifications for moving away from a run-to-failure strategy.”
Know your application
Just as all maintenance philosophies are not created equal, seal applications differ, as well, running from simple to difficult or impossible. “Large agitator shaft applications are difficult because of varying operations conditions,” explains Towse. “If vessel levels are allowed to run low and shaft RPM is too high, shaft wipe can cause seal damage.” He also mentions extreme conditions such as heavy solids/slurry, high pressure, high temperatures and operating outside of the prescribed pump curve as potential hazards to seals.
“The most difficult applications are those with a large number of unknown factors in the application,” says Huebner. “This might include unknown or under-communicated fluid properties, equipment design, operating procedures or customer expectations. There also are some applications for which the environment simply isn’t suitable for a mechanical seal, and these might require modifying equipment or the operating conditions. If all the factors are known up front, most applications can be sealed effectively.”
Seal failures occur, and catastrophic failures can be dangerous. “Shaft seals contain a vast assortment of processes and chemistries, at a wide range of pressures and temperatures,” says Steuernagle. “Seal failures can result in release of toxic, corrosive, flammable or explosive substances into the workplace, posing risks of explosion or conflagration, in addition to personnel health risks. Even more benign process materials, if leaked uncontrollably, can produce severe damage to bearings and gearboxes, requiring costly shutdowns and repairs.”
The most catastrophic seal failures are generally tied to a failure of some other component in the host equipment, says Huebner. “The most common could be seal failures caused by failed pump bearings or a broken pump shaft,” he says.
“Catastrophic failure is possible if a motor doesn’t trip out on amperage loads,” advises Towse. “The resulting effect could cause severe damage to the seal body, thereby increasing the possibility of fluid containment loss.”
Obviously, maintenance teams don’t want to be forced to deal with catastrophic seal failures. The idea is to prevent them. To that end, those maintenance teams can take affirmative action.
Prevention precedes failure
“Preventive maintenance is the first step,” says Wehrle. “Depending on the type of seal installed, any of a variety of predictive technologies might apply, but an effective one should be adopted and practiced. The seal manufacturer's instructions should provide guidance. Monitor the seal performance and anticipate leakage with adjustment or repair. Don't wait for the seal to leak. Once leakage is observed, process material has already passed between the sealing elements, and some damage to the seal — with associated risk from any hazards inherent to the process material — has likely already occurred.”
The best approach to preventing any seal failure is to understand the root cause of failure and take direct action to prevent the cause from recurring, adds Huebner. “In many cases, an end user simply puts in another seal hoping for a better outcome,” he says. “Identifying and correcting underlying causes are the most critical aspects to preventing failure. The biggest obstacle to being successful with this is that most failure causes ultimately have their roots in procedures and behaviors. It’s fairly easy to fix one problem; it’s much more difficult to build a culture and infrastructure to prevent problems from occurring in the first place.”
Using a shaft seal of any type results in a torsional drag the application’s prime mover must overcome. That’s the ante needed to get into the game. In the case of an electric motor, this means you’re paying for power consumption that isn’t being used to move process fluid.
“Torque loading is application-specific,” advises Wehrle, “and its effect can vary from slight to significant. Mechanical seals generally produce less torsional drag than packing. Measuring current draw on the drive motor will often prove informative when changing from one type of seal to another. Changes in seal design have, in some cases, resulted in more than a 30% reduction in motor current.”
All power consumption attributable to a mechanical seal is ultimately related to its torsional drag, adds Huebner. “This comes from two components: the contact between the seal faces and the viscous drag of the rotating components in the seal chamber,” he explains. “In most typical applications, the seal face-generated power is significantly greater than the fluid shear power requirements from the rotating components. As the seal gets larger and the rotational speeds get higher, the power requirements of the rotating components can also become significant.”
To minimize torsional drag, choose a seal with face loading appropriate to the process, where possible, suggests Steuernagle. “With mechanical packing, apply only enough compression to prevent leakage,” he advises. “When using packed gland seals with fluids, a slow, steady weepage of seal water or process fluid is usually required to minimize friction. In fluid sealing, maintaining proper barrier fluid flow and pressure in mechanical seals is required.”
Viscous shear is proportional to velocity, area and fluid viscosity, and inversely proportional to the film thickness, explains Huebner. “Changes in any of these variables affect the viscous drag on the seal,” he says. “In some cases, it might be beneficial to use seals with a flexible stationary element because they tend to have smaller rotating components. In other cases, it might be useful to use a lower viscosity barrier fluid on dual seals. On the seal faces, it might be beneficial to use a hydrodynamic face design which would reduce contact pressure and therefore face friction. Using seal face materials with an inherently low coefficient of friction also is useful.”
One of the most significant reductions in torsional drag can come from using dual gas seals, says Huebner. Gas seals eliminate fluid contact from a larger part of the seal and in the film between the seal faces.
Best practices and training
Apply a holistic methodology to PM/PdM on mechanical seals and operate continuously within the OEM-specified curve, recommends Towse.
Proper mounting and installation are important, too, says Wehrle. “Provide the correct pressure and flow of any required barrier fluid, use predictive techniques to monitor seal performance and anticipate leakage,” he says.
“A plant’s best practice for mechanical seals or any other equipment is to set goals of continuous improvement and track performance to those goals,” says Huebner. “This will, out of necessity, require that aspects such as procurement, installation practices, operating procedures, failure analysis and equipment upgrades be reviewed to ensure they support these objectives,” he says. “This approach systematically addresses specific problems, as well as helps prevent new ones. Fortunately seal OEMs developed proven programs to support these efforts. There’s a large base of customers who have benefited from this approach.”
Flowserve has an extensive training program both for employees and clients, says Huebner. “We have dedicated educational resources to train plant personnel in our modern training laboratories or at the plant site,” he says. “Effective training is a fundamental aspect of any reliability and continuous improvement program.”
The seal industry is evolving, and maintenance teams should be aware of trends. “There are more options being considered than the traditional packing,” reports Wehrle. “Increasing restrictions on water use offer increasing challenges to water-flushed seal designs.”
The most significant change is the focus on overall system reliability, adds Huebner. “This changes the scope from just mechanical seals to the entire pump system — pump, seal, seal support systems, pump hydraulics,” he says. “This has allowed realizing benefits not only from improved seal performance, but also from improved pump reliability and reduced overall operating costs for energy and maintenance. This trend should become more evident as more users become aware of these benefits.”