by Matt Migliore
The world of sealing solutions is an ever-changing one, with materials and technologies constantly evolving to meet a wider range of end-user needs. And while many seals may be marketed as the same or similar, the subtle differences in the quality of a sealing solution can have a significant bottom-line impact. As such, it is important for end-users to maintain a solid understanding of current-generation sealing technologies and to employ installation and maintenance best practices to ensure the proper operation and longevity of their sealing systems. To that end, the following article identifies key technology trends and best practices for sealing systems with the aim of providing end-users a high-level understanding of core issues in this important technology segment.
Sealing Technology Today
Today’s seal manufacturers are on a never-ending quest to extend the chemical resistance of seals, as well as their high and low pressure and temperature support. For example, Colin Macqueen, director of technology for Trelleborg Sealing Solutions Americas, says some fluorocarbon elastomers have emerged in recent years that reduce the occurrence of brittleness in sealing systems at lower temperatures. In addition, he says Trelleborg has recently released a perfluoroelastomer that is more affordable than some of the higher-end perfluoroelastomers currently on the market. He says the aim of this new technology is to bring perfluoroelastomer seals to end-users that have typically balked at the material in the past because it was too pricey. He says while the lower-cost perfluoroelastomer may not offer the same upper-level temperature support as the higher-end solutions on the market, it does provide end-users the same chemical resistance.
In the area of rotary seals, Macqueen says rubber seals have evolved in to support high-pressure applications that in the past would have required a much more sophisticated sealing system.
Jim Drago, P.E., for Garlock Sealing Technologies, cites the rise of graphite and PTFE (Polytetrafluoroethylene) as key trends in the area of sealing solutions. He says these materials have taken on a larger share of the sealing applications due to their chemical resistance and temperature resistance. Graphite can handle a wide array of media, with the exception of strong oxidizers and temperatures upwards of 1,200 F depending on the media it’s sealing. Meanwhile, PTFE can handle the media that graphite can, as well as strong oxidizers. It supports temperatures up to 500 F.
In addition, Drago says, “Technology advances in oxidation inhibition, corrosion inhibition and design have given sealing products the ability to last longer, protect the equipment they’re contained in, and seal in a wider range of applications than before.”
The material advances that are currently underway in the sealing segment are enabling a consolidation of technology, so end-users can, in many cases, use the same seal for a range of fluid types. This has taken some of the complexity out of the sealing specification process, as end-users can, in some cases, rely on a single type of seal for application requirements that in the past may have called for a number of different sealing solutions.
|There are sealing systems available today that are capable of relieving pressure in dynamic process environments like the hydraulic cylinder application pictured here.
Photo courtesy of Trelleborg Sealing Solutions
On the systems side, Trelleborg’s Macqueen says there are solutions currently available on the market that provide pressure-relieving capability to protect seals from damage in applications that are susceptible to pressure buildup. For example, he says applications such as injection molding, where the system produces asymmetrical motion (i.e., opens quickly and closes slowly), a pressure-trapping situation can develop and put the seals at risk of damage. With current-generation, pressure-relieving sealing technology, this pressure buildup can be vented to protect the seals from damage.
In addition, Macqueen says there has been significant progress in the area of seal surface coatings to reduce friction on assembly. Elastomers have a high coefficient of friction, which means they tend to stick, either to each other during automated assembly or to mating surfaces when inserted into a housing. Macqueen says there are now many surface treatment options available to the end-user to improve the friction characteristics of elastomer seals by lowering the insertion force required and thereby allowing the operator to reduce the occurrence of damage during the assembly process.
Today’s regulatory environment is also playing a role in the sealing technologies that are coming to market today. “Emissions requirements have become more and more stringent, and to keep pace with these requirements, many products have been developed to seal systems well below required emission levels,” says Drago. In the past, he says, “If it wasn’t dripping, it wasn’t leaking.” Now, however, the threshold for what is considered a leak has been magnified significantly, and the penalties for emissions violations can be steep.
Sealing solution providers are also being pushed by end-users to develop products that can provide a longer meantime between maintenance. As end-users extend their maintenance cycles from the traditional 12 months to 24, 36, and even 48 months in some cases, Drago says they are looking for sealing systems that can last for extended periods of time without the need for maintenance and/or replacement. In turn, he says there are now some compression-packing products on the market that provide up to 10 years of service before they need to be replaced.
Specification, Operation & Maintenance
When specifying a sealing system, there are some obvious pieces of application information the end-user should provide, such as pressure range, temperature range, and the fluids being handled. However, there are also some less obvious details end-users should consider.
Macqueen says it is important to have a realistic understanding of how the seal will be used before engaging with the supplier. Often, he says end-users will come to the supplier with a “wish list” that may not be the best representation of the actual application requirements. For example, he says the end-user may ask for a high-end temperature that is significantly above what the application will realistically experience. By doing this, the end-user runs the risk of over-engineering the sealing solution and/or specifying a solution that sacrifices needed low-end temperature capability. “The more closely the end-user can provide real-world application data, the better we will be able to help them specify the appropriate solution,” says Macqueen. Further, he says it is good to have information on previous failure modes the end-user may have seen in the application under consideration. “We find that customers are sometimes reluctant to discuss failures, but this information is really helpful in identifying high-risk scenarios,” says Macqueen.
When working with end-users to specify sealing systems, Garlock employs the acronym TAMPSS, which stands for:
• Temperature: The first consideration should be the temperature of the fluid contacting the seal, which in rotating equipment will increase due to frictional heat. Knowing this temperature will immediately reduce the number of viable materials.
• Application: How is the seal component to be used — rotating pump shaft, raised face flange, valve stem, etc.? Defining the parameters of the application itself requires what is often difficult-to-obtain, but necessary, information about the equipment in which the seal will be installed. This information also helps determine installation procedures to optimize seal performance.
For gaskets, you need to know the type of flanges on which they will be installed, as well as material and bolting information to determine the amount of compressive force available. These are extremely important factors, since more than 70 percent of gasket failures can be attributed to insufficient load.
Choice of compression packing for pumps and valve stems can depend on whether the motion is reciprocating, helical or rotating.
• Media: Either the common or chemical name of the gas, liquid or solid that will come into contact with the seal can be used to determine its compatibility with the seal material. Also considered should be any secondary media to which the seal may be exposed, such as fluids that are intermittently present during chemical or steam/hot-water flushing. Sometimes the sensitivity of the media to color contamination or extracted materials that may leach from the seal must also be considered.
• Pressure: Most systems operate at fairly consistent pressure, but it is important to take into account any severe spikes or surges that may occur.
• Size: ASME B16.5, B16.20, B16.21 and B16.47 provide standard dimensions for flanges and gaskets. Required information includes nominal pipe size and flange class. Most pumps and valves conform to API/ANSI standards.
Otherwise they must be field measured.
• Speed: Surface speed in FPM at the seal-shaft interface is crucial in selecting pump packing. (Note: The surface speed of two pumps operating at the same RPM with different shaft diameters will differ.) Surface speed indicates how much frictional heat will be generated; high speeds call for materials that can withstand and effectively dissipate heat.
Drago says end-users should be wary of specification requirements that call for “or equal” sealing solutions. He says, “Provider A, Provider B, and Provider C are often not created equal, and you generally get what you pay for.”
During the installation phase, Macqueen says end-users should pay close attention to design recommendations, lubricants, and the required assembly tools. Where possible, he says end-users would also be wise to look for symmetrical sealing solutions that provide installation flexibility, as compared to a seal that can only be installed in one way.
Macqueen says end-users should always consider seal specification before they get too far into the application design. He says waiting until the end of the process to consider the sealing solution is a common mistake that often results in a less-than-optimal solution due to space and/or some other constraint on the sealing system.
In addition, Macqueen says end-users should be mindful of the seals when considering an increase in the duty-cycle of the application. He says a reputable supplier can provide performance thresholds for the sealing solution, and the end-user should document those limits so they can be referenced whenever changes to the duty-cycle are being considered.
When it comes to seal selection, the criticality of the application generally determines how much an organization is willing to spend on a seal. While most organizations evaluate the cost-benefit of reliability versus the up-front purchase price of a sealing solution, Drago says the ultimate decision usually plays out with the critical application trending toward reliability through engineering and the less critical application trending toward the low-cost solution.
Preventing Seal Failure
|The primary cause of seal failure is insufficient load due to improper product selection or installation. The under-compressed PTFE gasket pictured here extruded at the lowest point of compression as internal pressure forced it out of the flange assembly.
Photo courtesy of Garlock Sealing Technologies
Regardless of the criticality of the application, Drago says end-users should work with a supplier that can act as an application partner. He says a tight relationship of this sort can provide significant long-term benefit to the end-user, particularly in the area of application specifications.
For example, Drago says Garlock recently contracted with a customer who was experiencing seal failures several times a month on a single process line. Upon further investigation, Drago says it became apparent that the specification for the application had been modified over the years to bring down the up-front cost of the seals. As a result, the customer was in a situation where the failure of a two-dollar gasket was costing the company nine thousand dollars in lost material. Drago says Garlock was able to work with the customer to not only identify a more reliable sealing solution, but also to tighten up their application specification to prevent a similar problem from occurring in the future.
According to Drago, many seal failures can be traced to poor installation practices. He says end-users would be wise to train their technicians on the subtleties of seal and gasket installation.
A key item to consider when installing seals is the condition of the equipment, such as the finish on flanges, bolting conditions, and stem wear or damage. Drago says when flanges are improperly designed, they are incapable of proper sealing. With oil seals in particular, he says the texture and finish of the shaft must be considered, as any oddities in this area can cause the seal to run dry and/or fail.
Operators generally assume that bigger is better when it comes to gasket thickness. However, Drago says there are many scenarios where a 1/16” gasket is better than a 1/8” gasket. Ultimately, the size of the gasket is not typically the root cause of failure, but rather the failure can be traced back to improper loading techniques. For example, Drago says pumps require a delicate touch to ensure the proper load. “Too many users think ‘more is better’ and tighten the packing set too much, creating a lot of heat through friction,” he says. “A soft touch is required for pump packing.”
In his experience, Macqueen says the primary reason for seal failure
and/or poor performance is that the seal is used in conditions for which it was not originally intended. Also, he says fluid contamination is a big reason for seal failure. For example, he says a closed-loop system that is circulating the same fluid over and over can be particularly susceptible to contamination that would damage seals. To protect against contamination damage, Macqueen recommends end-users periodically sample the process fluid to ensure its integrity. Further, he says it is a good practice to check the seals carefully whenever disassembling the equipment for maintenance and/or upgrades.
According to Drago, end-users should be careful not to be misled by the look of a seal and assume that just because two seals look the same that they will provide the same level of performance. “Many seals look the same or about the same, but [they] perform radically different,” says Drago. “If, for example, a filled, skived, blue PTFE gasket is used in an application where just enough load is available to create an initial seal, the flange will probably leak because skived PTFE has a significant amount of cold flow.” However, he says Garlock’s Gylon gasket, which looks very similar to a filled, skived, blue PTFE gasket, is designed to provide reduced cold flow for low-load applications.
Drago also advises end-users to be cautious when considering overseas-produced graphite and braided products. He says many of these solutions are marketed as low-cost alternatives to established sealing solutions, but they offer a significantly lower level of performance.
Sealing Solutions of Tomorrow
Going forward, Macqueen says end-users can expect to see sealing solutions continue to extend chemical resistance and high and low temperature and pressure limits. Also, like most other fluid handling technology segments, he says smart technology figures to gain prominence in the sealing space in the years to come. He says these smart seals will be capable of, for example, monitoring their own lifecycle and testing for leaks, as well as reporting their cycle count history and any potential failure modes.
Finally, Macqueen says end-users can expect to see more side-by-side partnerships between equipment suppliers and seal suppliers to optimize sealing solutions for given applications and take some of the complexity out of the seal selection process for the end-users.
In the years ahead, Drago says end-users can expect to see consolidation of the available sealing solutions, as technologies for effectively handling a wide range of fluid types emerge.
Another ongoing trend Drago cites is a continued drive toward lower-load sealing requirements. He says the emergence of nonmetallic pipe and lightweight flanges will require solutions that are capable of generating an effective seal under lower loads.
Finally, he says the trend toward green technology and stricter environmental regulations may impact the way seals are manufactured going forward. Whether it’s because they have their own internal green initiatives or they are being forced by stricter environmental regulations, there may come a time when end-users are looking at the carbon footprint of the seals they use. While he acknowledges that this is not really a factor for most end-users at this point, he says there could come a time when the end-users look at their sealing solution and ask, “Are the products we’re using contributing to [greenhouse gas] emissions.”
Matt Migliore is the editor in chief of Flow Control magazine. He can be reached at 610.828.1711 or firstname.lastname@example.org.