Seal the Deal on Gasketing Performance

The right joint sealant can yield significant cost savings.

Technical developments in the fluid sealing world have resulted in a growing number of gasket product options on the market. Some truly innovative products have been introduced in an attempt to meet worker safety needs and contribute to a cleaner environment with reduced emissions. These products also address increasingly important economic considerations.

This proliferation of products presents opportunities for users to achieve better sealing performance to meet specific needs. At the same time, however, the gasket qualification and selection process has become more challenging than ever. Organizations like the American Society for Testing & Measurement (ASTM), the Pressure Vessel Research Council (PVRC) and the Fluid Sealing Association (FSA) have developed gasket performance test methods to help manufacturers and end users work together to find the best sealing solutions.

Studying these test standards provides insight into the expected performance of a gasket in "real world" situations with regard to gasket tightness (how well a gasket seals initially) and long-term mechanical integrity (how reliably a gasket will seal). Even so, there is no single test that can point conclusively to the best gasket solution.

What Causes Seal Failure?

Before addressing solutions to sealing failures, it is worthwhile to review their root causes. The traditional perspective about what causes leaks includes the beliefs that bad gaskets cause leaks, using a bigger gasket is more effective and applying more torque will guarantee a tighter seal.

More users today are aware of the importance of other sealing dynamics aside from the gasket, such as the flange and bolt hardware and proper installation techniques. In fact, the discovery that most seal failures are not gasket-related was documented in a study commissioned by the PVRC. Furthermore, the majority of the cases cited as "gasket defective" involved improper gasket selection for the application-either incompatibility with process media, or operation outside the gasket's recommended temperature range.

Joint Sealants-Reliable and Affordable

One gasket category merits increased attention today-joint sealants. Joint sealants are expanded-PTFE gasketing materials sold in continuous lengths on spools. These "cords" or "ropes" of gasketing are formed in place on the flange surface during installation, with the aid of an adhesive backing. The end is overlapped near a bolt hole, and the product is cut to the proper length.

 Joint sealants represent one viable option for achieving a tight seal. Sometimes they are disregarded because of performance misperceptions. A common misperception of joint sealants is that they are not really a "true" gasket. Some maintenance engineers suspect that if a gasket is not a continuous, one-piece design, the gasket will leak.

Joint sealant is installed by forming the "rope" into a ring on the flange surface and overlapping the ends. In this process, the overlapped section of joint sealant, which is soft and conformable prior to compression, "meshes" with itself upon compression in the flange, thereby forming a thin, wide, continuous seal. The conformable nature of this material allows the gasket to fill in all the areas of micro-deviation across the flange surface, resulting in an extraordinarily tight seal.

One benefit of this tight seal is the control of fugitive emissions. These trapped emissions not only reduce the risk of environmental pollution and workplace hazards, but they also represent meaningful savings in terms of recaptured "lost" product. As J. Ronald Winter says in Gaskets and Gasketed Joints, "Ethylene gas at a price of $0.58 per liter would cost the user $905 per year at a leak rate of 3 cc/min. If this leak rate were reduced to 0.25 cc/min, the cost would be cut to $75 per year, a cost reduction of $829.50. Savings will typically more than justify the use of a more expensive gasket and/or flange."

Less Waste, More Savings

The higher initial cost of joint sealants is cited by some maintenance engineers as a negative factor. Of course, other negatives-employee health hazards, environmental pollution, product losses-can add up to significant expenses as well, usually far in excess of the cost of the original gasketing material. These kinds of savings, however, are difficult to quantify, so they are not usually factored into cost comparisons. Consequently, pressures to reduce costs still continue to focus attention on product price as the deciding factor.

Even without considering the "intangibles" cited above, the economic benefit from using joint sealant can be justified from a pricing perspective. None of the costs associated with scrap from large cut gaskets exist with joint sealant, often giving the form-in-place gasketing products a price advantage right from the start. Accordingly, as flange diameter increases, the upfront cost savings provided by joint sealant are even more pronounced.

"Creepy" Considerations

Some maintenance engineers believe that joint sealants will not stand up to use because of the inherent composition of PTFE. The beauty of PTFE lies in its chemical structure as a fully-fluorinated backbone carbon chain. The strong bonds between the atomic components that make up PTFE provide high chemical resistance properties. Unfortunately, however, other properties must be sacrificed to provide these gains. Without chemical cross-linking or electrochemical attraction to itself, PTFE does exhibit a flow characteristic, especially when subjected to high stress and extreme temperatures.

Back in the late 1940s, when generic PTFE gaskets were first introduced to seal highly aggressive media, they quickly earned the reputation of requiring constant retorque. In the ensuing years, this stigma cast a shadow on the credibility of PTFE as a gasket material, despite a number of technological innovations developed by various manufacturers to address this challenge.

One approach that dramatically lowers the long-term creep characteristic of PTFE is the expansion process, resulting in a product known as expanded PTFE (ePTFE). This process mechanically generates longitudinally-oriented fibers within the PTFE structure, adding strength. The newly added strength reduces the level of gasket creep in a flange. Lower creep means the bolts maintain a higher load during operation, minimizing the risk of seal failure.

Of course, not all ePTFE gasketing materials are the same, nor do they perform equally well. Many industrial users have learned the hard way that there can be dramatic performance differences among various joint sealant products. They have come to realize that the costs associated with seal failure always outweigh initial price concerns. Gaining a $10 savings upfront is not worth the risk of a $100,000 shutdown!

Choosing a joint sealant can be difficult, because these products generally look and feel the same. In fact, there are large variations in both the degree and quality of expansion among the various ePTFE sealants on the market. Products that are not fully expanded have less fibril structure, which means they are generally weaker, and have greater risk of long-term creep problems in operation. Extreme variation in product density is often found in lower-grade or lower-priced joint sealants-not only from spool to spool, but also within each spool itself. Variations in density result in inconsistent performance and can produce uneven loading within a flanged joint, with leaks likely to occur in areas of lower gasket density.

Choosing the Right Joint Sealant

Most maintenance engineers who study the available options discover that by incorporating joint sealants into their gasket program, they can achieve improved sealing reliability with significantly improved tightness and long-term performance gains. Installation time and cost are generally reduced. However, there are several important factors to remember when selecting a joint sealant that is best for a particular application and will provide reliable performance:

• Buy from a trustworthy manufacturer-one that responds to questions and needs for technical support.
• Review the technical differences among the products, particularly third-party test results. Ask for the original reports; do not settle for the manufacturer's "interpretation" of third-party test data.
• Because joint sealants look and feel similar, seek out a product that is brand-identifiable.
• In addition to variations in ePTFE quality, there may be significant differences in adhesive tack. Ensure the chosen joint sealant guarantees ease of installation.
• Be wary of products primarily promoted as "lower priced." Users find that the true cost-the risk of seal failure-will outweigh this otherwise enticing attribute.
• Always evaluate the levels of support and expertise, as well as product availability, from a local supplier.

Additional evolution is taking place in the gasket category. Manufacturers continue to educate users about thinner gaskets that can seal more effectively, as long as enough thickness is available to compensate for flange surface micro-deviation. There are also specialty form-in-place gasket tapes that incorporate fibers oriented in multiple directions. This "next generation" of form-in-place gaskets promises to provide even greater creep resistance and dimensional stability. As industry awareness of these innovations continues to grow-and more industrial users benefit from the enhanced performance and cost savings they deliver-the result will be lower overall sealing costs.

Pumps & Systems, May 2009