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Joint sealants to the rescue

Sept. 14, 2005
They represent a more viable gasketing option than ever before

Technical developments in fluid sealing have resulted in a growing number of gasketing options. Truly innovative products have been introduced to increase worker safety, reduce emissions for a cleaner environment and improve life cycle costs.

While users can now achieve better sealing performance to meet specific needs, the gasket qualification and selection process has become more challenging. Organizations such as the American Society for Testing & Measurement (ASTM), the Pressure Vessel Research Council (PVRC) and the Fluid Sealing Association (FSA) have developed a new generation of gasket performance test methods to help manufacturers and users work together to find the best sealing solutions.

Studying these new test standards provides insight into the expected performance of a gasket in real world situations such as gasket tightness (how well a gasket seals initially) and long-term mechanical integrity (how reliably a gasket will seal over time). However, there is no single test that points conclusively to a universal gasket solution.

Seal failure causes

Before addressing solutions to sealing failures, review the root causes. Traditionally it was believed that:

  • Bad gaskets cause leaks.
  • A bigger gasket is more effective.
  • Applying more torque guarantees a tighter seal.

More users are becoming 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, a study commissioned by the PVRC indicates most seal failures aren't gasket-related. 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. Figure 1 shows the PVRC's root causes of seal failure.

A permanent gasket

One gasket category that merits increased attention is joint sealants. They're expanded polytetrafluoroethylene (PTFE) gasketing materials sold in continuous lengths on spools. These cords or ropes of gasketing are looped on the flange surface and overlapped near a bolt hole before being cut to length. An adhesive backing holds the material in place.

Joint sealants represent a viable option for achieving a tight seal. However, at times they aren't considered seriously because of misperceptions about performance. For example, one persistent erroneous notion is that joint sealants aren't really a true gasket. Some maintenance professionals still believe a discontinuous gasket is more apt to leak.

Overlapping the ends renders the joint sealant continuous because the soft and conformable material meshes with itself upon flange compression. The result is a thin, wide, continuous seal. In addition, the conformable nature of expanded PTFE allows the gasket to fill in voids and micro-deviation across the flange face, resulting in a tight seal.

Tight seals control fugitive emissions. Trapped emissions reduce the risk of environmental pollution and workplace hazards, and represent meaningful savings in terms of recapturing lost product. To quote J. Ronald Winter's example 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 was reduced to 0.25 cc/min, the cost would be cut to $75 per year, a cost reduction of $829.50. Savings typically will more than justify the use of a more expensive gasket and/or flange."

Speaking of expenses

Some maintenance engineers cite the cost of joint sealant as a negative. But the costs associated with employee health hazards, environmental pollution and product loss are usually far in excess of the cost of the original gasketing material. Unfortunately, because these savings are difficult to assess, they usually aren't factored into cost comparisons.

Despite the gray areas, the economic benefit from using joint sealant can be confirmed from a pricing perspective. Joint sealant has none of the cost associated with scrap from large, cut gaskets. And as flange diameter increases, the up-front cost savings joint sealants can provide are even more pronounced.

"Creepy" considerations

Some maintenance engineers believe joint sealants won't stand up to use because of the inherent composition of PTFE. The chemical structure of PTFE is a fully fluorinated backbone carbon chain. PTFE's strong atomic bonds give the sealant its characteristic chemical resistance. Unfortunately, other properties are 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. Figure 2 illustrates PTFE's molecular structure.

Figure 2: PTFE's molecular structure

Generic PTFE gaskets were first introduced in the late 1940s for sealing highly aggressive media. They quickly earned the reputation of requiring constant retorquing. This stigma has cast a shadow on PTFE's credibility as a gasket material, despite a number of technological innovations.

One approach that reduces PTFE's tendency for long-term creep is the expansion process, resulting in a product called expanded PTFE (ePTFE). This process produces longitudinal fibers within the PTFE structure, which adds mechanical strength and reduces gasket creep. Flange bolts then can maintain a greater stabilized loading, minimizing the risk of seal failure.

Of course, ePTFE gasketing materials are not identical, nor do they perform equally well. Many industrial users have learned bitter lessons about the dramatic performance differences among joint sealants. The cost associated with seal failure always outweighs initial price.

Deciding which joint sealant to use can be difficult, because they generally look and feel the same. There are variations in both the degree and quality of PTFE expansion among sealants. Products that aren't fully expanded have less fibril structure. They're generally weaker and exhibit a greater risk of long-term creep.

Lower-priced joint sealants also exhibit extreme variation in product density — not only from spool to spool, but within a spool. Density variations result in inconsistent performance and uneven loading, with leaks likely to occur where gasket density is lowest. The magnified cross sections in Figure 3 illustrate the difference between a premium grade sealant and an alternative.

Comparative tightness
Figure 3. Jim Payne, "Bolted joint improvements through gasket performance tests," 1992 NPRA, MTI Publication #36, WRC Bulletin #391, HOTT test run by the TTRL.

The right sealant

Most maintenance engineers who study the options discover incorporating joint sealants into a gasket program achieves improved seal reliability, improved tightness and long-term performance gains. Installation time and cost are generally reduced, as well.

Specifiers should consider several important factors when selecting a joint sealant that will provide reliable performance over time:

  • Buy from a manufacturer you believe in--one that stands on integrity and follows through by responding to questions and requests for technical support.
  • Review the technical differences among products, particularly third-party test results. Ask for the original reports--don't settle for anyone's interpretation of third-party test data.
  • Because joint sealants do look and feel similar, be sure to seek out a product that is brand-identifiable.
  • In addition to variations in ePTFE quality, there may be significant differences in adhesive tack. Make sure the joint sealant you use guarantees ease of installation.
  • Be wary of products promoted primarily as lower priced. Users find the true cost — the risk of seal failure — greatly outweighs this otherwise enticing attribute.
  • Evaluate the level of supplier support and expertise, as well as product availability.

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

Rob Haywood is product specialist at W. L. Gore & Associates, Inc., Elkton, Md. Contact him at [email protected] and 410-392-3200.

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