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By Rob Haywood
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.
Before addressing solutions to sealing failures, review the root causes. Traditionally it was believed that:
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.
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."
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.
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.
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.
PlantServices.com is an MRO (maintain, repair, replace, retrofit, overhaul and operations) resource site that features problem-solving articles and editorials for plant maintenance professionals.