Introduction to Perfluoroelastomers - Part 3

Gallagher recently released our Introduction to Perfluoroelastomers White Paper, available for download on our site.  This was written by Russell Schnell, a current contracted employee of Gallagher Fluid Seals, and more importantly, a former Senior Application Engineer with the Kalrez® perfluoroelastomer parts business at DuPont.  The following is the third and final excerpt from the White Paper, discussing seal design and a cost-benefit analysis of using perfluoroelastomer seals.

Seal Design with Perfluoroelastomer Seals
Care must be taken when designing and using seals made of perfluoroelastomers. These elastomers typically have a higher coefficient of thermal expansion when compared to other elastomers; plus, they are often used at higher temperatures. If the seal gland design is not correct, seal extrusion will occur, resulting in seal failure. For example, a fluoroelastomer seal is scheduled for replacement with a perfluoroelastomer seal, due to high application temperatures. Shortly after this substitution, the FFKM seal fails due to extrusion. The probable cause is that the seal gland volume was too small to accommodate the thermal expansion of the high performance perfluoroelastomers, a factor that many of today’s seal design handbooks do not adequately take into account.

There are other subtleties of FFKM seals that must be considered. For example, manufacturers often have numerous FFKM products to choose from. Each product formulation is optimized for specific chemicals and/or application temperatures. Proper material selection is critical for optimum performance. When switching from a lower performing elastomer to an FFKM seal in existing equipment, or for new seal designs, it is critical to verify the design with knowledgeable engineers to assure the best results.

Cost-Benefit Analysis
In industry today, there is always the need to drive down costs. As process systems become more demanding, the equipment and seal requirements become more stringent. Unfortunately purchasing may not fully appreciate and understand all the nuances related to material selection.  Their primary consideration may simply be to drive down costs on parts such as elastomeric seals. In this case, perfluoroelastomers will almost always be the most expensive material of choice. So it is imperative to run a cost-benefit analysis in order to choose the correct elastomer seal for an application.

As an example, a plant may decide to use a fluoroelastomer seal, at a cost of $2, over a perfluoroelastomer seal at $20 each. The material selection may be influenced and guided by the purchasing department, which has the task of reducing expenditures. However, many other factors must be considered in material selection. These include: maintenance cost to replace the seal, lost production due to downtime while the seal is replaced, and environmental and safety considerations if there is product leakage. Taking these considerations into account, even a small increase in performance for an FFKM seal may more justify the increase in part cost compared to an FKM seal. Close communication between plant engineering and purchasing is necessary to ensure that a cost-benefit analysis is used for elastomer seal purchases, as well as other equipment purchases.

This chart illustrates the issues that must be considered when evaluating the total cost involved in seal purchase and application.
A comment often heard during material discussions is, “I can’t use a perfluoroelastomer seal because it is too expensive.” The more enlightened comment should be, “Can I afford not to use a perfluoroelastomer seal in this application?”

About the Author
Russell Schnell spent more than 37 years as an engineer with DuPont, the last 26 years  as a Senior Application Engineer with the Kalrez® perfluoroelastomer parts business. Recognized for his expertise in elastomer applications, seal design and failure analysis, he provided technical support for a wide range of industries including: chemical processing, aerospace, oil and gas, pharmaceutical and semi-con. He created and conducted hundreds of training seminars and workshops in this field and was solely responsible for the development of the Kalrez® Application Guide software tool.

Russ received a Bachelor of Science in Chemical Engineering from Columbia University in New York and MBA from the University of Delaware.