Fluid seals for chemical process equipment are commonly made from a host of elastomeric and plastic materials. Selecting a seal material requires careful consideration of several criteria. Presented here are short descriptions of the major criteria for seal selection and a table outlining the properties of several of the most common seal materials.
Seal selection criteria
There are many possible factors that may play a role in which seal materials would perform best in a given application, but here are four major ones.
Temperature capabilities. Each seal has a range of temperatures within which it is designed to be used. Near the service limit for each seal, the performance becomes less certain. At low temperatures, elastomers become harder and less pliable. Also, elastomer seals lose their rubber-like properties with decreasing temperatures. The changes experienced by elastomer seals at low temperatures are physical changes, and are generally reversible.
Fluid compatibility. Seals must be chemically compatible with the fluids they will encounter in the process. Significant swelling and rapid deterioration can occur if not. In addition, factors such as concentration, operating pressure and temperatures, seal geometry and design, must be taken into account. Chemical-resistance guides, which are offered by most seal manufacturers, can be good resources.
Resistance to abrasion and tearing. Depending on the requirements, seals may be required to resist scraping and tearing, as well as avoid small nicks and cuts, which can lead to seal failure. Compounding elastomers with internal lubricants can enhance abrasion resistance.
Ability to accommodate differential pressure. Many seals will experience pressure differentials between the process interior and the external environment, so they should resist extrusion. Harder materials are generally more resistant to differential pressure.
Cost. Although costs can vary widely depending on compounding and processing, relative prices begin with nitrile and chloroprene as the least expensive, followed by EPDM and silicone. Polyacrylate, butyl rubber and HNBR are the next most expensive, then the fluorocarbons, TFE/P and finally fluorosilicones, which are at perhaps 15 times the relative cost of the least expensive materials.
1. Parco, Inc., Elastomer Selection Guide, Parco, 1999.
2. Rogers Corp., Technical Sealing Guide, Rogers, 2012.
3. Chemical Retrieval on the Web, Polymer Properties Database, www.polymerdatabase.com, accessed May 2018.
4. Technical materials guide, www.allsealsinc.com, accessed May 2018.
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