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There are some applications where polymer seals or elastomeric seals just don’t provide the leak resistance, resilience, and reliability that is needed.
  

Such challenges can come from a variety of sources, including combinations of extreme pressure, extreme temperatures, and extremely corrosive media. The answer in those situations lies in metal seals.  Chemical and plastic processing is one of the major areas where metal seals have proven invaluable, especially in valves and pumps.  The focus of this blog post is metals seals for valves and pumps, where we will discuss three commonly used types of seals and their respective capabilities.

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C-Rings

C-rings are internally pressurized static face seals.  Also known as c-seals, they work well in situations where there are significant pressure fluctuations combined with extreme temperatures, from cryogenic to extreme heat. By using a c-shape, these seals can be designed to provide significant spring back and good leak tightness.  Leak resistance can be increased by using a plating or coating on the c-ring. These seals are an excellent choice for valve assemblies, and can be either internally or externally pressurized.

C-rings work very well in high temperature valves, plastics injection molding, blow molding, hot runners, extrusion, and pressure vessels.  They are considered a moderately priced sealing technology and make use of the system pressure to maintain a seal during separation of the joint of thermal fluctuations.

Spring Energized C-Rings

Another sealing option takes the C-ring a step further by spring energizing it. These spring energized c-rings are designed for applications that combine high loads with high leak tightness. Their spring back, however, is low compared to other metal seal designs.  These seals provide excellent performance even in the presence of cryogenic temperatures, and they work from low pressures to extreme vacuum pressures.  By combining the right seal material and coatings, these seals can be designed to resist corrosion and other forms of degradation.  

Spring energized c-rings are a common sight in chemical processing systems because of their ability to perform well in extreme conditions.  They are a good choice for situations where the mating surfaces are non-flat and provide one of the lowest leak rates of all metal seals.  They are also one of the best choices for preventing leaks in high vacuum applications.

O-Rings

O-ring seals are limited to static applications, but perform extremely well in situations where elastomeric or polymeric seals would likely fail due to outgassing or deterioration.  These seals are typically made from tubes of high-temperature nickel alloys or stainless steel.  They work for cryogenic through high temperatures, and both high pressure through vacuum pressures.  O-ring seals provide for medium to high leak tightness.

These seals are often the lowest price option, and are probably one of the oldest options because their usage can be traced back to the early 1900s.  They are often used with polymer melt stream flanges, plastics hot runners, melt stream filters, polymer extrusion seals, and combustion seals.

Conclusion

When designing a seal for an aggressive environment, don’t forget that metal seals are a viable option.  They can provide the sealing power needed in some of the most challenging combinations of extreme pressures, extreme temperatures, and corrosive media.  In the plastics industry, the most common metals seals include c-rings, spring energized c-rings, and o-rings.

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