^ Elastomeric seals such as rubber gaskets and O-rings

Article by Lucien Joppen

Although elastomers have been widely used, they do have limitations. In a fluid containment system, interfaces between different components (for example, the joint between two pipes) present an opportunity for fluid to escape from the system. Therefore, any fluid containment system – whether a gas pipeline or a central heating system – typically requires sealing.

The conventional sealing solution is to sandwich a layer of soft elastomeric material in the interface between any two parts of the system. A simple example is using a rubber O-ring to seal the joint between two metal pipes: place an O-ring in a groove at the end of one pipe, then connect the pipes using a compressive force (e.g., via a bolted flange or screw connection). Compression deforms the soft elastomeric ring, causing it to conform to the shape of the interface between the two pipes and acting as a load-bearing component.

Greater range

Non-metal-to-metal seals such as these are typically easy to fit, require low compressive forces, and provide a water- or air-tight seal in typical applications. For many years, these seals were versatile and provided sufficient performance to meet the needs of the industry.
However, over the last 50 years or so, process requirements have evolved beyond the capabilities of elastomeric seals, with metal-to-metal seals taking their place in high-performance applications.
“Metal-to-metal seals offer a much greater range of operating temperatures than elastomeric seals.
If too cold (below around 50°C/32°F), elastomeric materials undergo a glass transition, losing elasticity and mechanical stress resistance”, a representative of Saint-Gobain Seals states. “Too hot (250°C/482°F) and an elastomer can melt or degrade. This is not the case with metal-to-metal seals, which can operate at temperatures above 1,000°C/1,832°F.”
Elastomers also have strict pressure limits. If the differential pressure across a seal is too high, elastomers are prone to destruction by extrusion. “Metal-to-metal seals easily outperform elastomers in this regard, providing effective seals beyond 25,000 psi.”

Applications of metal-to-metal seals

As metal-to-metal seals offer required fatigue resistance and sealing characteristics, they are widely used for the most demanding high-pres sure and high-temperature applications.High operating temperatures in jet engines may pose a challenge to polymer seals in aerospace applications. Instead, metal-to-metal seals are selected for these applications, which offer low leakage rates and resistance to thermal cycling.
Metal-to-metal seals can easily handle the rapid thermal cycling that renders elastomer seals incompatible with many pressure vessels, pumps and valves in the petrochemical, chemical and plastics industries. Thanks to plating, metal-to-metal seals also offer excellent chemical compatibility, providing a durable and corrosion-resistant seal.
The nuclear industry places a significant demand on metal-to-metal seals which are exposed to high temperatures and radiation. Therefore, these seals must exhibit superior performance to prevent dangerous radioactive leaks. Metal-to-metal seals are a proven solution for the many valves, fittings, piping systems and large enclosure vessels used in nuclear reactor systems.

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