Metal-To-Metal Seals for High Temperature & Pressure
When You Need Resilience & Performance
By Rebecca Phan (August 2021)
Elastomeric seals such as rubber gaskets and O-rings have long been the “default” sealing solution, finding use in virtually all industries as well as domestic applications. However, despite their ubiquitous presence, elastomeric seals are prone to failure at high temperatures and pressures. For the most demanding applications – such as in cryogenic systems, nuclear plants and aircraft – only metal-to-metal seals can provide the necessary resilience and performance. This article takes a closer look at the benefits and applications of metal-to-metal seals.
The Limits of Elastomeric Seals
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 to this problem 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, as well as acting as a load-bearing component.
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. Indeed, for many years, elastomeric seals provided sufficient versatility and performance to meet the needs of 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 such as below 50°C (32°F), elastomeric materials undergo a glass transition, losing elasticity and mechanical stress resistance. Too hot around 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 in excess of 1,100°C (2,000°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
Since metal-to-metal seals offer required fatigue-resistance and sealing characteristics, they are widely used for the most demanding high-pressure and high-temperature applications.
High operating temperatures in jet engines may pose a challenge to polymer seals in aviation applications. Instead, for these applications, metal-to-metal seals are selected, which offer low leak rates and resistance to thermal cycling.
Chemical, Plastics and Petrochemical
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 the best chemical compatibility, providing a durable and corrosion-resistant seal.
The nuclear industry places huge demand on metal-to-metal seals: as well as exposing them to high temperatures and radiation, 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.