Using metal seals can be a viable alternative to polymer and rubber seals. They are used extensively in extreme applications, such as rocket engine nozzles and missile guidance housings, as well as in ultra-high-vacuum equipment and cryogenic transfer systems. This article focuses on where metal seals are used and the pros and cons of their use.
Where Using Metal Seals is a Good Approach
Metal seals do their best work in extreme environments where polymer and elastic seals typically fail, including extremely high temperatures (above 200°C) and pressures.
For example, in the oil and gas industry, they are used with wellhead equipment, subsea Christmas trees, valve bonnets, and high-pressure pipework, which require metal ring gaskets (such as API ring joint gaskets) because they can withstand extreme downhole pressures and resist corrosive fluids.
In aerospace and defense, metal seals are found in rocket engine nozzles, spacecraft propulsion systems, and missile guidance housings. Such applications rely heavily on metal seals to maintain integrity through thermal cycling, vibration, and vacuum conditions.
The nuclear industry uses metal seals in reactor pressure vessels, containment flanges, and coolant piping. Metal seals (often manufactured from Inconel or stainless steel) perform extremely well because they resist radiation embrittlement and provide reliable long-term sealing without the problems associated with outgassing.
Semiconductor and vacuum systems employ metal seals in ultra-high-vacuum (UHV) equipment such as particle accelerators, electron microscopes, and chip fabrication chambers. These applications often use knife-edge metal seals (CF flanges) to achieve near-zero leak rates without risking contamination in clean environments.
Metal seals are also used in cryogenic applications, including liquid nitrogen or liquid hydrogen storage and transfer systems. Polymers become brittle at cryogenic temperatures, while soft metals like aluminum or copper remain ductile, making them an ideal solution for use at cryogenic temperatures.
Engineers also use metal seals extensively in chemical processing applications, such as reactors and heat exchangers, where aggressive acids, solvents, or oxidizers (which use metal spiral wound or ring gaskets) are used. Metal seals work well where the chemical compatibility is a significant concern.
Benefits of Using Metal Seals
Metal seals used for several reasons. First, metal seals do not experience creep or relaxation when exposed to heavy, sustained loads, which can be a major issue with some elastomers and polymers. Depending on the material chosen, metal seals can also be engineered to be chemically inert to most process fluids, including strong acids, solvents, alkalis, and oxidizers. In addition, they are not subject to permeation or outgassing issues that can occur with some elastomers. They are also inherently fire safe because of their high melting point, and can handle some of the most aggressive cleaning and sterilization processes.
Metal seals can also survive extreme thermal cycling without experiencing permanent set. In fact, metal seals exhibit excellent temperature resistance, with some metals capable of withstanding temperatures from -270°C to over 1000°C, which cannot be achieved with a polymer or elastomer. They can also handle extreme pressures, from ultra-high vacuum pressures to tens of thousands of psi. Finally, they experience a long service life in static applications and do not have a limited shelf life or aging concerns.
Things to Keep in Mind when Using Metal Seals
Metal seals do require stricter surface finish requirements than polymer and elastomeric solutions, and a high seating stress is required to achieve a reliable seal. Most metal seals are single-use, and the cost of their replacement can add up quickly over the equipment’s life cycle. Metal seals are also more sensitive to misalignment and vibration, which makes them unsuitable for applications involving thermal distortion across joints of movement.
The raw material costs of metal seals (e.g., Inconel, stainless steel, titanium) and the precision machining required to manufacture them make them more expensive than their polymer and elastomeric options. In addition, soft metals (e.g., copper, aluminum, soft iron) can gall or seize against the flange during installation, which can lead to expensive rework. Metal seals can also be difficult to disassemble and are not suitable for low-pressure applications. Finally, metal seals have limited self-healing properties.
Conclusion
Metal seals have applications where they significantly outperform their elastomer and polymer counterparts, including those involving temperature and pressure extremes. If you are in the process of selecting a metal seal for your design, contact the seal experts at Advanced EMC today.
