SPACE | Resilient Metal Seals

Elastomeric (e.g. Rubber O-rings) and PTFE seals have limited physical properties that are not suitable for many space-based sealing applications. Although some can withstand temporary excursions to high temperature or pressure, they are incapable of being selected as a viable solution for many mission requirements.?Extreme or rapid changes in temperature or pressure, contact with corrosive media, extreme levels of radiation, shock load, vibration, persistent cycling of parameters, or length of mission, often precludes their being excluded from service. The names given to the associated failures modes include thermal degradation, compression set, explosive decompression, cold flow, out-gassing, nibbling, extrusion, absorption, swelling, localized wear, abrasion, stress concentration (e.g. where a weld of a metal spring energizing a PTFE seal creates an uneven amount of sealing pressure), etc.

Extremely low cryogenic fluid or hot gas leakage can be accomplished through the selection of machined metal seal materials and carefully engineered hardware configurations. The effectiveness of the machined metal seal solution is a function of the selection of a sealing material that is compatible with the environment and having sufficient strength properties to withstand pressures and temperatures without failure. As an example, the seal materials selected for cryogenic/gaseous hydrogen service should be resistant to hydrogen embrittlement. The material must also be compatible with any cleaning or purge media that may come into contact with the seal.

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Examples of Machined Metal Seal Materials

• Inconel 718 |?excellent corrosion resistance and physical properties, -450°F to +1,300°F
• Inconel X-750 | corrosion resistance > 300 SS, excellent high temp oxidation resistance, -450°F to +1,200°F.
• A-286 | high heat and moderately resistant to corrosion, -320°F.

Purpose and examples of coatings

?To provide an effective leak-tight joint, a seal must block all leak paths between two mating surfaces. Since no two surfaces are perfectly flat or smooth, deformation of the mating materials must take place to produce an effective machined metal seal. The coating provides a thin deformable surface to fill the micro-asperities in the mating surface.

The selection of a suitable coating is dependent on its compatibility with the system environment and its ability to deform and create an effective seal. In general, the coating which has the highest deformability and is still compatible with the system environment will give the best performance and life with maximum reusability.

• Gold | excellent corrosion resistance | most effective high temperature coating | -450°F to 1,500°F.
• Silver | good corrosion resistance | will oxidize and tarnish in the presence of oxygen | -450°F to 1200°F.
• PTFE | inert to most chemicals| effective seal on rough surfaces | - 450°F to 480°F.

Hardware design considerations

Engineering of a very low leak sealing location requires careful attention to not only the seal and coating but also to the hardware, flanges and bolting/fasteners. Additional application considerations include any supplemental action that may occur after the seal has been achieved. (e.g. disassembling of the sealing location, access the location for leakage measurement, purge or cleaning impacts, etc.) The key aspect of a successful sealing location is that the seal maintains constant flange contact under all operating conditions through the application of appropriately preloaded fasteners/bolting.

The cavity surface finish is the single most important item in seal performance. The machined metal seal must be designed with sufficient loading to cause seal coatings to flow into cavity micro-asperities and create a seal, but that flow is limited. Pressure and temperature fluctuations cause small movements of the seal relative to the cavity, resulting in wear on the seal and cavity. Proper cavity finishes limit the wear to the ability of the coating to flow and maintain a seal.

• For optimum performance finishes should be 12 rms or better.
• Coated seals will operate on a 16 – 32 rms finish with circular lay, but with reduced performance.
• PTFE coated seals will perform satisfactorily on a 32 rms finish.

Lack of parallelism in a cavity can cause excessive loading on one side of the seal and insufficient loading on the other resulting in poor performance and reliability. For optimum performance with metallic plated seals, cavities should be parallel within .0005”. For general service with metallic plated seals and for PTFE coated seals, cavities may be parallel within the tolerance of the cavity height.

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