Cryogenic

A cryogenic metal seal is specified where the joint must stay leak-tight after cooldown, not only at room-temperature assembly. LNG, liquid hydrogen, and liquid helium each shrink the flange stack, change bolt load, and leave elastomers and many polymers as the weak link. Sonkit treats the seal, groove, plating, and compression as one system.

This page is an application overview for those four duties. For LNG and liquid-hydrogen valve joints, use the industry article Cryogenic Valve Leakage Is Back in Focus: What LNG and Liquid Hydrogen Projects Should Specify in Metal Seals. For liquid-helium MRI and CT hardware, use the technical article Sealing Liquid Helium: Low-Preload Racetrack Metal C-Rings for MRI and CT Scanner Hardware.

Where metal seals are used

LNG valves and cold-box hardware — bonnet and body joints, instrumentation covers, and other static interfaces that see thermal contraction and a long shutdown cost if the joint leaks. Sonkit’s published direction for these locations is a Metal C-Ring or spring-energized Metal C-Ring. Related LNG valve-seat hardware is covered separately in the cryogenic disc / metal seat ring note (LNG carriers, FSRU, FSU, plants, and terminals).
Liquid hydrogen (LH2) valve interfaces — the same cooldown problem, with a smaller, more mobile molecule. The LNG/LH2 article points to C-rings where resilience under cycling is the need, and to a Metal O-Ring where the hardware can take a high seating load and a robust circular geometry. Pressure-assisted valve seats are a Metal U-Ring discussion, because orientation, cavity, and pressure direction control the result.
Liquid helium (LHe) in MRI and CT hardware — MRI magnets, high-end CT assemblies, cryogenic medical equipment, superconducting hardware, and transfer interfaces. The published solution on that page is a low-preload racetrack Metal C-Ring, used so the joint can follow a non-circular land without overloading a soft aluminum flange.

Published temperatures and media

The figures below are media temperatures already published by Sonkit on the two cryogenic articles. They are not a catalog seal envelope and not a leak-rate rating.

Medium / duty	Published temperature	Source
LNG	Normally handled near −162°C	LNG / LH2 valve article
Liquid hydrogen (LH2)	Near −253°C	LNG / LH2 valve article
Liquid helium (LHe), MRI-CT	Near 4 K	LHe MRI-CT article

The C-ring and U-ring product pages list cryogenic service among their working conditions. Those catalog ranges are product-family ratings. Specify the seal from the actual medium, temperature cycle, and groove — not from the family minimum.

Which profile to start with

Metal C-Ring

A Metal C-Ring is the usual starting point for static cryogenic valve joints. The open C section gives elastic recovery when cooldown and warm-up change the flange stack-up, and it needs a lower seating load than a solid metal gasket. Soft plating is part of the contact, not decoration. The standard C-ring page also lists racetrack and other non-circular shapes, which is the geometry used in the LHe MRI-CT article.

Spring-energized Metal C-Ring

Where ordinary springback is not enough — thermal cycling, pressure fluctuation, or relaxation in an LNG or LH2 valve location — a spring-energized C-Ring adds an internal spring to support contact stress. It does not remove the need for a correct compression window.

Metal U-Ring

A Metal U-Ring is useful when system pressure helps energize the lips, typically on a valve seat or another directional-pressure cavity. The U-ring product page lists cryogenic service. The LNG/LH2 article warns that U-rings are sensitive to orientation, cavity dimensions, and pressure direction, so they should be chosen from the load path, not from a catalog image.

What the joint has to survive

The two Sonkit articles make the same engineering point: a seal that is helium-tight at room temperature is not automatically helium-tight after cooldown. Differential contraction changes contact stress. Bolts relax. A scratch or out-of-flat land that passed at ambient can become a leak path after the cycle. On MRI-CT hardware the extra constraint is a soft aluminum land and a racetrack path, so preload has to reach metallic contact without marking the flange for the next maintenance opening.

Confirm the temperature at the seal location, not only the process-fluid number on the datasheet. Extended-bonnet and cold-box valves do not expose every joint to the same cold.
Set the compression window from the real bolt pattern and flange stiffness.
Specify groove finish, flatness, and corner radius with the seal. Helium finds machining marks.
Review repeated cooldown and warm-up, not a single minimum temperature.
For racetrack LHe joints, check straight-section support and corner-radius compression, not only perimeter length.
Do not drop a metal seal into an elastomer groove unless that groove was already designed for the metal profile.

Specification note. “Zero leakage” is not a usable acceptance line unless it names the method, the unit, the test temperature, and the boundary (seat, bonnet, or assembly). Sonkit’s cryogenic articles discuss helium leak testing as the usual verification method. They do not publish a single leak-rate number for LNG, LH2, or LHe service. Send the required leak rate with the drawing.

Industry article: Cryogenic Valve Leakage Is Back in Focus (LNG and liquid hydrogen)
Technical article: Sealing Liquid Helium — racetrack Metal C-Rings for MRI and CT
Company news: Cryogenic disc / metal seat ring for LNG valve service
Metal C-Ring product family — standard and spring-energized pages both list cryogenic service
Metal U-Ring — product page lists cryogenic environments
Contact Sonkit — send the groove drawing, flange stack-up, medium, temperature cycle, available bolt load, surface finish, and required leak rate. Email: johnny@sonkit.cn