Valves for cryogenic and liquefied-gas applications: LNG, hydrogen and ammonia
At cryogenic temperatures the rules change. Material toughness, contraction, sealing and operability behave differently than at ambient, and the valve must stay tight and operable from ambient temperature down to deeply sub-zero — often in an environment where leakage is not an option. This applies to LNG, liquid hydrogen, ammonia and other liquefied gases — media whose importance is rising sharply with the energy transition.
Different conditions at cryogenic temperature
At cryogenic temperature the material behaviour changes fundamentally. Materials that are tough at ambient temperature can become brittle; liquids evaporate at the slightest heat ingress; and the whole valve contracts. A design that performs at ambient temperature is therefore not yet suitable for cryogenic service — the behaviour at temperature is decisive.
Material selection: toughness at low temperature
The key requirement is that the material stays tough and does not fracture in a brittle manner at cryogenic temperature. Austenitic stainless steels retain their toughness where carbon steel turns brittle. Material selection for body, disc, shaft and seal therefore becomes a safety matter, not only a strength matter.
Thermal contraction and the extended neck
Everything contracts on cooling, and different parts differently. The design must accommodate that contraction so the valve stays tight and operable. An extended bonnet or neck also keeps the packing and operation away from the cold — so the stem seal stays in a workable temperature range and gas formation remains manageable.
Sealing and operability from ambient to deeply sub-zero
The valve must be tight not only when cold, but throughout the temperature range — from commissioning at ambient to operation deeply sub-zero, and back. The seating concept, the stem seal and the operating forces must all be right across that whole range. A valve that performs well only cold or only warm is not suitable.
Why the more common metal-to-metal design leaks at cryogenic temperature
Metal contracts predictably as long as a component has a uniform wall thickness everywhere. If the sealing geometry is not uniform, the parts contract differently — where one contracts most, the other contracts least — and the seal leaks in deep cold, even though the same valve is bubble-tight at ambient temperature. Commonly used graphite-laminated sealing is then no help: below about −196 °C graphite is no longer suitable to bridge the difference. EURAD therefore chooses a butterfly-valve design (5-offset) whose sealing surface is a true circle that contracts uniformly, regardless of valve size or temperature — so the metal-to-metal seal stays tight even with LNG, liquid nitrogen and other cryogenic media. With, where desired, an Inconel O-ring seal for a fully metallic, graphite-free seal.
Leak tightness, flow and maintenance
The 5-offset design seals bidirectionally to leakage rate A — zero leakage in both flow directions — and does so with a lower required closing torque (Md) and a higher flow capacity (Kv) than previous designs. In a top-entry design the valve can moreover be fully maintained in the line: replacing the seal and visual inspection are possible without removing the valve from the pipeline — ideal when the valve is welded into the line.
Hydrogen and ammonia
Liquid hydrogen, at around −253 °C, is even colder than LNG; that sharpens the demands on material toughness and thermal contraction further. Hydrogen is moreover a very small molecule, which places high demands on leak-tightness — preferably a metal-to-metal seal with demonstrable zero leakage, and a material selection that prevents hydrogen embrittlement. Ammonia, emerging as a hydrogen carrier, is handled in liquefied form — refrigerated at around −33 °C or under pressure, so not cryogenic but low-temperature — and is toxic and corrosive: that calls for material compatibility (no copper alloys) and likewise leak-tightness for safety. The sealing and material principles in this note apply to both.
Testing and certification
For cryogenic service, low-temperature testing is standard — covered among others by BS 6364 (valve testing for cryogenic service) and ISO 28921 (cryogenic isolating valves); leak-tightness is tested to EN 12266-1 (rate A). Where the medium is flammable — LNG, hydrogen — fire-safe requirements are added; the combination of cryogenic and fire-safe sets requirements that must not get in each other’s way. EURAD supplies the corresponding testing and documentation for its scope of supply.
Selection per situation
Temperature, medium, pressure, tightness and safety requirements and the required certification together determine the choice. For liquid oxygen (LOX), ignition requirements apply on top of these cryogenic requirements; these are set out in the oxygen note. EURAD advises and supplies on the basis of the application.
