The short answer
The core of an LNG cryogenic valve is cryogenic material + extended bonnet + cryogenic type testing. Selection commonly follows BS 6364 (general requirements for cryogenic valves), with materials stepping down from low-temperature carbon steel to cryogenic nickel steel and austenitic stainless steel, and a minimum design temperature down to −196 °C.
Temperature and materials
LNG is stored at approximately −162 °C at atmospheric pressure; liquid nitrogen is around −196 °C. Materials are selected step by step as temperature falls:
- Low-temperature carbon steel: ASTM A352 LCB/LCC and A350 LF2 forgings, approximately −46 °C, requiring Charpy impact testing at the corresponding temperature.
- Cryogenic nickel steel: A352 LC1/LC2/LC3 (1.5/2.25/3.5Ni), approximately −59 / −73 / −101 °C respectively.
- Austenitic stainless steel: A351 CF8M / A182 F316, suitable to −196 °C, with good low-temperature toughness and widely used for LNG.
- Duplex / nickel-base alloys: for highly corrosive or special cryogenic duty.
- All cryogenic pressure-retaining parts must undergo low-temperature impact testing at the corresponding temperature, with material certificates (EN 10204 3.1/3.2) verified.
Design requirements
- Extended bonnet (cryogenic extension): lifts the packing and actuator away from the cryogenic medium into their permissible operating temperature range, preventing icing, seizure and excess fugitive emissions.
- Top entry: allows the ball and seats to be serviced in line; common in LNG terminal tank areas.
- Anti-blowout stem and anti-static devices.
- Cavity overpressure protection: cryogenic media expand when warmed, so the cavity needs a pressure relief path (DBB or a relief hole).
- Sealing: soft seats (PTFE/PCTFE, PEEK) can achieve ISO 5208 Rate A zero leakage; for wide thermal cycling or fire-safe requirements, use a triple-offset metal-seated design.
Standards and testing
- BS 6364: general requirements for cryogenic valves, covering cryogenic materials, structural design and cryogenic type testing; a commonly referenced standard on LNG and cryogenic projects.
- ISO 21010 (valves and related products for cryogenic service) may be used as a supplement; the Chinese equivalent is GB/T 24925-2019 cryogenic valves.
- Type testing typically covers: operation and sealing cycles at cryogenic temperature, verification of sealing at ambient temperature, plus ISO 15848 fugitive emission and API 607 fire tests where required.
- For demanding duties, helium leak testing can be added to verify micro-leakage.
Typical application points (LNG terminal)
Loading arms, tank inlet and outlet, submerged pumps, BOG compressors, vaporisers and export metering — each point must be selected by medium, temperature, pressure class and size, with the actuator's cryogenic suitability checked at the same time.
Common misconceptions
- Using ambient-temperature valves in cryogenic duty: no extended bonnet and no low-temperature impact data, leading to frozen packing and severe external leakage in service.
- Testing only at ambient temperature without cryogenic cycles: this cannot demonstrate sealing and operational reliability at low temperature.
- Ignoring actuator suitability for low temperature: lubricants and seals in pneumatic and electric actuators must also suit the cryogenic environment.
Vexval cryogenic valves can be supplied with materials, design, impact testing and type test documentation organised to BS 6364, suitable for LNG and cryogenic service.