Cryogenic Valve Specification

Temperature classification for low-temperature valves

There is no single definition of cryogenic, but the industry generally uses the following temperature ranges: Low-temperature service: -29°C to -100°C — covered by A352 LCC (Charpy tested to -46°C) for carbon steel, or A351 CF8M (316 SS) which is ductile to -196°C. Cryogenic service: below -100°C — typically LNG (-162°C), liquid nitrogen (-196°C), liquid oxygen (-183°C), or liquid ethylene (-104°C). At these temperatures, standard carbon steels undergo brittle fracture (ductile-brittle transition). The Charpy impact transition temperature for A216 WCB is approximately -10°C to -25°C — well above LNG operating temperature.

Extended bonnet design

The defining feature of a cryogenic valve is the extended bonnet (also called cryogenic neck or cold extension). The extension raises the packing gland from the process temperature to ambient temperature — typically the bonnet neck is sized so that the packing operates above -20°C regardless of the process temperature. Without an extended bonnet, the stem packing contracts at cryogenic temperatures, loses sealing contact, and leaks process fluid. The extension length is calculated based on the fluid temperature, ambient temperature, and allowable packing temperature per BS 6364 or manufacturer's cryogenic design standard. Typical extensions: 100mm for -50°C to -100°C, 250mm for LNG service, 500mm or longer for liquid hydrogen (-253°C).

• -Packing temperature must remain above -20°C per BS 6364 / manufacturer design standard
• -Extension length calculated from thermal gradient through neck — typically 250-300mm for LNG
• -For cold-box mounting, top-entry or angle valves are preferred to avoid cryogenic liquid trapping in body cavity
• -Quarter-turn (ball, butterfly) cryogenic valves use anti-static device and seat design for cold operation

Material selection for cryogenic service

Austenitic stainless steels (A351 CF8M, A182 F316) remain ductile to -196°C and are the first choice for LNG and liquid nitrogen service. They do not require Charpy impact testing below -196°C as ductility is inherent in the FCC crystal structure. For -46°C to -100°C service, A352 LCC (nickel-bearing low-temperature carbon steel, Charpy impact tested at -46°C) is commonly used as a lower-cost alternative. For -100°C to -170°C, A352 LC3 (3.5% nickel steel, impact tested at -101°C) is the correct carbon steel choice. Duplex stainless steel (A890 Gr.4A) has limited low-temperature ductility and is generally NOT suitable below -46°C without specific qualification. Aluminium alloys (6061, 5083) are used in some cryogenic piping but are not common in flanged industrial valves at process pressures.

• -A351 CF8M (316 SS): ductile to -196°C, no Charpy test required — preferred for LNG
• -A352 LCC: Charpy impact at -46°C — suitable for -46°C to -100°C
• -A352 LC3 (3.5Ni): Charpy impact at -101°C — suitable to -162°C
• -A352 LC1 (0.5Mo): Charpy at -59°C — limited cryogenic application
• -Duplex 2205: NOT recommended below -46°C without full qualification

Seat and trim requirements

PTFE soft seats become rigid and brittle below approximately -50°C, losing the resilience required for sealing. Cryogenic service valves should use metal-to-metal seats (SS316 + Stellite 6 hard-facing) or PCTFE (polychlorotrifluoroethylene) seats which remain flexible to -200°C. Ball valve seats for cryogenic service typically use PCTFE in a spring-energised design that maintains seating force regardless of thermal contraction. For butterfly valves in LNG service, metal seat designs (double-offset or triple-offset) per API 609 are standard — elastomeric seats (EPDM, NBR) become rigid and fail below -50°C.

Testing — helium leak test vs hydrotest

Hydraulic hydrotesting (water) of cryogenic valves is problematic because: (1) water and moisture contamination of cryogenic systems causes ice formation and potential blockage; (2) water hydrotest does not replicate cold-temperature sealing performance. BS 6364 requires gas leak testing with helium or nitrogen at low temperature. Standard procedure: ambient temperature gas test at 1.1 × MAWP to verify body integrity; low-temperature test at service temperature with gas medium to verify seat and packing performance at cryogenic conditions. Helium is preferred for sensitivity — it detects leakage rates as low as 10^-7 mbar.l/s using a helium mass spectrometer detector.

• -Ambient gas test: 1.1 × MAWP with nitrogen or helium
• -Cold test at -196°C (LN2) or -170°C: seat and packing leak test with helium
• -Acceptance: zero visible leakage; helium leak rate per BS 6364 Class specification
• -Document: test temperature, test medium, pressure, duration, and measured leak rate

Key Specification Decisions

Material Selection Reference

Frequently Asked Questions