Cryogenic Valve Selection Guide for LNG & Low-Temperature Service | BS 6364 | Vajra

What Is Cryogenic Service for Valves?

Cryogenic service is typically defined as any service below −29°C (−20°F) in process engineering. The most common cryogenic services encountered in industrial plants are: LNG liquefaction and storage at −162°C (methane boiling point at atmospheric pressure); liquid nitrogen at −196°C (standard industrial cryogen for purging and inerting); liquid oxygen at −183°C (industrial gas plants, LOX medical and aerospace applications); liquefied CO2 at −78°C (food-grade CO2 storage and fire suppression); ethylene storage at −104°C (ethylene cracker cold boxes and storage terminals); and propylene at −47°C (propylene refrigeration systems in refineries). At these temperatures, standard carbon steel undergoes a ductile-to-brittle transition and becomes fracture-prone, elastomers solidify and lose sealing ability, and standard packing arrangements allow cryogenic liquid to migrate up the valve stem and leak. Specialised cryogenic valves with extended bonnets, fully impact-tested materials, and appropriate stem sealing systems are mandatory for safe and reliable operation.

Why Standard Valves Fail in Cryogenic Service

• Carbon steel becomes brittle below −29°C: the ductile-to-brittle transition temperature (DBTT) of standard A216 WCB carbon steel is typically between 0°C and −29°C — Charpy impact energy drops below 27 J, making the material susceptible to catastrophic brittle fracture under dynamic loading.
• PTFE seats lose compliance below −100°C: PTFE (Teflon) becomes increasingly rigid as temperature drops, losing the elasticity needed to conform to the ball surface — seat leakage increases significantly in LNG and LN2 service.
• Standard elastomeric stem packing solidifies: EPDM and NBR O-ring stem seals solidify and become non-functional below −40°C, allowing cryogenic liquid to leak up the stem to the outside of the valve.
• Ice and condensation on the stem: without an extended bonnet moving the stem seal to ambient temperature, atmospheric moisture condenses and freezes on the cold stem — ice build-up increases stem friction, can seize the valve, and damages the packing.
• Liquid oxygen service — explosive contamination risk: standard valve grease and hydrocarbon-based lubricants ignite explosively in contact with liquid oxygen. All valves for LOX service must be oxy-cleaned (solvent degreased and inspected per ASTM G93) and assembled with oxygen-compatible lubricants only.

Extended Bonnet Design — The Core of Cryogenic Valves

The extended bonnet (also called cryogenic bonnet or cold box extension) is the defining feature of cryogenic valves. It moves the stem packing gland and actuator connection a significant distance above the process flange — far enough that the packing temperature remains above −29°C (for standard cryogenic) or above 0°C (for LNG and LN2 service). The extension length is calculated based on the process temperature, stem diameter, heat conduction through the bonnet material, and ambient conditions. BS 6364 specifies minimum bonnet extension lengths as a function of these parameters. In practice: 100–200 mm extension is sufficient for −46°C service (propylene, CO2); 250–400 mm is typical for −162°C LNG service; 500 mm or longer is required for −196°C liquid nitrogen or liquid oxygen. The extended bonnet also provides a 'cold trap' — a vent hole in the bonnet above the seat allows any trapped liquid cryogen to evaporate and escape (rather than building up pressure and ejecting the bonnet). This vent hole is a mandatory feature per BS 6364.

Standards for Cryogenic Valves

Material Selection for Cryogenic Service

• ASTM A352 LCC (Low-temperature Carbon Steel Cast, rated to −46°C): most economical cryogenic body material — used for propylene, ethylene (with caution), and CO2 refrigeration service.
• ASTM A350 LF2 (Forged Carbon Steel, rated to −46°C): common for flanges, bonnets, and forged valve bodies in the −29°C to −46°C temperature range.
• SS 304 / SS 316L (austenitic stainless steel, rated to −196°C): the standard cryogenic material for LNG, liquid nitrogen, liquid oxygen, and liquid argon service — austenitic (FCC crystal structure) stainless steel retains ductility and toughness at all cryogenic temperatures with no ductile-to-brittle transition.
• Aluminium Alloy 5083 (rated to −196°C): used for some LNG valve bodies and cryogenic tank fittings — lighter than stainless steel but limited to lower pressure classes.
• Monel 400 (rated to −196°C): nickel-copper alloy for special corrosive cryogenic service — compatible with seawater and hydrofluoric acid (HF) at low temperature.
• IMPORTANT — Free-machining sulphur grades prohibited: Never use SS 303 (sulphur-added free-machining grade) or similar sulphur-bearing stainless for cryogenic bodies or trim — sulphur inclusions create notch sensitivity and brittle failure at low temperature.

Testing Requirements for Cryogenic Valves

• BS 6364 vent hole functionality test: the bonnet extension vent hole must be tested to confirm free drainage and prevent trapped liquid cryogen pressure build-up.
• Cold shock test: some LNG project specifications (Shell DEP, Woodside, TotalEnergies) require thermal shock testing of the complete assembled valve by immersing in liquid nitrogen or liquid helium — the valve is then tested for seat leakage and stem operation at cryogenic temperature.
• Charpy impact testing per EN ISO 148-1 or ASTM A370: all low-temperature body and bonnet materials must demonstrate ≥27 J Charpy V-notch impact energy at the minimum design temperature — witnessed test with EN 10204 3.1 or 3.2 certification.
• Helium fugitive emissions seat leak test: many LNG and cryogenic project specifications require a helium tracer gas test at ambient temperature — helium is 1,000× more sensitive than standard air/water tests for detecting micro-leakage.
• Liquid oxygen service — oxy-clean process: all LOX valves must be chemically cleaned per ASTM G93 (degreased with approved solvents, visually inspected under UV light for hydrocarbon residue, and sealed from recontamination before assembly).

LNG Liquefaction — Cryogenic Valve Applications

• LNG storage tank suction/discharge isolation: DN50–DN400, SS 316L body, extended bonnet per BS 6364, flanged Class 150–300, manual handwheel or pneumatic actuator.
• BOG (Boil-off Gas) compressor suction isolation: DN50–DN200, SS 316L, Class 300–600, cryogenic extended bonnet — must be fire-safe per API 607.
• LNG cold box isolation and bypass: DN15–DN100, SS 316L, very long bonnet (400–600 mm) for process temperatures approaching −160°C inside cold box heat exchangers.
• LNG sendout pump isolation: DN80–DN300, SS 316L or A352 LCC, flanged Class 150–300, quick-opening for pump protection and bypass.
• LNG marine loading arm and jetty isolation: DN100–DN400, Class 150–300, quick-closing ESD actuation (pneumatic spring-return fail-close) per ISGOTT and local port authority requirements.