High-temperature and cryogenic valve service

Carbon steel WCB is the standard industrial valve body material and carries a rated pressure at temperature that decreases above approximately 200°C. At 400°C, the allowable pressure for a WCB valve is roughly half its ambient rating. Above 400°C, WCB enters the creep range, where the material deforms slowly under sustained load, and must be replaced with higher-alloy steels such as 1.25Cr-0.5Mo (ASTM A217 WC6) or 2.25Cr-1Mo (ASTM A217 WC9).

Chrome-molybdenum alloy steels maintain useful strength at temperatures up to approximately 550°C. Above that, austenitic stainless steel (CF8 or CF8M) provides adequate strength to approximately 600°C. For very high temperature service above 600°C — combustion systems, furnace headers, superheated steam — high-alloy grades, nickel superalloys or specialised valves are required.

Check the pressure-temperature derating table for the specific material before finalising the pressure class. The ASME B16.34 P-T rating tables are the standard reference for common materials.

At elevated temperatures, thermal expansion of the valve body and internals must be accommodated in the design. Gate and globe valves for high-temperature steam service have extended bonnets (long-bonnet or finned-bonnet designs) that put the packing gland further from the hot process fluid, keeping the packing at a temperature where it seals effectively and lasts a reasonable service life.

PTFE packing is not suitable above approximately 260°C. Graphite packing is the standard for high-temperature service: it is thermally stable to above 500°C, self-lubricating, and provides reliable sealing under the cycling loads of temperature-induced valve body expansion and contraction.

Fire-safe design for high-temperature valves requires that the valve provides a defined level of shutoff after the soft seats or seals have been destroyed by fire. Metal-to-metal seats or fire-safe graphite seals back up the primary soft seal. Specify fire-safe to API 607 or ISO 10497 (for quarter-turn valves) if the application is in a fire-risk area.

At cryogenic temperatures, the mechanical behaviour of materials changes fundamentally. Carbon steel becomes brittle below approximately -20°C: a sudden pressure spike or mechanical impact can cause catastrophic fracture with little or no prior deformation. This is the ductile-to-brittle transition, and it is the reason that carbon steel WCB is not suitable for cryogenic service.

Low-temperature carbon steel (ASTM A352 Grade LCB, impact-tested to -46°C) extends the lower limit for carbon steel bodies. For temperatures below -46°C and down to approximately -104°C, ASTM A352 Grade LC3 (nickel steel) is used. For liquid natural gas (LNG) service at -160°C and liquid nitrogen or oxygen service at -196°C, austenitic stainless steel CF8 or CF8M is the standard, because austenitic grades remain ductile down to cryogenic temperatures without embrittlement.

Always specify the minimum design temperature (MDT) on valve orders for cold service. The MDT drives the material selection, impact testing requirements and design considerations. Stating only the operating temperature is not sufficient if the system can reach lower temperatures during startup, shutdown or failure scenarios.

Cryogenic valves have extended bonnets for the opposite reason to high-temperature valves: the extension keeps the stem sealing area (packing or O-ring) at a warmer temperature, where the seal material retains its properties, and prevents ice formation on the stem from condensation. The extension also provides the thermal gradient that prevents cold from conducting up to the handwheel or actuator.

Ball valves for LNG service use PTFE or filled PTFE seats (which remain flexible at -160°C), with a vented ball design that relieves any pressure build-up from trapped fluid in the ball cavity as it warms on isolation. Without cavity relief, a cryogenic liquid trapped in the ball cavity can pressurise to extreme levels as it vaporises, potentially exceeding the body pressure rating.

All cryogenic valves should be cleaned for oxygen service if there is any possibility of oxygen contact: hydrocarbon and particle contamination in oxygen at high pressure can cause ignition and explosion. Cleaning to ASTM G93 or equivalent involves degreasing and particulate removal under controlled conditions.