Cryogenic Valve Design: Considerations for LNG Service

Introduction

Cryogenic valves operate at temperatures from minus 50 degrees C down to minus 196 degrees C for liquid nitrogen service and minus 162 degrees C for LNG. At these temperatures, standard materials become brittle, lubricants freeze, and conventional soft seats lose their sealing properties.

Body and Trim Materials

Austenitic stainless steel (316L, 304L) remains ductile at cryogenic temperatures9% nickel steel for large bore cryogenic storage and transfer valvesAluminum alloys (6061-T6) used for lightweight cryogenic valve bodiesMonel 400 for chlorine and some cryogenic chemical servicesAvoid carbon steel, ferritic stainless, and most cast irons below minus 29 degrees C

Extended Bonnet Design

Cryogenic valves use an extended bonnet (cold box extension) to keep the packing and stem at near-ambient temperature. This prevents packing from freezing and maintains operator safety. Extension length is calculated to maintain the packing temperature above minus 20 degrees C even when the body is at process temperature.

Seat and Seal Materials

PTFE seats maintain flexibility down to minus 200 degrees CPCTFE (Kel-F) for oxygen cryogenic service compatibilityFilled PTFE compounds for improved wear resistance in cycling serviceMetal-to-metal seats for ultra-high purity service (minimizing non-metallic content)Avoid elastomeric O-rings below minus 50 degrees C unless specifically rated

Pressure Relief and Thermal Expansion

Liquid cryogens trapped in a valve cavity can vaporize when the valve is warmed up, creating extremely high pressures. Cavity relief valves or thermal relief ports must be provided per applicable codes. Most cryogenic ball valves include a body vent hole to relieve trapped liquid.

Testing and Qualification

Cryogenic valves must be tested at operating temperature per BS 6364 or equivalent. Type testing requires multiple cycles at cryogenic temperature with seat leakage checks. Shell pressure tests are performed at ambient; leakage tests at cryogenic temperature with nitrogen.