What Is a Cryogenic Valve?

Quick Answer

A cryogenic valve is designed to operate reliably at temperatures below -46 degC (-50 degF), down to -196 degC (-320 degF) for liquid nitrogen and -269 degC (-452 degF) for liquid helium. The defining feature is an extended bonnet that creates a thermal column between the cold process fluid and the stem packing. This prevents the packing from freezing and ensures leak-free operation. BS 6364 governs cryogenic valve design, testing, and qualification.

Key Specifications

Feature	Details
Function	Isolation and control at cryogenic temperatures
Temperature range	Down to -196 degC (LN2) or -269 degC (LHe)
Key feature	Extended bonnet (150-500 mm thermal column)
Design standard	BS 6364 (cryogenic valves)
Pressure-temperature	ASME B16.34 with low-temp material curves
Valve types	Gate, globe, ball, butterfly, check
Body materials	ASTM A352 LCB (-46 degC), LCC (-46 degC), CF8M (-196 degC), A351 CF3M, Inconel
Stem packing	PTFE or flexible graphite (must remain above 0 degC via extended bonnet)
Testing	Helium leak test, cryogenic shell test per BS 6364
Sizes	1/2” to 60”+

How the Extended Bonnet Works

At cryogenic temperatures, conventional stem packing materials freeze and lose elasticity, causing leakage. The extended bonnet solves this by adding a vertical pipe section (typically 150-500 mm long) between the body and the packing gland. Ambient heat conducts down the bonnet while cryogenic cold rises from the body. The temperature gradient ensures the packing zone stays above freezing.

The bonnet length depends on the minimum design temperature and the insulation thickness:

Service Temperature	Typical Bonnet Extension
-46 to -101 degC	150-200 mm
-101 to -196 degC (LNG, LN2)	250-350 mm
Below -196 degC (LHe)	400-500 mm

Body Material Selection

Material	ASTM Standard	Min. Design Temperature	Application
LCB (low-carbon steel)	A352	-46 degC	Mild cryogenic, LPG
LCC (low-carbon steel)	A352	-46 degC	Similar to LCB, higher strength
CF8 (304 SS casting)	A351	-196 degC	LNG, LN2
CF8M (316 SS casting)	A351	-196 degC	LNG, LN2 (corrosive)
CF3M (316L SS casting)	A351	-196 degC	LNG (low carbon, weldable)

All materials require Charpy V-notch impact testing at or below the minimum design temperature to verify ductility. Carbon steel becomes brittle below its ductile-to-brittle transition temperature, which is why austenitic stainless steels are mandatory for temperatures below -101 degC.

Typical Applications

LNG plants: cryogenic ball and gate valves on liquefaction trains, storage tanks, and loading arms operating at -162 degC.

Air separation units (ASU): liquid oxygen (-183 degC), liquid nitrogen (-196 degC), and liquid argon (-186 degC) piping systems. Note: liquid oxygen service also requires oxygen-clean valves.

Ethylene plants: cryogenic separation of ethylene at temperatures down to -104 degC.

LPG storage: pressurized and refrigerated LPG tanks at -42 degC (propane) and -46 degC (butane).

Cryogenic Testing

BS 6364 requires prototype cryogenic testing: the valve is cooled to the design temperature using liquid nitrogen or helium, pressurized, and cycled through open-close operations while measuring seat and stem leakage. This qualifies the valve design for the specified cryogenic service.

Common Mistake

Using carbon steel (A216 WCB or A105) valves in cryogenic service. Carbon steel has a ductile-to-brittle transition around -29 degC. Below this temperature, the body can fracture catastrophically without warning. Always specify impact-tested low-temperature or austenitic stainless steel per ASME B16.34 material requirements.

Read the full guide to valve types