Liquefied gases — LNG at −162 °C, liquid nitrogen at −196 °C, liquid oxygen, ethylene — are among the least forgiving services a valve can see. A standard ball valve installed on a cryogenic line will leak, seize, or worse. Here is what actually changes at cryogenic temperatures, and what to look for when you specify.
Why Standard Valves Fail Below −50 °C
- Materials go brittle. Carbon steels like WCB lose impact toughness; cryogenic service needs austenitic stainless (CF8/CF8M/CF3M, F316) or specific low-temperature grades, with impact testing at design temperature.
- Soft parts shrink and harden. Standard PTFE seat designs must be adapted; differential contraction between body, ball and seat changes sealing loads.
- Trapped liquid expands 600×. Liquid trapped in a ball cavity vaporizes as the valve warms — without a cavity relief hole or self-relieving seats, the body can overpressure.
- Stem icing. Without a gas column between the fluid and the packing, moisture freezes the stem solid.
The Extended Bonnet — the Signature of a Real Cryogenic Valve
Cryogenic valves per BS 6364 (and ISO 28921) carry an extended bonnet, installed vertically, so a vapor column forms between the cold fluid and the stem packing. The standard defines minimum extension lengths — this keeps the packing near ambient temperature, where it can actually seal.
What BS 6364 Type Testing Covers
A prototype is tested at cryogenic temperature (typically immersed in liquid nitrogen at −196 °C) for seat leakage with helium, external tightness, and operability. When you buy, ask for: the type-test certificate covering your size/class, materials with low-temperature impact certs, degreasing for oxygen service if applicable, and cavity relief direction clearly marked.
Typical Applications
LNG terminals and bunkering, air separation units (LIN/LOX/LAR), CO2 capture, hydrogen and ethylene plants, and cryogenic tank truck loading. Ball, globe, gate, check and triple-offset butterfly designs all exist in cryogenic execution — see our cryogenic & LNG valve range and triple-offset butterfly valves.
What BS 6364 Type Testing Involves
A cryogenic type test is what separates a genuinely qualified valve from one that merely uses cold-tolerant materials. The valve is cooled to service temperature using liquid nitrogen, cycled while cold, and leakage is measured with helium — both across the seat and through the stem seal — because helium will find paths that air and water do not.
The test qualifies a design across a stated size and class range, so the certificate must be checked against the valve actually ordered. Ambient-temperature testing, however rigorous, proves nothing about behaviour at −196 °C: materials contract at different rates, clearances change, and a seal that is tight at 20 °C can open a path when cold.
Why the Extended Bonnet Length Is Not Arbitrary
The extended bonnet exists to keep the packing and stem seal warm enough to function, by placing them far enough from the cold fluid that a stable vapour space forms in the neck. Two consequences follow for installation: the valve must be mounted so that the bonnet rises above horizontal — the vapour space depends on orientation — and the extension must not be buried in insulation that defeats its purpose by drawing cold up to the packing.
Shortening a bonnet to fit a space constraint invalidates the qualification. If the space is tight, the answer is a different valve arrangement, not a modified bonnet.
Cavity Pressure Relief
In a closed cavity — between the seats of a ball valve, for example — trapped cryogenic liquid that warms will expand dramatically and can generate destructive pressure. Cryogenic valves therefore need defined cavity relief, usually a vent hole drilled on one side of the ball. That makes the valve directional: it must be installed with the relieved side toward the correct pressure source, and the flow arrow followed. This is among the most common and most damaging field errors in cryogenic installations.
Frequently Asked Questions
Can a cryogenic valve be installed horizontally?
The extended bonnet must point upward (within the angle allowed by the manufacturer, commonly ±45° from vertical) so the vapor column is maintained. Horizontal stem orientation defeats the design.
Is 316 stainless always required?
Austenitic stainless is the default. Bronze and Monel appear in oxygen service; 9% nickel steel and aluminum appear in LNG static equipment more than in valves.
What seat leakage class should I specify?
Specify the BS 6364 / ISO 28921 type-test acceptance with helium at design temperature — not just an ambient API 598 test. Ambient tightness says little about −162 °C behavior.
Specifying valves for LNG or industrial gas projects in Canada, Europe, the Middle East or Türkiye? Send us your datasheet — we respond with compliant options and full documentation.
