When the service condition, not the mechanism, drives the specification — and the details that turn a standard valve into a compliant one.
Every other pillar on this site starts with a mechanism — a ball, a wedge, a disc. This one starts with a condition. "Cryogenic valve" is not a valve type; it is a ball, gate or globe valve to which a defined set of modifications has been applied so it survives at −162°C. The same is true of fire-safe, oxygen-clean and high-temperature service.
That has a practical consequence for procurement: you still have to pick the base type first using the normal criteria, then layer the service requirements on top as separate, individually verifiable line items. Skipping the second step is how a technically correct stainless ball valve ends up being the wrong valve for LNG.
Cryogenic generally means below −50°C, the threshold used by BS 6364. The common duties sit well below it: LNG at about −162°C, liquid oxygen at −183°C, liquid nitrogen at −196°C.
The defining feature. Lengthening the bonnet puts a column of cold vapour between the liquid and the gland, so the packing operates at a temperature it can survive and no ice forms on the sealing surface of the stem. The length is prescribed by the standard as a function of size and design temperature — treat it as a compliance dimension.
This is the requirement most often missed when a "stainless ball valve" is substituted for a proper cryogenic one. With the valve closed, liquid is trapped between the two seats. As it warms it vaporises and expands by roughly 600 times, and there is no path out. Cryogenic ball valves are therefore drilled with a relief hole through the upstream side of the ball, or fitted with an external body relief valve.
Field note: the relief hole makes the valve unidirectional. The flow arrow now defines which side the cavity vents to. Installing a cavity-relieved cryogenic ball valve backwards re-creates the trapped-liquid hazard it was drilled to eliminate.
Stem vertical, or within roughly 45° of vertical and pointing up. Any other orientation floods the extended bonnet with liquid and cancels the thermal break the bonnet exists to create.
Wherever hydrocarbons are present, an isolation valve is expected to keep isolating after a fire has destroyed its soft seat. Fire-safe designs provide a secondary metal-to-metal seating surface behind the primary soft seat, plus an anti-static device and a blowout-proof stem.
Field note: "fire-safe" on a datasheet with no standard cited and no certificate attached is marketing. Certification is granted to a tested design family within a stated size and class range. Ask for the fire test report, then check that your size and pressure class actually fall inside its scope.
Above roughly 250°C the soft materials give out before the metal does.
Work the numbers against the pressure-temperature rating chart and the high-temperature valve selection guide before committing to a class.
Oxygen service is a cleanliness problem more than a mechanical one. Any hydrocarbon residue — machining oil, ordinary grease, a fingerprint — can ignite in a high-pressure oxygen stream.
| Family | Role | Where it wins |
|---|---|---|
| Safety relief valve | Automatic overpressure protection | Any vessel or line with a credible overpressure case — see relief valve sizing |
| Diaphragm valve | Isolates media from all moving parts | Sterile, ultrapure and corrosive duty — see diaphragm selection |
| Pinch valve | Squeezes a flexible sleeve shut | Abrasive slurry and powder where any metal wetted part would erode |
| Plug valve | Quarter-turn tapered or cylindrical plug | Dirty service, frequent operation, double block and bleed configurations |
The extended bonnet moves the stem seal and gland packing far enough away from the cold body that a column of cold vapour, rather than liquid, sits under the packing. That keeps the sealing elements above their embrittlement temperature and above the frost line where ice would otherwise form on the stem and shred the packing on every stroke. The required length is set by the applicable standard — BS 6364 and MSS SP-134 both prescribe it as a function of size and design temperature — so it is a compliance dimension rather than a free choice.
A closed floating-ball valve traps liquid in the body cavity between the two seats. If that cryogenic liquid warms, it vaporises and expands by roughly a factor of 600, and the cavity has no relief path — pressure can rise until the seats, body or seals fail. Cryogenic ball valves therefore have a relief hole drilled through the upstream side of the ball, or an external body relief valve, so the cavity always vents to one side of the line. Ordering a standard ball valve in stainless steel for cryogenic duty without this feature is a genuine safety defect, not a specification nicety.
No. Ordinary carbon steel undergoes a ductile-to-brittle transition well above cryogenic temperatures and can fail suddenly without plastic deformation. Cryogenic bodies and trim are austenitic stainless steel (CF8 / 304, CF8M / 316), certain bronzes, or nickel alloys, and the material is normally required to pass Charpy impact testing at the design temperature. Impact test certificates are part of the documentation package, not an optional extra.
With the stem vertical, or at least within about 45° of vertical and pointing upward. That orientation keeps the cold liquid in the body and leaves vapour in the extended bonnet, which is exactly the condition the extended bonnet was designed around. Installing a cryogenic valve with the stem horizontal or downward floods the bonnet with liquid, defeats the thermal break, and freezes the packing.
It proves that a representative valve of that design family passed a defined burn test — typically API 607 for soft-seated quarter-turn valves, API 6FA for API 6D pipeline valves, or ISO 10497 — in which the valve is exposed to flame for a set period and then measured for through-seat and external leakage within permitted limits. It demonstrates that after the soft seat burns away, a secondary metal seating surface limits leakage. It says nothing about a different size, class or seat material from the same manufacturer, so always confirm the certificate scope covers what you are buying.
PTFE packing and soft seats reach their limit, so the specification moves to flexible graphite packing with carbon end rings, graphite or spiral-wound gaskets, and hard-faced metal trim. Where fugitive emissions matter, a bellows-sealed bonnet removes the dynamic stem seal entirely. Bolting must be reviewed for creep and relaxation, and the pressure rating itself falls with temperature under the ASME B16.34 tables — a Class 300 valve does not deliver Class 300 pressure at 400°C.