Guide
Cryogenic containers explained: open dewars liquid cylinders and VIE tanks, their relief devices and Indian rules
Cryogenic containers come in three main types. An open dewar holds liquid at atmospheric pressure behind a loose neck plug. A liquid cylinder is a pressurised vacuum-insulated vessel, commonly about 1.5 bar for liquid service or 16 to 24 bar for gas. A VIE tank is static bulk storage, typically 1,000 to 125,000 litres.
- Liquid cylinder pressure classes
- About 22 psig
- Liquid cylinder safety devices
- Pressure gauge
- Static VIE scope
- 1
- Twin relief devices
- Where a change-over valve serves two relief devices
- Indian rules
- Static and mobile vessels under SMPV
A cryogenic container keeps a liquefied gas such as nitrogen, oxygen or argon below its boiling point by stopping heat getting in. None of them stops it completely, so every cryogenic container is designed around the gas that boils off: where it goes, how fast, and what happens if its path is blocked. The three types in everyday use in India are the open dewar, the pressurised liquid cylinder and the static vacuum-insulated evaporator (VIE) tank. This page compares them and sets out the relief devices and Indian rules that apply. Properties of the liquids themselves are on liquid nitrogen, liquid oxygen and liquid argon.
Dewar, liquid cylinder or VIE tank?
| Feature | Open dewar | Liquid cylinder (pressurised dewar) | Static VIE tank |
|---|---|---|---|
| Typical use | Laboratories, semen banks, dermatology, small decanting | Workshops, laboratories, hospitals, micro-bulk welding gas | Factories, hospitals, steel and glass plants |
| Capacity | Small, hand-carried or on a trolley | Transportable, up to 1,000 L under IS 18719 | 1,000 to 125,000 L (AIGA 030/13 scope) |
| Working pressure | Atmospheric | About 1.5 bar for liquid service; about 15.9 or 24.1 bar for gas service (LBL) | Set by the plant or pipeline it feeds |
| Relief path | Loose neck plug; gas vents continuously | Relief valve plus at least one bursting disc; manual vent valve (LBL) | Duplicated relief devices on a change-over valve (AIGA 030/13) |
| How product leaves | Poured, or drawn by dip tube | Gas through a regulator, or liquid through a transfer hose | Gas from a vaporiser into a pipeline; liquid to fill smaller vessels |
| Boil-off | Highest per litre stored | Lower; holds for weeks to months (LBL) | Lowest per litre stored |
| Filled by | Decanting from a larger vessel | Transfer from a tank or tanker | Road tanker |
LBL is Lawrence Berkeley National Laboratory's environment, health and safety guidance. The pressures convert from its 22, 230 and 350 psig classes.
How the insulation works, and what boil-off means
Every type uses the same principle as a vacuum flask: an inner vessel holding the liquid, an outer jacket, and a space between them that is evacuated and often filled with multilayer insulation or perlite powder. The vacuum stops conduction and convection; the insulation cuts radiation. What heat still leaks in boils some liquid. Makers state this as a normal evaporation rate, usually given as a percentage of the full contents lost per day for a container standing idle. It is the figure to compare when choosing between containers, and it is why a container that is half-used and then left for a fortnight delivers less than its rated contents.
Boil-off changes as a container ages. AIGA 030/13 notes that voids in perlite insulation slightly increase the natural boil-off rate without making the vessel unsafe, provided the vacuum is intact. Frost or sweating on the outer jacket of a dewar or tank, away from its valves and vaporiser, is a different matter: it usually means the vacuum has been lost, boil-off will rise sharply, and the container should be taken out of service and returned to the supplier.
Rules that matter
Cryogenic liquids expand hundreds of times as they vaporise: about 694 times for nitrogen, 840 for argon and 860 for oxygen at 21 °C and 1 atm, from NIST densities. A container without a working relief path will keep building pressure until it ruptures.
Pressure-relief devices
- Open dewars rely on their loose neck plug. Never replace it with a tight stopper, tape or a screwed lid, and keep ice from sealing it.
- Liquid cylinders carry a relief valve set to the container's rating and a bursting disc as back-up, and some have more than one disc. LBL notes that both must work at cryogenic temperature without losing performance.
- VIE tanks carry two relief devices on a change-over valve, so one can be serviced while the other protects the tank. AIGA 030/13 requires that, whatever the change-over valve's position, at least one device is always open to the tank at full bore. It asks that relief valves be tested to EIGA Doc 24 and set to match the system's maximum permissible operating pressure, and notes that bursting discs can deteriorate and may need periodic replacement.
- Pipework that can trap liquid between two closed valves needs its own thermal relief valve.
Indian rules for cryogenic containers
| Container | Instrument | What it covers |
|---|---|---|
| Static VIE tanks, road tankers | Static and Mobile Pressure Vessels (Unfired) Rules 2016 (PESO) | Design, licensing and inspection of bulk pressure vessels; a Gas Cylinders Rules licence for bulk storage asks for the SMPV(U) receipt |
| Transportable vacuum-insulated vessels up to 1,000 L | IS 18719 (Part 1):2024 design and construction; IS 18719 (Part 2):2024 operation | Liquid cylinders and similar transportable vessels |
| Large transportable vacuum-insulated vessels | MED 16 draft, modified adoption of ISO 20421-1 | In draft at BIS |
| Valves for cryogenic vessels | MED 16 standard under print | Not yet published |
| Gas quality from liquid | IS 309:2023, IS 1747:2023 cl 6.1 | Both point to the SMPV(U) Rules and Gas Cylinders Rules 2016 for packing and transport |
There is no general Indian Standard for a static cryogenic storage tank; static vessels are dealt with under the SMPV(U) Rules. PESO's layout rules for acetylene plants put cryogenic storage vessels at least 15 m from the plant, and the AIGA distance table keeps acetylene cylinder storage 6 m from cryogenic vessels. The licensing pattern for gas cylinders more generally is on Gas Cylinders Rules 2016 for fillers and users.
Rules that matter
Handling cryogenic containers
- Keep them upright Liquid cylinders and dewars are designed to stand; never lay a liquid cylinder on its side or use it tilted on a cart.
- Move them properly Use the container's own wheeled base or a purpose-made trolley. Full liquid argon containers weigh far more than nitrogen ones, about 1.4 kg per litre of liquid against 0.81.
- Ventilate Store and use indoors only where ventilation is adequate, and fit an oxygen monitor with a low-level sensor in rooms holding nitrogen or argon containers. EIGA's cryosauna guidance sets its alarm at 20 % oxygen. Never take a container into a lift with people, and never carry one in a car's passenger compartment.
- Check the safety devices Before connecting a liquid cylinder, confirm the relief valve and bursting disc are present and undamaged and the gauge reads within range.
- Use the right connection Oxygen containers need oxygen-clean, oil-free fittings; nitrogen, argon and oxygen connections must not be interchangeable.
- Wear the right equipment for decanting: face shield, loose cryogenic gloves, long sleeves and closed shoes with trousers over them.
- Watch for frost on the jacket It means lost vacuum; stop using the container.
Entry into any space where nitrogen or argon from a container may have collected is covered on inert gas confined space entry.
How KIGL fits in
KIGL fills compressed gases in high-pressure cylinders at its own PESO-approved works at Noonmati, Guwahati, and Howrah, Kolkata. KIGL also operates an air separation plant and supplies liquid oxygen and liquid nitrogen. For sites that want high-purity nitrogen without handling cryogenic liquid at all, KIGL builds PSA nitrogen plants. Cylinder grades for nitrogen are on the nitrogen page.
Cryogenic containers explained — frequently asked questions
What is the difference between a dewar and a liquid cylinder?
An open dewar works at atmospheric pressure. Its neck is closed only by a loose plug, gas escapes continuously, and liquid is poured or drawn by a dip tube. A liquid cylinder is a sealed, pressurised vacuum-insulated vessel with a relief valve, a bursting disc, a gauge and separate liquid, gas and vent valves. It can deliver gas through a regulator or liquid through a transfer hose, and is moved on its own wheeled base.
What is a VIE tank?
A vacuum-insulated evaporator: a static bulk tank with an inner stainless steel vessel, an outer jacket and an evacuated, insulated space between them. It is filled by tanker, keeps pressure with a pressure-building coil and feeds a vaporiser that supplies gas to a pipeline. AIGA 030/13 covers VIE storage of oxygen, nitrogen and argon from 1,000 to 125,000 litres.
Why does a liquid cylinder vent gas when nobody is using it?
Because heat always leaks in. Some liquid boils, pressure rises, and when it reaches the relief valve setting the valve lifts and vents. That is normal and is the reason the valve exists. The rate depends on the insulation, stated by makers as a normal evaporation rate. A cylinder that vents far more than usual, or frosts on its outer jacket, may have lost vacuum.
Can I carry a liquid nitrogen dewar in a car?
Not in the passenger compartment. A dewar vents nitrogen continuously, and in a closed car the oxygen level can fall quickly enough to impair the driver. Use an open or ventilated goods vehicle, secure the dewar upright so it cannot tip, keep the vent unobstructed and drive with no passengers in an enclosed space shared with the container.
What licence is needed for a liquid nitrogen tank in India?
A static or mobile cryogenic pressure vessel is regulated under the Static and Mobile Pressure Vessels (Unfired) Rules 2016, administered by PESO, and a Gas Cylinders Rules licence for bulk storage asks for the SMPV(U) receipt. Smaller transportable vacuum-insulated vessels up to 1,000 litres are the subject of IS 18719 Parts 1 and 2, published in 2024. Confirm the exact licence category with the PESO circle office.
Why must a liquid cylinder never be laid on its side?
Its valves and internal pipework are designed for an upright vessel: the gas valve must draw from the vapour space and the relief valve must see vapour, not liquid. Lying down, liquid can reach the relief device and gas outlet, and the inner vessel supports are loaded in a way they were not designed for. Laboratory guidance from Lawrence Berkeley National Laboratory says never to operate one tilted.
References
- Asia Industrial Gases Association. (2013). Storage of cryogenic air gases at users' premises (reproduced with EIGA permission) (AIGA 030/13). https://asiaiga.org/uploaded_docs/AIGA%20030_13_Storage%20of%20cryogenic%20air%20gases%20at%20users'%20premises.pdf
- EIGA Safety Info 19/26, Nitrogen Hazards in Cryosaunas https://www.eiga.eu/uploads/documents/SI019.pdf
- IS 18719 (Parts 1 and 2):2024 Transportable vacuum-insulated vessels, Bureau of Indian Standards
- Lawrence Berkeley National Laboratory EHS — pressurised dewars and cryogenic liquid cylinders https://ehs.lbl.gov/service/cryogenic-liquid-safety/pressurized-dewarscryogenic-liquid-cylinders/
- Linstrom, P. J., & Mallard, W. G. (Eds.). (n.d.). NIST Chemistry WebBook (NIST Standard Reference Database 69). National Institute of Standards and Technology. [cryogen saturation properties] https://webbook.nist.gov/chemistry/
- Static and Mobile Pressure Vessels (Unfired) Rules 2016 and Gas Cylinders Rules 2016, Petroleum and Explosives Safety Organisation
Enquiries
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State the gas or equipment, the quantity or capacity, and your location. KIGL will confirm the grade, the cylinder or plant size, and whether the location can be served from Noonmati, or through the agent network.