Compressed gas storage: requirements and compatibility

The container is part of the hazard

Every other class on this site is defined by chemistry. This one is half-defined by engineering.

OSHA’s criteria classify gases under pressure by the receptacle, not the molecule (Appendix B, B.5): gases “contained in a receptacle at a pressure of 200 kPa (29 psi) (gauge) or more at 20 °C (68 °F)”, divided into four groups:

Group	Definition
Compressed gas	”A gas which when under pressure is entirely gaseous at −50 °C”
Liquefied gas	”A gas which when under pressure, is partially liquid at temperatures above −50 °C”
Refrigerated liquefied gas	”A gas which is made partially liquid because of its low temperature”
Dissolved gas	”A gas which when under pressure is dissolved in a liquid phase solvent”

The practical consequence is that a cylinder of nitrogen — chemically inert, non-flammable, non-toxic — is still a stored-energy device capable of becoming a projectile if its valve is sheared off. That hazard belongs to the packaging and is identical for every gas in the store.

The flammable-gas categories

For the gases that are also flammable, the classification is numeric and was restructured in 2019 (Regulation (EU) 2019/521, replacing CLP Annex I section 2.2). A flammable gas is one “having a flammable range with air at 20 °C and a standard pressure of 101,3 kPa”, and the categories run:

Category	Criterion	Statement	Signal word	Pictogram
1A	Ignitable at 13 % or less by volume in air, or flammable range of at least 12 percentage points	H220 — extremely flammable gas	Danger	flame
1A pyrophoric	Ignites spontaneously in air at 54 °C or below	H220 + H232	Danger	flame
1A chemically unstable	Able to react explosively even without air or oxygen	H220 + H230 or H231	Danger	flame
1B	Meets the 1A flammability criteria but has a lower flammability limit above 6 % by volume, or a fundamental burning velocity below 10 cm/s	H221 — flammable gas	Danger	flame
2	Any other gas with a flammable range in air	H221	Warning	none

Two things in that table catch people out. Category 1B is new — it did not exist before 2019, and it carries Danger and the flame pictogram despite sharing the H221 statement with Category 2. And the same hazard statement appears with two different signal words, so H221 alone does not tell you the category.

How we classify this page’s substances — read this before the list

This class is anchored on four hazard codes: H220, H221, H280 and H281. In practice, only two of them do any work.

No substance in our database carries H280 or H281. Not one. The reason is the definition above: “gas under pressure” is a property of the package, and harmonised EU classification under Article 36(1) of CLP normally covers health and environmental hazard classes, with others harmonised case by case. ECHA states the same scope plainly: harmonisation is for “hazards of highest concern (carcinogenicity, mutagenicity, reproductive toxicity (CMR) and respiratory sensitisers) and for other substances on a case-by-case basis”. The pressure classification is therefore the supplier’s to assign for the product as sold — and it never appears in the harmonised list this site is built from.

What is left is the flammable-gas anchor: 174 substances carry H220 and exactly one carries H221 — ammonia.

And 148 of the 175 are petroleum refinery gas streams. They sit under EU index numbers beginning 649-, they are UVCB substances rather than defined chemicals — “gases (petroleum), catalytic cracker”, and 147 more like it — and every one of them carries H350 (may cause cancer) and H340 (may cause genetic defects) alongside the flammable-gas classification. That is why 156 of the 175 substances in this class carry the GHS08 health-hazard pictogram, a figure that makes no sense until you know the composition of the list.

So the honest description of this page is: 27 named gases, plus 148 refinery streams. If you are storing cylinders, the 27 are your list.

And three ordinary cylinder gases are missing from it entirely. Oxygen and chlorine are classified as oxidizing gases and live in the oxidizers class. Nitrogen, argon and carbon dioxide are neither flammable nor oxidizing nor acutely toxic, so they carry no harmonised entry that this class can anchor on — but an inert-gas cylinder is still a pressure vessel, and in a confined space it is still an asphyxiation hazard. The absence of a substance from this list says nothing about whether it is safe to store.

The 27 named gases

Where you meet it	Gas
Fuel gases	methane, ethane, propane, ethylene, propene, dimethyl ether, cyclopropane, neopentane
Welding and cutting	acetylene, hydrogen
Refrigeration, fertiliser, cleaning	ammonia (the only Category 1B/2 entry here), chloromethane, chloroethane
Semiconductor and specialty	arsine, phosphine, ethylene oxide, methyl vinyl ether, ethyl methyl ether, ethylamine
Polymer feedstocks	1,3-butadiene, vinyl chloride, bromoethylene
Odorants and sulfur gases	methanethiol, hydrogen sulfide
Combustion and process gases	carbon monoxide, cyanogen, ethyl nitrite

Several of these are dangerous for reasons that have nothing to do with burning. Arsine, phosphine, hydrogen sulfide and carbon monoxide are acutely toxic — eight substances in this class carry GHS06. Ethylene oxide, 1,3-butadiene, vinyl chloride and bromoethylene are carcinogens. Ammonia is corrosive. A flammable-gas store designed only around ignition control is designed around one of the hazards present.

What the law actually requires

Generally: 29 CFR 1910.101, and it is short. Three duties: the employer “shall determine that compressed gas cylinders under his control are in a safe condition to the extent that this can be determined by visual inspection”; in-plant handling, storage and utilisation “shall be in accordance with Compressed Gas Association Pamphlet P-1-1965”; and safety relief devices per CGA pamphlets S-1.1 and S-1.2. Note the dates — this standard incorporates consensus documents from the 1960s by reference, and the current editions of those pamphlets are the ones actually used in practice.

Specific gases have their own sections: acetylene (1910.102), hydrogen (1910.103), oxygen (1910.104), nitrous oxide (1910.105).

The one hard separation number is in the welding subpart rather than the hazardous-materials one — 29 CFR 1910.253(b)(4)(iii):

“Oxygen cylinders in storage shall be separated from fuel-gas cylinders or combustible materials (especially oil or grease), a minimum distance of 20 feet (6.1 m) or by a noncombustible barrier at least 5 feet (1.5 m) high having a fire-resistance rating of at least one-half hour.”

The preceding subparagraph widens it: oxygen cylinders “shall not be stored near highly combustible material, especially oil and grease; or near reserve stocks of carbide and acetylene or other fuel-gas cylinders, or near any other substance likely to cause or accelerate fire”.

For bulk oxygen, 1910.104(b) is far more demanding: 50 feet from combustible structures, 25 feet from fire-resistive or sprinklered buildings, 50 feet from rapidly burning solids such as paper, and 50 or 90 feet from above-ground flammable-liquid storage depending on quantity.

Segregation: no prohibitions, twelve separations

Verdict	Classes
Never store with	none
Keep separate	all twelve other storage classes
Generally compatible	none

This is the only class on the site with that profile, and it is not an oversight. While a cylinder is intact, the contents cannot react with anything on the next pallet; the barrier is steel, not distance. The failure mode a store has to plan for is release — a valve sheared in a fall, a relief device operating in a fire, a leak from a regulator — and once that happens the relevant question is what the escaping gas reaches, which is a question about the whole room rather than about the adjacent shelf.

That is why every other class sits at keep separate rather than at either extreme, and why the practical controls for this class are physical rather than chemical: cylinders secured upright, valve caps fitted in storage, away from heat sources and from any place a vehicle can strike them, full and empty segregated, and — where a flammable gas is stored — ventilation at high level, because most of these gases are lighter than air. The exceptions that sink, propane among them, need it at low level instead.

The flame pictogram is GHS02, the health-hazard pictogram carried by most of this class is GHS08, and the gas-cylinder pictogram is GHS04. Class-level segregation is a starting point rather than a verdict on any individual substance: check SDS sections 7 and 10, or look the substance up in the storage compatibility matrix. Related pages: flammable liquids, oxidizers — where oxygen and chlorine live — and acute toxics.