There are three subclasses in Class 2: flammable (2.1), non-flammable and non-toxic (2.2) and toxic (2.3).

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2.1 Flammable gases

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These are gases that burn in the right mix with air.  The mixture needs to be in the explosive or flammability range. 

 

Below the Lower Flammability Limit and above the Higher Flammability limit, the gas will not burn (either too much or too little oxygen).  Between the LFL and the UFL the gas can burn or explode.  Different gases have different flammability ranges.  The flammability range is sometimes also called the explosive range, with explosive limits rather than flammability limits.

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Liquified flammable gases are liable to bleve when the storage container is heated.  A bleve (boiling liquid expanding vapour explosion) occurs when:

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• heat acts on the cylinder, causing the contents to expand and the pressure to rise;

• the safety devices (eg. bursting disc, pressure relief valves) activate, venting gas out;

• the liquid level inside the cylinder drops;

• the heat now acts on the vapour space above the liquid, which weakens the metal;

• the cylinder fails structurally, and the gas explodes out and catches fire.

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Flammable gases can be ignited by several sources, including sparks and static electricity.  

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Examples include propane, butane and hydrogen.

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2.2 Non-flammable, non-toxic gases

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These gases are dangerous by pressure, possibly cold temperatures (cryogenic) or asphyxiating.  Examples include carbon dioxide, argon and oxygen.

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Oxygen cylinders may be marked both green and yellow - green to indicate it is a non-toxic, non-flammable gas and yellow because it is oxidising.

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2.3 Toxic gases

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Toxic gases can cause death to humans and/or animals.  They can kill quickly (acute) or slowly (chronic).

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The level of toxicity is measured by the LC50 (lethal concentration that kills 50% of a test sample).

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Examples include chlorine, carbon monoxide and phosgene.

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Properties of Class 2:

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Gases occupy a large volume, so are often compressed to transport them more easily.  These pressures can reach over 3000psi (over 20 atmospheres), and pose a danger by this virtue alone.  This is why gases are not given packing groups, rather they allocated transport categories.  Toxic gases (2.3) are in TC 1, flammable gases (2.1) are in TC 2 and non-flammable non-toxic gases (2.2) are in TC 3.

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Some gases will liquify at these pressures and at temperatures above -50C, though some need refrigerating beforehand.  These liquid gases are cryogenic and pose dangers because of the extremely low temperatures colder than -153C.

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Some gases are safer transported when dissolved in a solvent.  Acetylene dissolved in acetone (and then in a porous ceramic mass) is an example.

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Adsorbed gases are those which are adsorbed onto a solid porous material (such as activated charcoal).

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Aerosols come under this class of dangerous goods.

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Compressed gases transported in cylinders need to be segregated from:

• heat - this causes the cylinder internal pressure to increase, possibly causing the cylinder to fail

• corrosive substances - these attack the metal cylinders and weakens them, possibly causing cylinder failure

• oxidising substances - in the case of flammable gases

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PPE required for this class include eye and hand protection, high visibility vest and a respirator (compulsory for 2.3).  The hand protection may need to be special thermal ones if handling cryogenic liquids to prevent frostbite.

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The majority of compressed gases are heavier than air and displace air, causing asphyxiation.

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All cylinders of liquified gases are filled nearly to capacity.  This left over space is known as ullage and is the allowance for the liquid expanding due to temperature variations.