Hazard Knowledge

Gas is one of four states of matter.Gases and vapours have no size or volume. They expand to fill their container or, in the open, spread out until they are equally distributed throughout the space available to them. For more information on the gases and the other states of matter, see the Foundation for Hazardous materials Physical properties of matter and The gas laws.

Most industrial gases occur naturally and are extracted from the atmosphere. Transporting gases at their ordinary temperatures and pressures is not a practical or economically viable option for the chemical industry due to the size of containers that would be required. Viable storage and transport options rely either on cooling, applying pressure or dissolving gases.

The favoured option is to use pressure to liquefy a gas, but for every gas there is a critical temperature. Gases below their critical temperatures are often called vapours and can be liquefied by applying pressure so they can be transported or stored as liquids at ambient temperature. Above this critical temperature, gases cannot be liquefied by applying pressure alone. When a pressurised liquid is warmed above the critical temperature it will exert a critical pressure on its container.

Gases with a critical temperature below ambient temperature can be stored or transported as compressed gases in small quantities at ambient temperatures. When bulk quantities are required they are usually cooled to below critical temperature and transported as refrigerated or cryogenic liquids see Hazard – Cryogenic materials.

For further information see A foundation for hazardous materials Transportation, packaging and supply.

Emergency responders can encounter gases under pressure at many different locations, including transport networks, as gases are often compressed for transportation. The Carriage of Dangerous Goods Regulations set out the safe conditions for transporting gases by road.

Properties of gases that should be considered when dealing with incidents involving pressurised containers include:

• Gases and vapours exert an increasing pressure on their containers as they are heated
• When a gas or vapour expands, such as when it escapes its container, its pressure falls

Gases under pressure, or compressed gases, pose different hazards to chemical liquids or solids and can often be more dangerous due to:

• The potential source of high energy, particularly in high pressure cylinders
• Cylinders containing compressed gases that may fail if over-pressurised or weakened by the application of heat
• The low boiling point of some liquid contents, resulting in the potential for ‘vapour flashing’
• Ease of diffusion of escaping gas
• Leakage of flammable and/or toxic gases that can cause dangerous conditions especially if they are confined
• Low flashpoint of some highly flammable liquids
• Absence of visual aural (odour) detection of some leaking materials
• Unsecured heavy and bulky containers that may topple over, cause injuries, become damaged themselves and cause contents to leak.
• Cylinders that may ‘rocket’ if the regulator and valve assembly shears off acting like a projectile or ‘torpedo’
• Liquefied gases, for example butane and propane, that respond more rapidly to heat than the permanent gases such as nitrogen or oxygen
• Low boiling point materials that can cause frostbite on contact with human tissue
• Cylinders protected by pressure relief valves, fusible plugs or bursting discs that may not work correctly in a fire situation, or if damaged
• Other physical hazards that stem from the high pressure of a cylinder’s contents (for example, accidental application of a compressed gas or jet into eyes or into an open wound, whereby the gas can enter the tissue or bloodstream, is particularly dangerous).

All cylinders represent a potential hazard if directly involved in a fire. Cylinders are pressure vessels, designed to withstand high internal pressure – but if that pressure increases with heat they may fail. This is particularly important if cylinders are directly impinged with flame as, in addition to the increase in internal pressure, the cylinder shell itself starts to lose its strength because of excessive heat.

The nature of the failure and its consequences depends on the combination of cylinder design and gas type. Flammable gases clearly represent a greater risk but all failures will have significant consequences.

If flammable gas cylinders burst in a fire they will release combustion energy. Potential effects of this release when a cylinder of up to 1000L or 40kg bursts include:

• Blast pressure wave
• Fireball of up to 25 metres
• Cylinder thrown up to 150 metres
• Flying fragments may travel up to 200 metres with high looping trajectories
• Flying glass and other structural material
• Structural damage to buildings in the vicinity

Cylinder valves and fittings

To prevent the interchange of fittings between gases, cylinder valve outlets are left hand threaded on flammable gas cylinders and right hand threaded on other gases.

Acetylene

When dealing with acetylene cylinders, the following hazards should also be considered:

• Self-sustaining decomposition, producing heat and pressure
• Acetylene cylinders are normally used in conjunction with oxygen cylinders, thereby increasing the potential hazard

Acetylene is distinguished from other flammable gases by its ability to continue to ‘self-heat’ after the fire has been extinguished. When involved in a fire, acetylene can begin to 'decompose', that is, break down into its constituent elements of hydrogen and carbon. The decomposition reaction is exothermic – it produces heat. Acetylene cylinders are designed to contain and inhibit decomposition, but decomposition in a heated cylinder could lead to the failure of the cylinder if left unchecked

Exothermic decomposition does not produce as much heat as acetylene burning in air. Initiating decomposition requires a significant input of energy from direct flame contact on a cylinder. Mechanical shock to a cold cylinder cannot initiate decomposition.

This means that unlike other compressed gases, acetylene may continue to be a hazard after the fire has been extinguished; it requires specific operational procedures.

For further information on acetylene see Foundation for Hazardous Materials Acetylene