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Hazard

Cryogenic materials

Hazard Knowledge

Cryogenic materials are liquefied gases that are maintained in their liquid state by a combination of cooling to very low temperatures (typically below –150C) and increased pressure. Key features of all cryogenic materials are their low temperature, including the gases that are produced through evaporation of the cooled liquid. These gases will typically be sufficiently cold to condense the water in air producing a visible fog. Also, a small volume of released liquid can produce a significant volume of gas; for example, one litre of liquefied nitrogen will result in 695 litres of nitrogen gas when warmed to a temperature of 21oC.

Cryogenics can be separated into three groups:

Inert gases – these will not typically undergo chemical reaction or support combustion. In some cases, these gases will be used to provide an inert environment to increase the safety of an industrial process (e.g. nitrogen).

Flammable gases – the gases released from these cryogenic materials will burn in air (e.g. methane as liquefied natural gas (LNG), natural gas liquids (NGLs) or hydrogen).

Oxygen – a release of liquefied oxygen creates a very high concentration of oxygen in the spillage area. This results in materials that would not typically be considered as flammable being able to support combustion. Organic materials can also react explosively when in contact with liquefied oxygen.

Specific properties of cryogenic materials must be considered during an incident where they are released:

Cold – cryogenic materials are cold enough to cause a thermal burn to the skin. This may be further characterised by a lack of initial pain, but intense pain when the affected area thaws. Unprotected skin can stick to surfaces that have been cooled by cryogenic materials, creating a problem in removing the skin without damaging it. Some materials subjected to such extremes of cold may also become brittle and break; during an operational response, this is particularly relevant to personal protective equipment (PPE) and other equipment.

Asphyxiation – the gas formed by evaporation from a cryogenic liquid is initially very cold. It will therefore typically be denser than air and accumulate close to ground level. When combined with significant volumes of gas produced from a small volume of spilt cryogenic liquid, it results in the displacement of oxygen from air. Therefore, even where the gas is not toxic, there is a danger to health due to reduced oxygen levels.

Fire hazard – flammable cryogenic material will easily form mixtures with air that are between the upper and lower explosive limits following a release, creating a fire and explosion hazard.

Oxygen-enriched air – liquid hydrogen and helium are sufficiently cold to liquefy air. Nitrogen will evaporate more rapidly from the liquefied air than oxygen. This will leave a liquid air mixture behind, which has a higher concentration of oxygen than normal air; when this evaporates, an atmosphere with a higher concentration of oxygen is created.

An increase in the temperature of a contained cryogenic liquid will lead to evaporation and an increase in pressure in the container. Cryogenic storage vessels will typically include pressure relief valves enabling gases to be released and the pressure reduced.

However, in an emergency such as a loss of thermal insulation, fire impingement on the vessel or ingress of another material (e.g. water) will lead to an immediate rise in pressure that is too significant for a pressure relief valve to reduce. In this case, the pressure rise can lead to a catastrophic failure of the container and an explosion.