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Cryogenic Engineering & Cryomagnet Technology Section
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HAZARDS
Cold burns, frostbite and hypothermia
- Contact of the skin with cryogenic liquids (or even cold gas) can cause severe cryogenic burns; the tissue damage that results is similar to that caused by frost bite or thermal burns. While the cold itself can reduce the feeling of pain, the subsequent thawing of tissue can cause intense pain.
- Contact with non-insulated parts or equipment or vessels containing cryogenic liquids can produce similar damage. Unprotected parts of the skin may stick to low-temperature surfaces and flesh may be torn upon removal.
- Inhalation of cold vapour can cause damage to the lungs and may trigger an asthma attack in susceptible individuals.
- Hypothermia is risk due to the low temperatures arising from the proximity of cryogenic liquids. Risk is dependent upon the length of exposure, the atmospheric temperature and the individual; those exposed for prolonged periods should be warmly clothed.
- The low viscosity of cryogenic liquids means that they will penetrate woven or other porous clothing materials much faster than, for example, water.
Oxygen deficiency and asphyxiation
Whilst not toxic themselves (excepting CO2 which is mildly toxic), the cryogenic gases are capable of causing asphyxiation by displacing the air necessary to support life.
A reduction in atmospheric oxygen results in loss of mental alertness and distortion of judgement and performance. This occurs in a relatively short time period and without the person being aware it is happening.
An oxygen shift as low as 3% below 20.9% (normal air concentration) is potentially dangerous and atmospheres containing less than 10% oxygen can be fatal.
Upon evaporation, the volume of cryogenic liquid expands approximately 700 - 900 times its volume in the gaseous form. If this occurs in a room that is inadequately ventilated, atmospheric oxygen will be displaced. This will result in the oxygen content of the air being reduced to such an extent that it will not sustain life.
The onset of oxygen deficiency problems is often not apparent to the individual involved as there are few warning signs. In going to assist unconscious colleagues, rescuers themselves are often overcome by the lack of oxygen, resulting in further fatalities.
Oxygen Enrichment
Although itself not flammable oxygen, when present in higher concentrations, can significantly increase the chance of fire or an explosion.
The boiling point of oxygen is above those of nitrogen and helium. In closed systems (such as cold traps cooled with liquid nitrogen) these liquids can cause oxygen to condense on their surface (resulting in a bluish liquid on the surface). This can lead to the ignition of normally non-combustible materials and the flammability limits of flammable gases and vapours are widened. Oil and grease may spontaneously ignite and as such should not be used where oxygen enrichment may occur.
Pressurisation and Explosion
Cryogenic liquids vaporise with a volume change ratio of 700-900 and can thus cause violent changes in pressure, particularly if this occurs in a confined space. This in turn can result in an explosion. Vent systems must be in place to allow gas to escape from confined spaces. Pressurisation can occur due to the following:
- Ice forming on the venting tube, plugging it and preventing gas release;
- Damaged equipment resulting in cryogenic fluids leaking into small areas. Upon vaporisation the cryogenic liquid vaporises and causes pressure build up;
- Loss of vacuum inside a cryostat or dewar;
- If a liquid helium-cooled super-conducting magnet "quenches" (changes spontaneously from a super-conducting state to a normal state);
- Liquid nitrogen having permeated through sealed cryo-tubes containing samples which then return to room temperature;
- Direct contact of the cryogenic liquid with water in a tube results in rapid vaporisation of the cryogenic liquid and can cause the tube to explode.
Damage to Equipment
The very cold temperatures of cryogenic liquids can damage equipment and materials, which can result in danger.
- Spilled liquid nitrogen can crack tiles and damage flooring such as vinyl;
- Rubber tubing may become brittle and crack during use;
- Condensation of water around electrical cables may result in an electrical shock hazard.
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