Cryogenic gases
Nitrogen, oxygen, argon, carbon dioxide and helium can all be supplied as cryogenic liquids.
In many applications, cryogenic liquid supply is appropriate where the customer requires large volumes of gas: storing the product in its liquid form takes a lot less space.
Liquid helium and, frequently, liquid nitrogen are used in liquid form for their cooling properties.
Cryogenic liquids are stored in vacuum-insulated vessels and, usually, the customer does not have to handle the liquid. A common exception is liquid nitrogen, where the product is often decanted into dewars for laboratory use, for example.
Properties of low-temperature liquefied atmospheric gases
The hazards associated with the low-temperature liquefied gases relate to their physical properties. These properties are described in 'Care With Cryogenics' which can be downloaded as a PDF from this page.
Hazards
The main hazards arising from the use of low-temperature liquefied gases are:
- asphyxiation in oxygen-deficient atmospheres
- fire in oxygen-enriched atmospheres
- liquid oxygen condensation
- cold burns, frostbite and hypothermia from the intense cold
- over pressurisation from the large volume expansion of the liquid
Asphyxiation - nitrogen, argon and helium
Releasing nitrogen, argon or helium may produce local oxygen-deficient atmospheres, which will produce asphyxia if inhaled.
Release may be caused by a leak, a spill, or simply during the process in which the gas was used.
BOC recommend that, as a precaution, oxygen deficiency monitors should be used.
Asphyxiation - carbon dioxide
Carbon dioxide is essentially an asphyxiant gas but also has mild toxic properties.
Schedule 1 of the 2007 Code of Practice for the Safety, Health and Welfare at Work (Chemical Agents) Regulations 2001, indicates that the recommended exposure limit for carbon dioxide is 5,000 ppm (0.5%) by volume - calculated as an eight hour time-weighted average concentration in air - or 15,000 ppm (1.5%) for a 15 minute period.
For these reasons, a carbon dioxide monitor should be used when there is a risk of CO2 exposure, rather than an oxygen deficiency monitor.
Carbon dioxide exposure symptoms
| Carbon dioxide content (vol%) |
Effects and symptoms |
|---|---|
| 2-4 | Slight feeling of suffocation and an increased breathing rate |
| 5 | Headaches, dizziness and sweating can occur after 30 minute exposure |
| 5-9 | Breathing becomes laboured, judgement impaired |
| 5-9 | Breathing becomes laboured, judgement impaired |
| 9 | Fatal after approximately four hours exposure |
| 12 | Immediate unconsciousness; fatality may occur after a few minutes |
Oxygen deficiency symptoms
| Oxygen content (vol%) |
Effects and symptoms (at atmospheric pressure) |
|---|---|
| 11-14 | Diminution of physical and intellectual performance without person's knowledge |
| 8-11 | Possibility of fainting after a short period without prior warning |
| 6-8 | Fainting within a few minutes, resuscitation possible if carried out immediately |
| 0-6 | Fainting almost immediate, death ensues, brain damage if rescued |
Fire hazards from oxygen-enriched atmospheres
If the atmosphere is enriched with oxygen the likelihood and potential intensity of fire are increased.
Many materials which are not usually combustible in air will burn fiercely in an oxygen-enriched atmosphere.
They can also be ignited with minimum energy resource that would not, in normal atmospheric air, be considered sufficient.
Liquid oxygen condensation
Liquid nitrogen and helium are cold enough to condense oxygen from the air, leading to a risk of combustion. Vessels should be insulated to minimise this risk. It is recommended to exclude combustible materials.
Cold burns, frostbite and hypothermia
Contact with extremely cold surfaces can cause cold-burns and frostbite. Low air temperatures can also cause hypothermia or provoke asthma.
It is essential that protective clothing (such as cryogenic gloves and goggles) is worn where there is a risk of contact with the cryogenic liquid or cryogenically-cooled materials.
Further detail on all of these conditions and preventative measures can be found in the 'Care With Cryogenics' PDF download.
Over pressurisation
Cryogenic systems must be designed with adequate pressure relief measures such as valves. This prevents a dangerous build-up of pressure as the liquid condenses into a gas in a sealed system.
Preventative measures
The 'Care With Cryogenics' PDF contains preventative measures for dealing with low temperature hazards.
View pages on 'Inert Gas Risks' and 'Oxygen Risks' for preventative measures for the gas hazards.
