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This information specifies the dangers and the simple precautions to be taken to ensure that every user employs oxygen with confidence and without danger.
Composition and behaviour of air
The approximate volumetric composition of air is:
- atmospheric gases are non-toxic but alterations in their concentrations - especially that of oxygen - have an effect upon life and combustion processes
- if good practice is not observed accidents may happen as changes in concentration cannot be easily detected by the human senses
- oxygen is not flammable but it does support combustion
- nitrogen and argon inhibit combustion
- when these gases are in the liquid state it is necessary to bear in mind the very low temperatures involved (below -180°C). They can rapidly cause cold burns and make certain materials sufficiently brittle to lead to structural failure
Fire hazards from oxygen enrichment
Oxygen reacts with most elements. The initiation, speed, vigour and extent of these reactions depend in particular upon:
- concentration, temperature and pressure of the reactants
- ignition energy
- mode of ignition
Combustibility of materials
Oxygen enrichment of the atmosphere, even by a few percent, considerably increases the risk of fire.
Sparks which would normally be regarded as harmless can cause fires. Materials which do not burn in air, including fireproofing materials, may burn vigorously or even spontaneously in oxygen-enriched air.
Oil and grease
Oil and grease are particularly hazardous in the presence of oxygen as they can ignite spontaneously and burn with explosive violence.
They should never be used to lubricate oxygen or enriched-air equipment. Special lubricants which are compatible with oxygen must be used under certain conditions.
In oxygen-enriched atmospheres, or where oxygen enrichment can occur, smoking must be strictly forbidden.
Causes and avoidance of oxygen enrichment
Oxygen enrichment of the atmosphere is best guarded against by careful attention to the following points:
- newly assembled equipment for oxygen service should be thoroughly leak-checked by a timed gas pressure drop test. This must be supplemented by testing with an approved leak test fluid which is compatible with the equipment for which it is being used. Alternatively a solution of 1% Teepol in demineralised water may be used
- all equipment, for instance welding and cutting nozzles and hose connections, must be properly fitted. Hoses and other equipment should be kept leak-tight and be protected from damage
- all maintenance and repair work should be carried out by experienced and fully skilled personnel
- when the work period is over, the cylinder valve or oxygen supply stop valve must be turned off. This is to avoid possible oxygen leakage between the end of one working period and the beginning of the next
- the gas valves on blowpipes or cutting torches should not be relied upon for turning off the oxygen supply
- gas cylinders in use should be protected against being knocked or dropped
Excess oxygen in metallurgical processes
In many metallurgical processes such as gouging, cutting, scarfing and thermic lancing, a surplus of oxygen escapes into the atmosphere and can lead to an oxygen-enriched atmosphere. Such workspaces must be sufficiently ventilated.
Incorrect practice in the use of blowpipes
- care should be taken, especially in confined spaces, to avoid any delay in lighting the blowpipe after opening the valves
- when flame cutting, oxygen is required for the preheating flame, as well as for burning the material and blowing out the slag. This leads to an excess of unused oxygen, the amount of which will increase if the pressure employed is too high, or if the nozzle is too big for the item being cut. It is therefore important to select the correct nozzles and pressures.
Improper use of oxygen
Apart from the hazards of oxygen enrichment already described, the following misuses of oxygen are particularly dangerous and must be strictly forbidden:
- driving pneumatic tools
- inflating vehicle tyres, rubber boats etc
- cooling or freshening the air in confined spaces
- cooling the person
- dusting benches, machinery and clothing
- starting diesel engines
This list is by no means complete.
Oxygen and oxygen-enriched air is used in medical equipment such as oxygen masks, oxygen tents, incubators and hyperbaric chambers.
Even in the vicinity of exhausts from equipment in which oxygen is used, the air is liable to become enriched. The resulting oxygen enrichment leads to a greatly-increased fire hazard.
To reduce the possibility of ignition, the following precautions should be taken:
- naked flames and spark producing equipment must be avoided
- smoking must be strictly forbidden
- the use of cosmetic oil and grease should be strictly forbidden
- precautions should be taken to avoid static electricity
Detection of oxygen enrichment or deficiency
These indicate increases and decreases in the oxygen concentration of the ambient atmosphere and have a measuring range from 0 to 40% by volume of oxygen.
Various measuring techniques are available, giving visible and/or audible warnings and can provide continuous or discontinuous measurement. The equipment must be easy to handle and offer a high degree of reliability of operation. BOC can advise on the suitability of instrument to be used.
The accuracy of the measuring method should be such that, when indicated 21%, the real value is between 19.5% and 22.5%.
Using the measuring instruments
The manufacturer's operating and maintenance instructions must always be carefully followed.
In confined work environments the measuring instrument should be located as near as possible to the worker.
It is recommended that the worker has a portable measuring instrument attached to his working clothes which gives an audible and visual alarm if the oxygen content of the atmosphere is likely to deviate more than 2% from that of normal air.
This method uses the addition of a foul-smelling substance to enable the easy detection of the escape of fuel gases which can explode when mixed with air.
Although this process can be employed to detect oxygen leaks, the following points should be noted:
- the method cannot be used in processes where oxygen excess is normal, for example, oxygen cutting
- odorised oxygen may mask the escape of other odourised gases
- because of the toxicity and combustibility of the odourants currently available, specific installations and special safety regulations are called for when users employ odourisation
- even with odourisation, users must continue to apply all their normal safety measures
The safety of a workspace does not depend on oxygen content alone, but can be affected by other gases such as fuel gases or carbon dioxide. These should be analysed as necessary.
Apparatus used for the manufacture, distribution and utilisation of oxygen must be installed and identified in accordance with the recommendations of the industrial gas industry, and must comply with the applicable regulations.
Any leak must be dealt with by adequately trained staff using specific equipment.
Information should be available to all personnel on the actions to be taken by staff, first aiders and fire-fighting teams in the event of an incident.
Operating personnel must at all times obey works rules and regulations and, where called for, protective equipment must be worn.
- many 'non-flammable' textiles will burn fiercely in air containing as little as 30% oxygen
- no material should be considered safe unless it is known to have been subjected to professional fire safety testing
- clothing should be well fitting yet easy to remove
- clothing must be free from oil and grease
- people who have been exposed to an oxygen-enriched atmosphere must not smoke or go near naked flames or sparks until they have properly ventilated their clothes in a normal atmosphere. A ventilation period of 5 minutes minimum, with movement of the arms and legs and with coats unbuttoned, is recommended
Breathing equipment is not required for oxygen-rich environments.
Under oxygen-rich conditions the best fire-fighting materials are water or extinguishers containing dry chemical powder or carbon dioxide.
Fire-fighting equipment should be properly maintained and all workers should know where it is located and how to operate it.
Where personnel have to work in confined spaces which may become subject to atmospheric oxygen enrichment or deficiency, a watcher must be stationed immediately outside the confined space entrance.
The watcher should hold the rope of a rescue harness attached to the person working in the confined space and should, if necessary, have a winch available.
Where the risk of accidental oxygen enrichment exists, adequate fire-fighting equipment must be available.
Before people enter a space which may be subject to oxygen enrichment or deficiency the atmosphere should be analysed for oxygen. Free entry is permissible only if the oxygen concentration is between 20 and 22%.
If there is any possibility of a change in concentration, anyone entering such a space must be issued with a personal continuous oxygen-measuring device giving an audible alarm when the oxygen concentration in the atmosphere varies outside the safe limits.
Information and training
All people who work in spaces where oxygen deficiency or enrichment can occur should be given adequate instructions as to the risks involved, special attention being drawn to the nature of the risks, the rapidity of their effects and that the operator may be unaware of the potential danger he is exposed to.
Practical risk-reduction training should be given.
Blanking and ventilation
- any vessel which is connected to a gas source other than air containing 21% oxygen must be disconnected from such a source by the removal of a section of pipe, by the use of a spectacle plate or by inserting blanking spades
- the space should be thoroughly ventilated so as to maintain a normal atmosphere before and during entry
- reliance on the closure of valves to prevent oxygen enrichment or deficiency is not sufficient
- permission to enter such a space may be given only after the issue of a permit certificate signed by a responsible person
Isolation of equipment
When an oxygen pipeline enters a building an isolation valve should always be provided outside the building.
Disused oxygen lines should be completely severed from the supply system.
Oxygen enrichment - first aid
- in the event of a major escape of liquid or gaseous oxygen, switch off all electrical appliances in the area concerned and extinguish all naked flames. Experience has shown that in the case of liquid oxygen spillage, substantial oxygen enrichment occurs only within the visible cloud associated with the spill
- fire-fighting materials should be confined to water or extinguishers containing dry chemical powder or carbon dioxide
- a person whose clothing catches fire should be thoroughly soaked with water from a hose or series of fire buckets and removed into the fresh air as soon as possible
- it is very dangerous to attempt to rescue a person on fire in an oxygen-enriched atmosphere as the rescuer will most probably catch fire himself. In certain cases it may be possible to enter such a space if the rescuer is totally deluged with water and protected by constant water hosing
Entry into confined spaces
Guidance is given in the HSE Approved Code of Practice called 'Safe work in confined spaces', available from HSE Books. It contains the Confined Space Regulations 1997.
In certain procedures fuel gases used with oxygen can cause an explosion hazard.