Oxygen Risks
Introduction Oxygen enrichment can arise when using oxygen unless good practice is observed. This information specifies the dangers and the simple precautions to be taken to ensure that every user can employ oxygen with confidence and without danger.
The atmospheric gases are non-toxic, but alterations in their concentrations - especially that of oxygen - have an effect upon life and combustion processes. It is essential to have sufficient oxygen in atmospheres being breathed. Although not itself flammable, oxygen does support combustion, whereas nitrogen and argon inhibit combustion. If good practice is not observed accidents may happen because changes in concentration cannot be detected in good time by the human senses. When these gases are in the liquid state, it is necessary to bear in mind the very low temperature involved (less than -180C at atmospheric pressure). 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:
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 and materials which do not burn in air, including fireproofing materials, may burn vigorously or even spontaneously in enriched air. Hydrocarbon 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 can be used under certain conditions). Smoking Many burning accidents which occur are triggered off by the lighting of a cigarette, therefore it is impossible to over-emphasise the danger of smoking in oxygen enriched atmospheres or where oxygen enrichment can occur. In such areas smoking must be forbidden. Causes and avoidance of oxygen enrichment Oxygen enrichment of the atmosphere is best guarded against by careful attention to the following points: Leakage of equipment Newly assembled equipment for oxygen service should be thoroughly leak checked by a timed gas pressure drop test, 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.
Click here for more information on how to perform a leak test All equipment, for instance welding and cutting nozzles and hose connections, should 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, in order to avoid possible oxygen leakage in the time 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 knocks or pulling over. 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. Therefore the ventilation in areas where such processes are undertaken must be sufficient to ensure that oxygen enrichment does not occur. 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 workpiece being cut. It is therefore important to select the correct nozzles and pressures. Improper use of oxygen Apart from the hazards of oxygen enrichment of the air already described, the following misuses of oxygen are particularly dangerous and must be strictly forbidden:
This list is by no means complete. Respiratory oxygen For many applications in the medical field, oxygen or oxygen enriched air is used for treatment in 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, all open flames and spark producing equipment must be avoided. Smoking and 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 Measuring instruments The equipment must be easy to handle and offer a high degree of reliability of operation. BOC can advise on the suitability of actual instruments. 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 giving visible and/or audible warnings are in use, the important difference being whether they are suitable for continuous or discontinuous measurement. When working in rooms where the oxygen content can change to a dangerous extent during the working time, continuous measuring methods must be used. Discontinuous measuring methods may be used only if the time between two measurements is such that the tendency for dangerous change of oxygen can be detected quickly enough. Accuracy 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 directions of the manufacturers for the use and maintenance of the measuring instruments should be carefully observed. The measuring instrument should be located as near as possible to the worker, in confined spaces. In confined spaces, it is recommended that the worker has a portable measuring instrument attached to his working clothes, giving an audible and/or visual alarm if the oxygen content of the atmosphere deviates more than 2% from that of normal air. Odourisation This method, which involves the addition of a foul smelling substance, is sometimes used to detect the escape of fuel gases because these form explosive mixtures with air. There is no similar incentive to use the method with gases such as oxygen and, although it can be employed to detect oxygen leaks, the following points should be borne in mind:
Other gases The safety of a space 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. Preventative Measures General considerations 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 whatever regulations are applicable. Any leak must be dealt with by people who have been adequately trained and who have the proper equipment. Information should be provided on actions to be taken by personnel, first aid and firefighting 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. Clothes Many so-called non-flammable textiles will burn fiercely in air containing as little as 30% oxygen, and no material should be considered safe unless it is known to have been subjected to a proper test. Untreated glass fibre and asbestos, which are the only truly non-flammable textile materials in 100% oxygen, are unsuitable for making clothes. Some synthetic materials including polyamides may be fire- resistant to some extent, but they can cause serious burns due to the adhesion of molten material. Fire-proofing with borax can be useful, but washing reduces the effectiveness of the treatment. Clothing should be well fitting, yet easy to remove and free from oil and grease. People who have been exposed to an oxygen enriched atmosphere must not smoke or go near hot spots 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 necessary. Breathing equipment Breathing equipment is not required for oxygen rich situations. Fire extinguishers Under oxygen rich conditions, the best fire fighting media are water or extinguishers containing dry chemical powder or carbon dioxide. Fire fighting equipment should be properly maintained and workers should know where it is located and how to operate it.
Double manning Where personnel have to work in confined spaces such as vessels 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 firefighting equipment must be available.
Analysis Before people enter a space which may be subject to oxygen enrichment, the atmosphere should be analysed for oxygen. Free entrance is permissible only if the oxygen concentration is between 20 and 22%. However, if there is any possibility of a change in concentration, anyone entering such a space shall 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 enrichment can occur should be given adequate instructions as to the risks involved, special attention being drawn to the insidious nature of the risks due to the rapidity of their effects coupled with the fact that an operator may be completely unaware of the potential danger he is exposed to. Practical training should be given in the means by which such risks can be minimised. 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 and 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. Steps to be taken in case of accidents 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 lights. As a rough guide, experience has shown that in the case of liquid oxygen spillage, substantial oxygen enrichment occurs only within the visible cloud associated with the spill. Firefighting materials under oxygen rich conditions should be confined to water or fire extinguishers as described in Section 5.4. A person whose clothing catches fire should be deluged 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.) 7 Entry into confined spaces Guidance is given in the HSE Approved Code of Practice called 'Safe work in confined spaces' L101 (ISBN 0-7176- 1405-0) available from HSE Books, cost £7.50. It also contains the Confined Space Regulations 1997. Flammability Ranges Fuel gases used with oxygen in certain procedures can cause explosive hazards. For more information on flammability ranges please click on the link below.
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