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Atmospheric Imbalance
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.
An understanding of the approximate volumetric composition of the main components of air can be helpful in preventing accidents and are as follows:
| Gas |
|
Symbol |
|
% |
| Oxygen |
|
O2 |
|
21% |
| Nitrogen |
|
N2 |
|
78% |
| Argon |
|
Ar |
|
1% |
Atmospheric Imbalance relating to Welding & Heating Process
The use of welding and heating processes can lead to an imbalance in atmospheric gases and can result in the following hazardous conditions:
- oxygen enrichment
- oxygen deficiency
- fuel gas accumulation
- toxic fume build up
Oxygen Enrichment
Definition
Oxygen enrichment occurs in an atmosphere having more than the normal percentage of oxygen found in normal air. The normal oxygen content of the air is 21%, but a small increase in oxygen can have a dramatic effect on your environment. Causes:
- Leakage of gases other than oxygen
- Spillage of liquid gases
- Incorrect storage of vessels or open dewars
Dangers
Oxygen is essential to life, and it is therefore vital to ensure that adequate oxygen is present in any atmosphere being breathed. While a healthy person may survive a short exposure to an oxygen content as low as about 16%, no one should ever be asked to endanger his/her life by breathing such an atmosphere. An insidious feature of oxygen deficiency is that it cannot readily be detected by the senses, and victims are usually unaware of the danger they are in and may even have a feeling of well-being.
Oxygen Deficiency
Definition
Oxygen deficiency occurs in an atmosphere having less than the normal percentage of oxygen found in normal air. Normal air contains 21% oxygen at sea level.
Causes
- Leakage of gases other than oxygen
- Spillage of liquid gases
- Incorrect storage of vessels or open dewars
- Welding and heating processes. All gas welding and heating processes involve taking oxygen from the air and can tend to cause a deficiency unless the volume of workspaces and their ventilation is sufficient.
Dangers
- Oxygen is essential to life, and it is therefore vital to ensure that adequate oxygen is present in any atmosphere being breathed. While a healthy person may survive a short exposure to an oxygen content as low as about 16%, no one should ever be asked to endanger his/her life by breathing such an atmosphere. An insidious feature of oxygen deficiency is that it cannot readily be detected by the senses, and victims are usually unaware of the danger they are in and may even have a feeling of well-being.
Fuel Gas Accumulation
Definition
The dangerous accumulation of fuel gases, when mixed with an oxident and provided with an ignition source will burn.
Causes
- Ignition occurs within a range of concentrations and the ignition limits of the most important fuel gases when mixed with air are:
The lower limit is particularly important, as that is the one that is reached first. There is hardly any danger that these ignition limits will be reached in large welding shops having good natural ventilation or in the open air Inside containers or in confined spaces. However, even small quantities of escaping fuel gas are sufficient under certain conditions to form an ignitable mixture. Fortunately acetylene can be recognised by its distinctive garlic like, smell in very small concentrations well below 2%. Acetylene in the air is therefore very easily detected by a good sense of smell, allowing corrective action to be taken.
Dangers
- Fuel gases that escape may form mixtures with the surrounding air and lead to fires and explosions.
- Because of its high specific gravity, propane flows to the ground like a liquid when there is little air movement; thus it may remain for a very long time if there is no way for it to flow away. It is then possible for ignitable concentrations to arise
Toxic Fume Build Up
Definition
The dangerous build up of toxic fumes from welding and cutting processes. Welding fume is an unavoidable by-product of welding. It consists of particulate fume, the part you can see and gaseous fume, which you cannot see, but can sometimes smell. All welding processes generate welding fume, the type and amount varying from process to process; MMA, MIG, FCAW, and plasma cutting tend to produce most, while TIG, plasma welding, laser welding and Submerged Arc give very little under normal circumstances, although TIG can give high levels of gaseous fume.
Causes
Cutting and welding metals by flame or arc processes produces fume. The fume is in two parts:
- Particulate fume, usually visible as smoke, and consisting of fine metal oxide powder. The composition of this fume depends upon the metal being cut or welded and, in the case of welding, the composition of the filler rod or wire.
- Gaseous fume, which is not visible and is usually composed of combinations of oxides of carbon, oxides of nitrogen and ozone.
Dangers
- It is important to understand that welding fume is hazardous to operators. The Health and Safety Executive lists the Occupational Exposure Standard (OES) of each fume component (in EH 40). For further information on this consult your local Health and Safety Executive Office. The overall OES for welding fume sets a maximum concentration of 5mg of fume per cubic metre of air.
Preventing Atmospheric Imbalance
Ensure Good Ventilation
- Wherever cylinder gases are used constant and thorough ventilation should be maintained. This is particularly important when they are used in confined spaces.
- In order to avoid exposure to fumes, general and local extraction is essential. Local ventilation should be arranged to suck the fume away as it is formed. A number of companies market such units. General ventilation should ensure that the whole atmosphere of the workshop is changed a number of times during a shift. A range of fume extraction equipment is available from BOC Gas & Gear.
- Take special precaution when welding or cutting coated material as their coatings can produce toxic fumes.
Avoid Leaks
- Assemble equipment carefully
- Ensure that valves are closed when apparatus is not in use. Hazardous situations can arise through negligence such as failure to close valves. For example, leaky equipment or torches with the fuel gas valve not properly closed have been left in small shops for long intervals, and when work was resumed serious accidents occurred.
- Conduct Leak Testing
Observe correct use of blowpipes
- Care should be taken, especially in confined spaces, to avoid delay in lighting the blowpipe after opening the valves.
- When flame cutting, besides the oxygen for the preheating flame, a considerable amount is required 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.
Remove gas equipment from confined spaces when not in use
- Gas equipment connected to a supply must not be allowed to remain in confined spaces or vessels during intervals or meal breaks, but must be taken out at that time.
Check the properties of filler rods and wires
- Some filler rods/wires used in welding or brazing may contain particular toxic materials, and the suppliers of these rods or wires should be contacted regarding the hazards and any special fume removal requirement.
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Safety in the Workplace:
The information contained herein is provided as a general outline and is not intended to be a definitive statement on the subject matter. Professional advice should be sought before any action is taken in relation to safety in the workplace. |
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