LEV and Welding
Inhalation of welding fume can result in serious ill health and occupational diseases such as asthma and cancer. The principal recommended method of fume control is by means of local exhaust ventilation (LEV), commonly an extracted flanged welding hood attached to a flexible arm or a mobile unit.
What is welding fume?
During welding, tiny particles are emitted along with gases, and this combination of particles and gases is referred to as "welding fume".
Welding fume is considered a "hazardous substance" under the Control of Substances Hazardous to Health Regulations 2002 (as amended). These regulations require employers to assess the risks and implement suitable control measures to ensure that employees (and others) are not exposed to unacceptable levels of hazardous substances at work.
Fume from welding may cause:
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dryness of the throat, coughing, tightness of the chest and difficulty in breathing
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an acute flu-like illness (metal fume fever)
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long-term changes in the lungs.
The fume from welding will vary greatly in the composition and concentration depending on the task being undertaken. Therefore the levels of exposure to different substances will vary accordingly. The amount of fume generated and the hazards to welders depend on factors such as:
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types of materials being welded
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process parameters
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duration and frequency of the actual process in operation
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operating position and location
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general ventilation and air movement in the area.
The processes that release the greatest quantities of harmful fume include:
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work on metallic coatings such as cadmium or zinc plating and chromium, manganese, cobalt and/or nickel hard surfaces
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work on painted surfaces that contain lead, zinc, chromium or cadmium pigments
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all types of cutting and gouging
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manual metal arc welding
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flux cored arc welding
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higher current metal inert gas shielded welding, particularly on stainless steel, aluminium, copper, nickel and their alloys.
Measurements of actual welding fume concentrations can be used to determine whether exposures of welders and others in the vicinity are likely to mean that fume extraction is required. Extraction may also be needed to prevent general build up of fume in the workshop, eg where a number of welders are working, even if individually they may be exposed only to low amounts of fumes.
In some instances it is possible to minimise the risks by using an alternative joining process (eg bolting parts together), removing grease and surface coatings first, using safer filler materials, adopting good posture (ie head out of plume), etc.
The welder, the work area and the environment
Generally, there is a choice between "high volume, low velocity" and "high velocity, low volume" systems.
High volume, low velocity systems use large diameter ducting and capture hoods or nozzles and are suitable for fixed bench or booth installations. They are also used with flexible extraction hoses, typically 150mm in diameter, with smaller portable capture nozzles that the welder can position close to the weld. At times, repositioning by the welder can be difficult and time-consuming, particularly with stiff, bulky hoses.
Low volume, high velocity systems use much higher extraction speeds with correspondingly low diameter flexible hoses, typically 30-40mm. However, the capture distance in front of the nozzle is also reduced, and it is usual to fit specially designed nozzles that must be positioned very close to the arc. Because the nozzle and hose are smaller and more flexible, they are more easily positioned and repositioned so this system should be easier to use. However, if the nozzle is not positioned to capture the fume, there is so little air removed from the local area that pollution can easily build up.
The high velocity, low volume system is also used "on gun" for semi-automatic welding, particularly with flux-cored wires. This eliminates the welder having to reposition the nozzle. The main disadvantage of such systems is a risk that the essential shielding gas is disturbed, giving defective welds if the nozzle is not carefully adjusted or is too close to the arc. If it is too far from the arc, fume is not collected. In the past, welders complained that the on gun extraction systems were bulky, heavy or awkward to use but their design has much improved and a variety of suitable systems exist.
Portable extraction units can be moved close to the area being welded and can reduce the length of the flexible hose required. Flexible hoses are expensive and can introduce serious inefficiencies by leaks or obstructed airflow resulting from wear, damage or misuse.
Portable extraction units are normally suitable only for one operator and can be quite noisy. Filters are expensive and need careful monitoring to ensure they have not become blocked, rendering the extraction system ineffective and possibly discharging polluted air into the workshop. There is also the hazard of power cables trailing in the working area.
Fixed fans with rigid ducting are the best way of providing effective fume extraction to a number of locations. Each welding position must be easily reachable by a short suitable connecting flexible hose from inlets built into the ducting.
Training for welders should include information about the health risks from welding fume. They should be trained to work in the right way by using the controls for exposure control properly. They should also be adequately supervised to ensure that the controls are used correctly.
It is therefore necessary to devise and implement a "safe system of work" to cover the collection, removal and handling of the dust from welding fume filters and collection units. It may be necessary to provide respiratory protective equipment, goggles and protective overalls for operators collecting or transferring dust or loaded filters to the safety of a sealed bag or container.
If welding fume is not properly extracted, fume can quickly build up in an area affecting not just the welders, but also those in adjacent areas. High volume extraction systems have the advantage that, even if the nozzle is not collecting fume directly from the welding plume, there is a comparatively high volume of air being extracted from the welding area, which can prevent a local build up of fume.
Under current legislation, discharge of welding fume into the atmosphere is permitted provided it does not to cause a "nuisance" to others. Welding is not a "prescribed process" that has to be notified to the Environment Agency.
However, when welding fume is collected into a filter or another container, there is a hazard to people in the area if any dust is dislodged or could be allowed to escape into breathing zones, and this is particularly true when the welding fume contains significant levels of toxic dusts, such as hexavalent chromium.
A competent ventilation engineer must examine the local exhaust ventilation (LEV) system and test its performance at least once every 14 months (or 6 months for non-ferrous metals). These records must be kept for at least five years.
Design, installation and balancing of LEV systems is a task for competent and experienced professionals. The installation must always be operated as intended, because unapproved modifications such as adding extra lengths of hose can render the entire system ineffective.
Further information
Source: Croner
Posted Date: 04th Apr 2011