Joint assessment of the capture efficiency and the ergonomics of welding torches with extraction systems

Outline of reasons and objectives
Over 60,000 welders in France are exposed to welding fumes containing metals and metal oxides. The concentrations often exceed the occupational exposure limit value, even in the presence of a ventilation-type collective prevention system. For semi-automatic arc welding, the on-torch extraction system is often the only available prevention means. It offers the advantage of capture that is always close to the pollutant emission. Conversely, the extractor hose adds weight to the tool, and the closeness of the extraction can give rise to metallurgical defects. In the field, it is often observed that the capture efficiency of such systems is low and that they are discarded because of the ergonomic constraints they bring. The main objectives of the study were to identify the parameters influencing the capture efficiency. They had to make it possible to delimit the fields of applicability, to define guidelines for the air-flow characteristics, and to provide torch manufacturers with design information.
Approach
In order to test the parameters influencing capture efficiency, an automated welding test bench was created and fitted with instruments. To supplement the study, and in particular to address the issue of the metallurgical defects generated by the capture (porosities), modelling by numerical simulation of the flows was conducted. Interviews in the companies made it possible to define the situations to appraise and to obtain "trade" understanding of the intermediate results. A more detailed ergonomic analysis (video recordings and confrontation) was conducted for one of the companies in order to gain a better grasp of the reasons stopping workers from using torches with on-torch extraction systems.
Main results
The study of the various parameters that influence the capture efficiency highlighted the complexity of the problem. The discontinuity and the unsuspected significance of certain parameters explain the difficulties encountered in the past for establishing design/prevention rules on the basis of field measurements. The study confirmed that the current guidelines that set an extraction flow rate of 100 m3/h is not valid; the parameter to be considered is the speed induced at the point of emission of the fumes. In addition, the study on the extent to which capture interferes with the quality of the welding confirmed the reality of the problem for the welds in corners. From the extraction point of view, the equipment available on the market should be improved to enable satisfactory use to be made of it. As things stand, only welding configurations at moderate current (< 250A) can give full satisfaction. Replacing conventional torches with models equipped with extractor systems requires alterations to be made to the work stations, but the recommendations remain limited. At the margin of the work on the capture effectiveness, the study highlighted significant gains on the fume emission flow rates by modifying the parameters of the process.
Discussion
These results should be put into perspective with the characteristics of the equipment currently available on the market and with the technical possibilities for improving that equipment. A meeting with all of the torch manufacturers is scheduled in March 2015 in order to continue the exchanges and discussions begun in 2014 for improving the torches. In addition, definition of new guidelines for dimensioning/controlling torches with extractor systems is currently being discussed with the various stakeholders (occupational safety and health specialists, engineering research centres, and torch manufacturers).