Welding fume extraction

Welding fume normally relates to the very small particles emitted during fusion welding. However, gases are also generated or released and these also form part of the 'welding fume.' As extraction equipment captures polluted air containing both gases and particulate contaminants, the term 'fume' is used to refer to any atmospheric pollution arising from the arc.

All atmospheric contaminants are potentially injurious to health. Some substances, particulate or gaseous, are more toxic than others. The Health and Safety Executive in UK recommends that for welding fume, actual exposure in the workplace should be reduced by the use of fume extraction equipment.

The amount of fume generated, and the toxicity hazards to welders depend upon a wide range of factors such as: types of materials being cut or welded, process parameters (increasing energy input to the process normally increases fume), duration and frequency of the actual process in operation, operating position and location, and general ventilation and air movements in the area.

As a general guide, any shop-based welder arcing on steel on a bench, or in the flat or downward position for more than two minutes in every ten with MMA or MIG at currents in excess of 170A is likely to need extraction. Extraction may also be needed to prevent general build up of fume in the workshop, for example where a number of welders are working, even if individually they may only be exposed to low Time Weighted Average (TWA) fume concentrations. The usual limiting OEL for welding steels is 5mg/ m3, for the total fumes. For welding stainless steels and other alloys, the limiting safe OEL for one of the constituents may be so low that the total fume level has to be reduced.

Extraction equipment
High volume 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 diameter, with smaller portable capture nozzles that the welder can position close to the arcing point.

Generally the capture zone of a portable nozzle is a hemisphere with its base centred on the nozzle inlet and radius equal to the diameter of the nozzle inlet nozzle (if the nozzle has reasonable flange.) If the nozzle is simply the open end of a pipe, the capture zone will be even less. The capture zone in this context is a measure of the inlet nozzle's ability to capture the plume of fume rising from an electrode, when the nozzle is placed with its face vertical adjacent to the arc so the fume is drawn horizontally to the inlet.

For most linear welding deposits, the weld will be out of the capture zone for at least some of the time because the width of the capture zone will be less than the weldbead deposit length. For this reason it is obviously more efficient to position the nozzle above the mid-point of the weldbead to take advantage of the convection current causing the welding plume to rise.

If the nozzle is placed on the bench with its face vertical so as to extract horizontally, extraction therefore is a compromise and repositioning by the welder can be difficult and time-consuming, particularly with stiff, bulky hoses to be accommodated.

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 fish tail or slot nozzles that can and must be positioned very close to the arc. Such slot systems can permit extraction over the full length of most MMA deposits. 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 polAir Max 95 20th Anniversary