Fume Versus Dust Extraction

Welding operations require proper exhaust ventilation to protect workers from harmful exposure to toxic fumes. Different ventilation strategies may be needed in each case to remove air contaminants from the welder’s breathing zone. General guidelines have been published in CSA W117.2, Safety in Welding, Cutting, and Allied Processes, and the ANSI Z49.1 Standard, Safety in Welding and Cutting.

Local exhaust ventilation (LEV) is always the preferred method of removing welding fumes and gases. It exhausts or removes the toxic gases, fumes, dusts and vapours before they can mix with the room air.

A moveable hood is one example of an LEV. Flexible ducting allows the capture hood to be moved where required. An air velocity of at least 100 ft/min. across the welding arc and duct velocity of 2,000 ft/min. are needed to properly capture the fumes. The hood must be placed as close as practical to the work. The optimal location for the hood is about one duct diameter from the arc.

The purpose of the local exhaust is to ensure that exposure to hazardous substances (toxic materials) is below applicable occupational exposure limits. These toxic materials can include antimony, arsenic, barium, beryllium, cadmium, cobalt, copper, lanthanide metals, lead, manganese, mercury, osmium, selenium, thallium, silver, vanadium or zinc, and cadmium in hard (silver) brazing alloys or as a coating.

Exhaust ventilation must be designed by a trained designer in strict accordance with the specifications in the ACGIH Industrial Ventilation Manual.

During the welding process, operations such as grinding, buffing and polishing are often required as well. Grinding and polishing of aluminum creates dust that poses an explosion hazard.

A question that is commonly asked is: “Can a local fume extraction system be used to remove aluminum dust?” The answer is definitely “No!” Here are the reasons.

The NFPA 484 standard for combustible metals stipulates that such dust collection systems shall be dedicated to aluminum and aluminum alloys only. In other words, the dust collection system for aluminum must never be used with steel-grinding operations.

The special exhaust system for aluminum powders consists of branch pipes connected to hoods or enclosures, one or more header pipes, an exhaust fan, a means for separating solid contaminants from the air flowing in the system, and a discharge stack to the outside.

The rate of exhaust and recommended minimum duct velocity of 4,500 ft/min. in the branch will be sufficient to ensure that there is no accumulation of dust inside the ductwork. The air velocity for the fume extraction system is usually 2,000 ft/min. Accumulation of aluminum dust inside the ductwork presents a very serous explosion hazard.

Separation of solids in an exhaust system for aluminum powders is usually done by either a wet or dry dust collector. Both devices are quite different from a typical fume extractor.

A wet collector by itself may pose an explosion hazard due to the generation of hydrogen gas when aluminum powder is immersed in water, or from a dry powder explosion when, for any reason, there is no water inside the wet collector.

The dry-type dust collector must be located outside, again because it presents an explosion hazard. In Ontario, Pre-Start Health and Safety Reviews (PSRs) are required to ensure explosion safety issues have been addressed.

See Table 1 for the typical requirements for a grinder exhaust. In welding operations where grinding or polishing is required, two separate systems must be employed.

Click here to view Table:1 from this article

Simon Fridlyand, P.Eng., is president of S.A.F.E. Engineering, a Toronto-based company specializing in industrial health and safety issues and PSR compliance. He can be reached 416-447-9757 or simonf@safeengineering.ca. For more information, visit www.safeengineering.ca.