Dust-Hazard Control in Woodworking Plants

Relative danger of dust-hazard in woodworking plants is explained, and methods for preventing dust-hazard and minimizing dust explosion effects are presented. It has been estimated that dust hazard exists in more than 30,000 U.S. industrial plants, and dust explosions have resulted in over 600 deaths and approximately $90,000,000 worth of property loss. Dust explosions in woodworking plants have caused 37 deaths, injury to 156, and property damage of more than $7,000,000. Laboratory tests have shown wood dusts, regardless of species, to be as hazardous as cereal dusts and wood dust ignition temperatures as low as 190?F have been reported. However, most wood dusts tested in laboratories have ignited as layers of static dust at temperatures of 523? and 630? F. Ignition temperatures for bark dust layers are generally lower. Ignition temperatures for dust clouds are usually considered higher than for static dust layers. Birch-bark dust ignited as a layer at 482?F, required 842?F when ignited as a dust cloud. Minimum concentration of dust in air through which flame will propagate when ignition source is present, is generally accepted at 0.05 to 0.06 oz. per cu. ft. However, many factors affect dust explosibility such as the dust particle’s shape, size, moisture content, size uniformity and dispersion uniformity, and minimum explosion concentrations could be significantly lower than the generally accepted values. For example, Douglas-fir bark dusts of 0.03 oz. per cu. ft. concentrations have been ignited in laboratory tests. Explosion pressures following ignition of dust cloud samples of wood and bark have been measured experimentally at 36 to 85 psi at concentrations of 0.05 oz. per cu. ft. Even at the lower figure, explosion pressures extrapolated to a building would result in pressures of more than 2-1/2 tons per sq. ft., well beyond the design specifications of normal buildings. Several codes or standards contain recommendations on the prevention of dust explosions in woodworking plants. These include buildings of fire-resistive construction, keeping interiors clean and free of excessive dust build up, careful and efficient dust dispension at points of production, elimination of all possible sources of ignition and segregation of dangerous areas and machines, proper electrical equipment, prohibiting of smoking and open flames where dust may be present, installation of explosion vents, and use of controlled atmospheres.

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