Combatting O
George Hutcheson, John Deere Waterloo Works, Waterloo, Iowa Tom Slavin, Navistar Inc., Chicago
rdinary materials that are normally nonhazardous may become hazardous when finely divided. Combustible materials, including resins,
binders and certain metals, may pose an explosion risk in a metalcasting facility under some conditions. For example, phenolic resins that
are manufactured onsite may contain ingredients that can condense and form a layer of combustible dust. On the other hand, sand that is pre-coated with phenolic resin does not present
a problem. Combustible metals (e.g. aluminum, magnesium and titanium) must be handled with care to avoid accumulations of fine particles.
Understanding Dust Explosions While combustible dust explosions
are relatively rare, the first step in controlling them is understanding how they occur. The fire triangle (Fig. 1) demon-
strates that the essential elements in a fire are fuel, an oxidizer (usually air) and an ignition source. To incite a combustible dust explosion, two additional elements must be added: dispersion and containment (Fig 2). For dust to explode, it must be dis- persed in air in a quantity that will support combustion. In addition, the pressure that develops as the dust burns must be contained to build up to an explosion. The container can be a dust collector, ductwork or, if the volume of dust is great enough, the walls of a room or building. A combustible dust explosion
Fig. 1. The fire triangle dictates three ele- ments are required for combustion.
MODERN CASTING / February 2011
usually has two phases: an initial or primary explosion that may be a dust explosion and a secondary explosion
that occurs when dust accumulated on floors or other surfaces is lifted into the air and ignited by the primary explosion. In some cases, the initial explosion can occur in a dust collector located outside the building, with the pressure and blast wave from the first explosion traveling back through the ductwork and suspending settled dust inside the building and causing the second explosion. If you are working with a com-
bustible metal, such as aluminum or magnesium, you must follow the
Dust explosions in casting facilities can be devastating, and metalcasters must be vigilant to recognize and properly evaluate their risk.
ombustible Dust
Fig. 2. A dust fire can be described by the fire pentagon, with two additional elements.
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