Dust explosions
The results of a dust explosion are normally quite devastating. Arriving firefighting units usually find large amounts of destruction as well as a sizeable working fire.
What makes a dust explosion so dangerous?
Dust explosions occur when an accumulation of minute combustible dust particles suspended in air encounter an ignition source. The particles ignite and release combustion gases as they burn. The rapid pressure rise is due to the formation of a shock wave as a result of the expansion of these heated combustion gases. All this happens in a fraction of a second.
A dust explosion usually occurs as a series of explosions. A comparatively small primary explosion is sufficient to jar dust from machinery, beams, ledges, shelving, and other areas. As a result, a larger dust cloud forms and a much larger and much more destructive explosion results.
Several groups of materials can generate dust particles when they are processed, including: agricultural products, carbonaceous materials, some chemical and drug compounds, celiulosics, pesticides, plastic resins, and metals.
Different phases of material processing create dust particles:
Grinding, “cracking,” pulverizing, and size reduction
- Conveying
- Separation
- Drying and heating
- Screening
- Mixing
- Packing
- Storage
Different characteristics determine the relative explosibility of a dust (the relative susceptibility of a dust to ignite and explode). Probably the most important characteristic is the dust partide size. Generally, the smaller the dust particle, the more easily it can be ignited (a large surface to mass ratio). Moisture content of the dust particle is also very important: if a particle is damp, it is more difficult to ignite. However, in order to prevent ignition by this method, dust would have to be so wet that its weight would not allow it to be suspended.
Combustible dusts are similar to flammable gases and vapors in that they possess an explosive range very much like a flammable range for a gas/vapor. The most violent dust explosions seem to occur slightly above a “perfect mix” (slightly more oxygen is available than is needed for the reaction to take place—the reaction is not too lean or too rich).
Another area to consider is the destruction potential of a dust (how much damage a dust explosion can cause). This destruction potential is the result of several factors:
- One such factor is the maximum explosion pressure, the highest pressure an exploding dust exerts on its surroundings.
- The rate of pressure rise is also very important; the more quickly an explosion pressure increases, the more severe the explosion.
- Another important factor is the effect of confinement of the explosion. It is virtually impossible to design a building to contain the pressures of a dust explosion, which can exceed 100 psig. Consider that 10-inch thick concrete reinforced walls can withstand only about 3 psig laterally.
- Another point that must be considered is the actual ignition source, which may be one of several types. Overheated objects, open flames, static electricity, electrical arcs, spontaneous ignition, and cutting/ welding sparks are all ignition sources. Probably the most common are frictional sparks and sparks from “tramp” metal (metal that finds its way into the material processing).
- Various methods are available to provide solutions to eliminate or minimize the effects of a dust explosion:
- Probably the most common method is the dust collector, which removes dust as it’s generated during material processing. There are several types of collectors, including bag collectors, cyclones, wet collectors, vacuum cleaners and electrostatic precipitators. However, such devices themselves are subject to internal dust explosions. Some collectors are constructed of low-strength materials and should be located outdoors.
- Another method of dealing with a dust explosion is the use of vents. The vents must be large enough to relieve the high pressures of a dust explosion. They should vent to the outdoors, away from inhabited areas. Pressure relief panels and walls should be secured to allow them to relieve the pressure but not become projectiles.
- The use of explosion rated electrical devices (Class II in the case of combustible dusts) minimizes the chance of an electrical spark or overheated motor from initiating an explosion.
- Magnets are commonly used to pick out tramp metal from the various processing areas.
- Proper housekeeping and maintenance also go a long way in the prevention of dust explosions.
- Suppression devices are available to stop a dust explosion as it begins to occur. Pressure detectors sense the developing explosion and quickly discharge the suppressant. Suppressants include the use of chemical inhibitors such as the halons or inerting materials such as nitrogen and carbon dioxide, as well as the cooling/ inerting action of water spray.
