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Dust and Risks of Fire and Explosion

As evidenced by silo explosions in the grain-handling industry, dust explosions are very powerful and a very real risk. Consequently, extreme care must be taken to minimize this risk. For many dusts, a settled layer as thin as the thickness of a paper clip – only 1 millimeter (1/32 in.) – is enough to create an explosion hazard. A 6-millimeter (1/4-in.) layer is a bigger problem – big enough to destroy a plant.

For there to be a dust explosion, these factors need to be present: a confined combustible dust at the right concentration, a gas that supports ignition, and an ignition source. Many fine dusts, including chemicals, food products, fertilizers, plastics, carbon materials, and certain metals, are highly combustible, the first requirement for a dust explosion. By nature, any dust collection device contains clouds of these fine particles suspended in air, which itself is a gas that supports ignition, the second requirement.

Filter illustration showing the steel mesh under the filter material
Dust collector filters should include a conductive thread woven into the filter fabric to carry any static buildup safely to ground.

In any mechanical material-handling operation, there are a number of possible ignition sources, the third requirement for a dust explosion:

  1. Mechanical failures that cause metal-to-metal sparks or friction
  2. Fan blades that spark when they are struck by a foreign object
  3. Overheating from a worn bearing or slipping belt
  4. Open flames from direct-fired heaters, incinerators, furnaces, or other sources
  5. Welding or cutting causing a point source ignition or a hot-particle dropping (perhaps several floors) to a flammable atmosphere
  6. Static electricity discharge
  7. Migration of flammable dust into the hot region of a compressor or catalytic reactor

Categorizing Dust Explosions

There are several ways to look at dust related conflagrations:

  1. Flash Fire
    A flash fire is the sudden ignition of unconfined dust. A flash fire is usually localized and can cause significant damage or injury. A flash fire can create the conditions for a secondary explosion, which can cause catastrophic damage and fatal injuries.
  2. Explosion
    When dust is confined and ignited, an explosion is created. This rapid explosion of gases will generate significant and destructive over-pressures that can even demolish the building, leading to greater damage and injury.
  3. Primary or Secondary
    An initial or primary explosion can cause secondary explosions by disrupting, dispersing, and igniting new sources of dust removed some distance from the original blast. Secondary explosions can be more destructive than the primary explosion, and every explosion can lead to additional secondary explosions.
  4. Magnitude
    The speed and force of an explosion are direct functions of a measurable characteristic called the deflagration index. Dust explosions can be more hazardous than explosions caused by flammable gases.

Topics: Belt Conveyor Safety, Dust

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