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Dust & Fibres

Hazardous Area Zones

danger_master_electricianHazardous Areas are defined by three main criteria

-The type of hazard

-The likelihood of the hazard being present in flammable concentrations

-The (auto) ignition temperature of the hazardous material

The type of hazard (groups)

What is a Dust Explosion?

A dust explosion occurs when combustible dust is suspended in air and ignited.  This causes very rapid burning with a release of gaseous products and subsequent pressure rise.  The resulting explosive force can damage plant, property, and people.  Dust explosions can be categorized as either primary or secondary

Primary Explosion

A primary explosion takes place in a confined atmosphere such as a cyclone, storage silo, or enclosed part of the manufacturing plant.  After detonation, the shock wave can damage and often rupture walls, allowing burning dust and gases from the explosion to be expelled into the surrounding area.

Secondary Explosion

The primary explosion will disturb settled dust that may have accumulated.  Once airborne, this dust can support a larger explosion; this is referred to as a secondary explosion.  Secondary explosions can cause severe damage to surrounding plant buildings.  All large-scale dust explosions result from chain reactions of this type. There may be a chain reaction of many explosions caused by the initial explosion.

fire_triangle_dust_pentagon

Required Conditions

For a dust explosion to take place, several key conditions must be present:

  • The dust must be combustible.
  • The dust cloud must be in the Minimum Explosive Concentration (MEC) for that particular dust.
  • There must be sufficient oxygen in the atmosphere to support and sustain combustion.
  • The dust must be dry.
  • The dust must be confined.
  • There must be a source of ignition

There are three zones for dusts:

  • Zone 20  Dust cloud likely to be present continuously or for long periods
  • Zone 21  Dust cloud likely to be present occasionally in normal operation
  • Zone 22 Dust cloud unlikely to occur in normal operation, but if it does, will only exist for a short period

(The presence of dust layers does not automatically lead to the dust zone.  The likelihood of the dust layer being disturbed to create a cloud needs to be considered.  Dust layers also need careful consideration in terms of ignition temperature.  Because the dust layer can make the equipment under it hotter then normal, a factor of safety is applied to the layer ignition temperature.)

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