The majority of powders that are used in the processing industries are combustible (also referred to as flammable or explosible). An explosion will occur if the concentration of the combustible dust that is suspended in air is sufficient to propagate flame when ignited by a sufficiently energetic ignition source.

A systematic approach to identifying dust cloud explosion hazards and taking measures to ensure safety against their consequences generally involves:

  • Understanding of the explosion characteristics of the dust(s),

  • Identification of locations where combustible dust cloud atmospheres could be present,

  • Identification of potential ignition sources that could be present under normal and abnormal conditions,

  • Proper plant design to eliminate and/or minimize the occurrence of dust explosions and protect people and facilities against their consequences.

Conditions Required for Dust Explosions to Occur

Six conditions must exist simultaneously and in one location for a dust explosion to occur:

  1. The dust must be combustible (as far as dust clouds are concerned, the terms "combustible," "flammable," and "explosible" all have the same meaning and could be used interchangeably).

    The first stage of a dust explosion hazard assessment is to determine whether the dust will explode when dispersed as a cloud. The combustibility of a dust can be assessed by conducting an "Explosion Classification Test" on a representative sample of the dust. In this laboratory test the observation of flame propagation determines whether or not a suspended dust is capable of ignition and sustaining an explosion.

  2. The dust must be airborne.

    A dust deflagration is dependent on intimate contact between the particle surface and the available oxidizer, oxygen in air. This is because most dusts do not contain sufficient oxygen to support combustion. Without suspension and availability of oxygen around each particle a pile of combustible dust will only smolder and burn poorly.

  3. The dust concentrations must be within an explosible range.

    If the concentration of a dust cloud is below the "Minimum Explosible Concentration" (MEC) an explosion cannot propagate. This is because when the dust concentration is below the MEC the spacing between the particles will be too great and the energy from a burning particle will not be effectively transferred to the nearby particles. The ease of ignition of a dust cloud and also the resulting explosion violence increase as the dust cloud concentration is increased above the minimum explosible concentration until an optimum concentration is reached causing the highest explosion violence. At higher dust cloud concentrations the explosion violence decreases because there is insufficient oxygen available for the combustion process at the particle surface.

    The "Minimum Explosible Concentration" of dust clouds can be determined by laboratory testing.

  4. The dust must have a particle size distribution capable of propagating flame.

    As the dust particles get finer the total surface area that is available for oxidation will increase. Therefore, the sensitivity of a dust cloud to ignition and the resulting explosion violence (severity) increase with a decrease in particle size.

This content is only available via PDF.
You can access this article if you purchase or spend a download.