Any facility that handles combustible particulate solids, including dusts, fibers, flocks, flakes, chips, and chunks should be proactive in evaluating the extent of this hazard in their facility and processes.

NFPA defines a Combustible Particulate Solid as “Any solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition that, when processed, stored, or handled in the facility, has the potential to produce a combustible dust.” (NFPA 652-2016)

NFPA defines a Combustible Dust as “A finely divided combustible particulate solid that presents a flash fire hazard or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations.” (NFPA 652-2016)

Combustible Dusts

Types of combustible dusts include, but are not limited to, wood dust, paper dust, coal dust, metal dust, plastic dust, rubber dust, textile dust, and many organic food ingredients—such as flour, sugar, and corn starch.

OSHA’s Combustible Dust poster available here provides examples of materials that could lead to combustible dust explosions if processed in powder form.

Combustible Dust Explosions

There are five elements necessary to initiate a dust explosion, known as the “Dust Explosion Pentagon.”

  1. Combustible Dust (fuel)
  2. Competent Ignition Source (heat)
  3. Oxygen in air (oxidizer)
  4. Dispersion of dust particles in sufficient concentration
  5. Confinement of the dust cloud

If one of these five elements is missing, an explosion cannot occur; however, as indicated in the figure, there may still be a fire or flash fire hazard associated with the dust.

Combustible dust tragedies often begin with an initial (primary) explosion in processing equipment that damages a duct, vessel, or collector. The pressure wave resulting from this primary explosion can cause fugitive dust accumulations on horizontal surfaces to be dislodged or lifted into the air. This creates an explosive dust cloud that is just waiting for an ignition source, which in many cases, will be the flame front that follows the pressure wave from the primary explosion. Once the airborne dust is ignited, one or more (secondary) explosions will occur.

The secondary explosion(s) are generally more destructive and deadly than the primary explosion, and will propagate anywhere fugitive dust has accumulated in the facility. This is why proper housekeeping is critical for plant safety, because this one action will minimize the amount accumulated fugitive dust available to propagate a secondary explosion.

Loss Examples

Many devastating combustible dust explosions have occurred in the US and around the world. The Chemical Safety Board (CSB) is an independent U.S. agency, charged with investigating industrial chemical accidents in the U.S.

Information on historical combustible dust explosions can be found on the CSB website, or through the links provided below.

We hope that you found this information helpful. If you’d like to learn more about combustible dusts, visit the second part of this blog next week, where we’ll review applicable industry standards that may apply to facilities with combustible dust exposures and highlight a few recommendations.