The Triangle, the Quadrilateral, and the Pentagon

One of the tools used to teach people about the danger of fire is the fire triangle. The triangle is used because there are three elements needed for a fire to occur: fuel, oxidizing agent, and an ignition source. The fuel is what burns be it a solid (wood, coal) or a gas (natural gas, gasoline or alcohol vapors). (Liquids don’t actually burn; it is the vapor above the liquid which burns.) The oxidizing agent is typically the oxygen present in the atmosphere. The ignition source can be an open flame, an electric spark, or even a hot surface. Combining these three elements typically results in a fire. If one or two of these elements are taken away, a fire will be extinguished.

In order to have a dust fire, a fourth element is typically required: dispersion. If a dust is in a pile or a layer, it will typically not burn due to the limited amount of oxygen and surface area exposed to the oxygen and ignition source. This statement is generally true and depends upon the flammability of the particular powder that is being exposed to the ignition source. But when the dust or powder is dispersed in the atmosphere, the amount of surface area exposed to the oxidant and the ignition source increase by orders of magnitude. The large amount of surface area of the dust can experience a rapid oxidation reaction and take the form of a flash fire. Technically speaking, this flash fire is a deflagration. If deflagration occurs in an open area the heat and products of combustion are free to expand. This expansion creates a large amount of radiant heat and a pressure wave. But since it is uncontained, the effects are primarily localized to the vicinity of the flash fire.

In order to have a dust explosion, a fifth element is required: containment. When a deflagration occurs within a contained area, the rapid expansion of the heat and products of combustion cannot freely expand. This causes the pressure within the containment structure to rise rapidly. If the pressure rise is sufficient the containment structure will fail violently and rapidly. This failure is the explosion.

It is important to realize that if these five components are present and dust explosion is likely. In order to avoid a dust explosion one or more of these components need to be removed from the area.

In manufacturing plants that create or handle dusts or powders, these five elements are present in various amounts throughout the facility. It is the responsibility of the owner/operator of every facility to carefully consider the processes in their plant and determine how these five elements are to be controlled, both within the process stream and within the building itself.