A number of factors come into play during the ignition and propagation of fire in a dust cloud. These factors include; composition of the dust and moisture content, size, and shape of the particle and turbulence in the system. These factors determine the severity of an explosion and the type of precaution that should be taken.
When a solid material is heated, it burns slowly layer by layer. This is due to the limited surface area exposed to oxygen. The energy is released slowly and is dissipated as fast as it’s released. This is different when it comes to a dust cloud. The surface area that is exposed to the air is large and if ignition occurs, it burns rapidly and the energy is released suddenly.
Composition of Dust
The extent of an explosion varies with certain physical properties of the dust and chemical constitution. Some metal powders such as magnesium, aluminum, and alloys of the two metals generate pressures of 12 bar and maximum rates of pressure rise of more than 500 bar m/sec. Organic materials have a maximum pressure of up to 10 bar and rates of pressure rise below 200 bar m/sec.
Flammable materials that form dust clouds may be made up of two or more substances. Metal dust may be contaminated with oxide. Coal contains volatile parts and variable levels of incombustible ash. Chemicals may contain solvents from previous processes and certain dust may be made up of many substances.
Flammability of dust is reduced by incombustible matter such as inert solid materials, non-flammable parts, and moisture. High concentration of moisture also reduces the formation of a dust cloud. The presence of volatile components increases the risk of an explosion.
Composition of the Atmosphere
The oxygen content of the medium determines the flammability of a dust cloud. When the oxygen is reduced, the hot surface temperature necessary to light-up a dust cloud is raised albeit progressively. When the cloud of dust is exposed to a source of ignition at a given temperature, there is a critical percentage of oxygen below which the dust will not explode. The limit is determined by the inerting properties of other components of the atmosphere.
Size and Shape of the Particle
The finer the dust, the easier it will be dispersed into a cloud and the longer it will stay suspended in the air. The reduction of particle size translates to an increase of surface area per unit weight and a higher chance of an ignition. The shape of the particle also determines the specific surface area. A particle that is thin and flat ignites faster than a spherical object of the same substance.
Presence of Flammable Vapor and Gas
The presence of vapor from highly flammable liquid or gas raises the severity of a dust explosion. These combinations are known as hybrid mixtures and due to their severity, help from a professional engineer should be sort.
Use flash point testing to classify flammable and highly flammable liquids.