- Dust explosion
A dust explosion is the fast combustion of dust particles suspended in the air in an enclosed location. Coal dust explosions are a frequent hazard in underground coal mines, but dust explosions can occur where any powdered combustible material is present in an enclosed atmosphere.
Conditions for dust explosion
There are four necessary conditions for a dust explosion or deflagration:
- A combustible dust
- The dust is suspended in the air at a high concentration
- There is an oxidant (typically atmospheric oxygen)
- There is an ignition source
Sources of dust
Many materials which are commonly known to oxidise can generate a dust explosion, such as coal, sawdust, and magnesium. However, many otherwise mundane materials can also lead to a dangerous dust cloud such as grain, flour, sugar, powdered milk and pollen. Many powdered metals (such as aluminium and titanium) can form explosive suspensions in air.
The dust can arise from activities such as transporting grain and indeed grain silos do regularly have explosions. Mining of coal leads to coal dust and flour mills likewise have large amounts of flour dust as a result of milling. A gigantic explosion of flour dust destroyed a mill in Minnesota on May 2, 1878, killing 18 workers at the Washburn A Mill. A similar problem occurs in saw mills and other places dedicated to carpentry. Thermobaric weapons, depending upon their fuel, are also a potential and intentional source of dust.
To support combustion, the dust must also consist of very small particles with a high surface area to volume ratio, thereby making the collective or combined surface area of all the particles very large in comparison to a dust of larger particles. Dust is defined as powders with particles less than about 500 micrometres in diameter, but finer dust will present a much greater hazard than coarse particles by virtue of the larger total surface area of all the particles.
Sources of ignition
There are many sources of ignition and a naked flame need not be the only one: over one half of the dust explosions in Germany in 2005 were from non-flame sources. Common sources of ignition include
- electrostatic discharge
- arcing from machinery or other equipment;
- hot surfaces, including e.g. overheated bearings
However it is often difficult to determine the exact source of ignition post-explosion. When a source cannot be found, it will often be cited as static electricity. Static charges can occur by friction at the surfaces of particles as they move against one another, and build up to levels leading to a sudden discharge to earth.
Below a certain value, the lower explosive limit (LEL), there is simply insufficient dust to support the combustion at the rate required for an explosion. A figure 20% lower than the LEL is considered safe. Similarly, if the fuel/air ratio increases above the upper explosive limit there is insufficient oxidant to permit combustion to continue at the necessary rate.
Mechanism of dust explosions
Dusts have a very large surface area compared to their mass. Since burning can only occur at the surface of a solid or liquid, where it can react with oxygen, this causes dusts to be much more flammable than bulk materials. For example, a 1 kg sphere of a material with a density of 1g/cm3 would be about 27 cm across and have a surface area of 0.3 m2. However, if it was broken up into spherical dust particles 50µm in diameter (about the size of flour particles) it would have a surface area of 1600 m² This greatly increased surface area allows the material to burn much faster, and the extremely small mass of each particle allows it to catch on fire with much less energy than the bulk material, as there is no heat loss to conduction within the material. When this mixture of fuel and air is ignited, especially in a confined space such as a warehouse or silo, a significant increase in pressure is created, often more than sufficient to demolish the structure. Even materials that are traditionally thought of as non-flammable, such as aluminum or iron, or slow burning, such as wood, can produce a powerful explosion when finely divided, and can be ignited by even a small spark. Such metal powders are widely used in fireworks for their dramatic effects.
Dust explosions may be classified as being either primary or secondary in nature. Primary dust explosions occur inside process plant or similar enclosures and are generally controlled by pressure relief through purpose-built ducting to atmosphere. Secondary dust explosions are the result of dust accumulation inside the factory being disturbed and ignited by the primary explosion, resulting in a much more dangerous uncontrolled explosion inside the workplace. Historically, fatalities from dust explosions have largely been the result of secondary dust explosions.
Protection and Mitigation from dust explosions
Much research has been carried out in Europe and elsewhere to understand how to control these dangers, but explosions still occur. The alternatives for making processes and plants safer depend on the industry. In the coal mining industry, a methane explosion can initiate a coal dust explosion, which can then engulf an entire pit working. Stone dust is spread along mine roadways, or suspended from trays in the roof, so as to dilute the coal dust raised ahead of the combustion zone by the shock wave, to the point where it cannot burn. Mines may also be sprayed with water to inhibit ignition. Some industries exclude air from dust-raising processes, known as "inerting". Typically this uses nitrogen, carbon dioxide or argon, which are incombustible gases and so inhibit combustion. The same method is also used in large storage tanks where inflammable vapours can accumulate. Usage of oxygen-free gases however brings a risk of asphyxiation of the workers.
Good housekeeping practices, namely eliminating the build-up of deposits of combustible dust that may be disturbed and lead to a secondary explosion, also help mitigating the problem.
Best engineering control measures which can be found in the National Fire Protection Association (NFPA) Combustible Dust Standards include:
- Oxidant Concentration Reduction
- Deflagration venting
- Deflagration pressure containment
- Deflagration suppression
- Deflagration venting through a dust retention and flame-arresting device
- "AP Enterprise: Rules lag amid deadly dust blasts". Nashville, Tennessee: Associated Press. 14 November 2011. http://start.toshiba.com/news/read.php?rip_id=%3CD9R0JQ2O1%40news.ap.org%3E&ps=1011. Retrieved 14 November 2011.
For stories about incidents in France and the USA see
- csb.gov various completed investigations from the Chemical Safety Board
- OSHA case studies of dust explosions
For information on how to protect a process plants and grain handling facilities from the risk of dust hazard explosions, see
Wikimedia Foundation. 2010.
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