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Essentially, a combustible dust is any fine material that has the ability to catch fire and explode when mixed with air. Combustible dusts can be from:
Some of these materials are not "normally" combustible, but they can burn or explode if the particles are the right size and in the right concentration.
Therefore any activity that creates dust should be investigated to see if there is a risk of that dust being combustible. Dust can collect on surfaces such as rafters, roofs, suspended ceilings, ducts, crevices, dust collectors, and other equipment. When the dust is disturbed and under certain circumstances, there is the potential for a serious explosion to occur. The build-up of even a very small amount of dust can cause serious damage.
The technical definitions for combustible dust vary. In Canada, the Hazardous Products Regulation (WHMIS 2015) defines combustible dust as " a mixture or substance that is in the form of finely divided solid particles that, upon ignition, is liable to catch fire or explode when dispersed in air".
Another example is Alberta's Occupational Health and Safety Code which defines combustible dust as "a dust that can create an explosive atmosphere when it is suspended in air in ignitable concentrations".
The Occupational Safety and Health Administration (OSHA) in the United States defines combustible dust as "a solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition, which presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations".
Many materials can become combustible under specific situations. Examples include:
There are many, many more types of materials that may become a combustible dust. The US OSHA has created a poster which lists more examples.
Dust explosions have occurred in many different types of workplaces and industries, including:
Dusts are created when materials are transported, handled, processed, polished, ground and shaped. Dusts are also created by abrasive blasting, cutting, crushing, mixing, sifting or screening dry materials. The buildup of dried residue from the processing of wet materials can also generate dusts. Essentially, any workplace that generates dust is potentially at risk.
Any fire needs three elements. These elements are known as the "fire triangle":1. Fuel to burn
A dust explosion needs two additional elements - known as the "dust pentagon":
4. Dispersion of dust particles in the right concentration, and
5. Confinement of the dust cloud.
Dispersion means the dust particles are suspended in air. Confinement means the dust is in an enclosed or limited space. This restriction allows pressure to build up, increasing the likelihood of an explosion.
Figure 1 shows the dust explosion pentagon. Figure from Occupational Safety and Health Administration (OSHA).
Deflagration is a term often used to describe combustible dust explosions. A deflagration is an "ordinary" fire such as a gas stove, burning wood or paper, and even the burning of gasoline vapour inside the cylinder of an automobile. In a deflagration, a burning substance releases heat, hot gases, and energetic particles or sparks that ignite and spread the fire.
In a dust explosion, the deflagration processes happens so rapidly that the heated air and gaseous fire products (such as carbon dioxide) produce extreme air pressure that can blow out walls and destroy structures.
When combustible dusts ignite, there are often two explosions known as primary and secondary explosions.
The primary dust explosion is the first explosion. It occurs when there is a dust suspension in a confined space (such as a container, room, or piece of equipment) that is ignited and explodes.
Figure 2 shows events in a primary dust explosion. Figure from U.S. Chemical Safety and Hazard Investigation Board.
The primary explosion will shake other dust that has accumulated. When this dust becomes airborne, it also ignites. This secondary dust explosion is often more destructive than the primary one.
The basic requirements for a dust explosion to occur is that combustible dusts are suspended in air and are ignited. In practice, for a dust explosion to occur, a number of conditions must be met including:
There are many variables that must be considered - the particle size of the dust, the method of dispersion, ventilation system characteristics, air currents, ignition sources, confinement of the dust cloud, physical barriers, and so on. As a result, the often quoted "rule of thumb" about dust accumulation (such as being able to write in the dust, or the dust being the thickness of a paperclip, dime or quarter, or the amount of visibility through a dust cloud) is not always reliable.
Best practice is to keep the workplace as dust free as possible.
Conduct a risk assessment and look specifically for dust explosion possibilities. Below are some questions that may help.
Note: It is very important to research the materials and products used in your workplace. Stating the possibility of a combustible dust hazard is a new requirement on the Safety Data Sheet (SDS). Older Material Safety Data Sheets (MSDSs) may not provide the same information so additional research may be necessary.
Once you have identified the hazards, you need to take prevention measures.
Install smooth ceilings and other surfaces (instead of a rough finish) to minimize dust accumulation and to make cleaning easier.