What is combustible dust?
Essentially, combustible dust is any fine material that has the ability to catch fire and explode when mixed with air. Combustible dust can be from:
- most solid organic materials (such as sugar, flour, grain, wood, etc.)
- many metals, and
- some nonmetallic inorganic materials.
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.
What is the technical definition of combustible dust?
The technical definitions for combustible dust vary. In Canada, one example is Alberta’s Occupational Health and Safety Code which defines combustible dust as “dust that can create an explosive atmosphere when it is suspended in the 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”.
What are examples of materials that can be a combustible dust hazard?
Many materials can become combustible in specific situations. Examples include:
- agricultural products such as egg whites, powdered milk, cornstarch, sugar, flour, grain, potato, rice, etc.
- metals such as aluminum, bronze, magnesium, zinc, etc.
- chemical dust such as coal, sulfur, etc.
There are many, many more types of materials that may become a cloud of combustible dust. The U.S. OSHA has created a poster that lists more examples.
What workplaces are at risk for a dust explosion?
Dust explosions have occurred in many different types of workplaces and industries, including:
- Grain elevators,
- Food production,
- Chemical manufacturing (e.g., rubber, plastics, pharmaceuticals),
- Woodworking facilities,
- Metal processing (e.g., zinc, magnesium, aluminum, iron),
- Recycling facilities (e.g., paper, plastics, metals), and
- Coal-fired power plants.
Dust is created when materials are transported, handled, processed, polished, ground and shaped. Dust is 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 dust. Essentially, any workplace that generates dust is potentially at risk.
How do combustible dust explosions happen?
Any fire needs three elements. These elements are known as the “fire triangle”:
- Fuel to burn
- Ignition source (heat, spark, etc.)
A dust explosion needs two additional elements – known as the “dust pentagon”:
- Dispersion of dust particles in the right concentration, and
- Confinement of the dust cloud.
Figure 1 shows the dust explosion pentagon. Figure from the Occupational Safety and Health Administration (OSHA).
Dispersion means the dust particles are suspended in the 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.
What is deflagration?
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 vapor inside the cylinder of an automobile. In a deflagration, a burning substance releases heat, hot gases, and energetic particles or sparks that spread the fire.
In a dust explosion, the deflagration processes happen 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.
What are primary and secondary dust explosions?
When combustible dust ignites, 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 the 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.
What conditions are needed for a dust explosion to happen?
The basic requirements for a dust explosion to occur are that combustible dust is suspended in air and ignited. In practice, for a dust explosion to occur, a number of conditions must be met including:
- The dust must be combustible and release enough heat when it burns to sustain the fire.
- The dust must be capable of being suspended in the air.
- The dust must have a particle size capable of spreading the flame.
- The concentration of the dust suspension must be within the explosible range.
- An ignition source must be in contact with the dust suspension.
- The atmosphere must contain sufficient oxygen to support and sustain combustion.
How do I identify a combustible dust hazard?
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.
A best practice is to keep the workplace as dust-free as possible.
Document last updated on December 21, 2012.
The original source of this information is from the Canadian Centre for Occupational Health and Safety www.ccchs.ca. All rights reserved.