Combustible dust is deadly. Although in my opinion, that word isn't strong enough. The incidents I have reviewed and seen or investigated personally would better be described as catastrophic.
The explosive potential of combustible dusts should never be underestimated.
In this article, we'll go over why dust poses an explosion risk, what causes it to ignite, and what you can do to prevent these explosions from occurring in your workplace.
What Are Combustible Dusts?
"Dust" sounds innocuous, but let's be clear that we're talking about the byproducts of manufacturing or material handling processes that, by their nature, produce or use very fine particulate matter.
Examples of these fine particulate matters (or dusts) include:
- Titanium and aluminum alloy powders
- Aluminum wire used in a melt/spray-on application
- Grain dust
- Sugar dust
- Saw/wood milling dust
- Ink manufacturing
- Iron powder/dust
- Synthetic resins and powders used in plastics manufacturing
We could multiply examples, but combustible dust hazards exist in any workplace where materials that can readily burn or oxidize is present in powdered form.
Why Do Combustible Dusts Explode?
Combustible dusts explode for two basic reasons:
- The surface or contact area of explosive dust particles is large in relation to its mass, meaning that they will ignite very easily and burn very, very rapidly (i.e. explode)
- They ignite very easily when a source of ignition is present – or they can, if and when the accumulation is large enough and conditions right (high enough temperature), spontaneously combust
Any fire or ignition requires three elements to be present. These are collectively known as the "fire triangle." They are:
- Ignition source (heat, spark, etc.)
A dust explosion needs two additional elements. Collectively, the five are known as the "dust pentagon." These two elements are:
- Dispersion of dust particles in the right concentration
- Confinement of the dust cloud
Dispersion here means that there are dust particles suspended in air. Confinement means that the dust is in an enclosed or limited space, which allows pressure to build up, thereby increasing the likelihood of an explosion.
Unlike ordinary fires (like burning wood or paper), the deflagration (the fire or dust explosion) is so fast that heated air and fire by-products produce extreme air pressure. This pressure can blow out walls and destroy structures.
Another factor is that small dust explosions can almost instantly cause an exponential increase in the size of the event and a secondary explosion by distributing additional dust into the atmosphere, which is then ignited by the original event.
How Can We Prevent Combustible Dust Explosions in Our Workplace?
There are two basics to dust explosion prevention:
- Prevent the accumulation of explosive mixtures of dust
- If these formations cannot be avoided, prevent the dust from igniting
How? The National Safety Council's Accident Prevention: Engineering and Technology manual provides detailed methods. Here is a summary of them.
- Provide an inert atmosphere (if you can enclose the system and no workers need to go inside it)
- Limit oxygen concentrations below which flames do not occur
- Take precautions to prevent dust buildup (be sure not to miss out-of-the-way spots – our shop had 135 steel girders, each eight inches wide and 40 feet above the floor, and the dust build-up was four to six inches deep on the full 140-foot length of the girders)
- Use local exhaust ventilation and conduct careful, regular cleanings of systems and places where dust can collect
- Routinely look for dust buildup and make this a part of your regular inspection process
- Segregate and enclose dust producing operations
- Eliminate smoking, open flames, sparks from grinding, static electricity, welding, and sources of excessive heat
- Consider the particles that might result from conveyors and augurs, bad bearings, and so on – inspect and properly lubricate and maintain this type of equipment
- Use magnetic separators in your dust collectors to remove potential spark-producing foreign objects and debris
- Use only dust-tight wiring, fixtures, and motors
- In high hazard dust-producing environments like grain milling or plastics, use extensive dust collection methods
- Construct buildings so explosive pressure can "blow out" designed windows, walls, panels, and so on in a predetermined manner
- Minimize floor openings and seal openings for ducts and piping
- Maintain the appropriate firefighting equipment in proportion to the hazard (e.g. a fog water nozzle is more effective than a solid stream water hose nozzle)
OSHA's Hazard Communication Guidance for Combustible Dusts is a resource worth looking at for those who want to dive further into the topic. It also includes a helpful chart comparing the explosive power of various dusts.
And always remember that some dusts react with water. With certain particles, like magnesium, adding water to the equation makes things worse, not better.
Dust control begins at the design or retrofit design stage of your process. Safety professionals and industrial hygienists need to be involved at that point.
After the design? Then it's all about maintenance. Keep the system enclosed and clean. Keep the whole work environment clean and dust-free, every day.
Inspect for dust. Make it part of your programs and practices. Look in places you might think to look like internal shop roofs. Train everyone doing the work on how to look for and clean up dusts. Include dust on your inspection lists, in your hazard analyses and inventories, and in policy, procedure, and all safety communications.
Avoid sparks by using non-ferrous tools in dusty work environments.
Humidity doesn’t hurt in dust control, as long as your dust is not reactive to high humidity or water (condensation). Any hazard analysis you do has to include that question and answer.
Be sure to communicate your processes and control mechanisms throughout the entire organization.
Make sure you properly handle the dusts you're collecting or exhausting. Dust does not become less of a problem when you blow it outside the shop. In fact, you may be creating an even bigger hazard (learn more in 5 Best Practices for Dust Control in Manufacturing).
And finally, check the laws, regulations, and standards that apply to your workplace and speak with suppliers and manufacturers of dust collection and storage equipment.