HSE Professionals have a never-ending list of tasks and items to contend with on a daily basis to ensure operations run smoothly and that employee safety issues are addressed without delay. With only so many hours in a day and checklists galore it is not hard to overlook silent killers that may loom in the workplace, which in the right conditions can deliver catastrophic consequences. In this article, we will talk about the hazards presented by combustible dust and hopefully provide helpful hints on ways to eliminate this hazard.
Several countries have established combustible dust standards or guidance and the specific definitions of a combustible dust vary. The Canadian Centre for Occupational Health and Safety (CCOSH) defines combustible dusts as “any fine material that has the ability to catch fire and explode when mixed with air”. The UK Health and Safety Executive guidance document states that, in general a combustible dust is “anything which can burn, and which exists in a fine powdered form, unless tests show that particular hazards are not present”. Essentially, any combustible material can explode if it is in finely divided form and the conditions are right.
The U.S. Chemical Safety and Hazard Investigation Board (CSB) has reported that 281 combustible dust incidents occurred in the US between 1980 and 2005, not including primary grain handling or underground coal dust explosions. These incidents resulted in the deaths of 119 workers, injuries to 718 workers, and extensive property damage. No matter how it is specifically defined, today’s safety professionals must understand how to recognize the material characteristics and workplace conditions that can result in an explosion.
This includes understanding what combustible dust is and how it is ignited. In their Hazard Communication Guidance on Combustible Dusts, the US Occupational Safety and Health Administration (OSHA) states that there are 5 elements needed to initiate a dust explosion. OSHA calls this the “Dust Explosion Pentagon”:
- Combustible dust (fuel);
- Ignition source (heat);
- Oxygen in air (oxident);
- Dispersion of dust particles in sufficient quantity and concentration;
- Confinement of the dust cloud.
Most of the countries which have established workplace safety regulations require employers to conduct an assessment of the health and safety risks associated with hazardous substances so they can be eliminated or controlled. Both US OSHA and the UK’s Dangerous Substances and Explosive Atmospheres Regulations specifically require assessment of combustible dust hazards. In some jurisdictions, these assessments must be documented, but it is prudent that you document regardless of whether it is a regulatory necessity.
Employers should ask the following questions to begin the assessment process within their facility:
- Do I use materials that could become combustible in my process?
While not all potentially combustible materials are listed, US OSHA has published a list of the more common materials to be aware of on the Combustible Dust Poster, which is available at this link: https://www.osha.gov/Publications/combustibledustposter.pdf
Different dusts of the same material may present different explosion risks due to the size, shape and moisture content, but your first step is to determine if there is a potential. Without testing for specific particle characteristics, you should consider a dust combustible if the material itself is combustible.
- Can the process or ambient conditions disperse the dust into the air?
Look for leaks in process equipment and ductwork. Ensure that maintenance and operations tasks always include pre-and post-task inspections and cleaning to prevent dust accumulation. Of course, the best preventive mechanism is a good design. Dust dispersion can result from component failures, poor human interface design (locations where materials are added or removed from the process), or improper ventilation system seals.
- Are there areas in my facility where combustible dusts may accumulate?
Inspect around dust collection systems, ductwork, etc. It’s usually best to locate dust collection systems outside the main facility. Check and clean surfaces and floors using wet wipe-down methods to prevent dispersing dust. Accumulation of less than 1/32” of dust on horizontal surfaces can cause problems. Good housekeeping is critical.
- Are there hidden areas where combustible dusts may accumulate?
Inspect and regularly clean the tops of ductwork, lamp fixtures, structural beams and other areas that may not be easily seen from the standard work areas. Take steps to provide clear access to these areas and add them to your checklists to ensure they are executed regularly. Check closets and maintenance areas where tools or machine parts may be stored or serviced.
- Are there potential ignition sources?
Identifying and removing all possible sources of ignition near combustible material storage or processing areas is vital to preventing explosions.
Obviously, the assessment process is much more extensive than these 5 questions, but they give you a starting point. US OSHA and other regulatory agencies have developed Dust Explosion Risk Assessment Checklists which can help facilitate a self-assessment. The OSHA Checklist can be downloaded here