Few people are aware of fires and explosions caused by combustible dust, yet they occur regularly in the industrial sector.
These types of accidents have caused more than 119 deaths and 700 injuries in the U.S. since 1990 (1). In 2012, two explosions occurred within a few weeks of each other at two British Columbian sawmills, resulting in four dead and 40 injured. On August 2, there was an explosion at a car parts factory, a subcontractor of General Motors located in Kunshan in the Chinese province of Jiangsu. This explosion killed 75 and injured 200.
When can an explosion occur?
Wood, paper, cardboard, aluminum, iron, magnesium, silicon, sugar, flours, cornstarch, whey, rubber, coal, graphite, polyester, polypropylene and PVC are only a few of the many combustible materials that, in particulate form, become explosive. Generally, the following rule applies: particles of fewer than 500 microns (combustible material) are explosive!
An explosion occurs when the following three conditions are met:
1. Combustible particles are suspended in the air (cloud)
2. The cloud is sufficiently dense (minimum explosive concentration)
3. There is a source of ignition (spark, arc flash, hot surface, static charge, friction, etc.)
In industrial sectors, the first two conditions are often found in equipment (dust collectors, silos, conveyors, mixers, etc.) and even at times in the ambient air. All that's missing is the ignition source. This problem affects a large number of businesses, which must control risks in accordance with applicable laws and regulations, based for the most part on American NFPA standards.(2)
Risk assessment: A plant requirement!
For the last 10 years in Canada, and following adoption of Bill C-21, plant managers and administrators must prove reasonable diligence in controlling operational and work environment hazards. Their risk management obligations include foresight, efficiency and authority. When combustible dust is present, and in accordance with NFPA (3)(4)(5) standards, plant management must carry out a risk assessment no later than six months after activities start, or following each significant change, and every five years afterwards.
This assessment must be performed by a qualified engineer and the results must be presented in a report that is kept at the plant and available at all times. The report must include, but is not limited to:
- A review of applicable laws, regulations, standards and generally accepted practices
- The characterization of combustible powders, dusts and particles used to fuel the process, generated by the process or treated in the process
- A review of equipment and areas (buildings) that present a fire and/or explosion hazard
- The consequences of a fire or explosion
- The list of possible ignition sources - static charges, mechanically-produced sparks, intrusions, frictional heating, arc flashes, etc.
- Ignition probabilities and the level of protection required for each scenario
- The measures taken to control identified risks - equipment protection, dust control, electrical classification, etc.
- Follow-up and inspection deadlines as well as personnel training requirements
Risk assessments should not be viewed as a constraint, but rather an effective management tool that not only prevents accidents, but prioritizes actions to ensure facility safety and plant sustainability.
 Source: Occupational Safety and Health Administration (OSHA)
 NFPA: National Fire Protection Association (United States); the key applicable standards are NFPA-69, NFPA-654, NFPA-484, NFPA-61 and NFPA-664.
 NFPA: National Fire Protection Association (United States)
 NFPA-654 2013, section 4.2. Process Hazard Analysis and section 7.1.3. Risk Evaluation
 NFPA-664 2013, section 4.3. Process Analysis; NFPA-484 2015, section 5.2. Hazard Analysis