Combustible dusts are serious hazards that can potentially cause a massive explosion if not controlled properly. Here are some things employers and workers should know about combustible dusts, how to control them and prevent dust explosions.
In February 1999, a fire at a Massachusetts foundry extended into the ventilation ducts. A small, primary explosion within the ductwork dislodged some of the heavy deposits of phenol formaldehyde resin dust that had settled on the outside of the ducts. This produced a dust cloud that fuelled a secondary explosion powerful enough to lift the roof and cause wall failures. Three people were killed and nine were injured.
Another seven people were killed and 37 injured in February 2003, in a dust explosion at an acoustics insulation manufacturing plant in Kentucky. The report from the U.S. Chemical Safety and Hazard Investigation Board (CSB) said the blast was likely caused by a small fire extending from an unattended oven, which ignited a dust cloud created by a nearby line cleaning. A deadly cascade of dust explosions followed throughout the plant.
Because of these incidents, the CSB conducted a major study of industrial accidents involving dust explosions over the past 20 years. The CSB found that during this period in the United States alone, there have been more than 150 serious industrial dust explosions, resulting in 80 deaths. And the issue extends beyond the U.S. Statistics show that fire/explosion hazards exist in any facility or equipment that handles or processes a combustible dust.
A combustible dust explosion hazard may exist in a variety of industries, including: food (e.g., candy, starch, flour, feed), plastics, wood, rubber, furniture, textiles, pesticides, pharmaceuticals, dyes, coal, metals, and fossil fuel power generation. Most natural and synthetic organic materials, as well as some metals, can form combustible dust. The safety bulletin prepared by the U.S. Occupational Safety and Health Administration contains important information on how to assess the risk in your facility.
Elements of dust explosion
Experts agree that for a dust explosion to occur, five conditions must be present at the same time. The first three of these elements are needed to cause fire:??
1. combustible dust (fuel)?
2. ignition source (spark or heat)?
3. oxygen in air (oxidizer)
4. dispersion of dust (into the air forming a dust cloud)?
5. confinement of the dust cloud (building or ceiling)
These five elements make up the "explosion pentagon" that causes a dust explosion. If one of the elements of the explosion pentagon is missing, a catastrophic blast cannot occur.
A dust cloud that is ignited within a confined or semi-confined vessel, area or building burns very rapidly and may explode. This could cause fires, additional explosions, flying debris and the collapse of parts or all of the building.
An initial explosion that occurs in processing equipment or in an area where there is an additional accumulation of dust, may shake the renegade dust loose, or damage a containment system (such as a duct or vessel). The additional dust released into the air, if ignited, can cause one or more secondary explosions that can be even more destructive than the first.
Facilities should conduct a dust hazard assessment to carefully identify materials that can be combustible, processes that use, consume or produce combustible dusts, open areas where combustible dusts may build up, hidden areas where combustible dusts may accumulate, ways that dust may be dispersed in the air, and potential ignition sources.
The key factor to consider is whether the specific dust from your facility is a combustible dust hazard. Although there is currently no combustible dust hazard class under WHMIS, there is a requirement to declare all hazards of the product on the Material Safety Data Sheet (MSDS). Unfortunately, the dust explosion hazard is under-recognized and often not declared on MSDSs.
In CCOHS' CHEMINFO database, the potential for a material becoming a combustible dust hazard has been identified. As well, any reports of dust explosions involving the chemical are included. Laboratory testing of your specific dust will help tell you if there is a hazard at your workplace. The facility analysis must also identify areas requiring special electrical equipment classification due to the potential combustible dust hazard.
The following safety practices are recommended:
Dust control — the most important step towards prevention of dust explosions
• Minimize the escape of dust from process equipment or ventilation systems. Use dust collection systems and filters, and try to use surfaces that minimize dust accumulation and are easy to clean.
• Prevent the accumulation of dusts on surfaces.
• Inspect for dust residues in open and hidden areas, and clean them at regular intervals.
• Use cleaning methods that do not generate dust clouds (i.e. vacuum rather than blow or dry sweep). Only use vacuum cleaners approved for dust collection. Locate relief valves away from dust hazard areas.
• Develop and implement a program for hazardous dust inspection, testing, housekeeping and control program that establishes, in writing, the method and frequency of these steps.
• Use appropriate electrical equipment and wiring methods.
• Keep static electricity under control — this includes bonding of equipment to ground.
• Prevent smoking, open flames, sparks, mechanical sparks and friction. Use separator devices to remove foreign materials capable of igniting combustibles from process materials.
• Avoid contact between heated surfaces and dusts. Separate heating systems from dusts.
• Ensure the proper use and type of industrial trucks, and the proper use of cartridge-activated tools.
Facility owners, managers and supervisors are responsible for conducting a facility analysis (ideally before introducing a hazard) and for developing a prevention and protection plan that addresses the specific needs of their operation. Their responsibilities also include improving policies and procedures, and taking whatever action is necessary to prevent a dust explosion.
Employees are responsible for adhering to safe work practices, and as the people closest to the source of the hazard, should be trained and encouraged to take an active role in recognizing unsafe conditions, taking preventative action and alerting management. All employees should be trained in safe work practices that apply to their jobs, as well as on the overall plant programs for dust control and ignition source control.
A heightened awareness of the hazards of combustible dust is the first of many steps in everyone working together to prevent further injury and deaths to workers.
The Canadian Centre for Occupational Health and Safety (www.ccohs.ca) is Canada's national resource for the advancement of workplace health and safety. CCOHS promotes the total well being — physical, psychosocial and mental health — of working Canadians by providing information, training, education and management systems and solutions that support health and safety programs and the prevention of injury and illness.