What is Combustible Dust?
According to the National Fire Protection Association (NFPA), combustible dust is “a combustible particulate solid material that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape." Even materials that do not burn in larger pieces (such as aluminum and iron) given the proper conditions can be explosible in dust form. The conditions must contain 5 factors for an explosion hazard to exist: oxygen, heat, fuel, dispersion, and confinement. These five factors are known as the “Dust Explosion Pentagon” and without all five an explosion cannot occur.
These dusts can exist in facilities across multiple industries. The most prominent industries being wood products, food products, and metal. However, they can also accumulate in industries such as agriculture, fossil fuels, rubber manufacturing, plastics, and pharmaceuticals.
According to the Dust Safety Science 2020 Year-End Combustible Dust Incident Report; Food and wood products made up over 75% of the fires and explosions reported in 2020. Food products alone were responsible for almost 50% and were also the cause for 57% of injuries and 40% of fatalities. In 2020, storage silos demonstrated the highest percentage of combustible dust incidents with 30 fires and 13 explosions recorded.
What happens after an explosion?
If an explosion takes place inside a building, accumulated dust from beams, rafters, and other equipment can shake loose and fill the air and, if ignited, may cause one or several secondary explosions. These secondary explosions can be much more destructive and dangerous than the initial explosion and are the cause of most of the injuries and deaths from combustible dust explosions.
It is common for a fire to precede an explosion, especially if it takes place inside a piece of process equipment.
Any explosion, primary or secondary, can cause large amounts of damage to buildings, equipment, and products, and most importantly, can threaten the safety of employees.
Typical Timeline of a Primary & Secondary Dust Explosion
The first step in the timeline of a dust explosion is the accumulation and build-up of dust inside a facility.
The second step is the primary deflagration inside some part of process equipment which causes a shockwave. This shockwave is reflected by surfaces within the building and causes accumulated dust to become airborne. The primary deflagration breaks out of the equipment creating a source for ignition.
The secondary deflagration is ignited and gets propagated through the dust clouds.
This secondary deflagration bursts through the building and the damaged building remains on fire causing the majority of the damage.
West Pharmaceuticals Dust Explosion, January 2003
Recent Combustible Dust Explosion Cases:
Food Waste Recycling Plant Combustible Dust Explosion
A food waste recycling company in Rose Hill, North Carolina that creates animal feed products had an explosion caused by combustible dust on March 11, 2020. When the explosion took place there were contractors on site that are believed to have been performing hot work activities in the plant's finished products area. The space was described as a "dusty area" by company representatives and a hot work permit had not been issued prior to welding and cutting equipment.
The explosion was a flash fire that lit and blew out of the structure. As a result of this explosion, five workers were injured and three medical helicopters were brought in. All five were airlifted to a regional burn center and some had as bad as third-degree burns. The property was significantly damaged.
The build-up of dust in this facility was caused by insufficient ventilation. With proper ductwork and dust collection systems the explosion may have been prevented.
Valley Proteins Dust Explosion, March 2020
Grain Dust Explosion at Rice Processing Facility
A rice miller and marketer reported a grain dust explosion on October 16, 2020, at a plant in Stuttgart, Arkansas. A plant supervisor met first responders and told them that there was an explosion and the sprinkler system was activated on the first floor but not the upper floors. At the top of the grain bins, there was an open room filled with smoke and dust. Fifteen ducts were separated from the bins and seriously damaged. The explosion also blew off the door to one of the bins. Ductwork had to be pulled down to eliminate the spread of flames when firefighters found multiple hot spots on the floor and on top of the ducts hanging from the ceiling.
The explosion appeared to have been caused by an undetermined spark igniting and contacting accumulated grain dust in the rice storage building. Three workers endured minor injuries and received hospital treatment and a building that was used to store rough and milled rice suffered significant damage.
The company did have equipment in place to minimize and capture grain dust, however, the equipment was outdated which led to inefficiency. Hazards of handling rough grain and the dust that comes along with it are always there but with preventative maintenance, some risks could be mitigated.
Controlling Combustible Dust
There are many helpful practices for controlling combustible dust at your facility. These consist of combustible dust awareness training, good housekeeping, Dust Hazard Analysis (DHA), and the three C's.
The objectives of combustible dust awareness training are to help employees identify the elements necessary for dust to explode, explain how to prevent dust from reaching combustible levels, and describe the difference between primary and secondary dust explosions. A link to OSHA's "Combustible Dust Explosion Hazard Awareness Training" can be found at the end of this article.
Good housekeeping requires immediately cleaning any surface whenever a dust layer of 1/32-inch thickness accumulates. When a surface area of at least 5% of the floor of the facility or any given room contains dust, this also requires immediate cleaning. Housekeeping also consists of acquiring hot work permits, lockout/tagout policies, design specifications for storage of flammable materials, emergency plans, alarm systems, and practices and procedures designed to minimize the spread of fire. Research has shown that facilities that are well-maintained experience fewer fires, explosions, and other accidents.
Dust Hazard Analysis (DHA) is both good practice and required by the National Fire Protection Association. (NFPA) 652: "Standard On the Fundamentals of Combustible Dust". Under these standards, a revalidated DHA is required every 5 years and a DHA is required before the startup of new processes. DHA assesses the level of risk in a facility and determines the effectiveness of the current safeguards by identifying where and when dust explosion and flash fire hazards can exist and their potential consequences.
The three C's are capture, contain, and clean. These three C's are considered the best practices for avoiding combustible dust explosions.
Capturing is gathering the dust particles at the point of generation before they are dispersed into the work area. This can be done using dust collection systems that are designed, installed, approved, and maintained to control the dust. A well-designed dust capture system will move dust away from any ignition source or heat source. Potential heat sources include the building's heating, ventilation, and, air conditioning (HVAC) system, forklifts, and employee smoking areas.
Dust Collection System Designed by JBW
Contain means that once the dust is captured it is stored within equipment, rooms, or systems that are designed for safe handling and disposal of hazardous dust. Safely designed containment systems keep combustible dust away from employee work areas by collecting it in bags or buildings outside the main work area or by separating them using barriers.
Dust Containment System
Cleaning means any dust that is not captured or contained must be prevented from accumulating in the work area. Overhead surfaces and concealed areas, in particular, must be identified and frequently cleaned. Cleaning practices should not generate dust clouds. Using an intrinsically safe vacuum equipped with a high-efficiency particulate air (HEPA) filtration system or wet methods over dry sweeping works best.
How We Help
JBW, located in Duluth, MN and Gillette, WY, has team members who have extensive knowledge and are specialized in combustible dust hazards with the ability to analyze the hazards present in your facility. We have the ability to:
Develop a dust sampling strategy for your facility
Evaluate your facilities combustible dust generation and handling processes (DHA)
Identify hazards related to combustible dust
Offer cost-effective recommendations intended to reduce risk
Design systems to mitigate hazards
Our experience reaches many industries and we go beyond simple prescriptive measures to mitigate hazards with the intent of developing the most efficient solutions; solutions that work for your facility.
NFPA "Recommended Practice for the Classification of Combustible Dusts"
"NFPA Standard On the Fundamentals of Combustible Dust"
OSHA "Combustible Dust: An Explosion Hazard"
OSHA "Combustible Dust Explosion Hazard Awareness Training"
OSHA "Combustible Dust: Safety and Injury Prevention Awareness"
OSHA "Combustible Dust: Risks Associated With Combustible Dust Hazards"
Dust Safety Science "2020 Combustible Dust Incident Report"