Dust explosions are devastating safety incidents that may occur in a wide array of industries such as petrochemical, food, paper, and pharmaceutical. These severe incidents may result in numerous casualties and cause irreparable damage to the process industry facilities. According to the Chemical Safety Board (CSB), an independent agency that conducts severe chemical incidents investigations, there were 111 cases of dust explosion between 2006 and 2017 in the United States, leaving behind 66 fatalities and 337 injuries. Another CSB study from 2006 showed that the food industry has the highest incident rate, with 24% of all dust explosion cases occurring between 1980 and 2006. Therefore, dust explosions have been recognized as a major safety risk that needs to be tackled with utmost caution and attention.
In order to protect industry workers’ lives, OSHA devised and started implementing a Combustible Dust National Emphasis Program (NEP) in 2008. This comprehensive safety plan provided procedures for dust hazard inspection, accompanied by policies that industrial plants need to apply to reduce dust explosion risks, and guidelines on complying with the NFPA standards.
Industry safety experts can rely on other useful resources when tackling dust explosions problems. Rolf K. Eckhof, a Professor Emeritus of Process Safety Technology in the Department of Physics and Technology at the University of Bergen, Norway, shared his knowledge on the topic in his book “Dust Explosions in the Process Industry.” This book’s 3rd edition provides invaluable advice on identification, assessment, and control of dust explosion hazards. As the book description says, “It is imperative that practical and theoretical knowledge of the origin, development, prevention, and mitigation of dust explosions is imparted to the responsible safety manager.” This book also offers an updated evaluation of prevalent activities, testing methods, design measures, and safe operating techniques. Industry safety consultants and students may find a detailed review of all the significant phases relating to the hazard and dust explosion control. As stated in the “Journal of Loss Prevention in the Process Industry” review, “This book is an account of all dust explosions aspects and will be an invaluable source of information to all safety or loss prevention specialists involved with dust explosion hazards.”
What is a Dust Explosion
This potentially deadly incident happens when dust particles build up in the air and combust rapidly, creating a violent pressure wave. Dust explosions present serious safety risks to processing and manufacturing industries, like food processing, coal mining, wood processing, pharmaceutical components manufacturing, and others. Several factors are required for a dust explosion to happen:
- Combustible dust
- Enclosed space
- Ignition source
- Besides these, one of the decisive factors for whether or not the dust explosion will occur is the dust particle concentration. Namely, a dust explosion is highly unlikely to happen when the dust concentration level is not high enough to fuel it. When the concentration of combustible dust is too high, there is not enough oxygen to induce an explosion. Combustible dusts containing finer particles are more dangerous because they disperse easily, covering larger surfaces, making them highly flammable. According to the OSHA Fact Sheet, when the right combustible dust concentrations are combined with enough oxygen in a confined space, near an ignition source, the so-called Dust Explosion Pentagon is formed, and a severe incident occurs.
Anything from a cigarette to a spark coming from an overheated wheel bearing can ignite a dust explosion. Once ignited, the dust combusts rapidly, creating a high air pressure wave. Frequently, the first explosion stirs another dust cloud, leading to the second explosion. Detailed case studies showed that these second explosions are more violent and destructive and occur shortly after the first ones, causing irreversible damage to facilities and endangering workers’ lives.
For this reason, this article will focus on describing dust explosion hazards in the process industry. It will also present dust explosion prevention and mitigation measures, along with safe operating practices that companies need to implement to ensure a safe work environment.
What Are Dust Explosion Hazards?
Some dusts, like coal, grain, and fertilizer dusts, are easily recognizable as explosion risks. The problem is that there is a wide range of other powders and dusts that may present serious fire or explosion hazards in industrial plants.
Unfortunately, catastrophic dust explosions at Imperial Sugar Company in 2007 were unprecedented safety incidents for those unfamiliar with dust explosion risks. The high concentration of combustible sugar dust incited a series of devastating explosions, causing 14 deaths and injuring 38 workers, 14 with life-threatening burns. The question that puzzled the OSHA inspectors and workers after this severe incident was, “How can a powder, a solid, typically non-hazardous material like sugar or plastic become a fire or explosion, i.e. a deflagration risk?”
The in-depth understanding of the finely ground materials’ nature and how fires work is necessary to realize why these dust explosion hazards exist. First, a fire is an oxidation reaction, meaning that all materials prone to oxidation may present a significant fire or dust explosion risk. An excellent example of such a material is an iron bar. When exposed to the air, an iron bar will slowly rust, and this oxidation process may take days or weeks, or even months. While a single rusty iron bar does not present an explosion risk, a ground iron bar containing fine particles certainly does. This is because iron dust has a significantly higher surface area that will react with the oxygen much more rapidly.
The finer the particles, the faster and stronger the chemical reaction will be. This ignition of dust particles releases heat, causing the gas around the particles to expand and create a flame front. When the flame front moves at speed lower than the speed of light, this is referred to as deflagration. When deflagration occurs in an enclosed space, it results in increased air pressure because the enclosure walls confine the internal gas expansion. This leads to an explosion, with the expanding pressure wave causing irreparable damage to the enclosed area. This process may occur with any material that can oxidize, releasing heat. Therefore, most organic chemicals, foods, metals, plastics, carbon compounds, and pharmaceutical components in the form of fine powders may present dangerous dust explosion risks.
How to Reduce Dust Risks?
Understanding that dust may present serious fire and explosion risks enables safety consultants and other safety experts to determine what measures to implement to bring process industry facilities into compliance with OSHA’s NEP standards. The crucial goal is to eliminate the leading causes of potentially catastrophic explosions in these facilities, even though it may not be possible to remove all dust deposits. The first step towards this goal is to reduce combustible dust accumulation on exposed and hidden surfaces inside the plants. As already stated, secondary explosions can be responsible for massive devastation and loss of life within the facility. These explosions happen when the pressure wave from an initial explosion lifts dust accumulated on horizontal surfaces into the air, where they are ignited and form an explosive dust cloud. As this explosive dust extends quickly from the primary ignition source, it causes catastrophic damage, severe injuries, and loss of life.
According to the National Fire Protection Association (NFPA), dust concentrations of 1/32nd of an inch, similar to the thickness of a paperclip, are sufficient to cause a dust explosion when dispersed near an ignition source. For this reason, the removal of accumulated dust is the primary concern of OSHA’s NEP plan.
Therefore, three crucial measures need to be implemented to reduce the dust hazard by lowering dust concentration:
- Problem areas identification
- Combustible dusts identification
- Dust hazards elimination
A dust explosion hazard may exist in the process industry facilities where powdered materials escape the processing and conveying equipment, settling on various horizontal surfaces near the dust source. The dust particles are usually on the top of the equipment, stairs, railings, light fixtures, and so on. While the visible surfaces are cleaned regularly, the explosive dust may accumulate excessively on hidden ones that are often overlooked. These problematic surfaces include roof support components, equipment parts above the eye level, elevated ductwork, piping, and cable trays, and safety experts need to focus on their cleaning. Thus, regular housekeeping and cleaning of exposed and hidden surfaces within the facility are crucial factors in reducing dust hazards.
Identifying whether dust is hazardous or not is the second-most significant aspect in curbing the risk of a dust explosion. As we already concluded, not all dusts present a fire or explosion risk. While all dusts should be cleaned for safety reasons, the focus is on combustible dusts that may cause severe explosions.
Therefore, safety professionals need to make an inventory of all powder materials and review material safety data sheets to identify potentially risky powders. If these data sheets are incomplete, safety professionals may conduct a hazard testing. Several NFPA publications may provide valuable information on combustible dusts. Apart from regular housekeeping, you need to inspect dust sources and primary ignition sources to eliminate explosion risks.
Therefore, you need to review process stages and equipment, close openings the hazardous dust may escape through, and reduce various ignition sources like sparks or heat. The NFPA has provided a collection of codes, standards, and guidelines to alleviate this significant process and help process industry safety consultants tackle dust explosion risks.
Dust Explosions Prevention and Mitigation
Serious efforts have been made recently to devise and enhance measures for the prevention and mitigation of dust explosions in the process industry. These efforts resulted in the development of various methods for addressing dust explosion-related issues, like the real-time explosion development prediction or ignition sources prevention. Adopting a safer process design based on in-depth knowledge, technology, and continuous staff training are keys to successful dust explosion prevention and mitigation.
In-depth knowledge in powder science is an invaluable help in assessing and controlling dust explosion hazards. Within the last 20 years, more sophisticated and flexible methods have replaced simple schematic ones. During this time, systematic research, accompanied by practical implementations, has resulted in adequately-designed mitigation measures. For example, systems for dust explosion venting, suppression, and isolation stemmed from the understanding of flame propagation processes.
Dust explosion development prevention and mitigation
One of the significant issues related to dust explosions is the rapid propagation of flames in dust clouds. This happens because inertial forces in dust clouds can produce fuel concentration gradients. Also, flames may transfer a considerable amount of heat to the dust cloud due to thermal radiation. However, the role of thermal radiation in the development of dust explosions needs to be studied further.
Numerous scientists have dedicated significant time and effort to devise comprehensive mathematical models for measuring flame propagation in dust clouds. These models will help safety managers to better understand how dust explosions escalate. Scientists have applied a flat-flame propagation technique to precisely measure the burning velocity. According to the results, numerous factors, like particle size, dust concentrations, oxygen enrichment, and heat loss by radiation, may affect the propagation of flame in dust clouds. This experiment drew attention to the similarity between the flame propagation through dust clouds and fuel mists, and the significance of radiation heat loss. Results showed that if a dust cloud consisted of 10 μm graphite or aluminum particles, radiation loss from particles could reduce the flame propagation velocity up to 40%.
Preventing the generation of explosive dust
Everything started with Dr. Weber, one of the first experts who tackled the dust explosion in the process industry 130 years ago. He pointed out that particles’ dispersibility and concentration considerably affected the generation of dust clouds. Using the example of flour, and the devastating dust explosions in Poland and Germany due to the high-dispersibility of this material, he proved that the dispersibility of wheat flour particles increased as their moisture level decreased.
Primary explosive dust clouds are usually created inside the processing equipment when a sufficient concentration of dust is lifted. Handling and transporting various materials in mills, dryers, mixers, bucket conveyors, and other processing equipment usually leads to the dispersion of dust, forming dangerous dust clouds. These primary dust explosions usually cause a second, more devastating explosion wave, along with catastrophic consequences.
Therefore, it is important to have some information about the dust cloud structure to predict the cloud’s ignition sensitivity and the course of this primary explosion. However, scientists have only started to develop numerical models to tackle this issue, and examples of practical use are scarce.
People usually tackle the dust explosion problem by adding preventive measures to the existing process. However, applying an inherently safer process approach may be a better solution. This approach implies designing a production process where none of the dust explosion hazards exist. This should include opting for production, treatment, storage, and transportation solutions where dust cloud generation is kept at a minimum.
Another practical solution is adding inert gas like nitrogen or carbon dioxide to the air. In this way, a dust cloud becomes less susceptible to catching flame. However, this measure should be implemented with caution because adding nitrogen to the air may present a suffocation risk.
Preventing Ignition Sources
The minimum hot-surface temperature sufficient for dust cloud ignition was considered a universal constant for any dust cloud in question. However, these minimum ignition temperatures may vary considerably, depending on the scale and the hot surface geometry related to the dust cloud. Therefore, we can identify several ignition sources: electrostatic discharges between two electrodes, electrostatic one-electrode discharges, glowing or burning particles, or an electrical apparatus. Although significant efforts have already been made to devise efficient measures for preventing ignition sources, these measures and standards need to be developed further.
Protective and Mitigatory Measures
Crucial steps towards preventing dust explosions, which protects human lives and equipment, are:
- Keeping surfaces clean
- Preventing dust accumulation
- Reducing ignition sources
For this reason, safety consultants may consider investing in tubular drag cable conveyors that fulfill all these requirements and may be a perfect solution for dust explosion prevention in the process industry.
First, these conveyors are designed to transport various raw materials through different process stages and have reduced the possibility of accumulating food residue or dust on the cable. Namely, the tubular cable surface exposed directly to materials is 80% less than the cable chain surface. Furthermore, this uniform and smooth tubular cable conveyor is nylon-sealed, preventing dust accumulation within the cable strands.
Secondly, tubular drag cable conveyors offer various cleaning options, like brush boxes, urethane wipers, in-line sponges and bristle brushes, and automated CIP wet cleaning, to ensure complete dust removal.
Finally, these tubular drag cable conveying systems use energy-efficient motors running on low horsepower. This way, the risk of generating sparks as a potential dust explosion source is minimized.
Simply put, by implementing the Cablevey Conveyors system in your production process, you will minimize dangerous dust explosion risks by conveying various materials in a hygienic, safe, and energy-efficient way.