Unfortunately, combustible dust explosions have caused catastrophic damages and loss of life in process industries across America recently. According to the Chemical Safety Investigation Board, there have been 111 dust explosion incidents in industrial plants between 2006 and 2017, causing 66 fatalities, while 337 workers suffered severe injuries. The CSB report stated that almost a quarter, 24% to be precise, of these devastating explosions affected the food industry. However, an array of other process industries that handle combustible material, like coal mining, wood processing, pharmaceutical components manufacturing, petrochemical, and paper process facilities, are exposed to dangerous explosion risks.
By recognizing the importance of assessing and controlling these combustible dust hazards, safety experts will be able to implement adequate dust explosion prevention and mitigation measures. In 2007, a devastating sugar dust explosion in the Imperial Sugar refinery in Georgia caused irreparable damage, leaving 14 fatalities and 36 injured employees behind. After conducting a thorough investigation, the CSB concluded that this severe explosion was “entirely preventable.”
After this severe safety incident, the US Chemical Safety Board incited the Occupational Safety and Health Agency to join forces with the National Fire Protection Agency and devise a series of combustible dust standards. Companies need to comply with these standards and ensure the prevention and mitigation of dust explosions in the process industry.
As professor Eckof, the author of The Dust Explosions in the Process Industry emphasizes in the 3rd edition of this invaluable resource, “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.” For this reason, this article will focus on the factors that lead to the ignition of dust clouds and the dust explosion escalation within a process plant. Furthermore, it will offer practical solutions for the prevention and mitigation of dust explosions.
What Is a Combustible Dust Explosion, and How does it Develop?
This potentially catastrophic safety incident happens when dust particles build up in the air and combust rapidly, creating a high air pressure wave. Five decisive factors are required for a dust explosion to happen:
- Combustible dust particles that serve as a fuel
- Oxygen in the air that boosts the flame propagation
- Enclosed space; initial dust explosions usually start inside confined equipment parts
- An ignition source – anything containing something hot enough to cause an ignition
- Specific concentration of combustible dust particles.
Besides these, dust dispersion levels play the important role in determining whether an explosion will occur and escalate. Namely, a dust explosion is highly unlikely to happen when the dust concentration level is not high enough to fuel it. Also, when the concentration of combustible dust is too high, there is not enough oxygen in the air to induce an explosion. Combustible dusts containing fine particles are more dangerous because they easily disperse and have a larger surface area, making them highly flammable.
When the specific explosible concentration of highly dispersive combustible dust combines with the oxygen in a confined piece of equipment near an ignition source, the so-called Dust Explosion Pentagon is formed, causing a primary dust explosion. This initial explosion in the processing equipment, or in the area with a sufficient concentration of accumulated “fugitive” dust, produces a shock wave that causes accumulated dust to become suspended in the air. It can also damage a duct, vessel, or dust collector, causing a secondary explosion wave.
These secondary explosions happen shortly after the original ones and are more dangerous and devastating, causing catastrophic damage and loss of life. For example, the origin of the initial explosion in the Sugar Imperial refinery was in the central building. Still, due to 15 subsequent secondary explosions, it expanded to the three adjacent silos, completely destroying them. This dust explosion demonstrates the devastating power of secondary dust explosions. It also shows that particle size can contribute significantly to the rapid flame propagation during the dust explosion. The NFPA study showed that an amount of accumulated dust as thick as an average paper clip is sufficient to cause explosions.
Combustible Dust Risk Assessment
Before you implement safety management practices and guidelines provided by OSHA and NFPA to prevent dust explosions from happening,identify and reduce existing combustible dust risks. You will achieve this by taking the following steps.
- Identifying problem areas
- Detecting combustible dust types
- Eliminating dust hazards
The hazard of a potential dust explosion is high in process industry facilities where powdered materials escape the processing and conveying equipment easily, settling on the surrounding horizontal surfaces. The dust accumulates on 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 the hidden ones that are often overlooked. Thus, regular housekeeping and cleaning of exposed and hidden surfaces within the facility are crucial in reducing dust hazards.
Identifying whether dust is hazardous or not is the second significant aspect in curbing the risk of a dust explosion. While cleaning the equipment for safety reasons, try to focus on types of combustible dusts that may cause severe explosions.
Several NFPA publications may provide valuable information on combustible materials. Apart from implementing effective cleaning methods, inspect the dust sources to eliminate the generation of explosive dust. You can also check the primary sources of ignition to eliminate explosion risks.
Therefore, you need to review process stages and equipment, close openings to avoid the accumulation of fugitive dust, 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 explosion risks.
Dust Explosions Prevention
We have already stated that dangerous dust explosions happen when the so-called Dust Explosion Pentagon is formed. By removing one of these crucial elements, you will prevent these safety incidents from happening. You can do this by implementing the following measures.
- Combustible dust removal
- Combustible dust concentration reduction
- Oxygen level reduction
- Ignition sources detection and control
Storing combustible dust outside the processing facility is an inherently safer approach that will prevent dust explosions within facilities
Make sure you keep the dust level below the minimum explosible concentrations by regularly cleaning or opting for equipment that prevents dust from accumulating or escaping from it. Tubular drag cable conveyors are the perfect solution for tackling the issue of fugitive dust. These tubular cable conveying systems transfer various materials through different process stages in completely sealed tubes. They use circular disks attached to a cable to push the material through the tube. Cablevey Conveyors offer completely enclosed tubular systems that will prevent dust particles from escaping the equipment.
You can reduce the level of the oxygen in the atmosphere by injecting an inert gas, like nitrogen, into an enclosed system. This will prevent the initial dust explosion.
By detecting, controlling, and preventing ignition sources, you will eliminate the ignition element from the Dust Explosion Pentagon, thus avoiding a safety incident. A single spark can become an ignition source when sucked into a dust collector. Heat, radiation, and smoke are indicators of this dust explosion hazard. Once you detect it, activate control methods like an abort gate that keeps the ignition source out of the processing line. You can also use suppression systems that quench the ignition source before it incites a flash fire or a primary dust explosion.
Dust Explosion Protection
The difference between preventive and protective measures is that you apply the latter once the explosion has started. First, you can try to increase the confinement to diminish dust explosion impact. In addition, you can use dust explosion vents. Most of the traditional dust explosion venting systems open at a predetermined level pressure. When an explosion happens, and the pressure builds up, the vents open, releasing the pressure into the surrounding area. Bear in mind that the location is essential when applying these traditional venting systems. They should never be directed towards areas where workers are present. Also, never place venting systems close to walls where the pressure cannot escape the confinement. Flameless venting is similar to removing the confinement process. Here you apply mass to quench the flame and prevent it from escaping through the vent panel. This way, flameless venting simultaneously quenches and diverts the flame.
Dust explosions in process industries have become common and costly safety incidents. However, OSHA and NFPA have made serious efforts and devised safety management policies to prevent dust explosions from happening and protect people and facilities once these explosions occur. The first step towards preventing the adverse effects of dust explosions is a better understanding of how they happen.