Dust explosions are severe safety issues affecting process industries that handle materials that generate combustible dust. According to the data collected by the US Chemical Safety Board, explosions of dust are frequent incidents. They may cause fatalities, severe injuries, and catastrophic damages to facilities, leading to considerable economic loss. For this reason, the Occupational Safety and Health Agency joined forces with the National Fire Protection Association and devised a series of safety measures and standards for safety professionals to implement in order to prevent, mitigate, and control explosive dust.
First of all, let’s discuss the difference between “prevention” and “protection,” which are terms that will be used frequently in this article. Prevention focuses on recognizing dust explosion hazards and eliminating them before a dust explosion occurs. Once the incident occurs, it is essential to protect people and equipment from potentially devastating flash fires, fires, and combustible dust explosions.
The fact that 70% of dust found in a wide variety of process industries is combustible emphasizes the importance of implementing adequate dust explosion prevention, protection, and control measures.
How Dust Explosions Occur and Develop
Combustible explosions of dust happen when five elements coincide, forming the so-called Dust Explosion Pentagon. These elements are:
- Combustible dust particles that act as the explosion’s fuel
- Dispersion of dust particles, which happens when accumulated dust is suspended in air, forming the explosive dust cloud.
- Ignition sources capable of setting combustible dust clouds on fire
- Confined spaces where the pressure builds up, leading to the vessel rupture, facility destruction, and potential structural collapse.
Dust concentrations are also a significant factor that may lead to an explosion. Namely, only specific combustible dust concentrations can cause dust deflagration. Therefore, an insufficient concentration of dust particles will not fuel the explosion. Also, the increased concentration of combustible dust will lower the oxygen level, thus preventing the explosion. Once the minimum explosive level of dust disperses in the air near an ignition source in an enclosed space, the dangerous explosion occurs.
This explosion develops in two stages. The above-described process causes a primary explosion inside the piece of equipment and is not as intense as the second explosion wave. The rapid dust particles combustion increases the air pressure, creating the pressure wave that can stir dust accumulated on horizontal surfaces, causing the secondary dust explosion. These secondary explosions happen shortly after the initial ones and are more dangerous and devastating, causing catastrophic damage and loss of life.
Dust particle size can contribute to the rapid flame propagation during the dust explosion. Fine dust particles are more dangerous because they can easily disperse through the air and have a larger surface area. A study conducted by the NFPA showed that an amount of accumulated dust as thick as an average paper clip was sufficient to cause devastating explosions. For all these reasons, it is important to focus on the NFPA recommendations on how to avoid explosion sequences and keep these events under control.
How to Avoid Second Dust Explosion Escalation
According to the NFPA, you need to take the following steps to avoid dust explosion escalation that may leave irreparable damage and endanger people’s lives.
First, you need to isolate equipment from other pieces of equipment. When an explosion happens, it causes flame propagation through the ductwork into the other parts of equipment. Therefore, it is essential to isolate explosions so that the flame cannot spread to the adjacent equipment. In case the explosion takes place in an outdoor dust collector, you need to make sure it does not spread through the ductwork into the processing facility and endanger workers.
Second, safety professionals need to be aware of the “fugitive” dust particles hazard. These are particles that escape the processing and conveying equipment, settling on visible or hidden horizontal surfaces. Implement the following measures to avoid fugitive dust accumulation.
Check whether all ducts are airtight and make sure the dust cannot escape when the machine starts working. Select a hood that captures dust properly and diverts it into the dust collector. Finally, effective and regular cleaning plays a crucial part in keeping the accumulated fugitive dust below the minimum explosible levels.
How to Comply with the NFPA Combustible Dust Standard
Apart from these recommendations, the NFPA has devised a 9-step plan you need to implement to further ensure explosion prevention, protection, and control.
- Identify the combustibility and the explosibility level of the materials you are handling.
- Detect and assess fire, flash fire, and explosion hazards.
- Pay attention to the safety of building and equipment design, house-keeping, personal protective equipment (PPE), dust control, explosion prevention, protection, and explosion isolation when managing these combustible dust hazards.
- Enhance housekeeping procedures and do not apply cleaning methods that involve compressed air.
- Try using effective dust collection systems.
- Use venting systems that comply with the NFPA 68 standard and direct exhaust air outside.
- Make sure that all central vacuum systems are grounded and contain attachments made of static dissipative materials.
- Devise MOC (Management of Change) procedures you should implement before changing the materials, equipment, technology, or work tasks.
- Educate all employees about hazards and train them on protective measures
How to Ensure Explosion Prevention
You will prevent dangerous explosions by removing one of the elements from the Dust Explosion Pentagon. Here are some proven strategies you can implement to prevent an explosion risk in the process industry.
- Remove combustible dust. You will eliminate an explosion hazard by storing combustible dust outside the processing facility.
- Reduce the combustible dust concentration. You can keep the amount of dust below the minimum explosible concentrations by regularly clearing or opting for equipment that prevents dust from accumulating or escaping from it.
- Reduce the oxygen level. You will achieve this by injecting an inert gas, like nitrogen, into a confined system. This will lower the oxygen level and prevent the initial dust explosion.
- Detect and control ignition sources. By detecting, controlling, and preventing ignition sources, you will eliminate the ignition element from the Dust Explosion Pentagon, thus preventing a devastating safety incident. Bear in mind that anything can be an ignition source, from hot screws to a burning ember sucked into a dust collector. The heat, radiation, or smoke are indicators of this dust explosion hazard.
Once you detect it, you need to activate control methods like an abort gate that keeps the ignition source out of the processing line. You can also use a suppression system that quenches the ignition source before it causes a flash fire or a primary dust explosion.
Also, bear in mind that welding and cutting can generate sparks, which means that hot work should not be done near accumulated dust. Clean up the dust or remove the equipment from the dusty environment before hot work begins.
How to Protect Workers
Unfortunately, combustible explosions of dust and fire have become common safety incidents in the processing industry over the years, taking numerous lives and leaving many workers with severe injuries. For example, 14 workers lost their lives, while 36 were injured, in a devastating explosion in a sugar refinery in Georgia in 2007.
Even though these explosion prevention strategies may reduce dust explosion hazards, they cannot sufficiently protect workers exposed to combustible dust flash fires and explosions. On the other hand, personal protective equipment, like flame-resistant garments and ARC flashing clothing, will offer adequate protection in case of dangerous explosions.
Another way to protect the workforce in process plants is to eliminate the risk of static electricity. Static electricity can be a dangerous ignition source for fine dust in grain elevators or coal dust explosions that can regularly occur. You can eliminate this explosion risk by applying grounding devices devised to prevent static electricity from building up and control static charges in dusty environments.
How to Protect Facilities
Once the explosion has started, you can implement the following protection measures:
- Try to increase the confinement
You will achieve this by applying explosion-proof drum kits to a dust collector bottom because these do not need an isolation channel between them. In cases of explosions in hammer mills, the equipment is durable enough to withstand the explosion inside.
- Use a dust explosion venting system
The pressure relief venting system is designed to open at a set pressure. When an explosion happens, building up pressure to the predetermined level, 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. You should 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 fire.
Combustible explosions are severe safety incidents that may have devastating consequences. Suppose you implement and comply with the dust explosion standards devised by NFPA and OSHA. In that case, you may conduct a dust hazard analysis to control, prevent, and protect workers and processing facilities. Handling combustible material is a demanding task, and tubular drag cable conveying systems can prevent fugitive dust from escaping the equipment and accumulating on surrounding surfaces. For example, Cablevey Conveyors systems, with their completely enclosed tubing, prevent dust from escaping the system and settling in the working environment, while ensuring a high-level of dust removal due to the variety of cleaning methods.