Combustible Dust Fires and Explosions

By Ben Peetz

In August, three firefighters were injured in an explosion at an Oregon lumber mill two hours after they discovered a fire in a dust collection bin. According to a fire department spokesman, responders thought they had it “under control” when the explosion occurred. However, even with the initial fire under control, the conditions still existed within the dust collection bin that contributed to what could have been a deadly explosion.

More and more, firefighters are encountering incidents well beyond the routine single-story residence fire. Beyond basic suppression and rescue skills, firefighters must be trained to recognize hazardous conditions that present potentially deadly situations. Combustible dust hazards have recently come under heavy scrutiny from the fire and safety industry.

Combustible dust has been recognized as a defined hazard for many years. But a February 2008 explosion and fire at the Imperial Sugar refinery in Savannah, Georgia, left 14 dead and 38 others seriously injured. Massive accumulations of combustible sugar dust throughout the packaging building fueled the explosion. Although similar to others in the past, this event prompted a significant movement among regulatory agencies and standards organizations to inform and educate workers and managers in such environments about the dangers of combustible dusts.

Fire and rescue personnel must also have a working knowledge of combustible dust, or they may not recognize some very dangerous situations until it is too late. If you respond to a facility that generates combustible dust, you have a potential fire and explosion hazard that can cause serious burn injuries, fatalities, and severe property damage. Occupational Safety and Health Administration (OSHA) and Chemical Safety Board (CSB) statistics show that combustible dust incidents have killed scores of employees and injured hundreds over the past few decades. According to OSHA, since 1980, almost 150 workers have been killed and more than 850 injured in combustible dust explosions. Some of these include firefighters responding to facilities with operations that produce combustible dust. These numbers do not reflect the countless incidents where combustible dust may have contributed to a fire’s growth or presented a close call for firefighters.

Facilities with an operation that produces combustible dust are susceptible to two distinct hazards—fire and explosion. One hazard often leads to the other, but either may be the initial event. OSHA figures state that more than 80 percent of combustible dust incidents are fires. In many instances, these fires precede catastrophic explosions. In fact, a dust fire where an explosion has not yet occurred may present the most dangerous situation. This also means that firefighters responding to a combustible dust fire may in fact be putting themselves right in the middle of what is arguably one of the most dangerous situations in firefighting.

In many incidents, investigations revealed that employers and employees were unaware that a hazard even existed. If they don’t recognize the hazard, how will a responding firefighter?

COMBUSTIBLE DUST

Combustible dusts include fine particles, fibers, chips, chunks, or flakes that when suspended in air could potentially cause a fire or an explosion. Combustible dust hazards exist in a variety of industries—food processing, grain handling, plastics, forest products, furniture, textiles, pharmaceuticals, and metal fabrication, among countless others. Raw materials such as wood, flour, sugar, coal, and some metal dusts can present very dangerous conditions in the right situations.

Any combustible material, and even some materials typically considered noncombustible, can burn rapidly when divided into a small enough form. Generally speaking, the smaller the dust particle, the greater the potential hazard. The National Fire Protection Association (NFPA) recognizes that dust particles sized 420 microns or smaller cause the most concern. But for mixtures of varied particle sizes, research has shown that an explosive mixture exists if only two percent of the total dust concentration is sized 420 microns or smaller. For comparison, table salt and granulated sugar grains are generally between 150 and 850 microns in diameter. If such a dust is suspended in the right concentration within an enclosed area, a violent explosion can result. The force of such an explosion can destroy entire buildings and cause serious injury or death. Depending on the particle size and type, and the area of compartmentation during dispersal, a dust layer of only 1⁄32 inch deep (0.79 mm, about as thick as a paperclip) is enough to cause an explosion.

ANATOMY OF A DUST EXPLOSION

Dust fires in their essence are no different from any other fire involving ordinary combustibles. Elementary fire behavior tells us that every dust fire involves the basic fire triangle: fuel, ignition source, and oxygen. However, two additional conditions are somewhat unique to combustible dust explosions. The first is the dispersion of the dust particles, and the second is the confinement of the dust cloud, such as in a vessel, a room, a building, or ductwork. These five conditions make up the dust explosion pentagon.

In many dust explosions, the initiating event may be relatively minor. Although this could involve a small primary fire or explosion, anything producing sufficient vibration to cause dust dispersal could create an explosive situation. A forklift striking a support column or an overhead beam may be enough to trigger a dust cloud, as can something as benign as cleaning an area with compressed air. Once dust is stirred up and suspended in the air, the only thing needed for a deflagration is for the right concentration to reach an ignition source. Once even a small explosion occurs, this often stirs enough additional fuel load to trigger secondary events that can rattle the building enough to shake dust free from all overhead areas and begin a chain-reaction explosion event that can potentially level a building.

RECOGNIZING A HAZARD

Properly assessing combustible dust dangers involves looking at every aspect from raw materials to the packaging of the final product. Performing regular walkthroughs at industrial and manufacturing facilities can help you determine your exposure to combustible dust. Machining, conveying, or otherwise manipulating a material is likely to produce dust in some form and volume. Dust may be produced through one particular process, such as grinding or sanding (photo 1), or it may result from material breakdown during the handling of a particular product, such as grain. In some cases, such as flour or sugar, the product itself is the combustible dust.

(1) High-speed sanding and milling equipment provides great potential for a dust fire or explosion. Not only do these machines produce a large volume of combustible material, but they use sanding abrasives and metal tooling, which provide a ready ignition source when a process breaks down or machine components fail. (Photos by author.)

Pay particular attention to the fuel load suspended in overhead areas (photo 2). Remember to think about all the places where dust may not be seen, including the tops of machinery, in or on the building structural members, and above drop ceilings. Remember, any “acoustic event” that produces enough vibration to dislodge this dust could result in a flash fire or an explosion. If a firefighter is in the area at that time, serious injury or death is likely. Keep in mind that the threshold level of dust used for OSHA citation purposes is 1⁄32 inch over 5 percent of the total surface area, which is supported by NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids.

(2) Loss prevention personnel are often accused of always looking up, and firefighters should do the same when in an area that looks look like this. Remember, calculations show that a layer of dust only as thick as a dime (1.35 millimeters or 0.05 inches—less than 1⁄16 inch) can provide twice the needed fuel for an explosion when suspended in air. Additionally, consider the dangers of this burning fuel falling onto firefighters working in this area.

As you look at the fuel load throughout a facility, also consider ignition sources and related control programs. Be sure to examine areas where dust is regularly in close contact with ignition sources. Assess the conditions of electrical disconnects, control rooms, and motors (photo 3), as well as equipment involving bearings, chains, conveyors, and other moving parts that could produce a friction ignition. These are all areas where you will have to reasonably expect a response could be needed.

(3) Inspect electrical equipment thoroughly in areas where dust is generated. In this instance, sawdust had accumulated to the point at which the heat of the motor was blackening the sawdust in contact with the housing. Note the dust collection duct in the background, which could have drawn in any ignited material, potentially resulting in a dust system fire or explosion.

Prevention programs could include grounding and bonding blower motors, duct systems, dust collectors, and dust-producing machinery. Does the company have a hot work permit program to ensure safe procedures are followed for any cutting, welding, or grinding in dusty areas? What about properly rated industrial trucks and even the cleaning equipment itself? Are these approved for combustible dust locations?

Although a walkthrough will give you a much better idea of what to expect, it may still be inconclusive. For example, the level of housekeeping within a facility may be a clue indicating whether a hazard exists, but it may not mean that there is no hazard. A January 2009 combustible dust explosion in a Jasper, Indiana, furniture facility resulted from an initial explosion within the interior of the dust collection equipment. This high-end manufacturing facility performs spray finishing that requires filtering the interior air down to the tens-of-microns level. From my own experience, a walkthrough of that facility would have impressed even the cleanest of neat freaks. Despite extreme housekeeping efforts and fully established safety and control programs, an event still occurred. The mere existence of a combustible dust-producing process and the related collection equipment indicates the potential for an explosion.

Many facilities, like the one discussed above, use specialized dust collection systems because of the volume and types of dust they produce. Although getting inside the facility is the only way to truly know what you face, an easy way to initially recognize a potential combustible dust situation from the outside is to look for the equipment commonly used to manage dust. Also, the dust management equipment often represents one of the operation’s more dangerous aspects. Dust filtration units, cyclones, silos, ducting, and blowers are all telltale indicators that dust is present at a facility. A little bit of knowledge about the systems can also help you get a better idea of the risks you could be facing through a preincident plan or a quick size-up during an incident.

DUST MANAGEMENT SYSTEMS

Dust management systems have three basic components: a blower/duct dust collection system, an extraction device to remove the dust from the conveyed air, and a storage area for the collected material.

Blowers are sized according to the needed volume of air flow in cubic feet per minute (cfm). Blower systems may be configured for either a positive flow (i.e., a fan actually pushes material through the system) or negative flow (i.e., a fan draws the air through the filter). Positive systems inherently require all of the combustible dust material to pass through the fan as it is pushed into the filtration unit. Negative systems are designed with the fan on the clean-air side of the unit so that the fan creates a negative pressure on the entire system. This is preferable because this eliminates the risk of waste causing a spark as it passes through the fan. There is also significantly less risk of a fan malfunction (e.g., overheated bearing, damaged fan fin) causing an ignition within the system.

Duct systems can be found in sizes ranging from about eight to 10 inches in diameter up to 36 to 48 inches or more. Often, the ducts at individual machines may start at a three- or four-inch diameter and then gradually increase in size as they are directed toward a main trunk line (photo 1).

Extraction devices may be as simple as an open cyclone, which uses basic physics to extract the dust from the air (photo 4). As the air and dust are directed into the cyclone, the air changes directions rapidly, which slows the acceleration of the dust to a point where gravity causes it to fall out of the air stream. The clean air exits an opening near the top of the cyclone unit while the dust drops out the bottom. These units are open, and no pressure is created within the system.

(4) Open cyclones are somewhat less dangerous than pressurized filtration units. The open design allows air to flow out of the unit as quickly as it flows in, which eliminates the backpressure and reduces the compartmentation effect.

In contrast, dust filtration units, often called “baghouses,” operate similar to that of a large shop vacuum with a pressurized air system. The dirty air is directed into an enclosed filter house that has filtration bags or cartridges (photo 5). As the air is pushed through, the filter media retains the dust and the clean air exits the unit. The clean air may be vented to the atmosphere, or it may be returned back into the building to reduce negative pressure on the building and to conserve heat.

(5) An interior view of the filtration bags that hang inside of a typical bag filtration unit. These bags may hang eight to 10 feet long or more in the upper section of the filtration unit, representing the primary fuel in this area. In a properly operating system, no significant amount of waste material is stored within the unit. However, as dust is shaken from these bags during the blowdown sequence, this area would be filled with suspended dust; at that point, all that would be missing for an explosion to occur is an ignition source.

Within the filter unit, a blowdown sequence occurs in which a pulsed jet of compressed air is regularly directed through the filter media to remove the dust buildup. These pressurized filtration units can be particularly vulnerable to a dust explosion because each time the dust is blown from the filters, dust dispersal occurs within the confines of the filter house and only an ignition source is missing in the dust explosion pentagon. Because of this extreme danger, fire personnel must be vigilant about dust system exposures, particularly any indoor pressurized filtration equipment.

Waste can be stored through any number of options, ranging from 55-gallon drums to dumpsters of varying sizes up to massive steel bins or concrete silos (photos 6-8). Again, because the containment vessels are typically fully enclosed compartments, they provide the confinement of particles necessary to facilitate an explosion. And because even an empty silo can still contain sufficient dust to explode, the volume actually being stored at a given time plays little role in the risk of an explosion.

(6) Smaller filtration units are used where the dust volume is light enough to collect in drums. However, the danger of explosion is no less.
(7) Dust management systems come in hundreds of possible configurations. This system has multiple high-speed abort gates and storage in an elevated “clamshell” bin. Note the two explosion ventilation panels on the blue filtration unit at left. Such panels are attached with frangible (breakable) hardware and are designed to release during an explosion, thus reducing the risk of rupture to the unit itself.
(8) Concrete silos may be used to hold any number of products, including wood waste. The sawdust may be then conveyed directly from the silo to a wood-burning boiler or other point of use. This particular silo is configured to load waste material onto trucks. Note the two explosion ventilation panels on the blue cyclone unit atop the silo.

SAFETY PRACTICES AND EQUIPMENT

In operations where combustible dust may be present, there are two primary approaches to mitigating the hazard: control the fuel load, and control the ignition sources. NFPA 654 and NFPA 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities, both contain comprehensive guidance on controlling dusts and ignition sources to prevent explosions. In addition to operational practices, a number of engineered safety features are available for combustible dust operations and management systems. Some of the equipment is designed to reduce the risk of a fire or an explosion; other items are designed to redirect the energy of an explosion to reduce the extent of casualties and property damage.

Spark detection systems (photo 9) use photoelectric eyes mounted on key ductwork to watch for any spark or hot ember traveling through the system. On detection of a spark, the system triggers at a calculated distance downstream from a spray nozzle designed to flood the debris stream with water and extinguish the spark before it reaches the dust enclosure. Abort gates (photo 10) may then be programmed to trigger at the same instant or as part of a secondary detection zone. The abort gate equipment is a rapid-release device designed to instantly redirect the air flow through the system to an external vent, thus preventing the return of any fireball or explosive concussion back into the plant.

(9) Photoelectric eyes mounted on ductwork constantly “watch” the debris stream inside the dust system. If the eye senses a spark or burning ember, the system will instantly trigger safety devices downstream—e.g., water spray nozzles and high-speed abort gates. Inspecting and maintaining spark suppression components are critical to ensure their proper operation.
(10) High-speed abort gates allow material to move through a duct until a detection system activates the gate, which causes it to immediately divert the waste stream or the clean return air away from the building or other equipment. This allows the controlled release of any fireball or explosive concussion in an area deemed less hazardous to occupants and property.

At the instant that an explosion takes place, a backpressure will also occur on the dirty air infeed ducts, which may push a reverse flow of fire back into the building through the intake piping. This is why a gravity-activated swing-type damper is recommended between the building and the dust filtration unit’s air intake. Similar to a household dryer vent, any reverse flow will slam the damper shut, thus keeping the fire and explosion’s force on the exterior of the building. If properly engineered, the dust unit will release the force and fireball through specially designed explosion venting panels.

Explosion venting panels, or blow-out panels, (photos 7, 8, 11, and 13) are specially engineered panels on the sides of dust units that are attached with frangible hardware to allow an explosive force to release without destroying the unit. They should also be in place on silos and other large storage structures. Sometimes entire rooms subject to dusty operations will be constructed with blow-out panels on the roof or even entire walls designed to be sacrificial in an explosion, thus preventing serious structural damage to the building. Firefighters must remember never to position themselves or apparatus in the path of potential explosion venting. In addition to the engineered explosion release panels, any other access panel or opening could potentially become a means of explosion venting, so be sure to avoid them and be extremely careful with putting personnel at risk in any attempts to open an enclosed unit. Remember, even a small fire initially unrelated to the dust system could be drawn into collection equipment and quickly lead to ignition within the dust enclosure, thus creating a deadly scenario.

(11) Dust filtration units equipped with automatic sprinkler or deluge systems allow the application of water into a burning unit without endangering firefighters. Here, the sprinkler line provides water to both the areas above the bag plenum inside the unit as well as the area below the bags where the filtered waste remains prior to the transfer to storage. Note the yellow explosion vent panel.
(12) Automatic sprinkler or deluge heads installed in the interior of dust filtration units can provide firefighters with a safe means of supplying suppression water without endangering members.
(13) Large facilities with high dust volume may use multiple systems for dust management. Remember, since all of these systems will be tied together in some fashion, a fire in one unit could easily affect the other systems. Take special care to reduce the risk posed to firefighters from any other systems not immediately affected by an event.

SAVING LIVES AND PROPERTY

Unfortunately, despite every effort to reduce the risk of dust fires and explosions, they will still occur, and that’s when firefighters must understand the implications of every action they take. Once a combustible dust fire begins, it is extremely important to control the fuel load and avoid dispersal. A fire within a dusty building can quickly escalate if dust is dispersed and a flash fire causes rapid spread. Just the resulting expansion of heated gases as a flame front propagates through a structure can actually cause air gusts that disperse enough settled dust to result in an explosion.

If a fire occurs within a dusty operation or a dust management system without causing an initial explosion, the only thing stopping it from exploding is the lack of dispersal. In this situation, it is absolutely critical that firefighters know they should NEVER climb onto a dust filtration or storage unit or attempt to open the unit to gain better access for suppression efforts. Any activity that causes dust to shift within the unit could cause dispersal and result in an explosion.

Additionally, never place firefighters in aerial devices within close proximity to a unit. Even without direct contact with the equipment, burning material or components could shift within the fire compartment and cause dust dispersal, resulting in an explosion.

Automatic sprinklers or standpipes installed on dust filtration and storage equipment (photos 11, 12) provide a great way to reduce the risk of firefighter injuries by making water-based suppression to the entire unit quick and easy. Antifreeze-filled automatic sprinkler systems are preferable because the suppression effort occurs immediately. Dry-pipe systems may also be used, but they tend to be high maintenance and cost more.

Another alternative is an open-pipe deluge system tied to electronic heat detection within an enclosure. In areas where the water supply is inadequate, a preinstalled standpipe with a fire department connection may be the best option available to get water inside a unit while keeping firefighters off the unit and at a safe distance. These systems can be used for filtration units as well as storage silos and bins. Responders must simply ensure the water supply is adequate and uninterrupted until there is no question that the fire is completely extinguished.

When fires do occur within silos and storage structures, they often present serious challenges for responders. Although dust filtration equipment does not typically retain a volume of material sufficient to burn for any duration, a storage structure can be a different story, possibly involving hundreds of tons of material. Although there have been some industry attempts at developing alternative suppression methods for silo fires, such as the use of nitrogen to suppress oxygen within the structure, these systems are relatively rare right now. This is primarily because of the cost associated with them and the limited success rate seen so far in practical applications. This means the only way to address a fire in a silo will likely be the traditional application of water-based suppression.

Any efforts to empty a storage structure must be carefully calculated and performed with precision to avoid creating any type of dust cloud that could flash or explode. Even when the fire is believed to be extinguished, the tiniest embers could quickly ignite a dust cloud and cause a serious flash fire. Silos may have unloading systems that allow material to be removed from the bottom of the bin, or they may be designed for a truck to drive under and load. A fire within a clamshell-type bin (photo 7) may compel firefighters to open the bin and empty the material onto the ground. However, you must exercise extreme caution because of the potential to stir a dust cloud that could result in a flash fire, consuming firefighters and apparatus. Remember, any attempts to empty a storage structure from the bottom will mean the entire fuel load may move at one instant. Again, do not attempt to send firefighters onto the structure, and resist the urge to open any access hatches, even at ground level. Not only can this cause movement in the fuel load, but it also will allow oxygen to enter a burning silo, which can enable a smoldering fire to quickly grow. If there are open panels at the top of a unit or if explosion venting has released, it may be possible to direct an aerial stream into a unit, but only from a safe distance—and unstaffed, if possible. Again, applying fire suppression water through preinstalled plumbing is the safest and most effective option.

Not only is preinstalled fire suppression equipment safer and more efficient, but, in some instances, it may be the only viable and safe approach to extinguishing a fire. It is not uncommon to see dust filtration units on the tops of silos (photo 8). Additionally, the configuration of some systems may make it difficult to reach the affected equipment because of the tight clearances to other system components (photo 13). For this reason, if you have dust systems in your areas, ensure that fire response has been considered in the planning and engineering. Firefighter safety must be a top concern.

As with so many of today’s response challenges, with a combustible dust fire, firefighters must resist the initial enticement to “squirt wet stuff on the red stuff.” This is of particular importance with an enclosed dust filtration unit where explosion venting has not yet released. A dust fire within an enclosed area is a ticking time bomb just waiting to complete the explosion pentagon. I have encountered several instances where firefighters decided that a unit needed to be opened to allow for better suppression efforts. This has usually resulted in the unit burning hotter and more violently, with greater overall loss. Not only is the property damage often more severe, but this move can prove deadly. If the explosion panels have not released, the safest approach is to leave it closed up and allow it to burn. An understanding and evaluation of safety features in place for combustible dust operations may just prove to save your life or the life of one of your own.

REFERENCES

FM Global, Data Sheet No. 7-76, Prevention and Mitigation of Combustible Dust Explosions and Fire (2005 edition).

National Fire Protection Association 654, Standard for the Prevention of Fires and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids (2006 edition).

National Fire Protection Association 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities (2007 edition).

Occupational Safety Health Administration (OSHA), Hazard Communication Guidance OSHA 3371-08 2009, Combustible Dusts, 2009.

Occupational Safety Health Administration (OSHA), Instruction CPL 03-00-008, Combustible Dust National Emphasis Program (Reissued), 2008.

Occupational Safety Health Administration (OSHA), Safety and Health Information Bulletin SHIB 07-31-2005, Combustible Dust in Industry: Preventing and Mitigating the Effects of Fire and Explosions, 2005.

U.S. Chemical Safety and Hazard Investigation Board, Investigation Report No. 2006-H-1, Combustible Dust Hazard Study, November 2006.

BEN PEETZ, CFPS, is a loss prevention specialist and fire protection consultant for Lumbermen’s Underwriting Alliance, a commercial property insurance carrier specializing in the forest products and plastics manufacturing industries. He is a second-generation fire service veteran with the Napoleon (IN) Volunteer Fire Department, where he has served for 15 years. Peetz is an Indiana fire instructor II/III and a Fire Department Instructors Conference instructor and has degrees from Purdue University.
Ben Peetz will present “Combustible Dust Fires and Explosions” on Wednesday, April 18, 2012, 10:30 a.m.-12:15 p.m., at the FDIC in Indianapolis.

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