By Brian Ward
Recently, several incidents involving combustible dust emergencies have made national news because of the devastation they caused or because they were videotaped and shared through social media. The U.S. Chemical Safety and Hazard Investigation Board released a study in 2006 that identified 261 dust-related fires between 1980 and 2005 that caused 119 fatalities and 718 injuries. Furthermore, there have been nine major incidents since 2003 that have caused 36 deaths and 120 injuries. These numbers are exponentially increased if you include modernized countries outside of the United States.
As I thought back over my career experiences, firefighter certifications, classes, and even a fire safety and engineering bachelor’s degree, I only recall one class that discussed combustible dust and the horrific effects it can have (if handled improperly). Although these events may not occur every day, just about every department has a potential combustible dust situation in their response area, even if they do not realize it. The proactive approach is to review this article and then preplan your respective sites while the information is fresh in your mind. The notes you make in your preplan may be the difference in your crew or the next shift making the right decisions. This article will discuss the terminology, past events, various types of combustible dust situations, dust collection designs, and considerations for mitigating a combustible dust emergency.
Combustible dust. This is a finely divided combustible particulate solid that presents a flash fire hazard or explosion hazard when suspended in air over a range of concentrations. [See National Fire Protection Association (NFPA) 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids.]
Flash fire. This is a fire that spreads by means of a flame front through a diffuse fuel such as combustible dust, flammable gas, or ignitable vapor without the production of damaging pressure.
Deflagration. This is a propagation of a combustion zone at a speed that is less than the speed of sound in the unreacted medium (air).
Detonation. This is a propagation of a combustion zone at a velocity that is greater than the speed of sound in the unreacted medium (air).
Explosion. This is the bursting or rupture of an enclosure or a container from the development of internal pressure from deflagration.
The difference between a deflagration and a detonation may be explained simpler by describing an event. The deflagration may not result in an explosion, although it has the potential; it generally will consume the fire load as it spreads. The detonation event will create an explosion because of its rapid heat expansion.
Dust Explosion Pentagon
As firefighters, we are all aware of the “fire triangle” and the “fire tetrahedron.” We have gained additional knowledge concerning fire dynamics recently through the research from the International Society of Fire Service Instructors, the National Institute of Standards and Technology, and Underwriters Laboratories. This research has allowed us to further our knowledge of flow paths, vent-limited fires, flashover, and backdraft potential events. Using this knowledge, for a dust event to occur, we must still have an ignition source such as oxygen and fuel (in this case, dust). However, for a dust explosion to occur, the dust must be suspended into atmosphere and confined in a location where pressure exerts itself against the container.
Figure courtesy of the Occupational Safety and Health Administration/www.osha.gov.
This event is best thought of as being similar to igniting a vapor or gas cloud. However, once the dust ignites the pressure waves will further disperse dust within the container and/or facility causing secondary and tertiary explosions and/or flash fires.
If your station has a manufacturing facility in your first-in territory, it more than likely produces some form of dust as a by-product of its process. Other manufacturing facilities accumulate dust over time for various reasons or use fine, powder-like materials as part of its processes. Regardless of how it occurs or how it is used, the Occupational Safety and Health Administration (OSHA) describes any accumulation of more than 1/8-inch of dust as a dust deflagration hazard; in general terms, if you can write your name in the dust, consider it potentially explosive. The key is to preplan your facilities and work with the site responders to identify and comprehend the situation quickly; the hazard will quickly escalate if you do not react on it. A word of warning here: The correct action may be no action if your firefighters do not properly understand the hazards involved.
Not until recently has OSHA become involved in the combustible dust scenario. There currently is no OSHA regulation on combustible dust, but on July 20, 2011, it released a collaboration by combustible dust experts—the Expert Forum Summary Report. In addition, OSHA extended combustible dust to its National Emphasis Program. NFPA has released several documents on combustible dust as early as 1930, notably the Code on Wood Flour Manufacturing. NFPA has since developed several standards to include Explosion Prevention Systems (69), Fire Prevention and Control in Coal Mines (120), Combustible Metals (484), Prevention of Fires and Dust Explosions from Manufacturing (654), Processing and Handling Combustible Particulate Solids and Prevention of Fire and Dust Explosions in Wood Processing and Woodworking Facilities (664). Also, FM Global produced Prevention and Mitigation of Combustible Dust Explosion and Fire in October 2014, which addresses the engineering aspects.
(1) A firefighter trapped in a hopper flash fire. (Photo courtesy of OSHA.gov.)
Following are a few incidents that injured and/or killed facility employees and firefighters. The OSHA’s Firefighting Precautions at Facilities with Combustible Dust describes additional incidents with firefighter injuries and fatalities from coal dust in South Dakota, aluminum shavings in Wisconsin, sawdust in Oregon, and wood chips in Ohio. These events can occur in any type of manufacturing facility, and they are not tied directly to any one industry. It is important to remember that any fine material which is dispersed can be ignited (i.e., sugar, flour, metal, coffee creamer, wood, and so on).
- West Pharmaceuticals Polyethylene explosion; Kinston, North Carolina (2003): Six deaths, 38 injuries, and two firefighter injuries. Cause: An accumulation of polyethylene powder that was applied to rubber to reduce its stickiness. (Photo 2 courtesy of the U.S. Chemical Safety and Hazard Investigation Board.)
- Imperial Sugar Mill; Port Wentworth, Georgia (2008): Fourteen deaths, 38 injuries. Explosion caused by accumulated sugar dust. (Photo 3 courtesy of U.S. Chemical Safety and Hazard Investigation Board.)
- AL Solutions, New Cumberland, West Virginia (2010): Three deaths from ignition of titanium powder. (Photo 4 courtesy of U.S. Chemical Safety and Hazard Investigation Board.)
Dust Collection System Design
Dust collection systems are designed to contain, capture, convey, and collect dust. There may be multiple reasons such as to control air emissions or to reuse the dust later in the process. Regardless of intent, these systems possess inherent hazards which require a tremendous amount of investments to provide additional safeties to reduce the risk of a fire or explosion event from occurring. Figure 1 depicts a basic system where dust is vacuumed up from a source with the use of a blower or fan and conveyed into an air material separator (AMS). The AMS is a collection device which filters the dust. There are several types of similar looking devices, such as a cyclone, which collects larger sized particles or chips and is vented in the top to atmosphere. Generally, cyclones do not present explosion hazards because of their vent and by not allowing itself to become over-pressurized. Another type may be a baghouse device, which is not engineered with vents and can become over-pressurized, resulting in an explosion.
RELATED: Peetz on Combustible Dust Fires and Explosions ‖ Wenzel on the Kansas Grain Dust Explosion ‖ Vernon on Post-Blast Response
Hazards and Fire Response
These systems operate by pressurizing the duct work leading to the collection device by either pushing or pulling particles with the use of a fan or blower. One inherent hazard is a spark in the duct work that can create a duct fire. Another hazard is an event inside the collection device which becomes over-pressurized and creates the potential for an explosion. A third hazard comes from fugitive dust which is not captured by any device and tends to build up inside a building or on equipment. The fugitive dust will create a flash fire, consuming the fuels as it extends rapidly through the building. Each of these hazards requires a different thought process and strategy to mitigate.
Many manufacturing facilities have installed “detect and suppress” or “spark detection” systems inside their ductwork to prevent duct fires. These systems are installed prior to the collection device and are designed to detect sparks and then trigger spray heads inside the duct work, suppressing sparks only; they are not intended to extinguish fires. Also, if an explosion occurs and the fire travels backward through the ductwork toward the actual pickup point at the equipment, these systems would not extinguish the fire. Their primary purpose is to prevent a spark from reaching the collection device.
To prevent over-pressurization of collection devices, chemical suppression units (photo 5) can be installed which activate once a change in container pressure is identified. The chemical suppression units use a sodium bi-carbonate and will provide instantaneous burst. After this burst, they are deemed ineffective for the remainder of the incident. They look similar to the photo 6 (although different colors and configurations are available depending on the vendor).
(5) Photo courtesy of Fike Corporation.
Other manufactures have chosen to install manual and/or automatic deluge systems with either a rate-of-rise detector or a fusible link type of sprinkler. The primary concern here is that the deluge system will not prevent an explosion. Furthermore, some systems may have no protection at all, and firefighters should be extremely careful approaching an unprotected system.
Another area of caution is explosion vents (photo 6), which may be mounted on the AMS. Their intention is to relieve the internal pressure by blowing off during an event, so stand clear outside the “exclusion zone.” The exclusion zone is the immediate area around the collection device, which could range any distance depending on multiple variables (particle size, size of collector, and so on). There may be painted lines on the ground which dictate the facility’s best estimate for an exclusion zone. It is also important to note that some walls may be sacrificial walls intended for relief venting. Firefighters should not be in these areas.
(6) Photo courtesy of Donaldson Torit Corporation.
Fugitive dust fires arise from uncollected dust particles inside the building or equipment. Fugitive dust will create a flash fire event that will move rapidly through the building.
Arguably the greatest hazard is the potential for secondary and tertiary explosions after this dust becomes suspended in the air while firefighters or occupants are inside the building. Events such as these may damage the sprinkler systems and render them inoperable. The portion of the sprinkler system that remains intact will generally have limited water supply unless it is supplied by a municipal water system. This may create additional difficulties concerning water supply for fire apparatus. So, identify a facility representative who has knowledge of the fire protection system; he may be able to close certain post indicator valves (PIVs) to the affected area of sprinklers, thus retaining adequate pressure on the remaining sprinkler system.
An additional hazard involving a damaged sprinkler system includes determining if the electrical power is shut off, electrical shock hazards from standing water and consider what damage has occurred to the area involved such as in the ceiling from a structural integrity standpoint. Dust can settle anywhere, but the ceilings are, in many cases, “out of sight, out of mind.”
The first-arriving unit must determine if any of these protection systems are installed and if they have been they activated. If they are activated, allow the system to perform as intended and described above. The officer should locate a facility representative and have them explain what they believe is occurring. This information will help you determine if additional resources are needed and/or which task(s) need delegating. If there is any doubt if the fire and/or explosion protection systems have been activated, ask the facility representative if the system can be manually activated safely.
If the situation is unclear, the thermal imaging camera (TIC) comes in handy here. The TIC will allow you to trace the piping systems and determine if there is hidden fire or smoldering particles that could ignite instantly if oxygen is introduced.
FIGURE 2. Air Material Separator.
Additional considerations for firefighters include the following:
- Determine if this is duct work, a collection device, or a fugitive dust fire. This will dictate what actions you must take and determine if this is an offensive or defensive strategy.
- Determine what material is burning. Some materials cannot be put out with water and will require a special extinguishing agent (e.g., magnesium or lithium). Metal fires can produce the extremely elevated levels of high-energy explosions.
- Determine if power to all affected equipment is shut off. In an industrial setting, voltage is well over the 120 volts (V) seen in residential structures and which are controlled by motor control centers.
- Never disperse the dust into a cloud, which can be done by using a straight stream or smooth bore.
- Consider avoiding using a fire extinguisher because of its potential to disperse dust into a cloud.
- Always use a TIC to detect heat sources within the system, and trace the system from the collection point to the pickup point. If heat is noted, assume there is a fire hidden inside the system or ductwork.
- Use caution when opening any inspection door, and only do so after you have scanned the system with a TIC. It is best to gain a consensus with the site representative prior to opening any inspection door(s). Always stand on the hinged side of the inspection plate. Maintain door control and only open as far as necessary being careful to not shake or rattle the duct work, which would suspend the dust and potentially ignite it.
- Never attempt to open the pipe with an ax (because of the swinging motion) or a K12 saw. Both of these actions can create dust clouds, which produces sparks from the dust dispersion, increasing explosion potential.
- Never use a ground ladder to lean on the duct work; it may not be able to hold the additional weight. Also, do not overload the duct work with the weight of water. Firefighters must be cognizant of the potential of the duct work falling out of the ceiling because of the increased weight load. If possible, consider drilling holes, if this can be done safely. Your only option may be to evacuate these areas.
- A best option is using an aerial lift, which can be articulated as needed. All personnel should be completely dressed out, including their self-contained breathing apparatus (SCBA), even if the firefighters believe there is no hazard present. A firefighter wearing his turnout gear and SCBA should stand by at the base of the aerial lift in case a ground firefighter needs to override the lift controls.
- Use a low-pressure nozzle (if one is available) and roll on or “lob” water on the burning particles. Do not open the nozzle fully; this will disperse the dust.
- Attempt to extinguish the fire from as far away as possible and never stand under an AMS collector.
The best method for learning about the systems in your first-in territory is to conduct preplanning and discuss them with the facilities engineers or a fire protection specialist. The site representatives should be able to discuss or find the answers to all of the questions I pose in this article. Combustible dust emergencies are not a typical fire department training event, and they are best mitigated by a shared interest between the facility and fire department.
Items in your preplan should include:
- Layout of the building.
- Combustible dust duct work and collection device locations.
- Spark and explosion protection systems.
- Water supply, sprinkler systems, fire department connections, and standpipe locations.
- Post indicator valves.
- Manual deluge valve location.
- Determine any special considerations for the operating system.
- Dust specialist contact number for your facility (this may also be referred to as a compliance system owner).
Training for the Event
Another helpful action to take is to conduct tabletops or computer-based simulations with the facility stakeholders. Using the developing tactical decision game approach (which is being presented at FDIC International 2017 as a workshop), develop several potential situations which could occur at the given facility and develop a storyline for each. I use “Digital Combustion,” which allows me to take a picture of the facility and place smoke and/or fire in a particular area for realism. I then place the fire department officer and the facility representative together and allow them to run the scenario. This has helped us determine member knowledge and skill level gaps as well as build relationships between the organizations. These gaps are turned into action items, where the facility and fire department can work together to create the best outcome for all individuals involved. Another inherent advantage of this approach is that it is nice to see a familiar face at the facility when an event of this magnitude actually occurs.
For additional information, review “Developing Tactical Decision Games” in the July 2016 issue of Fire Engineering. During the FDIC Internnational 2017, I will discuss this very event and many others, from residential to commercial tactical decision games.
FM Global. Prevention and Mitigation of Combustible Dust Explosion and Fire. October 2014.
Occupational Safety and Health Administration. Preventing and Mitigating the Effects of Fire and Explosions. Retrieved from https://www.osha.gov/dts/shib/shib073105.html.
Occupational Safety and Health Administration. Firefighting Precautions at Facilities with Combustible Dust. Retrieved from https://www.osha.gov/Publications/OSHA_3644.pdf.
U.S. Chemical Safety and Hazard Investigation Board. Retrieved from http://www.csb.gov.
Brian Ward is an author for Fire Engineering‘s “Training Officer’s Toolbox” and Barn Boss Leadership. He facilitates programs around the country on emergency response, training, and leadership topics in the public and private sector. He is the founder of FireServiceSLT.com.