Editor's note: An edited version of this article is in the print and digital editions of December Wood & Wood Products. Below is the article in its entirety.
Combustible dust is a serious issue. It has become a top health and safety issue in the woodworking industry. While the first reported combustible dust fire occurred in a 1785 at a flour mill in Italy and over two hundred years later in 2008 a major sugar refinery in the state of Georgia exploded due to combustible dust, in 2012, the woodworking industry saw a major sawmill in British Columbia, Canada, launch a fire ball reportedly 60 meters high due to a suspected combustible dust explosion. Additionally a wood pellet manufacturer in New Hampshire suffer a third combustible dust related incident.
While the occurrences of combustible wood dust related fires have been increasing and at times catastrophic, the incidence of OSHA (Occupational Safety and Health Administration) inspections related to combustible dust in woodworking facilities has also increased dramatically.
What Are Combustible Wood Dust Particles?
Combustible dust, as defined by the National Fire Protection Assn. (NFPA), is “a finely divided combustible particulate solid that presents a flash hazard or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations.” Another definition, by OSHA, describes combustible dust as fine particles that present an explosion hazard when suspended in air under certain conditions. Common to the woodworking industry is wood dust otherwise known as “wood flour” an agricultural product that is potentially combustible under certain conditions and presents an explosive dust hazard. Other dusts such as stone dusts and granite; an igneous rock or common table salt which is sodium chloride are not explosive dust hazards as they cannot combust. However, it is not simply defining a dust; it is determining the explosibility of the dust. Important factors include size, shape, moisture as well as environment. If there is any doubt of combustibility, the dust must be sent to a certified facility to be tested.
Additionally, Kst value can be used as a determining factor in the deflagration of your dust. The higher the Kst value the greater the explosion characteristic of the dust. For example, wood flour (wood dust) has a Kst Value of >200 and < 300 meaning it has a strong explosion characteristic. The Hazard Communication Guidance for Combustible Dusts also lists a dust explosion class rating system from St 0 – St 3. The dust explosion class of wood flour is St 2. NFPA defines the size of “Deflagrable Wood Dust” as 500 microns (0.5 mm, 0.0196 inch) or less and has a moisture content of less than 25 percent. Another way to measure is to see if the material will pass through U.S. No. 35 Standard Sieve according to NFPA 664 (126.96.36.199) which is approximately the “size of fairly coarse sand.”
What exactly is “wood” the source of wood flour or wood dust? NFPA details wood as “cellulosic material derived from trees, and other cellulosic materials including, but not limited to, wheat straw, flax, bagasse, coconut shells, corn stalks, hemp, rice hulls, and paper or other cellulosic fiber used as a substitute or additive to wood. Additionally, ‘Wood-Derived Materials” are defined by NFPA as “sawdust, sander dust, planer shavings, hoggings, wood flour, and moulder waste.”
What Is a Combustible Wood Dust Explosion?
In order to have a fire you must have “Fuel,” “Ignition” and “Oxygen,” the three main elements of the “Classic Fire Triangle.” However, for a combustible dust explosion you must add “Dispersion” and “Confinement” which together creates the “Dust Explosion Pentagon.” Without these five elements an explosion is impossible but not a fire.
With many larger combustible dust explosions there are two issues, first the primary issue which is the initial combustible dust explosion. Additionally, the second issue is the secondary explosion which typically comes from stagnant undisturbed fugitive dust that has settled in hidden places which becomes dispersed and suspended due to the disturbance caused by the initial explosion. This is what happened at the Imperial Sugar explosion in Port Wentworth, GA, in 2008. The initial explosion occurred at one location within the facility and the explosion was so forceful that it literally shook other sections of the building releasing and suspending the previously settled fugitive dust, thus creating dust clouds in enclosed rooms that exploded as fire spread throughout the building. There is suspicion this is also what happened at the sawmill explosion in British Columbia, Canada, earlier in 2012.
Explosion and/or Inspection?
Under the right conditions a combustible dust explosion can occur. Under just about any condition OSHA can inspect a facility.
Combustible dust is on OSHA’s radar screen, and inspections have increased substantially as have the actual penalties. OSHA is taking this matter seriously using the general duty clause in classifying combustible dust violations. One combustible dust violation alone had a $5,000 penalty, according to OSHA “The employer did not furnish employment and a place of employment which were free from recognized hazards that were causing or likely to cause death or serious physical harm to employees in that employees were exposed to fire and explosion hazards caused by the presence of combustible dust.”
Another OSHA Citation for a woodworking facility stated “layers of combustible wood dust were allowed to accumulate to depths over surface areas in quantities that exposed workers to fire and or explosion hazards.” This citation references 29 CFR 1910.22(a)(1). CFR is the United States “Code of Federal Regulations.” Title 29 is regulations related to “Labor”. Part 1910 is the “Occupational Safety and Health Standards.” Section 22 is “General Requirements” (a) “Housekeeping” (1) “All places of employment, passageways, storerooms, and service room shall be kept clean and orderly and in a sanitary condition.” So the layers of wood dust in this citation are in direct violation of the current “housekeeping” regulation whereas combustible wood dust accumulated on I-beams, inside the trough of ceiling joists as well as on the floor.
The citation continues to state “when combustible wood dust was cleared from surfaces, the employer used cleaning methods that increased the potential for a combustible dust deflagration and or explosion” because “the employer use 30 psi compressed air to blow down and clear combustible wood dust.” The current NFPA Standard as referenced in this citation states 15 psi as being the proper low pressure. This citation specifically refers to the NFPA 664 (2012) 188.8.131.52 which states “surfaces shall be cleaned in a manner that minimizes the generation of dust clouds…only a low gauge pressure 15 psi…shall be used.”
Steps Toward Prevention
Studying past combustible dust explosions, OSHA citations, as well as NFPA standards provide guidelines for prevention. A few areas that should be highlighted include: Hazard Recognition/Assessment; Building Design & Engineering Controls; Administrative Controls; Housekeeping; and Worker Training.
â Hazard Recognition/Assessment includes area such as determining if dust is combustible via Dust Explosion Testing which may include Particle Size and Moisture Analysis, Explosion Severity Test which will tests the Kst value and Minimum Explosible Concentration (MEC) all as mentioned earlier. Hazard Recognition/Assessment also covers issues related to NFPA as well as potential application of State and Local codes including the role of the AHJ, also as mentioned earlier.
â Building Design & Engineering Controls should cover “fixed structures that are built into a facility or processing equipment designed to remove or minimize a hazard.” Building Design includes the building or facility which focuses on prevention of fugitive dust accumulation on surfaces, beams, etc. Flat surfaces are not good, surfaces including rectangular shaped ductwork, overhead beams, flat surfaced lighting fixtures, and all invisible areas such hung or suspended ceilings. This is where good housekeeping is imperative. Engineering controls focus on the equipment such as dust collection systems or prevention devises such are spark detection in dust collectors and ductwork and explosion venting and suppression.
â OSHA requires a great detail on documentation which is one of the most important roles in Administrative Controls. Just like with any other safety and health regulation, OSHA requires written rules and procedures and wants to and ensure policies are fully understood and practiced by employees. For example, is there a method to prevent escape of fugitive dust? If there is escape of dust is there a policy to remove fugitive dust from surfaces? In addition to documentation various NFPA Standards have detailed proper methods of operating procedures, inspections, testing and maintenance procedures as well as training and states “safe work habits are developed and do not occur naturally” enforcing the importance of a detailed training program.
â One of the most important things any facility can do is fully engage in housekeeping and fugitive dust control. If underlying surface colors are not readily discernible there could be a dust deflagration hazard as mentioned in another NFPA document.
If you can see the dust, do not ignore it. Clean it up, but do not blow off with an air gun as that simply releases and stratifies the dust — use a vacuum to collect dust. Then investigate to determine the source of the dust. For example, if the ductwork is not airtight, seal joints to prevent the release of dust. When inspecting the workplace for dust accumulations again consider all flat surfaces including rectangular shaped ductwork, overhead beams, flat surfaced lighting fixtures, and all invisible areas such hung or suspended ceilings.
So while a combustible dust explosion might occur, the likelihood of an OSHA inspection is much greater. OSHA’s role is to protect the worker. And with the increase of combustible dust incidents OSHA is visiting woodworking facilities with greater frequency. Proper understanding of the enormous destruction of a combustible dust fire with the knowledge that an explosion can be mitigated by ensuring the proper steps are implemented will protect the workforce, facility as well as the industry.
NFPA is the National Fire Protection Assn., an International Codes and Standards Organization that creates voluntary consensus standards used by various organizations including AHJ (Authority Having Jurisdiction), which can be anyone from a Building Inspector to a Fire Marshall. There are several useful standards covering combustible dust published by NPFA. Some of the most relevant are:
â NFPA 61: Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities. Current Edition: 2013
â NFPA 484: Standard for Combustible Metals, Current Edition: 2012
â NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, Current Edition: 2013
â NFPA 655: Standard for Prevention of Sulfur Fires and Explosions, Current Edition: 2012
â NFPA 664: Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities, Current Edition: 2012
âAnd the newly proposed - NFPA 652: Standard on Combustible Dusts. Released this Fall is the first draft of the proposed NFPA 652 Standard on Combustible Dusts. This new standard will be a companion to the other combustible dust related standards. The goal of the new standard as proposed in the preliminary draft “is to provide safety measures to prevent and mitigate fires and dust explosions in facilities that handle combustible particulate solids.” The development of this standard will take approximately three years with a proposed release date of Fall 2015.
About the Author:
Jamison Scott is executive vice president and a third generation member of family owned industrial ventilation manufacturing firm Air Handling Systems located in Woodbridge, CT. With over 20 years of experience, he serves on the Technical Advisory Board for Air Pollution Control and Chairs the Industrial Dust Task Force for the WMMA (Wood Machinery Manufacturers of America). He holds an MBA and is a licensed sheet metal contractor in the state of Connecticut. For information contact: 203-389-9595; [email protected]; airhand.com
To keep current on combustible dust related issues visit: airhand.com/combustibledust.aspx
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