What You Need to Know About ComDust

What You Need to Know About ComDust

From our friends at Wood Products – December 2012
What You Need to Know About ComDustby our friend Jamison Scott at Air Handling Systems

With the increase of combustible dust incidents, OSHA is visiting woodworking facilities with greater frequency

Combustible dust is a serious issue in the woodworking industry. Gaining national attention in 2008 when a major sugar refinery in Georgia exploded due to combustible dust, in 2012 alone, the woodworking industry saw a major sawmill in British Columbia, Canada, launch a fireball reportedly 60 meters high due to a suspected combustible dust explosion. Additionally a wood pellet maker in New Hampshire suffered a third combustible dust related incident.

While the occurrences and severity of combustible wood dust related fires have been increasing, the incidence of OSHA (Occupational Safety and Health Administration) inspections related to combustible dust in woodworking facilities has also increased dramatically.

Combustible Wood Dust Terms

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.

However, it is not simply defining a specific dust, it is determining its explosibility. Important factors include size, shape, moisture as well as environment.

The Kst value can be used to quantify the severity of a dust explosion. 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 being 500 microns (0.5mm, 0.0196 inch) or less and having a moisture content of less than 25 percent. Another way to measure, according to NFPA 664 (, is to see if the material will pass through U.S. No. 35 Standard Sieve, which is approximately the “size of fairly coarse sand.”

The word “wood” itself is defined by NFPA 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, the term “Wood-Derived Materials” is defined by NFPA as “sawdust, sander dust, planer shavings, hoggings, wood flour, and moulder waste.”

The Combustible Dust Explosion

In order to have a fire you must have “Fuel”, “Ignition” and “Oxygen,” the three main elements of the “Classic Fire Triangle.” For a combustible dust explosion, you must add “Dispersion” and “Confinement,” which together creates the “Dust Explosion Pentagon.” (See diagram page 34.)

With many larger combustible dust explosions there are two issues: the initial explosion and the secondary explosion, which typically comes from fugitive dust that becomes dispersed and suspended due to the disturbance caused by the initial explosion.

This is what happened at the Imperial Sugar explosion 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 earlier this year.

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 penalties. OSHA is taking this matter seriously, using the general duty clause in classifying combustible dust violations. One OSHA citation 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.” The layers of wood dust in this citation were in direct violation of the current “housekeeping” regulation in that it accumulated on I-beams, inside the trough of ceiling joists as well as on the floor.

The citation also found “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.” This citation specifically refers to NFPA 664 (2012) 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.

• Hazard Recognition/Assessment includes 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 the potential application of State and Local codes.

• Building Design & Engineering Controls cover “fixed structures that are built into a facility or processing equipment designed to remove or minimize a hazard.” Building Design focuses on the prevention of fugitive dust accumulation on surfaces, beams, etc., and all invisible areas such as hung or suspended ceilings. This is where good housekeeping is imperative. Engineering controls focus on the equipment such as dust collection systems or prevention devices such as spark detection in dust collectors, ductwork and explosion venting and suppression.

• OSHA requires detailed documentation, which is one of the most important roles in Administrative Controls. In addition to documentation, various NFPA Standards have detailed proper methods of operating procedures, inspections, testing and maintenance procedures, as well as training.

• 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 NFPA documents.

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. For example, if the ductwork is not airtight, seal joints. 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.

Jamison Scott is executive vice president and is a third generation member of family-owned industrial ventilation manufacturing firm Air Handling Systems 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 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; jscott@airhand.com; airhand.com. To keep current on combustible dust related issues visit: airhand.com/combustibledust.aspx

NFPA Standards

NFPA is the National Fire Protection Assn., an International Codes and Standards Organization that creates voluntary consensus standards used by various groups including AHJ (Authority Having Jurisdiction), which can be anyone from a building inspector to a fire marshal. There are several useful standards covering combustible dust published by NPFA. Some of the most relevant are:

• NFPA 61: Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities. Current Edition: 2013

• NFPA 484: Standard for Combustible Metals, Current Edition: 2012

• NFPA 654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, Current Edition: 2013

• NFPA 655: Standard for Prevention of Sulfur Fires and Explosions, Current Edition: 2012

• NFPA 664: Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities, Current Edition: 2012

• And 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 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.


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