A deflagration is the rapid propagation of a flame front through a fuel/air mixture at subsonic speed. An explosion is produced when the pressure from a deflagration ruptures an enclosure. In a woodworking facility, most deflagrations and fires are caused by woodworking, hot work, heating, and electrical equipment. These hazards are ever-present in woodworking facilities. They cannot be eliminated, but they can be mitigated by taking preventative action.
During a woodworking process, the five conditions required to produce a dust explosion are present (the first four are required to produce a deflagration):
- Combustible particulates having an equivalent diameter of 420 microns or less1,
- The suspension of such particulates at a sufficiently high concentration,
- An oxidant such as atmospheric oxygen,
- An ignition source, and
Frictional heat is generated when wood is processed (cut, milled, planed, sanded, etc.). Such heat can ignite combustible particulates being generated, creating a fire and explosion hazard. Dust collection systems are used to reduce employee health risks associated with dust inhalation. But they must also protect against ignited particulates. Requirements for these systems are detailed in the Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities (NFPA 664). Large dust collection systems are of noncombustible construction, located outside, and possess protective features that address the potential of deflagration and fire. Such features commonly include, but are not limited to, sufficient air velocities, deflagration relief vents and abort gates. Abort gates are used in conjunction with spark/ember detectors to prevent fire from entering a building through the return air duct.
Dust collectors are not completely efficient. The remaining dust accumulates on horizontal surfaces within the woodworking space. Thus, cleaning practices are required for all woodworking processes. To mitigate hazards, housekeeping should be implemented at a frequency that prohibits a layer of dust thicker than 1/32-inch (this depth can be increased under specific circumstances). At Such a thickness, enough particulates are available to become aggravated and billow, creating a hazardous dust cloud2. To further mitigate hazards, the surface of heaters should also be cleaned on a regular basis.
Hot Work Operations
Woodworking and combustible storage should be separated from hot work operations. When hot work is performed, pellets of molten metal can land on combustible material, creating a fire hazard. It can take hours for a fire to develop as the combustible material smolders. Thus, a fire watch should be maintained for at least two hours after the hot work is performed. Further guidance to safeguard hot work operations is detailed in the Standard for Fire Prevention During Welding, Cutting, and Other Hot Work (NFPA 51B).
Heating and Electrical Equipment
Heater inlet air should be ducted from an outside source. Otherwise, combustible particulates can become mixed with inlet air and ignite in a combustion chamber, possibly causing a fire that’s capable of affecting the woodworking space. This is especially true for gas-fired space heaters.
Electrical equipment should be appropriate for local hazards. The National Electric Code (NFPA 70) classifies three types of areas where fire or explosion hazards exist. These areas are categorized as Class 1, 2, and 3 locations. A Class 2 location is one that is hazardous because of the presence of combustible dust. It is further categorized into divisions and material groups. Generally, a Division 1 location is one which hazardous materials are present during normal conditions. A Division 2 location is one which hazardous materials are present during abnormal conditions. A Group G atmosphere contains combustible dusts not included in Group E or Group F, including wood. Thus, it’s common for equipment in woodworking spaces to be listed Class 2, Division 2, Group F. Dust-ignition-proof (Class 2) equipment is different than explosion proof (Class 1) equipment. However, equipment can be listed for both locations.
Fire and explosion hazards cannot be eliminated in woodworking facilities. But they can be mitigated by taking preventative action. Dust collection and proper equipment are crucial safeguards, but no more important than basic housekeeping. With mindful protocols, dangerous amounts of dust are kept within dust collection systems, and well designed systems control deflagrations and fires without endangering the lives of facility personnel.
Written By: Mark R. Richards, PE
1 Cholin, John M. “Woodworking Facilities and Processes.” Fire Protection Handbook. 20th Edition. Arthur E. Cote. Quincy: National Fire Protection Association, 2008. Page 9-13. Print.
2 NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids. Quincy: National Fire Protection Association, 2017. Page 14. Print.