Certain materials and products, such as paper, wood, flour, metal, coal, plastic, cotton, and rubber that may combust in powdered form are extensive and include a wide range of materials. Some materials, such as Zinc or Potatoes are not combustible in larger pieces but can be when pulverized to fine dust. The US Occupational Safety and Health Administration (OSHA) maintains a comprehensive list of combustible materials.
The NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, provides guidance on combustible dust suppression and dust control. The NFPA 654 Standard is referenced by OSHA’s Combustible Dust National Emphasis Program (NEP) to identify dust hazards and define mitigation strategies that help protect life and property. The standard provides industry-recognized safety practices for facility and systems design, process equipment protection, fugitive dust control and housekeeping, ignition source identification and control, fire protection, training and procedures, inspection, and maintenance.
The Occupational Safety and Health Administration (OSHA) in the United States defines combustible dust as “a solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition, which presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations. Understanding these characteristics is vital for identifying combustible dust hazards, managing the risks associated with combustible dust, ensuring workplace occupational safety and compliance, and preventing a combustible dust explosion.
Evaluating the Combustibility of Dust
Determining the combustibility of dust present in your facility requires professional evaluation through laboratory testing. This process involves igniting a sample of the dust in a controlled environment and analyzing key parameters to assess the potential explosion risks.
Two critical elements of this analysis are the Kst value and the Pmax value. The Kst value measures the severity of an explosion that could occur if the dust ignites, taking into account factors such as particle size, shape, and moisture content. Meanwhile, the Pmax value indicates the maximum pressure generated by a potential dust explosion.
The Dust Explosion Classes
Combustible types of dust are divided into four classes: St 0, St 1, St 2, and St 3. A high Kst value will rank highly on the class scale, indicating a potential explosion. Similarly, a Kst value of 0 means there is zero risk of a combustible dust explosion.
St 0: Kst value 0 – no explosion. Typical of silica dust, welding-generated dust, and thermally generated dust (from cutting processes, for example).
St 1: Kst value >0-200 – weak explosion. Typical of charcoal, powdered milk, sugar, sulfur, wood dust, and zinc.
St 2: Kst value >200-300 – strong explosion. Typical of cellulose, wood flour, poly methyl acrylate (PMA).
St 3: Kst value >300 – very strong explosion. Typical of metal dusts such as aluminum, magnesium, and titanium.
About Dust Safety Science
For more from Dr. Chris Cloney on Combustible Dust Suppression, visit this podcast episode: Dust Explosion And Combustion Test Methods With Dr. Ashok Dastidar.
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