DSS115: NFPA Definitions of Combustible Dust with Eric Brideau

In today’s episode, Eric Brideau, Project Consultant at Jensen Hughes, based in Halifax, Nova Scotia, discusses NFPA definitions of combustible dust.

Eric is also a Master’s student in Chemical Engineering at Dalhousie University, where he is involved in a project funded by the WorkSafeBC Innovation at Work program. This project, whose participants include Dust Safety Science, the Wood Pellet Association of Canada and the BC Forest Safety Council Group, is about the integration of inherently safer design into hazard analysis- specifically, using bow tie diagrams and bow tie type analysis.

Eric put together a short write-up on different combustible dust definitions within the NFPA guidelines. The idea arose from internal discussions about the technical definition of a combustible dust, particularly with respect to physical and chemical characteristics.

“We were trying to define the top event from a combustible dust hazard, and what the consequences of dust explosions are,” he says. “We were trying to figure out how exactly to define top event and how exactly to define what a combustible dust is.”

He pointed out that the broad definition is widely understood: small particulate solids that ignite under certain conditions. However, the definitions provided for combustible dust in various codes and technical standards vary to some degree.

“Most notably, there are some technical standards that use a specific particle size limit as a criterion, while others provide a broader, more general definition,” he says.

NFPA Combustible Dust Definitions

Eric found that most of the NFPA standards have similar if not identical definitions of combustible dust.

“Some of these would be the NFPA 652 standard on fundamentals of combustible dust and NFPA 654, 68, 69. They all define combustible dust without a particle size limit. So the exact definition given is a finely divided combustible particulate solid that presents a flash fire hazard or explosion hazard when suspended in the air or other process-specific oxidizing over a range of concentrations.”

In addition:

- NFPA 77 emphasizes that particle size is not a requirement. It states, “Regardless of a particle size or shape.”

- OSHA says, “Combustible dust can be combustible regardless of the size, shape or chemical composition.”

- NFPA 499 uses a particle size criterion of 500 microns or smaller.

Similar definitions are also provided in the U.S. National Electrical Code, which is NFPA 70, and the Canadian Electrical Code, which is CSAC22118. They also specify a 500-micron particle size.

“Really, there’s no single particle size limit that you can use to define combustible dust, chemical composition, and other physical properties, such as particle size and particle shape,” Eric says. “They do affect the upper limit for particle size that will be combustible, but it’s different for every dust. And when particle size distribution changes, when the shape changes from the chemical changes, this upper limit changes as well.”

NFPA 664, which is related to Eric’s project with the Wood Pellet Association of Canada, is the standard for the prevention of fires and dust explosions in wood processing facilities. This standard doesn’t provide an explicit definition for combustible dust. Instead, it refers to a deflagrable wood dust, which is defined as wood particulate that will propagate a deflagration flame when suspended in the air or other process-specific oxidizing medium in sufficient concentration.

International Combustible Dust Definitions

During his research, Eric reviewed a few international combustible dust definitions.

- The latest version of IEC 60079-10-2 defines combustible dust as finely divided solid particles, 500 microns or less in nominal size, which may form an explosive mixture with air and temperature, or atmospheric pressure in normal temperatures.

- The 2018 edition of the International Fire Code defines combustible dust as finely divided solid material, 420 microns or less in diameter – in which, when dispersed in the air in proper portions, could be ignited by flame, spark or other emissions.

“But then, weirdly enough, the International Fire Code 2018 says that the requirements of NFPA 652 apply to all new and existing facilities,” Eric says. “DHAs must be performed in compliance with NFPA652. So in that sense, there would be a difference in the definition for combustible dust, between the two standards.”

Conclusion

Eric recommended that all facilities get their dust tested, bearing in mind the type of dust that they produce.

“Perform a DHA and determine what the actual risks are for the facility and specific equipment,” he advised.

If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Eric Brideau directly:

Email: [email protected] or [email protected]
Website: https://www.jensenhughes.com/services/combustible-dust-safety
LinkedIn: https://www.linkedin.com/in/eric-brideau-b56256b8/

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