Defining combustible dusts: does particle size matter?
January 18, 2021
By Eric Brideau
In a processing or manufacturing facility that handles wood or agricultural materials, there can be solid particulates and dusts with varying particle size distributions that exist throughout the process, some of which may be combustible dusts that are explosible in dust cloud form.
But what exactly is a combustible dust, and how can you determine if combustible dust hazards exist in your facility?
Conditions for explosions
First, it’s critical to understand the conditions that can cause combustible dust explosions.
Many wood and agricultural materials are not easily burned in their raw form but may be explosible in dry particulate or dust form. This is because of the reduced moisture content and increased surface area available for reaction and diffusion mechanisms during the combustion process. These materials are known as combustible dusts and can cause dust explosions in processing facilities if:
- the dust particle size is small enough to propagate a flame front
- there is a means of suspending or dispersing the dust in air or other oxidizing atmosphere
- a sufficient quantity of dust exists to achieve the minimum explosible concentration
- a source of ignition exists, and
- there is a sufficient degree of confinement such that damaging overpressure may develop because of the rapid increase in temperature associated with the combustion process
If there is no or little confinement, a dust flash fire may occur, which can still cause injury or damage to equipment and property.
Defining combustible dust
Back to the original question – what exactly is a combustible dust, and how can you determine if combustible dust hazards exist in your facility?
Definitions for combustible dust are provided in various codes and technical standards, among other sources, but can vary across these sources making it somewhat difficult to clearly define what a combustible dust is.
Most notably, there are some technical standards that use a specific particle size limit as a criterion while others provide a broader definition. For example, combustible dust is defined without a particle size limit in NFPA 652 (2019), NFPA 68 (2018), and NFPA 69 (2019), along with other NFPA standards, as “a finely divided combustible particulate solid that presents a flash fire hazard or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations.”
Similarly, NFPA 77 (2019) defines a combustible dust as “a combustible particulate solid that presents a fire or deflagration hazard when suspended in air or other oxidizing medium over a range of concentrations, regardless of particle size or shape.”
Occupational Safety and Health Administration (OSHA) also uses a similar definition to NFPA 77 with slightly different wording, but emphasizes that a dust can be combustible regardless of size, shape, or chemical composition.
The definition provided in NFPA 499 (2021), however, uses a particle size criterion and defines combustible dusts as “dust particles that are 500 micron or smaller (i.e., material passing a U.S. No. 35 standard sieve as defined in ASTM E11-17), and present a flash-fire hazard or explosion hazard when dispersed and ignited in air.” Similar definitions with a 500-micron particle size limit are used in the U.S. National Electrical code (NFPA 70, 2020) and the Canadian Electrical Code (CSA C22. 1-18).
For facilities that process wood and agricultural materials, industry-specific NFPA standards exist that provide slightly different definitions for the specific combustible dusts handled.
For example, NFPA 664, which is an industry standard specific to the prevention of fires and dust explosions in wood processing and woodworking facilities, does not provide a definition for combustible dust and instead provides a definition for deflagrable wood dust. However, the term combustible dust is used interchangeably with deflagrable wood dust throughout the standard. NFPA 664 defines deflagrable wood dust as “wood particulate that will propagate a deflagration fame front, when suspended in air, or the process-specific oxidizing medium, in sufficient concentration, thus presenting a deflagration hazard.”
Similarly, NFPA 61, which is an industry standard specific to the prevention of fires and dust explosions in agricultural and food processing facilities, provides an industry-specific definition for agricultural combustible dust, which is defined as “any finely divided solid agricultural material that presents a flash fire hazard or explosion hazard when suspended and ignited in air.”
These industry-specific NFPA standards that are applicable to biomass materials do not specify a particle size limit when defining deflagrable wood dust and agricultural combustible dust, like many of the other technical standards that provide general definitions for combustible dust.
Does particle size matter?
Although a particle size limit of 500 microns is not used as a criterion to define a combustible dust in many of the relevant NFPA standards, such as NFPA 644 and NFPA 652, these standards explain that, typically, it is unlikely that particulates will be combustible when the particle size is greater than 500 microns due to the small surface-to-volume ratio.
However, these standards also recognize that when particles deviate from a spherical shape, such as for flat platelet-shaped particles, flakes, or fibres with lengths that are large compared to their diameter, they may not pass through a 500-micron sieve, but could still pose a deflagration or explosion hazard.
In reality, there is no single particle size limit that can define combustible dusts as chemical composition, in addition to physical parameters such as particle size and particle shape, affect the upper limit of the particle size in which a given dust will be combustible.
Therefore, from a dust explosion prevention and mitigation perspective, a definition of combustible dust that does not define a particle size limit is more appropriate for the identification of combustible dust hazards. Under such a definition, potential hazards are not erroneously screened out from further hazard analysis or risk assessment based on particle size.
For example, if a 500-micron particle size limit is used for preliminary screening of potential combustible dusts handled at a facility based on sieve analysis, a combustible material that does not pass through a 500-micron sieve could be wrongly classified as a non-combustible – even though the particulates are small enough to propagate a flame front and may be explosible in dust cloud form. If it is assumed that no hazard exists from the material during the hazard identification stage, further evaluations will likely not be performed to determine the prevention and mitigation measures required to manage the existing process-specific risks, which may leave process equipment and building areas vulnerable to dust deflagrations or explosions.
Identifying combustible dust hazards
So, if combustible dusts can’t be definitively identified based on particle size, how can you determine whether the specific wood or agricultural materials handled at your facility present combustible dust hazards and are explosible in dust cloud form?
As a starting point, the materials should be tested based on an informed dust sampling and testing strategy, using standardized equipment and procedures such as those developed by ASTM International.
By testing the dusts at your facility, you can determine whether your dusts have the potential to cause an explosion, as well as characterize the ignition sensitivity (i.e., how easily the dust is ignited) and explosion severity (i.e., how violently the dust will react) of the combustible dusts.
Once the combustible dusts within your facility are characterized, a Dust Hazard Analysis (DHA) should be performed to ensure that combustible dust hazards are appropriately identified and are being managed in accordance with the applicable codes, standards, and engineering guidelines with respect to explosion safety requirements. A DHA is a systematic review of potential fire, flash fire, and explosion hazards associated with combustible dusts that will help ensure equipment is installed in compliance with good engineering practice guidelines and that a proper level of protection exists to prevent combustible dust explosions from occurring and mitigate the severity and consequences of a dust explosion should one occur.
Eric Brideau is a project consultant in the Industrial Process Safety group at Jensen Hughes. His technical expertise is focused on industrial process safety and explosion protection engineering, including the performance of Process Hazard Analyses (PHAs), Dust Hazard Analyses (DHAs), and Hazardous Area Classification Studies (HACs), as well as the use of various bench-scale laboratory apparatuses used to characterize the explosion severity and ignition sensitivity of combustible dusts. Brideau is also currently completing a Masters of Applied Science (MASc) in chemical engineering at Dalhousie University with an ongoing research project being performed in co-operation with the Wood Pellet Association of Canada (WPAC) to improve process safety in wood pellet manufacturing facilities through the use of Inherently Safer Design (ISD) principles.
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