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Is Your Combustible Dust Risk Greater Than You Think? What You Need to Know About Particle Size

November 29, 2017

Wood SawdustAs part of our effort to identify knowledge gaps surrounding combustible dust, we asked several experts what they see as the biggest misconceptions about this hazard. By far the most common answer we heard was that companies don’t understand the hazards present in their facilities.

Lewellyn Technology Sr. VP Jason Reason said: “Companies don’t understand their hazards, which leads to everything else — not doing proper testing, not doing engineering correctly, not doing a dust hazard analysis (DHA). Everything comes back to the fact that many people just don’t know they have combustible dust hazards in their facilities.”

One reason for this knowledge gap, according to Dr. Ashok Ghose Dastidar, Fauske & Associate’s VP of Dust & Flammability Testing and Consulting Services, is that many people don’t fully appreciate the role of particle size. Let’s clear up the confusion.

What size dust is considered combustible?

The definition of combustible dust has changed over the years. In the past, the National Fire Protection Association (NFPA), from which OSHA takes its cues, considered 420 microns to be the threshold under which dust was considered combustible. More recently, the guidance has shifted higher, to 500 microns, and in some cases the size requirement has been removed altogether.

For example, 2016’s NFPA 652: Standard on the Fundamentals of Combustible Dust defines combustible dust 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.” No mention of size.

In practice, what this means is that while micron size is still a reasonable guideline (anything smaller than 500 microns should automatically be considered combustible), larger particles can also fit into the definition of combustible dust.

How does segregation of material affect particle size?

Dr. Dastidar notes that many processes produce dust of different sizes — some coarser, some finer —  and that the segregation of this material can result in different severities of risk.

He explains: “Dust isn’t like gases and vapors. Methane gas is methane gas. But in a wood processing plant, you may have coarse sawdust with wood flour mixed in. This material can segregate so that you have all wood flour in some areas, like settled on light fixtures and beams. If there’s ever a disruption in the processing environment, this dust could disperse, creating a combustible dust hazard.”

How can you determine your risk?

The only way to determine with certainty whether your dust is combustible is to have it tested. This is a requirement under NFPA 652.

There are two ways to test dust samples: “as received” and after sizing and drying. “Companies want their dust tested ‘as received,’” Dr. Dastidar says, “But they don’t realize that even if just 5% of that material is less than 500 microns, if they’re handling large quantities (e.g., 1 or 2 tons of material), then 5% can represent hundreds of pounds of dust, which is big enough to pose a risk.”

That’s why Fauske & Associates recommends both. The “as received” test will help you understand how the material will behave in the absence of segregation. Sizing and drying the material will give you more conservative results, which will help you assess the risk of fines accumulating in hard-to-reach areas like elevated surfaces. Learn more about combustible dust sampling and testing in this article by Fauske Senior Consulting Engineer Ronald L. Allen.

Do you have questions about the combustible dust hazards in your facility? Give us a call or leave them in the comments! We’ll tap our extensive network of safety professionals to provide the latest information available.

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