Minimum Layer Thickness Requirements for Combustible Dust With Jeramy Slaunwhite - Dust Safety Science

In this episode of the DustSafetyScience Podcast, we talk to Jeramy Slaunwhite, Explosion Safety Engineer at REMBE, Inc. about minimum layer thickness requirements for combustible dust.

Jeramy has appeared on the podcast before: in Episode #4, he shared information about the Nova Scotia dust collector safety program. Today, he discusses minimum layer thickness, a topic that originated during a conversation he recently had with Dr. Chris Cloney.

Dr. Cloney regularly gives presentations on combustible dust safety. One of the things he tells the audience is that if they can see footprints in the dust, there’s enough of an accumulation to blow up their facility. It catches their attention- fast. With dust, a little build-up can do a lot of damage.

In this interview, Jeramy talks about:

Where the minimum layer thickness requirement comes from
Evaluative criteria such as density
How mass-based measuring methods can dictate housekeeping schedules
Other safety issues to consider in relation to dust accumulation

Where Does the Minimum Layer Thickness Requirement Come From?

Jeramy explained that in North America, the requirement is largely based in NFPA 654, specifically Chapter 6, Section 6.1. This section, which covers facility and systems design and hazard assessment, offers methods for calculating the amount of dust capable of creating a hazard, such as a flash fire or even full deflagration and explosion.

The section has an equation that includes different variables. For example, a room that measures 5 x 5 x 5 meters contains 125 cubic meters. The minimum explosible concentration for dust is around 50 grams per cubed meter, so, in that room, you would only need just over 6 kilograms of dust.

The point is that the bulk density is quite high, so if it’s 1000 or 1500 kilograms per meter cubed, the result will be a thickness of around two millimetres. This means that two millimetres of dust lying on the ground will give you enough fuel to have a large explosion.

Jeramy has some videos on YouTube that depict small-scale testing. He uses a six-inch-tall coffee can that is also six inches in diameter, adding up to 0.1 cubic feet. According to the math for explosible concentration, it would theoretically need about two-tenths of a gram to reach minimum explosible concentration.

Although it would be very difficult to homogeneously suspend 0.2 grams of dust in that kind of volume, especially on an amateur basis, Jeramy managed to still get a high-energy ignition with a low 0.8 grams of cornstarch being used. He then weighed out the same amount and distributed it on a six-inch circle drawn on a piece of black paper to visualize what that amount of dust really looks like spread over that kind of that area. The results showed that the dust amount was not very significant, although the resulting explosion certainly is.

What Are The Evaluative Criteria?

Bulk density is a primary consideration. An equation based on coal dust, which is fairly dense, might not be accurate for all materials. If you have a material that is a lower bulk density, the minimum layer for having a full explosion may be quite a bit thicker than one millimetre.

Examples of this kind of material include fine paper dust, especially in the tissue manufacturing industry. The dust is light and fluffy, so the layers would have a larger minimum thickness to have an explosible concentration.

“By this, I don’t want to say that it’s OK to let a lighter bulk density dust accumulate more because there are certainly hazards associated with it,” Jeramy explained. “But with that being said, when evaluating and using the layer thickness to understand what the hazards are, it definitely should be treated with the properties of the actual dust.”

With some dust, a millimeter lying on the ground will be explosible. However, the bulk density of some materials like paper dust might be a factor of 10 lower. If you apply the regular thickness criteria, you may be over conservative by a factor of 10. So instead of 1/32 or 1/16 of an inch, you may be looking at a quarter of an inch as the allowable thickness. If you’re doing a dust hazard analysis and use the bulk density of coal, which is done quite a bit, you can be over-conservative by quite a high amount for meeting the minimal explosible concentration throughout the facility.

While being over-conservative is not necessarily a bad thing, it’s important to understand the hazards by using real data and for what they are as opposed to being overcompensating to an extent where it becomes almost debilitating to the operation.

Jeramy noted that the layer depth criterion is based on about 5% of the total footprint area up to a maximum of 1000 square feet according to NFPA. It takes into account all the surfaces -the ducts, pipes, rafters, beams, and cable trays- that could accumulate dust layers. If it all adds up to 5% of the equivalent base area of the whole of the open space then that’s the threshold factor for adequate concentration. This is similar to the coffee can example Jeramy talked about earlier.

How Can These Mass-Based Methods Determine Housekeeping Regimens?

The ultimate goal is to manage combustible dust so that it doesn’t accumulate until it creates a hazardous situation. In order to do that, housekeeping is going to be paramount, along with dust collection systems dust extractions and other methods. It’s a matter of managing the interval between clean-ups. Scheduling and understanding those intervals are critical in managing the hazard.

One company Jeramy worked with calculated the volume of their areas and the explosible concentration of their dust to develop a program that allowed them to shut down all their equipment at certain intervals throughout the year and do thermal scans to look for hot spots to make sure there were no ignition sources. They would then weigh all the dust that they could gather, correlate it to the volume of the space, and relate that to the minimum concentration.

If they were within even a 10-time safety factor (10 % of the explosible concentration), they increased their cleaning efforts and, at their next scheduled interval, weighed the dust again to make sure they’re within the accepted range. If they were within it, they could go an interval or two by just doing normal cleaning procedures, but then they were required to weigh again on the next interval to make sure they were well within those factors.

Are There Any Other Safety Issues to Consider in Relation to Dust Accumulation?

Jeramy explained that a lot of the layer accumulations that end up being the most hazardous are the elevated ones on pipes, rafters, and similar structures. In the event of an initial disruption, pressure wave, or some sort of vibration these layers are dislodged and then rain down, creating a dust cloud.

He added that even a small thickness of a low bulk density dust can be easily disrupted by a broom or other object dropping something on the floor. The resulting air current could lift dust up and create a very significant flash fire or a dangerous localized calibration.

With the Westray Mine, the explosion propagated three kilometres from the ignition point. It went underneath the highway 102 going up to Cape Breton and exited the entrance of the mine, all the while picking the dust up off the ground. With buildings that fail, as was the case with the Imperial Sugar Refinery, dust can fall down through cracks in the floors and create add fuel to the explosion.

Conclusion

“There are a lot of different rules of thumb and an assumption for minimum layer thickness,” Jeramy said. “Some are stated in NFPA 664 for the wood processing standard and 650 for the general combustible dust standard. NFPA 654 Section 6.1.31 specifies 1/32 of an inch of 0.8 millimetres as the threshold criteria. Whether you can see the colour of the surface below or write your name in the dust are also benchmarks. There are a lot of different evaluative criteria on how much dust is too much dust.”

With so much variance, the best rule of thumb appears to be regular DHAs, an understanding of what’s safe for your facility, and always erring on the side of caution.

If you would like to discuss further, leave your thoughts in the comments section below.

You can also reach Jeramy Slaunwhite directly:
Email: [email protected]
LinkedIn: https://www.linkedin.com/in/jeramy-slaunwhite-04b30019/

If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode