DSS054: Dust Explosion And Combustion Test Methods With Dr. Ashok Dastidar

In this episode of the DustSafetyScience Podcast, we interview Dr. Ashok Dastidar, Vice President, Dust and Flammability Testing and Consulting Services at Fauske & Associates in Chicago, Illinois, about the dust explosion and combustible dust testing methods used to improve process safety in industries handling combustible dust.

Ashok has been on the podcast many times. His most recent appearance was in Episode 50 when he discussed dust explosion hazards in the pharmaceutical industry. He has extensive experience in the testing and mitigation of combustible dust hazards and contributed a chapter to the book [Affiliate] Methods and Chemical Process Safety Vol. 3 on Dust Explosions, which we reviewed in Episode 53.

In this episode, he will talk about:

The different stages of combustible dust testing
What the dust deflagration testing process looks like
The various testing methods and how they apply.

What Are The Different Stages Of Combustible Dust Testing?

Stages of Combustible Dust Testing | DustSafetyScience.com

Ashok’s chapter on combustible dust testing includes a graphic that depicts the four stages, which are outlined below.

Stage One: Material Identification

The first stage is the material identification. Do you actually have a dust? This part is easy if you have a known input, such as polyethylene powder for your process. It becomes harder if it’s a fugitive material or a waste material coming off of a process, such as paint being blasted from a piece of steel.

Stage Two: Sample Identification And Characterization

The second stage is sample identification and characterization. In the bead blasting example above, it would involve pulling samples from the cyclone and baghouse and characterizing them. What’s the particle size distribution? What is the particle morphology? What is the moisture content? Are there volatiles associated with it?

Stage Three: Screening

Stage Three has two potential steps:

3A: Measuring the reactivity of the dust cloud. This is where you have to do a go- no go explosibility screening test. This is based on standards such as ASTM E1226 and ASTM E1515.
3B: The reactivity of a pile or layer of material is being tested.

There are two ways to do this:

A burning behaviour test that originated in the VDI standard 2263. Classes 1, 2, and 3 are combustible but non-propagating classes. Class 4, 5, 6 are propagating combustion classes.
A burn rate screening test with a slightly larger pile of powder ignited in a very similar way to see whether the material propagates or not.

Ashok said that Stage 3 is the most critical stage for determining if the hazard exists or not.

“This is a good part where if you want to do DHA of a facility, you have a lot of dust you need to look at,” he said. “You find out which of your powders or dusts deflagrate and which of your powders combust and then start your DHA based on that. Then, after the DHA is completed, you’re looking for mitigation steps.”

Stage 4: Testing

At stage 4, you’re aware that the hazard exists: you have a material that can burn or trigger a flash fire or explosion. Now you need to determine what parameters are needed to design safety systems.

There are certain times where you might have the cloud that’s deflagrable,” Ashok explained. “But the pile of material may not show a burning behaviour at all and the dust definitely does create a deflagration. And so you might want to then tailor your mitigation steps, maybe focus it more on the deflagration aspect of it.”

In the remainder of the episode we spent most of the time discussing dust combustion tests as we have covered deflagration testing before. There are several different combustion tests, each aimed at particular applications.

Burn Rate Test

The burn rate test started as a U.N. D.O.T. transportation test, but other standards have adopted it too. A trough of material, usually about a foot long, is ignited at one end. Then you monitor it and calculate the amount of time it takes to burn from one end of the pile to the other or from one end of the trough to the other end.

Depending on the speed at which it burns, the U.N. guidebook on transportation of dangerous goods would put it into a shipping classification. You can use that same methodology to assess your own safety parameters at your company. If the material doesn’t really propagate, it’s easier to contain. If it does propagate, you have to look at containment mechanisms like extra sprinklers, fire barriers, or firewalls.

Layer Ignition Temperature

With this test, you are looking for the minimum temperature that will ignite a layer of material. This test is usually done per ASTM E2021, although there are VDI and ISO methods that are equivalent.

“If you’re still using mercury vapour lights as opposed to L.E.D. lights, those mercury vapour lights get very hot,” Ashok explained. “The question is: if you have the powder deposited on these mercury vapour light shields, can it get hot enough that those that layer of material will ignite?”

A key part of your ignition control strategy is to ensure that surfaces and bearings don’t get hot enough (or operating temperatures don’t get hot enough) to ignite layers or a dust cloud. The AST method uses the default value of 12 mms for the LIT test and the European method uses 5 mms, but in the end, the way it works in terms of the chemistry of self-heating is that the thicker the layer, the lower the LIT value.

Auto Ignition Screening and Hot Storage Screening

Although these are separate tests, Ashok said that they are similar in form. They are typically placed in a hot airstream or oven so you can observe whether the materials accelerate in temperature.

“You can ramp the oven and then see whether the material exhibits an exotherm,” Ashok said. “Does it does it accelerate and heat up and give off heat or does it give off a flame?”

The auto ignition screening test, which uses the Grewer oven, tests a smaller volume while the hot storage screening test uses a much larger volume in a larger oven. Ashok said he recommended using the smaller method first to see how violent the exotherm will be.

“If it’s too violent, you don’t want to scale up to the larger test because that could end up having a very energetic flame in your oven, which could compromise your facility, or maybe you’d have explosive effects and might actually create a deflagration, just not a cloud bit of the pile of material of its own. So you want to be careful with that.”

Dr. Cloney pointed out that if you know a given sample is going to burn exothermically over a 24-hour period, it can give you some indication of how long that material can sustain increased temperatures before it starts to react. In other words, if you are transporting material from Maine to Los Angeles and it is going to take a week, you will know what temperature to avoid.

Smoldering Gas

The smoldering gas evolution is typically used in applications like a silo or spray dryer. The material is slightly oxygen-starved in a heated environment. Maybe it is in a 50-gallon drum and being heated by a fire or is simply in a hot storage environment, like the back of a truck. Does it give off aromatics or maybe lightweight hydrocarbons, or would it give off carbon dioxide because it’s smouldering?

“When we’re talking about powders and dusts, we tend to ignore the aspect that in that smouldering environment and that smouldering step, those vapours that are given off could pool inside of a piece of equipment or structure,” Ashok said. “We know that vapours and gases typically have MIEs that are measured in tenths of millijoules as opposed to dust, which are measured in millijoules. So maybe you have taken precautions at your facility to avoid against millijoule level ignition energies. But did you protect against fractions of millijoules?”

This situation is often seen in dryers and silos. You have a burning ember that gets transported along either a pneumatic or chain conveyor and dumped into the silo. That ember embeds itself in the silo because it’s surrounded by material. Oxygen can’t necessarily percolate through the material to keep a very rapid or energetic combustion, so the result is a prolonged smouldering combustion with a lot of carbon monoxide. If the silo is improperly ventilated, the carbon monoxide can pool, resulting in a carbon monoxide explosion in the headspace.

The Combustible Dust Incident Database includes an incoming charcoal shipment that caused an explosion in Hong Kong.

Conclusion

Having your facility tested for combustible dust hazards and identifying the level of danger are critical to properly mitigating the hazards. The approach you take will vary based on application, but the goal is the same: preventing explosions and saving lives.

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

You can also reach Ashok Dastidar directly:
Email: [email protected] [email protected]
LinkedIn: https://www.linkedin.com/in/ashok-dastidar-26aa1514/

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.