In this episode of the DustSafetyScience Podcast, Dr. Ashok Dastidar, Vice President of Dust and Flammability Testing and Consultancy Services at Fauske & Associates, and also a Fellow Engineer with Westinghouse Electric Company, has returned to talk about how to incorporate decomposition, off-gassing and hybrid mixtures into your dust hazard analysis.
Ashok has an extensive background in combustible dust testing and dust hazard analysis. He has chaired the ASTM E27 committee on hazard potential chemicals and is a member of several NFPA technical committees. In today’s discussion, he answers the following questions:
- What are some of the increased hazards of mixing flammable gas with combustible dust in a hybrid mixture?
- How do hybrid mixtures appear in industrial scenarios?
- What risks should be looked for during a DHA?
- How can managers tell if their materials might be prone to decomposition or off-gassing?
- How can the possibility of a hybrid mixture be incorporated into safety design or explosion protection?
What are some of the increased hazards of mixing flammable gas with combustible dust in a hybrid mixture?
Combustion occurs when a fuel reacts with an oxidizer, such as oxygen in the air, creating a flame that propagates. With vapors and gases, a molecule of methane, methanol, or benzene reacts with the equivalent number of oxygen molecules present, creating a flame front and reaction. So it can happen very rapidly because it’s a molecule to molecule reaction.
Ashok explained that with a dust explosion, a solid particle reacts with the oxygen. This oxygen has to come to the surface of the solid particle to react, or that surface material has to boil away, vaporize away, or react away, mix with the oxygen, and then combust.
A hybrid mixture is defined as the presence of a vapor gas greater than or equal to 10% of the LEL or LFL of that gas present and dust that is greater than 10% of the MEC of that particular dust. There is a combination of readily available volatiles, gasses reacting with the oxygen, and the mass loading of solid fuel liberating material from the surface, mixing with air and reacting.
“Your explosion severity is much greater than if you’re looking at the pure dust alone,” Ashok said. “Your ignition sensitivity, your MIE, is much lower than if you had the pure dust alone. Your MEC is much lower. Your LOC can also change as well or be modified by the fact that you have a fuel loading. Unless you know what to look for, you might miss it.”
How do hybrid mixtures appear in industrial scenarios?
Ashok admitted that people often misread hybrid mixtures, which can be introduced into industrial scenarios in a variety of ways. For example:
- A pharmaceutical product being crystallized out of solution that might have contained ethanol or methanol. Residual solvent could stick to the dry particle.
- During a precipitation and filtration process, solvent wet powder that dries could escape as fugitive material or enter into a dust collector.
- A resin powder is introduced into a vat of toluene in order to thicken it or to create a reaction.
One of the possibilities that people rarely consider is the powder itself undergoing partial combustion or some sort of decomposition. A lot of times, spray dryers develop crusts or hangings like stalactites at the very top. If a smoldering nest occurs inside them, it can generate gases, carbon monoxide in particular, that are then acting as a source for additional hybrid mixture possibility.
“Certain materials, if you store them long enough in a certain location, they have a tendency to self-heat,” Ashok explained. “There are a lot of chemicals that do self-heat just sitting out in the environment, maybe slightly elevated temperatures. Then you have an upset scenario where some of the powder spills, and now you have a hybrid mixture. So these things have to be theorized or looked at in a ‘what if’ type environment and ‘what if’ type of scenario when you’re doing a DHA.”
What risks should be looked for during a DHA?
Ashok recommended that anyone doing a DHA determine if solvents are present. If so, are they using a flammable solvent or a combustible solvent? Is it present at any time or are hybrid mixtures possible?
The next question is whether the facility tests for hybrid mixtures. Do they have an LEL meter in the room, anywhere in the vat, or in the dust collector? Do they monitor to see if there are any hybrid vapors or gases being given off?
Another consideration is thermal decomposition. Does the facility experience smoldering in its silos or storage bins? Are fires a frequent occurrence? Even if the answer is no, any scenario where smoldering or partial combustion could occur always merit further investigation.
Ashok recommended that facilities hiring outside consultants to do their DHAs work with the consultant to ensure a thorough review and analysis.
“You’re asking for an outside third party to come in. At that point, they’re not necessarily totally familiar with your specific process. They might have audited other processes that are very similar, but they don’t know exactly what your near miss scenarios have been in the past. You know all about those. So if you don’t bring those to the forefront when you’re going through one of these reviews, the assessor or the auditor would have no idea to bring that up.”
How can managers tell if their materials might be prone to decomposition or off-gassing?
Certain screening tests can be run to determine whether a material is prone to decomposition or off-gassing.
- The Lütolf Oven test puts the material in a sealed test tube and heats it up to test for an exothermic reaction. The off-gasses are collected and tested for flammability.
- A thermal gravimetric analysis (TGA) makes it possible to see where mass loss is occurring, capture the gases, and identify their chemical composition.
How can the possibility of a hybrid mixture be incorporated into safety design or explosion protection?
Ashok recommended that facility owners first get their dust tested as is.
“Then you might have to go back and say, “Looking at my upset scenario or my failure scenario, I expect that I’m going to have some solvent present and the solvent could be present in this concentration in the environment.””
After the appropriate calculations are done, the manager can construct the tests for KST, MEC, LOC, and MIE with that solvent atmosphere accounted for and measure the explosion or ignition sensitivity with solvent being present at that hypothesized concentration.
NFPA 68 recommends a placeholder of a Pmax of 10 bar and a KST of 500, but Ashok warned that these ratios could be overly conservative for certain systems.
“Your dust might normally be a low ST one, like a 100 or an 80 bar meter per second, and maybe your overpressure is six bar. Having a hybrid mixture of methanol present might, in a real world scenario, never approach a 10 bar overpressure or KST of 500. But if you’re going to use those placeholder values as NFP 68 recommends, you might be over-engineering your system.”
Conclusion
Ashok recommended that facility owners assemble the right team for a DHA that incorporates decomposition, off-gassing, and hybrid mixtures. With so many variables and potential upset conditions to account for, a well-rounded approach that includes technical knowledge and experience will yield the best results.
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Ashok Dastidar directly:
Website: https://www.fauske.com/
LinkedIn: https://www.linkedin.com/in/ashok-dastidar-26aa1514/
Email: [email protected]
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.
Resources Mentioned
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Dust Safety Academy
Companies
Fauske & Associates
Westinghouse Electric Company
Standards
ASTM E27
Previous Podcast Episodes
DSS007: Dust Hazard Analysis and Explosion Prevention with Dr. Ashok Dastidar
DSS050: Dust Explosion Hazards in Pharmaceutical Industries with Dr. Ashok Dastidar
DSS054: Dust Explosion And Combustion Test Methods With Dr. Ashok Dastidar
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