In this episode of the DustSafetyScience Podcast, we talk to Dr. Suzanne Smyth about a case study involving a grain dust explosion in a milling facility.
Suzanne is the managing engineer with Exponent’s Thermal Sciences Practice, based in Chicago, Illinois. She is a professional engineer, certified fire investigator and certified vehicle fire investigator. The case study that she discusses in this episode is covered in Lessons learned from a milling explosion, an article that she co-authored with Brenton Cox. Todd Hetrick and Dr. Russell Ogle, who is a previous guest on the podcast.
During the interview, she answered the following questions:
- What made you want to publish this incident as a case study?
- What were the processing operations at this mill?
- What caused the explosion?
- Why didn’t the dust collector properly vent outside?
- Why wasn’t isolation present?
What Made You Want to Publish This Incident as a Case Study?
Suzanne found this mill explosion investigation to be significant because the explosion caused a lot of damage but there was no evidence of fugitive dust being involved.
“While fugitive dust is important and can certainly cause large secondary explosions that result in significant damage, this was not one of those instances,” she said. “So it was interesting in that we were able to get these high levels of damage without any additional explosions outside the equipment.”
What Were the Processing Operations at This Mill?
The facility had two lines that milled wheat. The tempered wheat would come in, be diverted to these lines, and make its way to one of four sets of roller mills. There was also a hammer mill and an attrition mill. This area produced a wide variety of flour grades.
Each mill had its own cyclone up on the roof. The products would come down into a sifter (each line had one) that would size the particles and divert them either back into the process or into the finished product hoppers in the basement. A single large filter pulled off of all the cyclones on the roof while a different filter pulled off from the inlet temper bin area.
“They could route things all over the place depending on what specific product they were making,” Suzanne explained. “All of the controls for diverting the flow were manual. So slide gates and diverter valves, certainly the motors on the mills and that type [of] thing had a more complicated control panel system. But everything else as far as controlling flow was manual.”
What Caused the Explosion?
The chain of events leading to the explosion started the night before the incident. The miller on duty heard the belt squealing on the attrition mill and determined that it had choked, so he shut down the line, opened the mill up, and started cleaning out the choke.
When the shift changed, he left and the next miller continued with the cleaning process. Once he decided that it was sufficiently clean, he reassembled everything and went to turn the attrition mill back on.
At that time, someone else looked through a sight glass on the first floor and saw a flame traveling up the chute from the attrition mill, toward the cyclones. The explosion immediately followed.
Suzanne pointed out that when the attrition mill becomes choked, it imparts a lot of mechanical energy into the grain and heats it up. “They smelled an odor of burning. They had seen some discoloration. I don’t recall if they saw glowing or smoldering but they say they saw things that could indicate that heat was being transferred into the grain here.”
She added that the attrition mill was bulky and incredibly difficult to take apart and clean, especially if only one person was doing it.
“So given that we saw or we had this indication of flame moving out through the chute from the attrition mill, a possibility is that they didn’t get to a hot spot, that there was a smolder or an ember or something in that attrition mill that didn’t get cleaned out. And then when they energized it back and allowed fresh air to flow over it, that ember could get conveyed up into the cyclone and eventually the filter system.”
The investigation revealed discoloration and charring on the sifter as well as the filter system.
“We saw it on two of the cyclones: the attrition mill cyclone and one of the roller mill cyclones. There was some charred material product in the attrition mill cyclone, in the filter and in the cleaning house filter, which draws off the inlet bin.”
The distance between the attrition mill and the filter was significant. The mill was in the basement and the cyclone was on the roof, so the flame shot up five storeys to propagate.
“Then you’ve got the width of the building between all the different cyclones,” Suzanne added. “Then it comes back down. The filter is spanning the third floor. It’s between the second and third floors and so it’s got to come down another two, three stories from the cyclone. So there’s quite a bit of distance where this flame was able to propagate and speed up and kick up some more dust as it made its way.”
The filter itself had an explosion vent positioned at the exterior walls. It was sized appropriately based on the codes. However, the inspection door was blown off and there was evidence that flames had shot out. The floor was charred and one worker suffered burn injuries. Fortunately, he was able to eventually return to work.
Why Didn’t the Dust Collector Properly Vent Outside?
Suzanne pointed out that the system was sized for an event in the filter. When an event occurs away from the filter, the propagation can speed up and increase turbulence and pressure as it travels toward the filter.
“Then your initial pressure, if you’re doing the calculation in that filter, is going to be higher than normal, which is going to result in a much higher final pressure. After all, the dust within the filter can get involved. That was why, even though the explosion venting was up to code and sized appropriately, it was unable to fully vent this incident. It wasn’t sized for this incident.”
Why Wasn’t Isolation Present?
Suzanne acknowledged that isolation was important, but there were a lot of challenges involved in choosing an isolation or suppression technology for a food processing facility.
“Having a chemical suppressant enter into the food stream is not ideal. Even water can be really difficult and cause mold issues. So there are various choices to be made in that area.”
For her, the big takeaway was the need to find ways to minimize the likelihood of ignition events in cases where isolation is difficult. In this facility, there was no system to alert the miller of a flow blockage other than hearing the belts whine during routine checks. She recommended the use of flow or pressure sensors to detect blockages and alert the operators to a situation where the mills may be inputting too much heat to the product.
Conclusion
Given that this explosion occurred when the attrition mill was turned back on, there is a natural concern about all of the industrial equipment that is currently dormant due to the pandemic. Could something happen once the crisis passed and they are turned back on?
Suzanne recommended treating the reactivation like a startup. “Go through the checklist, go through the procedures. Think ahead about what each piece of equipment is going to be doing. A lot of these incidents do happen during start up or shut down or process upsets. It’s so important, especially now when places may be shut down unexpectedly, that the startups are done in a really thoughtful and systematic and logical way.”
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Suzanne Smyth directly:
Email: [email protected]
LinkedIn: https://www.linkedin.com/in/suzy-smyth-b384298/
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
DustSafetyScience:
Combustible Dust Incident Database
DustSafetyScience Podcast
Questions from the Community
2020 Digital Dust Safety Conference
Companies:
Exponent
Previous Podcasts:
DSS057: Performing Dust Explosion Risk Assessments with Dr. Russell Ogle
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DSS078: Case Study – Grain Dust Explosion in a Milling Facility with Dr. Suzanne Smyth
