In today’s episode of the Dust Safety Science podcast, we address a question that arrived at our Dust Safety Help Desk: can the fan be used as an explosion isolation device?
Moving forward, we will have a number of shorter episodes covering questions and answers originating from the Dust Safety Help Desk. We currently have over 200 questions logged and will be featuring answers in future episodes, along with our usual commentary and interviews.
Fans as Explosion Isolation Devices
This question came in via email. The requestor says, “I’m reaching out to you again in hopes that you can point me in the right direction for some information I’ve been trying to find on exhaust fans not being a reliable form of isolation for an explosion, for a flame propagating in the ductwork.”
This is coming from an explosion protection technician who understands that the fan is not an isolation device. However, they’re having a hard time convincing their client that this is not true. Therefore, they’re looking for support information; something demonstrating the fact that this is not an isolation device.
Let’s start with the basics. What does NFPA 69 say about fans?
NFPA 69 is the standard on explosion prevention systems. If you open it up and actually search for the word ‘fan’, it doesn’t come up as an explosion isolation device. Why aren’t they mentioned?
To answer that question, let’s look at what NFPA 69 actually says about passive explosion isolation.
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- Chapter 12.1.1 says that the technique of deflagration isolation by passive means shall be permitted for interruption or mitigation of flame, deflagration pressures, pressure piling and flame jet ignition between enclosures that are interconnected by pipes and ducts.
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- Chapter 12.1.3 states that the piping, the ducts and the enclosure protected by an isolation system shall be designed to withstand the estimated pressures as provided by the isolation system manufacturer.
Fans fail at least two of these criteria. They don’t stop the explosion propagating from one side to the other and they’re not likely to withstand the pressures necessary during isolation. Even if they manage to stop the flame from spreading, they’ll probably blow apart.
If a client isn’t convinced by the NFPA 69 argument, you might be able to claim pressure rating issues. If you can identify the pressure rating for the fan and show that an isolated explosion would have a pressure of so many bars and the fan isn’t going to withstand those pressures, the client may be persuaded.
Answers From the Dust Safety Community
When we presented the question to members of the Dust Safety Academy, we got some great responses.
Chris Giusto from Hallam-ICS wrote, “NPPA 69 is pretty clear on what the requirements are for various types of isolation devices, and a fan does not meet any of them.
“For the sake of argument, let’s assume that a fan could function as a passive mechanical device. NFPA 69 requires these devices to close automatically because of the pressure wave preceding the flame front. There’s nothing in a fan that’s going to close and provide a complete barrier against the flame front. In fact, a pressure wave is likely going to have the opposite effect on the fan. I can’t imagine how a fan might meet any of the requirements for flame front diverter or rotary valve either.”
He discusses tolerances for rotary valves and says that fans don’t achieve anything similar. It probably would cost a small fortune to build a fan with those kinds of tolerances. He said he would recommend just protecting the fan with an isolation device, not using it as one.
Another DSA member stated, “In the end, after explaining why a fan isn’t an acceptable mitigation measure, it might be best for your career to lose this client.” In other words, if you can’t convince them that fans are not built to isolate explosions, it may be better to part ways with them, as there could be some liability issues if you move forward with that design.
Related Safety Considerations
Another interesting discussion arose from the Dust Safety Academy on this issue. It revolved around the question of whether a dust explosion could propagate against process flow.
One person referred to a Fike white paper called ‘Dust Explosion Propagation, Myths and Realities.’ In that paper, they talk about some research published under the title ‘Flame Propagation in Pipes of Pneumatic Conveying Systems & Exhaust Equipment.’ It demonstrates that a dust explosion can propagate both with the flow and against the flow and show test results from the Fike Facility down in Kansas City, where they have a closed ducting system with flow being pulled through it or pushed through it.
The article states that if you have an extraction rate of 15 meters per second, a dust explosion might propagate around 10 meters a second depending on the conditions. What if it’s turbulent? What about when the fan shuts off? These are the kinds of reasons why a fan isn’t an appropriate safety device.
Jeramy Slaunwhite from REMBE mentioned that there are several cases of upstream flame propagation and things like fuel in the ducting can also cause an issue. If the fan does stop and you have all that fuel suddenly in the ducting, that can be an area where the explosion propagates up flow. Similarly, if you have fuel that’s actually just sitting there because you don’t have the correct flow velocities in your ducting, that’s going to be an issue as well.
One DSA member said, “This seems to me that the wrong fan, specifically the motor, could easily be an ignition source rather than an isolation device.”
Conclusion
It was really interesting seeing this question come in, and we were especially glad that we could answer it and, hopefully, make the facility in question safer. If you have a question, you can go to the Dust Safety Academy and submit it in the Help Desk there, send it through DustSafetyShare.com, or email Chris Cloney at [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
Dust Safety Professionals
Standards
NFPA 69
Companies
Hallam-ICS
Fike
REMBE
White Papers
Dust Explosion Propagation, Myths and Realities
Flame Propagation in Pipes of Pneumatic Conveying Systems & Exhaust Equipment
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