In today’s episode of the Dust Safety Science podcast, we talk about challenges using the rule of thumb method for explosion venting requirements.
As we said in last week’s podcast episode, in 2022 we’re going through some of the questions that we get through our Dust Safety Help Desk system and answering them on the show. This question came in from an engineer working with a dust collection company. He received a question from the explosion protection design engineer that’s working on the installation of their system. The question related to the discrepancy between local code and the approach that the designer used, which was based on the 2013 edition of NFPA 68.
In summary, the local code document stated that dust collectors shall be located outside the building and shall have explosion venting outdoors of no less than 0.1 square metre vent area for each cubic meter of the dust collector enclosure volume. These guidelines represent a rule of thumb method for explosion venting size requirements. What is the difference between this local venting requirement and what would be required by NFPA 68 for explosion venting?
For this question, we contacted an explosion protection expert who does a lot of work in the local area. In answering this Help Desk question, they discussed three challenges.
Challenge #1: Local Codes Always Lag Behind the Reference Standards
The subject matter expert (SME) stated that if you have a local fire code or a local building code, it’s always going to lag behind what it uses as its model or reference code standards. Some codes will reference or use the latest up-to-date version of the reference standard and other ones may not, depending on the sort of legal systems that are set up in place in the countries that have those local codes.
The SME encouraged the individual to look through the whole code. In this case, the code wasn’t very big, which was good. Almost immediately, you could see some clauses that would require the adoption of better information about handling combustible dust.
In particular: dust collection systems are required to be designed in conformance with good engineering practices. In other words, the explosion venting must conform with good engineering practice, such as those detailed in NFPA 68: explosion protection by deflagration venting. If you are a designer and you see these sorts of clauses, you need to use good engineering practices. Otherwise, you may run into liability issues if something goes wrong.
Challenge #2: The Rule of Thumb Method For Venting Requirements Is Generally Not That Accurate
The SME stated that “the rule of thumb ratio method of vent sizing is archaic, highly generalized, and honestly, could be highly inadequate. It doesn’t take into account KST, PMax, Pred or L over D, for example, and could have highly different vendor requirements depending on these parameters.”
An example calculation was sent using some sort of off-the-shelf software or internal software. For one cubic metre volume with a Pred of .2 bar and L over D of less than two, (basically a square or something that does not doesn’t have a high L over D ratio) a KST of 150 bar meter per second gives exactly 0.1 one meter square vent area. But what if those variables change?
If you have a Pred that’s different or an L over D that’s greater than two or a KST that’s 300 or 450 or whatever it’s going to be under these conditions, then you’re going to have undersized venting requirements which can be dangerous when an explosion happens.
That’s the challenge around the rule of thumb method for venting requirements – it’s just not that accurate. There are better methods available today.
Challenge #3: Using An Outdated Version of the Reference Standard Can Cause Difficulties
The SME mentioned that using an outdated version of the reference standard can cause challenges. He said that due diligence and good engineering practice would be to apply the methodology of the most current version of NFPA 68. So the question is: why are we referring to the 2013 version of NFPA 68 in 2021 or 2022?
In the 2018 version of NFPA 68, there are some new requirements – the volume occupied by the filters has to be included as part of the full dirty side volume, L over D be included irrespective of where the vent is placed, and there’s a 25% vent area correction when vents are located entirely beside flexible filters in case those filters flex and move and shut and close off the vent area.
This ties back into being in conformance with good engineering design practices – it generally means using the most up-to-date versions of the standards because those requirements matter.
Conclusion
This is another question that received an enlightening and practical answer from an expert in the field. 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
Dust Safety Share
Standards
NFPA 68
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