This episode of the Dust Safety Science podcast is Part Two of a three-part series on NFPA 652. Dr. Chris Cloney recently presented on this topic at the 125th Annual Conference of the NFPA. In the last episode, we answered three of the five common questions about NFPA 652, and in this one, we discuss the last two, namely:
- What does the DHA process look like?
- Who is a qualified person?
What Does the Dust Hazard Analysis (DHA) Process Look Like?
Dr. Cloney answered this question by going through a mockup of a facility. His example was derived from several different resources, including a great book from the Center for Chemical Process Safety (CCPS) called Guidelines for Combustible Dust Hazard Analysis.
Section 7.2.1 in NFPA 652 is an overview of DHAs. It says that the DHA shall evaluate the fire, deflagration and explosion hazards and provide recommendations to manage those hazards in accordance with Section 4.2, which is the compliance section.
This is the bare minimum: evaluate the hazards and provide recommendations. It doesn’t necessarily include risk ranking, an implementation plan, management systems, hazardous area classification or explosion protection design, which are all things you might see in a DHA.
Even at its most basic, a DHA is a six-step process:
- Step 1 is to determine if combustible dust is involved.
- Step 2 is to break the processing operation into a list of nodes.
- Step 3 is to add nodes for rooms, buildings and building enclosures.
- Step 4 is to evaluate the fire, flash fire and explosion scenarios at each node.
- Step 5 is to evaluate existing safeguards available at each node to prevent those hazard scenarios.
- Step 6 is to document the hazard scenarios, the existing safeguards and recommendations against prescriptive requirements.
As an example, we’re going to walk through a facility similar to the milling facility we discussed in Episode #78 with Dr. Suzanne Smyth. The explosion that occurred there started as a fire in the attrition mill. That fire was cleaned out after the process was turned off. Upon starting up again, a deflagration occurred and propagated through the ducting up to the cyclone on the roof back into the dust collection system, where it caused an explosion in the dust collector.
The dust collector was properly vented as per NFPA requirements, but it was designed for an explosion that starts within it. Because this explosion propagated five storeys upward, then a couple of hundred feet across the building, and then back down a couple of stories through ducting, it was occurring at a much more rapid rate than you would see from an explosion just in one processing vessel.
So what would a DHA look like at this facility? The six-step process would be:
- Determine if combustible dust is involved. In this case, you could do a go or no-go screening test in the dust collection system. Once you do that testing and determine that the material in the dust collector, proceed to the next step.
- Break the processing operation to a list of nodes. Node one is the feed hopper, two is the rotary valve, three is the first break mill, four is the attrition mill, five is the cyclone, six is the sifter, seven is the dust collector.
- Add nodes for rooms, buildings and building enclosures. Node eight is the building envelope for the building enclosure since it’s one single-room building.
- Evaluate fire, flash fire and explosion scenarios in each node. For example, with node one (the feed hopper), Scenario A might be static electricity accumulation igniting a dust cloud in the hopper, causing a flash fire or explosion. Scenario B might be an explosion in the first break mill propagating back into the feed hopper.
- Evaluate the existing safeguards at each node. For node one, safeguards may include bonding and grounding the unit as per NFPA 654 and the rotary valve between the feed hopper and the first break mill, which isolates an explosion upstream and downstream.
- Document the hazard scenarios safeguards and the recommendations against the prescriptive requirements. In this facility, some recommendations might include providing explosion protection on this feed hopper and ensuring that the design of the rotary valve complies with the requirements of NFPA 69.
This is a long answer to the simple question of what s DHA looks like.
Who Is A Qualified Person?
Chapter 3 in 652 defines a qualified person as a person who, by possession of a recognized degree certificate, professional standing or skill, and who, by knowledge, training and experience have demonstrated the ability to deal with problems related to the subject matter, the work or the project.
Appendix A.7.2.2 says that a qualified person who is leading or performing a DHA should be familiar with conducting a DHA. The qualified person should also be familiar with the hazards of combustible dust. The DHA is typically conducted by a team, but someone has to lead it at the end of the day, and that person must be qualified, meaning that:
- They should be familiar with conducting a DHA
- They should understand the fundamentals of combustible dust
- They should know the techniques for performing a DHA.
- They should have an ability to deal with the problems related to the subject matter, the work or the project.
At Dust Safety Professionals, we combined them into what we’re calling the three core components of a qualified person.
- Broad knowledge and understanding of combustible dust hazards
- Specific understanding of the industry processes and systems under review
- Demonstrable experience performing DHAs and or leading DHA teams
If any of these core components are missing, the DHA won’t be effective, as all the hazards aren’t likely to be recognized or quality recommendations made.
Conclusion
Next week, we’re going to talk about four challenges with applying NFPA 652 in practice. 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
Organizations
NFPA
Standards
NFPA 652
Previous Episodes
DSS078: Case Study – Grain Dust Explosion in a Milling Facility with Dr. Suzanne Smyth
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