In today’s episode of the Dust Safety Science podcast, we’re answering the Help Desk question “Do cyclones need explosion protection in the wood industry?”
The individual who submitted the question asked if, in the context of the NFPA 664 guidelines on combustible wood dust, a cyclone is essentially a sophisticated duct, similar to a drop box, and what implications that understanding has for explosion protection. As we review the subject, we answer the following questions:
- What does NFPA 664 define as a cyclone?
- What are the other stipulations in NFPA 664 concerning cyclones?
- How does the NFPA documentation approach explosion protection assessment?
- How is explosion protection evaluated for ducting?
There’s a reference suggesting that cyclones are treated similarly to ducts. So how should a duct be addressed in this context? This is a vital point of discussion. We’ll also reference an example from NFPA 652‘s Annex B, which provides a detailed dust hazard analysis, specifically examining the factors for a cyclone to determine the necessity of explosion protection.
How Does NFPA 664 Define a Cyclone?
Let’s begin by understanding NFPA 664‘s definition of a cyclone.
Within the initial definitions section, a cyclone is described under the category of dust collectors. Specifically, a cyclone is characterized as a cylindrical dust collector designed to segregate particulates from the airstream using centrifugal force. It possesses a circular cross-section, a tangential air and material inlet, an air and material exhaust outlet, and a material discharge area. This is nested within the broader definition of a dust collector, which is labeled as an air material separator (AMS) employed to isolate material from the airstream. This category encompasses not just cyclones but also media-type baghouses and enclosure-less units.
The first takeaway is that NFPA 664 distinctly identifies a cyclone as a subset of dust collectors. An illustrative diagram of a typical cyclone configuration, showcasing features like the air inlet, material discharge, and air exhaust outlet, can be found in Appendix A.3.3.12.1. It’s worth noting that while this schematic provides a standard design, other viable configurations may also be encountered.
These descriptions are housed in chapter three of NFPA 664, under subsections 3.3.12 for dust collectors and 3.3.12.1 for cyclones. This immediately implies the necessity of considering explosion protection when dealing with cyclones. Before diving into the explosion protection specifics, it’s important to highlight other stipulations that NFPA 664 sets for cyclones. Three primary requirements can be pinpointed:
- Spark detection in rotary dryers.
- General ignition sources, with an emphasis on lightning strikes targeting cyclones situated on rooftops.
- Provisions related to pre-cleaners and their air exchange mechanisms.
Spark Detection in Rotary Dryers
In section 9.3.9.2.2, which addresses rotary dryers, the standard mandates that a rotary dryer must be equipped with automatic spark detection and extinguishing systems. These systems should be placed between the drum dryer and any downstream material handling apparatus, like cyclones or wind boxes. This is in addition to the broader explosion protection measures, ensuring that rotary dryers have dedicated spark detection mechanisms, especially when paired with downstream cyclones.
Lightning Strikes on Rooftop Cyclones
Appendix 9.4.7 emphasizes the need for heightened awareness regarding the risk of lightning strikes affecting roof-mounted dust collector cyclones and ductwork. The appendix further discusses the implications of impulses generated by such lightning strikes. Therefore, for those employing roof-mounted systems, it’s imperative to evaluate the potential ignition scenarios resulting from lightning strikes and the subsequent impact it might have within the facility’s ducting and other systems. This is equally crucial for all other ignition sources.
Air From Cyclone Pre-Cleaners
The third provision, found in section 9.3.5.4.4, states that air from cyclone pre-cleaners positioned outside a building, with an air capacity of 5,000 cubic feet per minute or less, is allowed to be directed straight to enclosure dust collectors located inside the building. This specific mandate falls within the equipment design subsection of the hazard management chapter, emphasizing the recycling of air, material separator clean air exhaust, and the application of cyclones in such systems.
Cyclones and Performance-Based Design
Before delving into the specifics of explosion protection for cyclones and dust collectors, it’s worth highlighting that cyclones are also referenced in section six concerning performance-based design. Within the “Deflagration Scenarios” segment, it’s stipulated that any duct, enclosed conveyor, silo, bunker, cyclone, dust collector, or other vessel housing combustible dust in quantities or conditions capable of sustaining a deflagration flame front under typical operating situations should be accounted for as a potential deflagration scenario.
Under the performance-based design section, cyclones are explicitly highlighted for inclusion in deflagration scenarios. Given that NFPA 664 primarily categorizes cyclones as dust collectors, there are additional mandates for elements such as clean air exhaust and specifics like spark detection for rotary dryers and measures for lightning strikes targeting roof-mounted cyclones.
Shifting our focus to explosion protection, it’s clear that cyclones should be considered when addressing deflagration scenarios. The pertinent section is 9.7.1.2: “Explosion Prevention and Protection,” with the subsection specifically addressing “Dust Collectors with Deflagration Hazards.” This subsection details requirements for dust collectors (and by extension, cyclones) with a dirty side volume exceeding eight cubic feet. They must conform to one of several outlined options:
- Construct it to endure the peak unvented deflagration pressure.
- Integrate a deflagration suppression system, as per NFPA 69 guidelines.
- Equip the system with deflagration relief vents in line with NFPA 68.
- Position the dust collector outside, ensuring minimal risk to personnel and the general public.
While there’s a fifth note pertaining to enclosure dust collectors and their allowance without added explosion protection requirements, it’s a complicated subject. One would need to explore the specifics of when and how such dust collectors can be used effectively.
Ducting and Explosion Hazards Under NFPA 664
The initial question revolved around the treatment of cyclones akin to ducting. So, how does NFPA 664 address ducts concerning explosion hazards?
There are specific guidelines for ducts prone to deflagration hazards under “Explosion Prevention and Protection” section 1.1. Ducts identified with such hazards must be designed and built in accordance with one of the six outlined criteria, which largely parallel those for dust collectors:
- Design them robustly enough to endure the highest vented deflagration pressure.
- Incorporate a deflagration suppression system.
- Use deflagration relief vents, conforming to NFPA 68 guidelines.
- Opt for venting with an attached flame quenching device or a flameless vent.
- For ducting positioned outdoors, venting can be done without needing a flameless vent.
- Metal ducts placed outdoors with lighter construction can be utilized, but a risk analysis acceptable to the relevant authority is mandatory.
The crucial takeaway is that ducting, especially of lighter construction or with particular joins or flanges, must be factored into a dust hazard analysis. This is vital since in many cases, the joints or connections between ducts and the vessels they’re connected to might be vulnerable points. Here, flames can erupt, potentially leading to a significant fireball. Such scenarios, especially in populated zones, often result in detrimental impacts on personnel during a deflagration incident.
Ducting should be evaluated as a critical point in a dust hazard analysis to determine the presence of explosion risks. This analysis also identifies if the ducting at a facility is adequately protected against such hazards.
NFPA 652, particularly Annex B, provides a sample dust hazard analysis. In Section B or Appendix B.4.5.4, for the fourth location (the cyclone), it notes that cyclones use particulate inertia to segregate particles from the conveyance air, and deflagrations can arise within them. Cyclones may cause particles, especially larger ones, to accumulate near their perimeter. This accumulation, along with the cyclone’s inherent design of separating large particles from finer ones, heightens the chance that certain areas within the cyclone might achieve conditions conducive for a deflagration. Visualize the dust amassing around the cyclone’s outer edges, where it can exceed the minimum explosive concentration, leading to potential deflagration risks.
This example features important questions. Firstly, is the material combustible or capable of deflagration? If the answer is ‘yes’ for both questions – the material being combustible and suspended in the air – then we’re dealing with a potential dust hazard. If the material isn’t combustible, then this dust hazard analysis isn’t relevant.
Next, the question arises: is there a concentration ample enough to propagate a flame front? The answer leans towards ‘probably’, which can be interpreted as a ‘yes’. This determination relies on the proportion of fine combustible particles in relation to the total amount of particles present and the air volume responsible for their movement. Calculations should be undertaken to ascertain if the concentration of combustible particles, under varying operating conditions, surpasses the minimum exposable concentration, making the cyclone susceptible to explosions. This is vital for comprehending the potential explosion risks associated with a specific cyclone.
The next question focuses on hazard management. How are cyclones equipped to handle deflagration hazards? Common measures include venting and isolation, or alternatively, deflagration suppression coupled with isolation. It’s worth noting that a rotary air lock at a cyclone’s base might function as an effective isolation device. But what if, when the system halts, there’s smoldering material in the hopper at the cyclone’s base? How is this scenario addressed? It’s crucial to understand that most explosions stem from fires that result in deflagrations. The presence of fire detection mechanisms is essential. Moreover, in the event a fire is detected, there must be a clear action plan. For instance, simply offloading smoldering material into a silo isn’t a viable solution.
This overview offers a snapshot of considerations regarding cyclones and explosion prevention. For a comprehensive understanding, it’s recommended to study the cited example in detail. And if you’re venturing into performing these calculations, it’s prudent to collaborate with an expert to ensure accuracy and safety, rather than making assumptions.
Conclusion
Cyclones, which are treated like sophisticated ducts, play a significant role in dust collection and thus need to be meticulously evaluated for potential explosion hazards. The overarching recommendation is to engage with experts and to rely on thorough dust hazard analyses, ensuring that the safest, most informed decisions are made for both personnel and facility safety.
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
The resources mentioned in this episode are listed below.
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Dust Safety Academy
Dust Safety Professionals
Organizations
NFPA
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DSS241: Do Cyclones Need Explosion Protection in the Wood Industry?
