DSS248: Helpdesk Question: Considerations for Metal Dust Collection

In today’s episode of the Dust Safety Science podcast, we’re talking about dust collection considerations for metal dust. This is a help desk question that came in through our system.

A consultant working with a company reported an issue about a piece of equipment they use for handling aluminum powders. They conducted tests on the powders, and the results from the testing lab showed no problems. However, there was an explosion in the equipment’s filter system, even though the tests were negative.

After investigating, they think the sample might have oxidized before the tests, but it wasn’t as oxidized during actual processing. That could be why the tests didn’t predict the explosion. Now, they’re asking us for advice on dust collection for this metal dust, specifically about using wet dust collection systems and meeting the requirements of NFPA 44 to make the system safe.

We’ve discussed metal dust on our podcast before. In Episode #71, we examined a 2013 metal dust explosion in a 3D printing application. In Episode #140 we reviewed the history, severity, and practical aspects of metal dust explosions. We explored the challenges in understanding and preventing dust explosions, including issues like:

Low minimum ignition energies
Difficulties in isolating explosions
Handling metal dust fires without worsening them
Safely using inerting systems

We’ve generally addressed the complexities and requirements associated with metal dust. However, we haven’t specifically focused on particular applications of metal dust, which can be tricky to manage due to their unique subtleties, such as the material, process operation, temperatures, pressures, and explosion protection methods.

We usually don’t cover such applications. This is partly because the standards related to them are always evolving. If you need someone to assess your system for combustible metal dust hazards, or if you’re a consultant inexperienced in this area, it’s crucial to consult with an expert familiar with the NFPA 44 Standards for Combustible Dust, including past and present versions, and understand the changes over time regarding combustible dust safety.

At the time of this recording, we’re using the 2022 version of NFPA 44. But there have been many changes in previous standards, especially with new processes like additive manufacturing and other methods of handling combustible dust. This is a complicated field that requires experienced professionals to ensure safety. There are a few companies and industry experts who specialize in this area or have conducted hundreds of hazard assessments and risk analyses, but they are not very common.

If you need help with metal dust, don’t hesitate to contact us. We can connect you with these experts. However, remember that this podcast episode is not engineering advice. It’s meant to provide general information and pointers. For specific guidance, you should consult with a professional who has experience in combustible metal dust.

In this episode, we’re just going to highlight the main points we addressed in our response to a help desk query. And to clarify, the advice given in our help desk responses is not engineering advice either. We recommended that the person who asked the question seek out experts in this area. Now, let’s talk about some key points that came up regarding dust collection for metal dust.

What Does NFPA 44 Say About Metal Dust With A Greater Than 150 Bar Meter Per Second?

Question number one was what does NFPA 44 say about metal dust with a greater than 150 bar meter per second?

We reached out on LinkedIn for feedback from experts experienced in handling metal dust. We focused first on what NFPA 44 says about metal dust with a speed greater than 150 meters per second, specifically in the section about dry type air material separators (Section 12.13.2.4.4). It states that dry filter media air material separators should not be used for metal dusts like niobium, tantalum, titanium, zirconium, hafnium, or any dust moving faster than 150 meters per second, unless a dust hazard analysis acceptable to the Authority Having Jurisdiction (AHJ) supports their use.

The Appendix, which provides non-compulsory information, mentions that explosion protection, isolation, and venting can be challenging for many combustible metals at speeds greater than 150 meters per second. Some equipment has shown effectiveness in these situations, but success depends heavily on the dust properties and the specifics of the process. This underscores the need for an experienced professional to conduct a thorough assessment when dry dust collection is considered.

Regarding wet dust collection, NFPA 44 addresses its unique risks. We’ve seen cases where installing a protection method in one part of a process led to explosions in another part. For instance, adding inerting in one area might prevent explosions there, but then explosions occur further down the line. This was observed where wet dust collection was implemented to recover metal powder; the subsequent drying process for the powder led to explosions. These scenarios highlight the importance of having an experienced individual design a safe process. Their expertise is vital in ensuring safety throughout the entire operation.

What Does NFPA 44 Say About Wet Dust Collection?

The second question we’re addressing is about what NFPA 44 says regarding wet dust collection. In Section 13.2.4.5, which covers wet type air material separator requirements, there are various rules about return air signage, disposal of effluent, operational safeguards, and more, though this isn’t a complete list. A key concern mentioned is the generation of hydrogen.

NFPA 44 specifies that a wet type air material separator must be designed to vent hydrogen produced when metal contacts water. It allows indoor venting of hydrogen if the Authority Having Jurisdiction (AHJ) deems it acceptable and takes into account the risk of hydrogen accumulation in the building. The standard also provides additional rules for non-water-based wet material air separators.

In Appendix 13.2.4.5.10, it notes that most combustible metals react with water to produce hydrogen, a highly flammable gas. It warns that wetted combustible metal dust, when not submerged in water, can be extremely flammable and dangerous. Thus, effluent from the system, when drying, poses a hazard, as does hydrogen buildup during the wet dust collection of combustible metals.

Reflecting on our previous podcast episodes, we recall discussing similar challenges in episode 71, which focused on a metal dust explosion in a 3D printing application. They faced issues with inerting the powder bed and also with wet dust collection, possibly involving hydrogen buildup in a wet dust vacuum. However, it’s been almost four years since that episode, so we’d need to review it for accuracy.

What Does the Community Say About Wet Dust Collection for Aluminum Powders?

Question number three is what does the community say about wet dust collection for aluminum powders?

We’ve discussed the requirements for dry dust collection with a KST greater than 150 and for wet dust collection according to NFPA 44. We shared these insights on LinkedIn and invited feedback and comments on the best or alternative approaches for wet dust collection with aluminum to prevent hydrogen buildup.

One method is to design a system that can vent accumulating hydrogen. This involves considering factors like indoor venting and the challenges associated with it. Another strategy is to eliminate the hydrogen. Suggestions included using mineral oil or other types of oils, and there was talk about adding additives to the water, such as hydrosol or l-malic acid, to reduce hydrogen formation from the metal-water reaction.

An important point from Eric Potorski of Dust Safe USA is that if you use oil, you must ensure the correct velocities at your pickup points. Since oil is more viscous than water, it can clog traditional internal dispersion screens and demisting pads. This can cause additional friction loss, possibly reducing the flow and pressure of the collector, and consequently, lowering the velocity at your pickup points.

We also received advice on getting a compatibility report for all sealants and gaskets used in the wet collector. It’s also considered best practice to correctly size the demisting pad and dispersion screens for the liquids being used. There were also discussions about using mineral oils, such as shingle oil, with combustible metals to try and prevent hydrogen buildup.

Conclusion

This episode underscores the dynamic nature of metal dust safety, particularly in the context of evolving standards like NFPA 44. It emphasizes the necessity of continuous learning, consulting with experienced professionals, and staying informed about the latest practices and technologies in dust hazard management.

Remember, while this podcast provides valuable insights, it’s not a substitute for professional engineering advice. For specific guidance on combustible metal dust and related safety measures, consulting with experts familiar with the intricacies of this field is essential.

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.