In today’s episode of the Dust Safety Science podcast, we talk about metal dust explosions: their loss history, severity, and practical considerations. The goal is to answer the following questions:
- What has happened to date in terms of loss history?
- Why is it a challenge and a large problem?
- What causes the large severity for some metal dust when compared to some other materials, and what does this mean practically?
- What kind of things might someone need to look at if they’re coming up with a safety strategy for combustible metal dust?
This material is from a presentation that Dr. Chris Cloney recently gave at Brandfarlig Vara (Swedish for “Flammable Goods”), which is a conference hosted every year in Sweden. His presentation was titled “Metal Dust Explosions from International Perspective Research, Loss History & Practical Considerations.”
In the last few years, there have been some large dust explosions in Sweden, particularly at metallic pigment plants and other facilities that handle metal dust. Ken Nessvi, who discussed dust explosion loss history in Sweden in Episode #88, wrote a paper on the subject. His research uncovered 237 fires and explosions involving combustible dust in Sweden between 2012 and 2017. Of this total, 12 explosions and 32 fires involved metal dust.
In the United States, the U.S. Chemical Safety Board reports indicate that there were 281 dust explosions between 1980 and 2005. Sixty of these were metal dust explosions, resulting in 27 fatalities and 96 injuries. Aluminum and magnesium were involved in 60% of the explosions, but 86% of the fatalities and 82% of the injuries for metal dust.
All of this data begs the question: why does metal dust cause such severe events, in particular dust like aluminum and magnesium? There appear to be three primary reasons.
1. The Heat of Combustion and Flame Temperature
Iron is a metal with a relatively low heat of combustion: 530 kilojoules per mole of oxygen, which results in a similar flame temperature – about 2200 Kelvin. Magnesium and aluminum have over double the heat of combustion. With magnesium, it’s approximately 1250 kilojoules per mole of oxygen and for aluminum it’s 1100 kilojoules per mole. This results in 3300 Kelvin flame temperature for magnesium and 3800 Kelvin for aluminum, which represents a 60 to 80% higher flame temperatures compared to metals like iron.
2. Maximum Pressure and Rate of Pressure Rise
When carbon dust reacts, there may be 5 to 9 bar of pressure out of that reaction in a closed vessel, at a rate of 50 to 350 bar metre per second. This is the scale of the rate of pressure rise, bar per second being how fast the pressure increases in that vessel. Then it’s scaled by the cube root of the vessel volume.
With magnesium and aluminum, pressures may be as high as 17 bar or more and rates of pressure rise are upwards of 500 to 800 bar metre per second. With very fine aluminum, rates of pressure rise can exceed a thousand bar metre per second: two to three times faster with much larger output pressures than organic dust.
3. Additional Factors
There are additional factors that come into play during metal dust explosions. They include
- Turbulence
- Oxide layers from metal dust reactivity with other types of metal dust
- Water reactivity
- Metal/metal reactivity
Radiation has the biggest impact on explosion severity. When something ignites a dust cloud, the flame moves out from that center location. Radiation impacts the material outside that flame because it travels faster. It can preheat the dust that’s ahead of the flame. If that preheating is very high, when the flame actually reaches that dust, it’s already hot. It may already be starting to react, resulting in an acceleration effect. The bigger that cloud gets, the more feedback from radiation occurs and the more severe the explosion becomes.
Practical Considerations
In general, there are five challenges that require consideration.
1. Minimum Ignition Energy
Dust emission energy decreases with particle size. If magnesium gets below around 75 micron and aluminum gets below 40 micron, the ignition energy drops below 50 millijoules. This is where concerns arise, with spark and electrostatic being viable ignition sources for these dust clouds.
2. Different Particle Sizes for Different Processing Operations
Challenge number two is different particle sizes for different processing operations.
Dr. Enrico Danzi and Dr. Luca Marmo from Italy wrote a paper called “Dust Explosion Risk in Metal Workings.” They took dust samples from different unit operations (grinding, sanding, shot-blasting, welding, brushing and processing) and used SCM microscopes to measure how large the dust particles sizes were. They also measured the rate of pressure rise, or the KSt values, for these dusts. The rates of pressure rise were all over the place, making it difficult to come up with a safety strategy.
3. Dust Collection and Explosion Isolation
Challenge number three is dust collection and explosion isolation. It can be quite challenging to come up with mitigation strategies for metal dust because the explosions can be severe, especially if a large dust collection system is involved. It’s probably better to have multiple small dust collection systems instead of one large box containing metal dust because of issues like radiation. It becomes extremely difficult to protect these systems from combustible dust.
4. Safe Response to Dust Fires
If a company has a fire in an aluminum bin, how does a worker tackle that safely? What kind of extinguishing system do they need? How do they go about using this new system? Are they trained on it? These are all considerations for safer response to dust fires. The challenge is amplified for metal dust, which can be reactive with water and other fire-fighting agents as well as other metals.
5. Safe Use of Inerting Systems
The last challenge is the safe use of inerting systems. Because metal dust can have such high ignition sensitivity and severity parameters, the solution is often to inert the system- put it into a closed loop and apply different inerting solutions.
When an inerting system is used, it needs to be treated as a safety device, not as a case of removing the combustible dust hazard. Workers have to be really cautious, especially about ignition sources in the vicinity of the equipment. First responders and firefighters need to know what systems are in place near redundant sensors. Inerting systems are great when used and designed correctly, but the hazard is not being removed – there is simply a safety system that’s protecting everyone from the hazard.
Conclusion
Metal dust explosions present unique challenges that require further study. Understanding what makes them so severe compared to other explosions and knowing the practical considerations involved are the best avenues for coming up with an appropriate safety strategy.
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
Conferences
Brandfarlig Vara
Previous Podcast Episodes
DSS088: Dust Explosion Loss History in Sweden 2011 – 2017 with Ken Nessvi
Thanks for Listening!
To share your thoughts:
- Leave a note in the comment section below
- Ask a question to be answered on the show
- Share this episode on LinkedIn, Twitter or Facebook
To help out the show:
- Subscribe to the podcast on iTunes
- Leave a review and rate our show in iTunes to help the podcast reach more people
Download the Episode
DSS140: Metal Dust Explosion Loss History, Severity & Practical Considerations
