In this episode of the Dust Safety Science Podcast, we’re talking about the fundamentals of combustible dust explosion prevention protection and avoiding accident sequences. In the process, we’ll discuss key subjects like the following:
- Important concepts surrounding combustible dust explosion prevention safety, fire safety, and flash fire safety
- Examples of combustible dust prevention and protection strategies
- The five elements of a combustible dust explosion
- How to avoid accident sequences and escalations of a primary event into secondary and larger-scale disasters and catastrophic losses
There are three concepts we want to introduce before we get into the actual combustible dust prevention, protection, and action sequences. They are:
- Prevention and protection
- Incident sequences
- Explosion design parameters
Combustible Dust Explosion Prevention and Protection
This concept can be regarded in terms of prevention vs. mitigation.
Prevention refers to stopping an incident or incident sequence from occurring prior to their incipient stages. On the other hand, mitigation refers to protecting workers, equipment or the environment from an incident or incident sequence after the incipient stages.
The definition gets a little gray when we define the incipient stage of a combustible dust explosion, flash fire, or even a regular fire. You can think of it as a flame kernel that develops and reaches a critical mass or size that enables it to self-propagate. Anything that can stop the kernel from developing would be a prevention concept where anything that protects workers, the equipment or the environment after that stage is mitigation.
Incident Sequences
Incident sequences are typically documented as being primary vs. secondary incidents.
In the case of a primary deflagration or primary explosion, this is the first ignition of a dispersed combustible dust cloud in the incident sequence. It often takes place inside process machinery and, in most cases, occurs when all five sides of the Combustible dust explosion pentagon are present.
The secondary deflagration, which is initiated by the primary step, consists of the subsequent steps in the incident sequence. In most cases they include flash fires and explosions outside of process equipment. There are exceptions though: an explosion in a cyclone could propagate through the ductwork into a dust collector or into other process machinery for example.
Explosion Design Parameters
Explosion design parameters are from explosion testing. They include:
- Pmax: Maximum pressure, or how strong an explosion is in an enclosed vessel.
- Dp/dt max: Maximum rate of pressure rise. How fast does a material react? Is it fast enough to actually have a flash fire or explosion hazard?
Other design parameters include:
- KST: Scaled maximum rate of pressure rise. Rates of pressure rise are taken from different-sized vessels and scaled up to a common denominator, which happens to be the one cubic meter chamber.
- MEC: Minimum explosible concentration, or minimum amount of dust required to sustain flame propagation in a dust cloud.
- LOC: Limiting oxygen concentration, or the amount of oxidant required in that cloud to sustain combustion and explosion.
- MIE: Minimum ignition energy, or the minimum energy required to ignite a dust cloud. This could be in terms of spark energy or other type of energy releases.
- MIT: Minimum ignition temperature. This could be a hot bearing, hot surface, or anything else that’s heating up and submerged in the dust cloud. Is it hot enough to ignite the dust and cause an explosion?
These three concepts give us the fundamental building blocks needed to understand prevention, protection and avoidance of incident sequences.
Five Elements of a Combustible Dust Explosion
The five elements of a Combustible dust explosion, which we have covered on the podcast before, are typically referred to as the Dust Explosion Pentagon. They are:
- A fuel, which is the combustible dust.
- An oxidant, which is typically the oxygen in the air.
- An ignition source capable of igniting materials when they are dispersed as a cloud.
- Dispersion, which is when the accumulated dust is spread out and creates a dust cloud.
- Confinement, which leads to pressure rise and potentially vessel rupture, facility obstruction, or structural collapse.
In terms of definitions, ‘deflagration’ is used for both flash fires and explosions. A fireball deflagration as defined by NFPA 652 is a reactive front moving in an unreacted medium at a rate lower than the speed of sound. This happens with a dispersed dust cloud. It can cause an unconfined flash fire or, when confined, an explosion that ruptures the containment vessel.
Combustible Dust Explosion Prevention Options
Explosion prevention options generally arise when one side of the dust explosion pentagon is removed. Below are some possible strategies for preventing an explosion risk in a facility.
Remove Combustible Dust.
If you can go without storing combustible dust in your facility, you don’t have a dust explosion hazard. This approach is inherently safer than any other prevention technique because the fuel is no longer present.
Concentration Reduction
This is the process of keeping fuel below the minimum explosible concentration, or MEC. It can be done via regular cleaning or selecting equipment that doesn’t allow dust to accumulate inside it or in the ductwork.
Oxidizer Reduction
Oxidizer reduction is inerting the atmosphere to bring the oxygen level down below the limiting oxygen concentration, or LOC. This method involves the injection of an inerting gas like nitrogen into a closed system.
Spark Detection and Control
With this active system, possible ignition sources are detected. Examples include hot screws, smouldering piles, or a hot ember that has been sucked into a dust collection system. These hazards can be detected through the presence of smoke, radiation, and high temperatures, and must activate a control method such as:
- An abort gate that shunts a hot ember out of the processing line
- A suppression system that quenches it before it can get downstream and start the incipient stages of a flash fire or explosion
Proper Hot Work Systems
Hot work such as welding and cutting should not be done in a dusty environment or on tanks or hoppers containing combustible dust. Clean up the material and/or empty it from the equipment being worked on before hot work begins.
Avoid Self-Ignition
Self-ignition can happen in silos where smouldering combustion is deep inside the stored material and turns into flaming combustion when it reaches the surface. This event can ignite a dust or gas explosion in the headspace.
Self-ignition can also occur in equipment like spray dryers. Material sticks to the inside edge, heats up, and becomes an ignition source for a dust explosion. It is critical to remove this material safely- if you hit the vessel to dislodge the buildup, it could ignite an explosion.
Ignition Source Control
Making sure the ignition sources in your system or surface temperatures are below the MIE and MIT of the dust cloud is the final way to prevent and explosion from occurring by removing the ignition element of the dust explosion pentagon.
Explosion Protection
Protection kicks in after the incipient stages have started. Options are discussed in detail below.
Containment
Containment refers to increasing the confinement. There are explosion-proof drum kits that can be applied to the bottom of a dust collector to avoid the need for an isolation channel between them. In other cases, such as hammer mills, the equipment is built strong enough to withstand an explosion inside.
Venting
Venting is a passive approach. The explosion vent is designed to open at a set pressure. When it hits that set pressure (which happens when an explosion occurs), the vent opens and the pressure is expelled into the surrounding area.
With traditional venting, location is highly important. It should never be towards an area where people may be present. Vents should also never be bolted, painted over, or located so close to a wall that the pressure can’t escape.
Flameless Venting
Flameless venting is essentially the same process as removing confinement, except that a mass is introduced to quench the flame and prevent it from ejecting through the vent panel. It diverts and quenches the flame at the same time.
Suppression Systems
Suppression systems actively monitor for the beginning of an explosion. A pressure sensor detects an incident at its incipient stage and activates a control system. This system could suppress the flame kernel or inert the atmosphere so that the flame kernel can’t develop further.
Avoiding Incident Sequences And Escalation
Incident sequences and escalation cause damage to progress from a single piece of equipment to the entire facility.
Isolation
NFPA standards require equipment to be isolated from:
- Other equipment. When an explosion occurs, it can propagate through the ductwork into a second piece of equipment. By the time it gets into the second vessel, it is much stronger, so it is essential that explosions be isolated so that they can’t propagate from one vessel to another.
- People. If an explosion occurs in an outdoor dust collector, it must not be able to propagate back through the ductwork into the building, where people are working.
Avoid Fugitive Dust Accumulations
To avoid fugitive dust accumulation. Follow the three C’s:
- Contain: Make sure ducts are airtight and ensure that dust isn’t released when a machine is being operated.
- Capture: Select a hood that does a proper job of capturing dust and diverting it into the dust collector.
- Clean: Maintain an effective housekeeping schedule that keeps the dust below the minimum explosible levels. The NFPA has commodity-specific and industry-specific guidelines. With the Imperial Sugar incident, huge secondary explosions lasted for 15 minutes due to the high amount of fugitive sugar dust.
Conclusion
How do we start making facilities safer? If we focus on the incident sequences (in particular, the fugitive dust accumulations), it may be possible to prevent a single fire or explosion from escalating into a disaster. Perhaps the biggest component, however, is doing a dust hazard analysis. As we’ve mentioned on the podcast before, DHAs are going to be mandatory come September 7th, 2020, and they will likely be a key driver in outlining your specific facility risks and the recommended ways to move forward.
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
DustSafetyScience
Combustible Dust Incident Database
DustSafetyScience Podcast
Questions from the Community
Organizations:
OSHA
Standards:
NFPA 652
Incidents:
Imperial Sugar Refinery Explosion (2008)
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DSS048: Fundamentals Of Dust Explosion Prevention, Protection And Avoiding Incident Sequences