Combustible Dust: An Explosive Situation
December 16, 2019
The What and the How
Combustible dust is one of the biggest hazards confronting a wide variety of industries. Because it is mostly unseen or otherwise sensed, it is easily unrecognized for the damage and injury it can cause.
Combustible Dust Explosions
In February, 2008, a sugar company in Georgia was leveled by a chain of sugar dust explosions racing through the building. Fourteen workers were killed and 38 others injured. The resulting fines totaled almost 9 million dollars. (Imperial Sugar)
In December, 2010, three persons were killed and another injured from a metal dust explosion in a blender at a metal recycling plant in West Virginia. (AL Solutions)
In May, 2017, five people were killed and 12 injured from an explosion of grain dust at a grain milling operation in Wisconsin. (Didion Milling)
In 2018 alone, there were 194 dust fires and explosions in North America, an increase of 25 per cent over the year before.
And the one thing they all had in common is they could have been prevented.
What is Combustible Dust?
Combustible dust is a collection of fine, solid particles which, either from a single source or in a mixture with other materials, are liable to catch fire or explode upon ignition when dispersed in the air.
Ignition can come from a totally unexpected source. For example, superfine iron dust from a steel shop collected on a filter will spark each time its hit by a raindrop.
Not all combustible dust is the same. Each type of dust is assigned a Kst value, which provides an indication of the severity of an explosion resulting from the ignition of that dust in a confined space. Some materials, like cornstarch (208 Kst), are identified as extremely hazardous with a high Kst value. They explode quickly, creating a lot of pressure in seconds. Other products, like coffee creamer (28 Kst), burn more slowly so the pressure rise is slower. But an explosion may still result.
And with combustible dust, the whole is often greater than the sum of its parts. Knowing the Kst value of everything that goes into a mixture is not the same thing as knowing the Kst value of the resulting mix. It could be greater or lesser, depending on the content and quantities of the materials involved.
Dust in a confined space will attempt to escape, and that’s when damage to equipment and harm to people is likely to result.
The 5 Elements of a Dust Explosion
There are five required elements for a dust explosion. They are sometimes referred to as the Dust Explosion Pentagon:
- 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 a potential vessel rupture, facility obstruction or structural collapse
A term often used in connection with dust fires and explosions is ‘deflagration’ and is defined as combustion which propagates through a gas or across the surface of an explosive at subsonic speeds, driven by the transfer of heat. The term is used for both flash fires and explosions. With a dispersed dust cloud, deflagration can cause an unconfined flash fire or, when confined, an explosion that ruptures the containment vessel.
The Two Explosions
Combustible dust explosions often involve two explosions: primary and secondary. The primary explosion is the first to occur when dust suspension in a confined space is ignited and explodes. The first explosion will dislodge other dust that has accumulated which, when airborne, also ignites. A secondary dust explosion is often more destructive than the primary one.
Prevention & Mitigation
For all the harm that dust fires and explosions can cause, prevention and mitigation together can all but eliminate the risks to your business.
Prevention refers to stopping an incident from occurring prior to its incipient stages.
Mitigation refers to protecting workers, equipment or the environment from an incident after the incipient stages.
Imagine the start of an explosive incident. A spark and small flame develops and quickly grows to a critical mass or size that enables it to self-propagate. Anything that can stop the flame from developing would constitute prevention, where anything that protects workers, equipment or the environment after that stage is mitigation.
Steps for Prevention
Explosion prevention begins when one piece of the Dust Explosion Pentagon is removed.
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 safer than any other prevention technique because the fuel is no longer present.
Keeping fuel below the minimum explosive concentration, or MEC, can be done through regular cleaning of your present equipment or selecting new equipment that doesn’t allow dust to accumulate inside it or in the ductwork.
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, smoldering 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 or empty it from the equipment being worked on before hot work begins.
Self-ignition can happen in silos where smoldering 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; striking the vessel to dislodge the buildup could ignite an explosion.
Ignition Source Control
Minimum Ignition Energy, or MIE, is the minimum amount of energy required to initiate the combustion of a cloud of dust, vapor or gas. The lower the number, the more hazardous it is. Making sure the ignition sources in your system or surface temperatures are below the MIE of the dust cloud removes the ignition piece of the Dust Explosion Pentagon and is the final way to prevent an explosion from occurring.
Explosion mitigation - protecting workers, equipment or the environment from an incident or incident sequence – begins after the incipient stages have already begun.
Containment refers to increasing the confinement. Explosion-proof pressure vessels on a dust collector can 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 is a passive approach. The explosion vent is designed to open at a set pressure. When an explosion reaches the set pressure, the vent opens and the pressure is expelled into the surrounding area.
Flameless venting usually involves a passive explosion vent panel and flame-inhibiting device located within equipment such a dust collector. As an explosion begins, the explosion vent panel opens, permitting flame and dust to enter the flame-inhibiting device, reducing the flame’s temperature below the dust’s ignition temperature and thereby keeping the flame from spreading and causing an explosion.
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 or inert the atmosphere so that the flame can’t develop further.
Be Prepared with RoboVent
We understand air quality and guidelines and how to meet them making your facility safer for you and your employees. RoboVent has set the benchmarks in the industrial ventilation industry due to cutting edge technology and completely integrated solutions.
Together let’s create a Combustible Dust control strategy for your facility.
Contact RoboVent today to learn more about our innovative systems, processes and approach.