By Brice E. Ohl, EIT
Dust collectors are common pieces of equipment in any industrial and process building setting; however, there is no such thing as a “typical” dust collector design. Dust collectors vary in size, construction material, filter media, and more, all depending upon the application. In very simple terms, dust collectors are used to remove unwanted particulate matter, that is off-put from a process, from a space. This is achieved by pulling the particulates into an airstream, and then running the airstream over a filter media, once again separating the matter from the air.
One of the key design factors for dust collectors is the prevention and, potentially, the suppression of fires within the dust collector.
When you were a kid, you were probably taught the basic “Fire Triangle,” which consists of three components required to sustain a fire: heat, air, and fuel. Unlike most pieces of equipment, a dust collector can contain all of these components, which adds a wrinkle into the selection of dust collectors and the required options. This blog will discuss some of the considerations for designing safe dust collectors with regard to explosion protection and isolation, using the Fire Triangle as a guide to consider all the facets of explosion within dust collectors. We will discuss the particulate matter in the airstream acting as an ignition source, the collected material acting as the fuel, and how the air within the dust collector can be used both as an oxidizing agent and a pathway for the flame to travel.
Most importantly, we will discuss prevention and isolation of dust collector fires and explosions.
Part A: Ignition
Let us begin our walk through the Fire Triangle with the source of heat that can cause ignition. In the setting of a dust collector, the source of ignition is often a spark. This spark can come from several sources. The most common source is when the material being collected makes contact with the interior of the duct that it is being transported through. Often the friction of this contact can be enough for a few particles to ignite, causing a chain reaction that leads to the surrounding particles igniting, which can in turn potentially ignite the filter media and collected material. This can result in an explosion within the dust collector if the reaction is fast enough.
In many ways, the aforementioned chain reaction is like striking iron on flint to start a campfire. At first only a few sparks may appear, but with the right fuel and air mixture it can grow into a roaring fire.
There are a couple of precautions that can be taken to reduce the number of sparks in a dust collection system. First, it is best to minimize the number of bends and turns in the dust collection routing. Minimizing the number of bends and elbows reduces the product-on-duct contact, which reduces the heat gain in the product due to friction. In the analogy of iron on flint, this would be reducing the number of strikes of iron on the flint to reduce the chances of ignition.
The second design precaution that can minimize the potential for sparks is material selection of the dust collection duct. This choice is a twofold process. First, you must identify the material being conveyed through the duct. The second is to identify duct materials that are compatible. For example, metallic dust particles can be very reactive and ignite easily when striking other metals. It is important to select materials that will not react, but also do not wear easily. This would be analogous to replacing the flint with a piece of plastic. If the two objects striking each other do not produce sparks, there will be no ignition. Depending on the application, a spark-resistant fan or explosion-proof fan should also be considered. These fans are designed so that dust particles cannot come into contact with the motor, reducing the chance of ignition.
Part B: Air
The next side of the Fire Triangle is the hardest to deal with in the world of dust collection. Given that air is used to convey the collected dust from its source to the collector, it cannot be removed from the equation. The only real option with air, as far as explosion prevention is concerned, is to reduce the amount of airflow as low as possible while maintaining an appropriate velocity to keep the particulates in the airstream moving toward the collector. This is a tricky concept because reducing the airflow too low will cause the product to fall out of the airstream and likely block the dust-collecting duct, thus rendering the system useless. However, there are several benefits to properly adjusting the airflow. The lower the airflow, the lesser the speed the product will come into contact with the duct interior, especially at the bends. This will reduce heat gain due to friction and wear. Additionally, reducing the airflow will provide less air to feed a potential fire or to spread the fire. Using the iron-and-flint analogy, properly controlling the airflow would be akin to preventing someone from blowing on the sparks created by the flint and spreading the fire.
Although you cannot remove air from the dust-collection process, there are ways to prevent the air from being used as a pathway for the flames to spread. Explosion isolation valves, which act as a check valve for the incoming airstream, should always be considered to prevent flames from traveling back upstream the dust collection system. Additionally, a means of venting an explosion should be considered. For dust collectors located outdoors, explosion relief valves are often used. These valves provide a path of least resistance for the flames to travel down in an effort to protect the equipment from major damage.
For dust collectors located indoors, where an open flame would be dangerous, a flameless vent should be used. Flameless vents are similar to explosion relief valves, but with the addition of a series of layered meshes. These meshes combine to provide a large surface area for heat from the explosion to transfer to, and mitigate its effect. Rather than a large flame bursting out of the dust collector, large amounts of smoke do instead.
In the example of the iron and flint devices, the explosion relief valve would be like using a fire ring to prevent the flame from spreading too far.
Part C: Fuel
The final side of the Fire Triangle is the one that cannot be changed when it comes to dust collecting. The material being collected often acts as the fuel in dust collector fires and explosions. While some materials are more volatile than others, you cannot change them without changing the process from which the dust is being collected. Although you cannot change the material being collected, you can change how to suppress a fire if one occurs. A good example from the home is that you would stop a campfire with water, but a grease fire with baking soda.
Most major dust collectors can be fitted with a fire suppression system. These systems include a typical wet system, a dry chemical system, or even a CO2 system.
A wet system is just like it sounds. Water is supplied to the interior of the dust collector when a fire is sensed to douse the flames. Wet systems work well in many cases, but do come with some precautions, depending upon the application. Much like a grease fire, some products oxidize when in contact with water, so a wet system would make things worse, if used. Additionally, many products would absorb the water and solidify. A good example of this is concrete or cement dust. Using a wet system may put out the fire, but the product can solidify at the bottom of the collector, blocking the material outlet and rendering the collector useless.
In cases like the one above, a dry chemical system would be of better use. Like the wet system, it is important to make sure the chemical agent and the collected product are not reactive. If a dry chemical system is used, it is important to have a means of removing the agent once the fire is contained. The chemical itself can be collected and removed from the dust collector like the product, but it is important to have a disposal plan and any necessary personal protection equipment necessary to handle the chemical.
The last fire suspension option, CO2 systems, can be used when a wet system and dry system won’t be effective. Admittedly, CO2 Systems have more to do with the air-side than the fuel-side of the Fire Triangle. CO2 systems function by supplying CO2, an inert gas, to the dust collector, displacing the ambient air, reducing the oxygen level, and suffocating the fire. CO2 Systems are a great option when dealing with extremely volatile products or when little-to-no cleanup after a fire is desired.
The CO2 system does not fit the analogy of the iron and flint, but the wet and dry systems do. If you are trying to start a campfire you would always want some water or a fire blanket on hand, so things cannot get out of control.
The world of dust collecting may seem complicated, but it can become simple when you consider all the parts in terms of concepts you are familiar with. The biggest is step is to familiarize yourself with the product being collected and its application.
JDB Industry regularly designs dust collection systems for manufacturing and food processing facilities. Contact Jeffrey W. Pauley, PE, to learn more. Have questions about this blog post? Contact Brice E. Ohl, EIT.