Preventing dust explosions is essential for worker safety and site health. If you have dust in your facility, you have the potential for a combustible dust event and need to design your dust collection system accordingly.
In an effort to create safer work environments, regulatory agencies like OSHA are cracking down on operations with combustible dust, using standards such as NFPA 652 and requiring a dust hazard analysis (DHA) for applicable industries. If you are creating or handling particulate that is combustible, you need a DHA.
Since dust collection systems are a leading source of combustible dust incidents due to the concentration of particulates inside the equipment, the design of your collector will be reviewed during the DHA. Inspectors will take a close look at the housing, hopper, inlet ducting, and outlet or return ducting of your dust collector to ensure your system was properly designed for explosion protection.
While every application and dust collection system must be individually addressed to see how to best mitigate the risk of an explosion, there are some general tips that you can follow. Here are four steps you can take to design a more explosion-proof dust collector so that you can increase site safety and better adhere to regulatory guidelines:
1. Determine Your Dust’s Combustibility
The first recommendation from the NFPA is to analyze your dust and see if it is combustible or not. According to the NFPA, combustible dust can be any solid material comprised of particles less than 500 microns in size that are a fire hazard when suspended in air or a similar oxidizing medium.
Even if you are working with materials that do not seem combustible at first, as they break down and particulate falls off from handling, it can create combustible dust. Common examples of combustible dust include agricultural products like soy flour and powdered milk, carbonaceous dusts such as coal and charcoal, metallic dusts like aluminum and magnesium, and plastic dusts such as polyacrylamide and polyethylene. No matter the type of combustible dust, you will want to determine the Kst (the dust deflagration index) and Pmax (maximum explosive pressure) of your dust. The larger these values, the more severe the potential explosion.
This is also the time to determine if your dust has any other special qualities, like if there are oils present or if the dust is water-reactive. What is the dust loading? Does it flow easily, or is it a very fine and abrasive material? Understanding the nature of the dust is critical to designing your dust collector.
Sly TubeJet dust collector equipped with explosion venting and duct isolation valves
2. Choose the Correct Type of Dust Collector
When not properly designed, the inside of a dust collector is a prime location for a dust explosion to occur. This is due to the prevalence of several risk factors such as ignition sources like static discharges, the confinement of dust clouds, and the buildup of dust on ledges or in pockets.
For combustible applications, you need a dust collector that eliminates internal ledges and potential static buildup by proper bonding and grounding. If you handle a dust that is prone to self-ignition such as coal, you should avoid storing any dust in the hopper itself, as this creates a potential fire or deflagration risk.
Pulse jet baghouse with a suppression system to mitigate the risk of a dust explosion extending to other equipment
3. Choose the Right Location for the Collector
Based on your DHA, where you put your collector will influence your risk for damage and injuries, as well as what explosion-proof auxiliary equipment is required. Collectors that are inside a facility naturally have a greater safety risk due to their proximity to workers, and you are generally more limited in your explosion prevention options. Process dust collectors typically have to be inside a facility and integrated into your systems, while nuisance dust collectors can easily be at the end of a process line or outside of your plant.
4. Add Explosion-Proof Auxiliary Equipment
Auxiliary equipment that is integrated into your dust collection system can often help to reduce the severity of an explosion. The main three types of auxiliary equipment that you can use are flameless vents, suppression systems, and explosion venting.
Outdoor dust collectors can use any of the above, with explosion venting the go-to option as it is the least costly of the three. Indoor systems are limited to flameless vents and suppression systems, with flameless vents being the most expensive.
Other equipment, such as a sprinkler system, can also help if your dust is not water-reactive.
Mike Maxwell is engineering manager, Sly Inc. (Strongsville, OH). For more information, call 866-747-5219 or visit www.slyinc.com.