Page 40 - OHS, September 2024

COMBUSTIBLE DUST
NFPA classifies dust according to its potential explosibility, known as the Kst value, as follows:
■ ST0: Kst value of 0 = not explosive.
■ ST1: Kst value below 200 = weak to moderately explosive.
■ ST2: Kst value from 200 to 300 = strongly explosive.
■ ST3: Kst value exceeds 300 = very strongly explosive.
In addition, OSHA provides guidance on combustible dust ex-
plosion hazards and notes, “In many combustible dust incidents, employers and employees were unaware that a hazard even existed.4 It is important to determine if your company has this hazard, and if you do, you must take action now to prevent tragic consequences.”5
Explosion Protection Solutions for Dust Collection Systems
Let’s explore a few explosion protection devices and systems de- signed to ensure dust collection systems meet NFPA standards. Ac- tive systems aim to prevent explosions by detecting and responding to them before or during an incident. In contrast, passive systems ac- tivate in response to an explosion, focusing on controlling the event to safeguard employees and reduce equipment damage.
Active Devices
■ Chemical isolation systems are installed in inlet or outlet ducting to react within milliseconds of detecting an explosion. These systems typically consist of an isolation canister, explosion pressure detectors and a control panel. By creating a chemical bar- rier, they suppress the explosion within the ducting, halt the spread of flames and minimize the pressure surge that could affect process machinery connected to the dust collector.
■ Chemical suppression safeguards the dust collector itself. It is often used in conjunction with chemical isolation when safe venting of an explosion is not feasible or when the dust is hazard- ous or toxic. The system rapidly detects an explosion hazard, trig- gering the release of a chemical agent that promptly extinguishes the flame before an explosion can occur.
■ Fast-acting valves, which close within milliseconds of de- tecting an explosion, can be installed in inlet or outlet ducting. These valves form a physical barrier within the ducting, effectively isolating pressure and flame fronts from either direction, prevent- ing them from spreading further through the process.
■ High-speed abort gates integrated into the inlet or out- let ducting of a dust collection system reroute potential ignition sources away from the collector, thereby averting potential explo- sions and blocking flames and burning debris from entering the fa- cility via the return air system. A mechanical barrier then redirects airflow to a secure area. A spark detection system, positioned at an adequate distance upstream to ensure ample reaction time, sets the abort gate in motion.
Passive Systems
Passive systems respond immediately after an event to prevent the deflagration from spreading to other areas and causing further damage. Many facility operators prefer passive systems to active systems because they are significantly less expensive and do not require regular recertification. Passive systems control the speed of explosions by releasing pressure once it reaches a certain threshold. Below are commonly used passive explosion protection systems:
■ Explosion venting is the most commonly used method for protecting industrial dust collectors. When the dust inside the col- lector combusts, pressure rapidly increases. In this event, explosion vents open, allowing the pressure and flame front to exit in a safe
direction. NFPA 68 – Standard on Explosion Protection by Defla- gration Venting outlines specific criteria for these venting systems’ design, location, installation, maintenance and proper use.6
■ Explosion isolation valves installed in the inlet ducting serve as a mechanical barrier. Their primary purpose is to prevent the explosion flame and pressure from propagating through the ducts and reaching the process area. Normal airflow keeps the flap plate open. However, in the event of an explosion, the sudden pres- sure wave triggers the flap to close swiftly, effectively containing the flames and smoke within the duct system.
■ An integrated safety monitoring filter safeguards down- stream equipment and work areas. It is positioned on top of a dust collector, serving as a flame front barrier. In the event of an explosion in the dust collector, the filter retains the dust inside, preventing the flame front from entering the workspace. The iSMF complies with NFPA-mandated design options and effectively acts as a flame front arrestor for ST1 and ST2 combustible dusts.
■ Flameless vents can be placed over a standard explosion vent to suppress the flame front as it exits the vented area, preventing it from escaping the device. This enables the use of conventional venting indoors, even in situations where it might otherwise pose risks to personnel or lead to secondary explosions. However, it’s essential to establish a safe zone around the flameless vent owing to the release of pressure and dust and gases.
■ Backdraft dampers placed within the inlet ducting incorpo- rate a mechanical barrier that remains open by the process airflow but swiftly closes from the pressure of an explosion. When closed, this barrier effectively prevents pressure and flame fronts from ad- vancing further upstream in the process.
Conclusion
In industrial manufacturing and processing, controlling dust—es- pecially combustible varieties—is a complex task. To tackle these challenges, implement a tailor-made, high-efficiency dust collec- tion system equipped with appropriate explosion protection tech- nology that adheres to NFPA standards. This strategic approach will help safeguard against risks to humans and prevent damage to both equipment and the facility.
Todd Willey is the Environmental, Health & Safety Quality Assurance Manager at Camfil Air Pollution Control, manufacturer of dust and fume collection equipment for challenging industrial applications. For more information call 800-479-6801, email filterman@camfilapc.com or visit camfilapc.com/.
REFERENCES
1. NFPA 652 – Standard on the Fundamentals of Combustible Dust, tinyurl. com/289xea9y
2. NFPA 654 – Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids, tinyurl.com/524ndykz
3. NFPA 69 – Standard on Explosion Prevention Systems, tinyurl. com/544s3kuk
4. OSHA, Combustible Dust: An Explosion Hazard, tinyurl.com/3ude9yu3 5. OSHA, Hazard Alert: Combustible Dust Explosions, tinyurl.com/3ttarvmd
6. NFPA 68 – Standard on Explosion Protection by Deflagration Venting, tinyurl.com/2nuztcaf
38 Occupational Health & Safety | SEPTEMBER 2024
www.ohsonline.com