By Brian Richardson, Camfil Air Pollution Control
Any facility that handles, transfers, packages or processes dust-producing materials or powders could face an explosion risk from combustible dust. A dust explosion is caused by a concentrated, confined combustible dust cloud coming into contact with an ignition source. Regular housekeeping and operating a dust collection system that is properly designed for a specific operation can significantly reduce airborne dust in the work environment and help to mitigate the risk of a primary or secondary explosion. A primary explosion is the first point where an explosion occurs and is often an isolated incident. A secondary explosion occurs when the primary explosion pressure disturbs dust that has collected in the workplace, resulting in a much more extensive explosion.
Combustible Dust Inspections and Standards
In the U.S., the Occupational Safety and Health Administration (OSHA) and National Fire Protection Association (NFPA) regulate combustible dust issues, each with its own area of responsibility. OSHA, together with local authorities, enforces the NFPA’s combustible standards. OSHA’s Combustible Dust National Emphasis Program (NEP) outlines policies and procedures for inspecting workplaces that create or handle combustible dusts that have the potential to cause a deflagration, fire or explosion.
Under the NEP, OSHA inspects facilities where processes cause dust to be suspended in air or some other oxidizing medium, regardless of particulate size or shape—particles, fibers, fines, chips, chunks, flakes or a mixture of these types. Dusts of concern include, but are not limited to, metal dust; wood dust; plastic dust; biosolids; organic dust (such as sugar, paper, soap and dried blood); and dusts from certain textiles. Industries that handle combustible dusts are wide ranging and include agriculture, chemicals, textiles, forest and furniture products, wastewater treatment, metal processing, paper products, pharmaceuticals and recycling operations.
NFPA sets and updates safety standards regarding combustible dust. Most insurance agencies and local fire codes require facilities that generate combustible dust to follow NFPA standards, including:
- NFPA 652, the Standard on the Fundamentals of Combustible Dust, which covers the requirements for managing combustible dust fires and explosions across industries, processes and dust types. It requires the owner or operator of any facility where combustible dust exists to conduct a dust hazard analysis, develop a plan for managing the hazards and provide training for personnel potentially affected by the hazards.
- NFPA 654, the Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, an all-encompassing standard on how to design a safe dust collection system. This standard also points you to more direct standards that deal with different types of dust and explosion protection equipment.
- NFPA 61, the Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities, which covers facilities engaged in dry agricultural bulk materials or manufacturing and handling starch.
- NFPA 68, the Standard on Explosion Protection by Deflagration Venting, which applies to the design, location, installation, maintenance and use of devices and systems that vent combustion gases and pressures resulting from a deflagration within an enclosure.
- NFPA 69, the Standard on Explosion Prevention Systems, which extends beyond explosion venting to address the whole dust collection system—inlet and outlet ducting, spark-extinguishing systems and methods for preventing an explosion from traveling back into the building or to process locations upstream of inlet duct work.
Combustible Dust Hazard Analysis
NFPA 652 requires a combustible dust hazard analysis (DHA) to assess risk and determine the necessary fire and explosion protection. Facility operators can conduct the analysis internally or use an independent consultant. Ultimately, the authority having jurisdiction will review the findings and grant approval. This DHA must be completed by the fall of 2020 and demonstrate reasonable progress toward completion of the DHA in each of the years approaching the deadline.
A complete dust hazard analysis includes process machinery that produces dust and the dust collection system itself. Many dust collector fires and explosions occur after a fire outside the collector produces flames or sparks that are drawn into the collector. Appendix B of NFPA 652 provides guidance on performing a thorough DHA.
DHA considerations for the dust collection system include documenting all possible hazards associated with the dust collector and the controls used to mitigate them. Evaluate each segment of the dust collection system and ask:
- Is the particulate in this section of the system explosible?
- Is the particulate suspended in air?
- Is the concentration dense enough to support a deflagration?
- Is there an ignition source strong enough to ignite the dust cloud?
- Is there a hazard control already in place?
Cartridge-Style Dust Collectors and Explosion Protection
Fortunately, there are many types of devices and systems that help to protect dust collection systems from explosions and keep them in compliance with NFPA standards. These devices fall into two general categories: passive and active.
Active systems such as chemical suppression/isolation and fast-acting valves react prior to or during a deflagration event. Active systems require detection, control and a pneumatic or electrical response that creates an isolating barrier in response to a deflagration.
- Chemical isolation is designed to react within milliseconds of detecting an explosion and can be installed in inlet and/or outlet ducting. Typical components include an explosion pressure detector, suppressant canisters and a control panel. This system creates a chemical barrier that suppresses the explosion within the ducting and can reduce or eliminate the spread of flame through the ducting.
- Chemical suppression protects the dust collector itself, whereas chemical isolation detects and suppresses explosions within the ducting. The system detects an explosion hazard within milliseconds and releases a chemical agent to extinguish the flame before an explosion can occur.
- Fast-acting isolation valves are installed in inlet and/or outlet ducting and are designed to close within milliseconds of detecting an explosion. They create a physical barrier within the ducting that effectively isolates pressure and flame fronts from either direction, preventing them from spreading further through the ducting to the process equipment.
Passive systems react immediately following an event and create an isolating barrier in response to the deflagration pressure to prevent the flame and pressure wave from traveling to other areas and causing more damage.
- Explosion venting is the most commonly used passive system for dust collector housings. It is designed to safely vent the gases and pressures from a deflagration to minimize/eliminate damage and injury to workers in the area. When the dust inside the collector combusts, the pressure inside the collector can reach unsafe levels. The explosion vent opens when the pressure reaches a specified level that is lower than the design pressure of the dust collector. The vent opens, allowing pressure and flame front to exit to a safe area.
- Flameless vents can be installed over a standard explosion vent to extinguish the flame front as it exits the collector. This enables the venting to be installed indoors depending on the application. A safe area will need to be established around the flameless vent as specified by the manufacturer because pressure is still released from the vent.
- Explosion isolation dampers provide an additional layer of protection and are installed in the inlet ducting to create a mechanical barrier that keeps explosion flame and pressure from traveling through ducts into the process area. Normal airflow keeps the flap plate open. But if an explosion occurs, the pressure wave forces the flap closed, so flames and pressure are contained.
- Integrated Safety Monitoring Filters (iSMF) are considered an outlet protection device because they protect the downstream (outlet) equipment and work areas. Installed on the top of a dust collector, the iSMF acts as a flame front barrier. In the event of a dust collector explosion, the filter prevents the flame front from exiting the collector and entering the workspace or exiting the collector.
Brian Richardson is the Technical Departments Manager at Camfil Air Pollution Control (APC). He started with the company in 2008 and has held managerial roles in R&D, field service, production, quality & safety and training. Camfil APC, headquartered in Jonesboro, Arkansas, is a global manufacturer of dust, fume and mist collection equipment. For information, contact 1-800-479-6801 or 1-870-933-8048; email firstname.lastname@example.org; or visit www.camfilapc.com.