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18 / May 2021 powderbulk.com References 1. "1910.1000 TABLE Z-1 - TABLE Z-1 Limits for Air Contaminants," Occupational Safety and Health Administration, United States Depart- ment of Labor, accessed March 10, 2021, www.osha.gov/pls/oshaweb/ owadisp.show_document?p_table= STANDARDS&p_id=9992. 2. NFPA 654: Standard for the Preven- tion of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, Annex C: Informational Primer on Spark Detection and Extinguishing Systems, Section C.1.2.2: Detector Sensitivity and Speed, 2013 Edition, National Fire Protection Association (NFPA), 1 Batterymarch Park, Quincy, MA 02169-7471; 800-344-3555, fax 800-593-6372 (www.nfpa.org). For further reading Find more information on this topic in articles listed under "Dust collec- tion and dust control" in the article archive on PBE's website, www. powderbulk.com. John A. Constance, PE (215-300- 9563), is a consulting engineer at The Engineers Collaborative. He has more than 30 years of experience designing, engineering, and trouble- shooting dust control and industrial air systems for bulk solids process- ing and handling industries. The Engineers Collaborative Canandaigua, NY 215-300-9563 www.engcollab.com from the filters during the clean- ing cycle, resulting in a dust cloud exceeding the minimum explosible concentration (MEC), an explosion will occur. The MEC is the lowest concentration of a combustible dust in air that will propagate a flame. However, sprinklers can't extin- guish an explosion, so an explosion prevention and protection system will be necessary. Such explosion prevention and protection systems include spark or flame detectors within ductwork leading to a dust collector that are designed to protect the dust col- lector from a fire caused by sparks or burning embers upstream from the dust collector. These detec- tors actuate abort gates, dampers, or extinguishing systems to keep sparks or embers from entering the dust collector. Spark or flame detectors that actuate abort gates, dampers, or spark extinguishers are effective as long as the com- bustible dust concentration within the ductwork is consistently below the MEC. Having spark or flame detec- tors in place won't guarantee 100 percent safety, however. A spark detector coupled with an abort gate or spark extinguishing sys- tem won't stop a propagating flame front (fire and explosion debris) from entering your plant back through the dust collector's inlet or outlet return air ducts. For more detail on how to prepare for and handle this issue, refer to the National Fire Protection Associa- tion's NFPA Standard 654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combusti- ble Particulate Solids, Annex C, Section C.1.2.2. 2 The dust collector inlet and outlet return air ducts need mechanical or chemical isolation in case of an explosion within the dust collector. PBE and pickup points. If your system is designed using the static pres- sure balance method, there wouldn't be any air-balancing dampers in the system to adjust. Using this method, the dust collection sys- tem's duct sizes and system layout would've been engineered in the design phase to balance the airflow within the system's pickup points, hoods, and ducts based on the static pressure loss through each section. However, the system should still be checked at startup to ensure that design exhaust air quantities are flowing through the system. Most industrial dust collection exhaust systems are designed using the air balance by damper adjustment method. In this method, the duct diameters are sized to satisfy the desired dust transport velocities, and dampers are installed at each pickup point within the system and manually adjusted to balance the airflow. If this applies to your sys- tem, the airflow must be balanced by adjusting the dampers at sys- tem startup. Adjustable balancing dampers give you some flexibility with airflow changes that might be needed because of process changes that always occur down the road. However, be sure that the damper slides are securely locked in place after balancing. What protects personnel and property from an explosion inside your dust collector? If a fire starts inside the hopper of a dust collector handling combus- tible dust, fire protection sprinklers built into the dust collector may be able to extinguish the fire before an explosion takes place. However, if the dust collector goes through a filter cleaning cycle before the fire is extinguished by the water from the sprinklers, there's a very good chance that the fire will ignite the resulting dust cloud. Also, if a suf- ficient amount of dust is dislodged