Planning for recirculating from a dust collector
NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids provides general guidelines for combustible dust control in industry. The NFPA also has two standards that cover specific guidelines for explosion protection systems (NFPA 68) and explosion prevention systems (NFPA 69). There are also three other standards for specific industries that require slightly different (usually stricter) regulation. These include NFPA 61 that concerns agricultural facilities, NFPA 484 covering combustible metals and NFPA 664 covering woodworking facilities.
Since NFPA 654 is a broader guide to designing a safe dust collection system for combustible dust applications, let’s consider a few choice highlights as they apply directly to dust collection systems.
Prevention and Protection Strategies
The gist of the standard is that where combustible dust hazards exist, facilities must take steps to reduce the risk of an explosion occurring - and at the same time limit the severity of any conflagration that might occur. These are commonly called out as prevention and protection strategies (sometimes termed active or passive efforts).
Prevention strategies include methods of blocking combustion from taking place inside the system, such as spark arrestors, inerting systems or oxygen control.
Protection methods limit the damage caused by initial or primary conflagrations or prevent them from propagating throughout the system or facility where they can cause larger, more devastating secondary explosions. These include explosion vents, back draft dampers, isolation valves, high speed abort gates as well as suppression systems (both chemical and water based).
The NFPA standard requires a combination of prevention and protection devices. Generally, systems with more ductwork branches and pickup points require more prevention and protection devices than simpler systems. Additionally, some simple systems can achieve compliance simply by locating the dust collector outside in a restricted access area where an explosion would cause no harm to persons and not risk propagating the fire onward.
Good Housekeeping and Adequately Sized, Operated and Maintained Systems
Further requirements from the standard emphasize the need for properly designing the system to adequately control dust generated in the facility. These are required to achieve a clean environment within the facility, supplemented by good housekeeping practices to prevent even the smallest of dust accumulations from forming. (NFPA recently tightened its definition of hazardous surface dust in this standard, defining it as any dust layer of 1/64 inch (0.4 mm) or more.)
Further provisions direct attention to adequate operation of the dust collector to ensure no dust accumulates within the system, thus turning the dust collection system itself into the greatest potential hazard. For example, it specifically requires the ductwork to be sized properly to provide an “air velocity necessary to keep the duct interior clean and free of residual material.” This means matching fan CFM and vacuum pressure with properly sized ducts to achieve a sufficient air speed above the minimum conveying velocity of the material to avoid product dropout. Other parts of the standard imply a need to maintain the dust collector differential pressure within an acceptable range to prevent the loss of suction and the resulting loss airflow/air velocity
Finally, for plants wishing to recirculate exhaust back into the facility, NFPA 654 requires a filtration of at least 99.9% of PM 10 as well as prohibits recirculating of the exhaust if oxygen levels fall below 19.5 percent by volume or in the presence of any combustible gases or vapors or hybrid mixtures. In addition, there must be a means of knowing when any air separator (dust collector) filters are not performing, or starting to leak.
NFPA 654 is a must read for any facility handling combustible dusts. Nearly every industry, application and process has at least one or more potentially combustible material dusts present. With the potential for substantial property damage, regulatory fines and sanctions, and risk to health and safety of workers, plants can't ignore these hazards.
Auburn Systems’ triboelectric dust detection technology now has multiple uses in helping plants monitor these and other hazards. For example:
- Triboelectric Ambient Dust Monitors can provide early warning to plants of problems with ambient or fugitive dust within the facility allowing them to take corrective action BEFORE it becomes a major hazard
- Particle velocity detection can monitor particle speed in ductwork to ensure it is fast enough to keep particles entrained
- Using Triboelectric Broken Bag Detectors to monitor recirculating air to ensure that it satisfies requirements
Contact Auburn today for a consultation about how their technology can help you manage combustible dust hazards in your facility!
A note on the evolution of NFPA standards:
Some updates to standards since the release of NFPA 652: Standard on the Fundamentals of Combustible Dust. This was an important first step for creating a single overarching standard for dealing with fire and explosion hazards associated with combustible dusts of all types and in all industries. Main idea is to harmonize all the combustible dust standards. As such, NFPA 654 saw only minor revisions and clarifications to definitions of terms and procedures for conducting hazard analysis.