Air material separators - critical for dust collection and recirculation
Despite the well know dangers associated with combustible dust hazards there exist few specific OSHA/MSHA regulations covering them outside of a handful of specific industries (e.g. grain elevators) on a national level. Rather, local jurisdictions normally base their regulations on the guidelines found in the National Fire Protection Agency standards that outline best practice for eliminating or controlling these hazards.
The most widely used NFPA standard for combustible dust is NFPA 654 - Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids. This standard contains guidance that is applicable to most combustible dusts in general industry and is attracting new attention based on updates to the regulations around recirculating air from dust collectors and combustible dust monitoring.
The primary objective of this standard is stated clearly within the text itself: “The facility, combustible particulate processes, and human element programs shall be designed, constructed, equipped, and maintained to protect occupants not in the immediate proximity of the ignition from the effects of fire, deflagration, and explosion for the time needed to evacuate, relocate, or take refuge.”
Central Role Played By Air Material Separators
Much of the guidance in section 654 concerns the effective handling and control of dust(s) within the facility by means of an ”Air Material Separator”. What is an “Air Material Separator” according to the NFPA? An air material Separator is any device that separates air used for conveying from the material it conveys. NFPA 654-2013 states:“Air–Material Separator (AMS). A device designed to separate the conveying air from the material being conveyed.”
Air material separator can be a number of different systems, including a fabric filter dust collector (baghouses and cartridge collectors) and cyclone collectors among others. It technically also includes wet collectors, however these are subject to certain exceptions and have their own regulations due to their inherent advantages in dealing with combustible dust hazards.
Formerly, a distinction was made between primary air material separators used to remove the large majority of dust from the air stream (cyclones, knock out chambers, scrubbers, etc.) and secondary air material separators (fabric filter collectors, cartridge collectors, HEPA filters, etc.) used to capture remaining fines to achieve compliance with PM 2.5. However, in the latest edition of NFPA 654 this distinction has been removed and now both types of collectors are subject to the same requirements.
What the NFPA standard Means for Dust Collectors (Air Material Separators)
NFPA standards involve many separate and interlinked strategies to combat combustible hazards effectively. Air separators (dust collectors) are a core focus of these strategies. In general, the thrust of NFPA 654 and other standards for combustible dust require facilities to effectively capture any dust generated by their processes and to outfit collection systems in a way to minimize the potential for fires and explosions with them. As such, most requirements involve either preventing fires or explosions or implementing design strategies to protect systems and facilities in the event of a conflagration. These strategies can range from quite simple (e.g. locating the collector outside, including explosion venting, etc.) to more complex and costly (e.g. dry injection systems, inert gas systems, dilution with noncombustible dusts, oxidant concentration reduction, etc.).
Section 18.104.22.168.3.6 notes the requirement of a method for detecting malfunctions in the AMS that would reduce filter efficiency. These are usually classified as broken bags or filter failure, and can cause emissions of particulate to be released into the facility. Consider using triboelectric bag leak detectors - instruments that not only alert when a bag is broken, but predict an impending failure by tracking real time performance with a degree of sensitivity that anticipates a problem. An effective method to monitor the Air Material Separator performance.
With a more generalized definition of air material separators, the newest version of NFPA 654 has eliminated some exceptions that previously existed for some classes of dust collectors handling combustible dusts. Now, more of these systems fall under the same standards for combustible dust protection and prevention.