Operations in many chemical plants can pose the risk of dust explosions. One common potential source of such explosions is a dust collection system. It’s therefore timely to examine the latest complete revision of the “National Fire Protection Association (NFPA) 68 Standard on Explosion Protection by Deflagration Venting” to see what’s changed and how this impacts future dust-collection decisions. The standard, which can be purchased online via the NFPA web site ( http://www.nfpa.org ), applies to all closed-vessel dry-collection systems such as cartridge-style dust collectors. So, here, we’ll share our understanding of five key implications of NFPA 68 as they relate to cartridge dust-collection systems.
1. NFPA 68 has changed from a “guideline” to a “standard.” It provides mandatory, and much more stringent, requirements for collection applications involving explosive dusts. This echoes actions by the U.S. Occupational Safety and Health Administration (OSHA), which recently launched a National Emphasis Program focusing on the safe handling of combustible dusts ( http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=DIRECTIVES&p_id=3830 ). OSHA defines combustible dusts as “organic or metal dusts that are finely ground into very small particles, fibers, chips, and/or flakes…. Some of the industries in which combustible dusts are particularly prevalent include agriculture, chemical, textile, forest and the furniture industry.”
Simply stated, it’s the NFPA’s role to set the standard and OSHA’s and local authorities’ role to enforce it.
Most insurance policies and local fire codes state that NFPA standards shall be followed as code, so plants in nearly every town and county in the U.S. face a legal obligation to satisfy NFPA 68. The only exceptions would be where the authority having jurisdiction (AHJ) specifies another safety approach such as that of Factory Mutual (FM).
|Acrylonitrile butadiene styrene copolymer||200||147|
|Polyester resin with glass||14||182|
Table 1 -- Explosive Power of Various Dusts:
The new requirements hold major significance because many chemical plants now will have to install updated dust-collection/explosion-protection equipment to ensure regulatory compliance.
2. You now must determine whether a dust is explosive. In a closed vessel such as a cartridge dust-collection system, an explosion usually begins when a suspended cloud of combustible dust is present in high concentration within the collector. As the fan draws in large volumes of air, an outside spark or ember can be sucked into the collector, colliding with the dust cloud to trigger the explosion. The source of the spark may be a production process, a cigarette butt thrown into a hood (believe it or not, this really happens), or a static electricity discharge due to improper grounding of equipment.
To determine whether your dust is combustible, it must undergo explosibility testing in accordance with ASTM test methods. NFPA 68 stipulates that if a dust sample is available, it must be tested. The standard further states that it is the responsibility of the end-user (i.e., the plant or safety engineer) to commission the required testing and report results. Your dust collection supplier may ask you to supply a report of the test or, if unavailable, to provide in writing the explosive power (denoted as “Kst,” the pressure velocity) value that you know the dust doesn’t surpass.
Even if you believe you know the characteristics of your dust, a sample, if available, still must be tested under NFPA 68. After all, particle size may profoundly affect explosive properties. Take anthrachinon, for example: if its particle size exceeds 10 microns, the Kst value is estimated to be 91 bar-m/s. For particle size less than 10 microns, its Kst value is estimated to be 364 bar-m/s. This changes the classification of the dust.
Explosibility testing is the only way to know for sure and, therefore, is the only way to guarantee compliance. Several companies that specialize in explosion protection services offer this testing. You can opt to go directly to such a company or can commission the testing through your dust collection supplier.