Dust-processing systems such as dust collectors, filter receivers, and baghouses have historically been involved in many dust explosion events. In fact, according to a U.S. Chemical Safety and Hazard Investigation Board study of 281 dust explosions in the United States between 1980 and 2005: “…dust collectors are the equipment most often involved in incidents…” (Investigation Report Combustible Dust Hazard Study, Report No. 2006-H-1, November 2006).
Explosions in dust collection systems occur because facilities lack proper understanding of the “ignition sensitivity” and “explosion severity” of dust, do not identify and control all potential ignition sources, and do not use effective measures for controlling or eliminating dust explosions.
To minimize explosion risks, every dust collection system—indeed, any operation involving combustible powders—requires a Basis of Safety. A Basis of Safety from dust-cloud deflagrations can include measures to avoid explosions (explosion prevention) or facility and equipment designs that protect people and equipment in the event of an explosion (explosion protection). The selection of explosion prevention or protection measures is usually based on the following criteria: how much information is available on the powder’s ignition sensitivity and explosion severity; the nature of the plant’s processes and operations; how well plant personnel understand and appreciate the consequences of dust explosions; how a facility manages preventive measures; and the effects of explosions on personnel, the environment, and the company’s business.
Explosion prevention measures include:
• Avoidance of Ignition Sources. All ignition sources capable of igniting the dust cloud are identified and eliminated. Depending on the dust’s ignition sensitivity characteristics, measures such as electrostatic bonding and grounding, regular maintenance and cleaning of ducts and filter units, spark and ember detection, etc. can reduce the probability of ignition in dust collectors. It is noteworthy that the elimination of ignition sources as a sole Basis of Safety is not commonly considered for dust collection systems.
• Inert-Gas Blanketing. The atmosphere is sufficiently depleted of oxygen or oxidants to prevent combustion. Inert-gas blanketing or purging is practical when a dust collector is connected to equipment that is maintained under inert conditions and forms a closed-loop system. If pulse-jet cleaning is used, an inert gas should be employed instead of air.
If explosion prevention cannot be ensured, measures must be taken to protect people and minimize damage to facilities. In such cases, explosion protection measures should be considered. In addition to taking all reasonable steps to reduce the possibility that dust clouds will form or spread, facilities should remove ignition sources. Explosion protection measures include:
• Explosion Containment. In this method, equipment is constructed to withstand the maximum explosion pressure resulting from the deflagration of the dust that is present within the process. Although possible, explosion containment is generally only feasible for new system designs. In addition, there is a high cost associated with ensuring that the dust collector unit, ducts, collection bins, and upstream equipment are strong enough to withstand the maximum explosion pressure (typically 8 to 10 bar) throughout the system’s life cycle.
• Explosion Relief Venting. Explosion venting is the most commonly used protection measure for dust collector units. It relies on relieving explosion products (pressure and a fire ball) from the equipment to a safe location. Explosion isolation should also be provided to ensure that an explosion cannot propagate back through the duct to the work areas or to any interconnected equipment.
• Explosion Suppression. Explosions should be detected early on and suppressed using a suitable suppressant. Explosion suppression is commonly used when the location of a dust collector prevents the use of relief venting or if powder material represents a health or environmental hazard.
More information on dust explosion hazard assessment and control, including the OSHA Combustible Dust National Emphasis Program, can be obtained from Chilworth Technology Inc. (Plainsboro, NJ), 609-799-4449, [email protected], or www.chilworth.com.