DS 7-76 Prevention and Mitigation of Combustible Dust ... - FM Global

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7-76 Combustible Dust Explosion Page 32 FM Global Property Loss Prevention Data Sheets p o is ambient pressure, absolute (Pa) ρ is density of ambient air (1.18 kg/m 3 ) c is the speed of sound in air = 347 m/s γ is the specific heat ratio = 1.4 This formula is valid for flame speeds up to 500 m/s (1640 ft/s). To estimate the speed of flame propagation down a duct connected to a vessel in which an explosion has originated, (p - po) is assigned the value of P red for vented explosions or P max for vessels protected by explosion containment. If the flame is propagating in a system where air has been flowing before the time of ignition, add the velocity of the flow, or subtract it (as appropriate) from the value given by this equation. As an example, a vessel protected with explosion venting with a P red value of 0.21 barg (3 psig) has a duct flame front propagation velocity of approximately 45 m/s (148 ft/s). 3.1.16 Rapid-Action Float Valves (2.3.3.3.5) These devices actuate either on a differential pressure caused by the approaching flame front or the gas velocity in the duct. As a result, the vent relief pressure (P stat ) of the equipment experiencing the explosion must be high enough, about 0.2 barg (3 psig), to ensure the valve will shut. Where a lower P stat is needed to protect the equipment, an additional explosion sensor and helper gas are needed to actuate the valve. Data on the Ventex device indicate that specified minimum and maximum separation distances are needed to ensure the device closes properly and no transition to detonation has occurred. These distances are determined by test but are approximately valid for a broad range of operating conditions. If hybrid mixtures are involved, both distances will be reduced and the exact figure depends on the valve/pipe size. Manufacturer’s data would supersede this general guidance. Ventex valves are either uni- or bi-directional, i.e., will actuate from a pressure event coming from one direction only or from either direction. Once closed, they lock in that position and need to be manually reset. 3.1.17 Vacuum Operation (2.3.3.6) Because the final pressure after an explosion is proportional to the initial (pre-explosion) pressure, a dust explosion occurring in an environment that is at less than 0.1 bara (1.5 psia) will produce an absolute explosion pressure of less than 1 bara (14.5 psia). Thus, damage potential from that explosion does not exist. In addition, if a process operates at a pressure of less than 50 mbar (0.73 psi) (absolute), usually, an explosion could not be ignited. 3.1.18 Spark Extinguishing Versus Explosion Suppression (2.3.4.1) Losses have revealed that a misconception might exist as to the capabilities of a spark-extinguishing system. Spark-extinguishing systems are very effective in reducing the frequency of combustible dust explosions. However, they do not affect the severity of an explosion, and are not an alternative to explosion venting, explosion-blocking systems, or explosion-suppression systems. A spark-extinguishing system, also known as a spark-suppression system, detects and extinguishes sparks or glowing embers upstream of dust-collection equipment, to prevent these ignition sources from traveling to the dust collector(s) and initiating an explosion. The detection system uses an infrared sensor to look for particles passing by at elevated temperatures. The extinguisher is a water spray located downstream from the detector. The detector is expected to function regardless of the size of the hot particles passing by. It detects hot particles as small as sparks or as large as a piece of broken sander belt. However, the extinguisher might not work if the particles are large. Sparks should all be extinguished, but a large burning or smoldering piece of material might not be, in which case an explosion could occur if there is an ignitable dust cloud in the downstream dust-collection equipment. The spark-extinguishing system will not decrease the severity of the resulting explosion, so the same level of explosion protection or mitigation needs to be provided for the equipment with the dust explosion hazard. ©2008 Factory Mutual Insurance Company. All rights reserved.
Combustible Dust Explosion 7-76 FM Global Property Loss Prevention Data Sheets Page 33 A spark-extinguishing system extinguishes an ignition source but cannot suppress an explosion once it has begun. By preventing an explosible cloud from ever igniting, spark-extinguishing prevents the explosion altogether. An explosion-suppression system detects the early phases of an explosion and quenches the explosion to prevent pressure from rising to a level at which equipment may be damaged or destroyed. The explosion-suppression system reduces the severity of an explosion, whereas the spark-extinguishing system only reduces the frequency. A spark-extinguishing system is only intended to eliminate one ignition scenario: small, hot particles conveyed to a dust collector where a combustible dust cloud can be ignited. There are other ignition scenarios which the spark-extinguishing system cannot influence. For example: • Tramp metal (e.g., a screw or nail) aspirated into the dust collection system. The metal creates sparks as it impacts against ductwork or metal equipment downstream of the spark-extinguishing system. • Ignition sources produced downstream of the spark-extinguishing system; for example, hot surfaces, cutting and welding on/around dust collection equipment. For most processes, conveying small, hot particles from the dust-producing process to the dust-collection system is by far the most common potential dust-explosion ignition source. Installed spark-extinguishing systems can actuate weekly or more often without incident. This attests to the system’s ability to consistently detect and extinguish even the smallest hot particles. There are losses where burning material passed through the spark-suppression zone into the dust collection equipment and triggered an explosion. The burning material possibly got through because of its size or form or because of a defect in the spark-extinguishing equipment or its installation. Because other damagelimiting features, such as explosion venting or blocking systems may have been lacking or were not fully adequate, the ensuing explosions damaged the collection equipment. In addition, burning material was forced (ejected) into the work area either forwards through a warm air return or backwards to the dust pickup points. Adequate explosion venting combined with safeguards on the warm-air return (recommended here and in Data Sheet 7-73, Dust Collectors and Collection Systems) could have prevented any burning material from being ejected into the facility. 3.1.19 Minimum Ignition Energy (MIE)(2.3.4.2) The vast majority of dusts have MIE values above 10 Mj, so it is not necessary to routinely have dusts tested for MIE. Such testing is normally pursued only when there are reasons to suspect a dust might be particularly susceptible to ignition from static. There are currently at least two recognized test standards for MIE, ASTM E2019 and EN BS 13821. Any test result reporting an MIE of 10 mJ or less should be interpreted as proving static ignitability. 3.1.20 Foreign Material Separators, Magnetic or Other (2.3.4.3) Using separators upstream of all equipment that mechanically impacts with the process material prevents metal and other foreign objects from entering the equipment. Without the separators, tramp metal or other material entering the equipment can create impact or friction sparks capable of igniting a dust cloud. 3.1.21 Clean Versus Dirty Side of Dust Collectors (2.4.2) The distinction between the clean and dirty air sides is not important when considering explosion scenarios. The pressures generated in an explosion frequently rupture the filter media as the explosion propagates throughout the clean and dirty sides. In addition, there may be a breach of the filter media even before an explosion (e.g., a bag break), thus allowing dust to enter the clean side. The issue of where to locate vents relative to the clean and dirty side is both theoretical and practical. Most testing is based on an enclosure with no internal obstructions and with the vent remote from the ignition. Bags or other obstructions can change the explosion and the venting process. Bags generally fail early in the explosion and have a minor effect on the venting process, but the tube sheet dividing the clean and dirty sides can be more of an impediment. The equation specifying the minimum amount of explosion venting that must be on the dirty side is based on providing fully adequate explosion venting for a dirty-side explosion if the bags do not get damaged and do not allow any gases to vent out through the clean side of the collector. ©2008 Factory Mutual Insurance Company. All rights reserved.
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DS 7-76 Prevention and Mitigation of Combustible Dust ... - FM Global

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