wall. During a dust explosion’s incipient stage, the vent ruptures and directs the explosion’s overpressure, flame, burnt and un-burnt material, and other combustion by-products away from the vessel to a safe location. The explosion relief vent is designed to ensure that the explosion’s pressure rise doesn’t exceed the vessel’s pressure shock resistance. These vents are designed according to procedures in NFPA 68: Standard on explosion protection by deflagra-tion venting. NFPA provides an equation for estimating the vent-ed fireball’s size; from this information, you can calculate the safe distance required in front of the vented vessel to protect workers, equipment, and the building structure from the ejecting fireball. Flameless venting protects indoor equipment from dust ex-plosions by combining an explosion relief vent’s weak membrane with a mesh trap that arrests flame and retains particles. Like an explosion relief vent, the flameless vent’s membrane, installed on the process vessel, ruptures during a deflagration. But unlike with a relief vent, the deflagration’s overpressure, flame, and materi-al discharge through the membrane into the mesh trap, which prevents the flame and material from discharging into the sur-rounding area. Instead, the flameless vent discharges hot gas and overpressure. A safety perimeter must be established around the flameless vent to protect workers from this discharge. Like the explosion relief vent, the flameless vent is designed according to procedures in NFPA 68. To ensure that the flameless vent can successfully protect the vessel, the ratio of room volume to vessel volume (that is, the ratio of the volume of the room in which the vented vessel is located to the vessel’s volume) must also be kept below the flameless vent manufacturer’s recommen-dations. Since any dust or dirt blocking the openings in the mesh trap would compromise the vent’s operation, the mesh must also be regularly cleaned to ensure that the surface is free of dust or dirt at all times. Some vent manufacturers offer low-inertia, fire-resis-tant fabric covers to help keep the mesh surface clean. It is important to understand that explosion venting only re-lieves the deflagration pressure from the protected vessel. It does not stop flame from propagating to any interconnected vessel, nor does it address the post-explosion fire in the vented vessel. Other protection measures are needed to deal with these threats. EXPLOSION SUPPRESSION Explosion suppression systems are often installed in applications where it’s not possible to safely vent an explosion away from pro-cess equipment. The system detects an incipient dust explosion very soon after ignition and discharges a chemical extinguish-ing agent quickly enough into the developing fireball inside the equipment to extinguish the deflagration before a destructive overpressure develops. Major components in a typical explosion suppression system are one or more explosion suppressors, one (or more) explosion pressure detector, one (or more) flame de-tector, and a control panel. Explosion suppression systems are designed according to NFPA 69: Standard on explosion prevention systems. Typically, explosion suppression systems also include Canadian BIOMASS 23
Canadian Biomass March/April 2014: Page 23
