|Vahid Ebadat, Ph.D.|
There have been many explosions in dust collection systems in industries that handle combustible dusts. This is often because the ignition sensitivity and explosion severity of the dusts that are present in dust collection systems are not adequately appreciated and as a result explosion safeguards are less-than-adequate. This column reviews the conditions that could result in dust cloud explosions in dust collectors and discusses measures commonly used to ensure safety.
The Fuel - Because of the inherent nature of most dust collection units, explosible dust clouds are often present within the “dirty side” volume routinely or at least during filter-media cleaning by mechanical shaking or compressed air blow-back. It should be noted that the powder within dust collectors is generally the finest and perhaps the driest seen in the process, which can result in an increase of the ignition sensitivity and explosion severity of the dust.
The Ignition Source - In dust collection systems there are several different ignition sources that can occur. Impact and friction sparks can develop due to foreign objects entering the system. Electrostatic charge generation and accumulation on electrically isolated metal filter support assemblies and insulating ductwork can lead to incendive discharges. Accumulation of thermally unstable powders within the dust collector can lead to self-heating and ignition over time. Transfer of smoldering or burning material from upstream equipment/operations to the dust collector can ignite the dust already in the dust collector.
The Consequence - A dust explosion occurring in an unprotected collector could result in destruction of the vessel and generation of a fireball, pressure wave, and even missiles. If no explosion isolation is provided to the dust collection unit propagation of the explosion could occur through the interconnected ductwork back to the shop floor or connected equipment, potentially amplifying the destructive power of the initial explosion.
The Hazard Assessment - The most common options for ensuring safety of dust collection systems may be summarized as follows:
Elimination of potential ignition sources: Obtain information on sensitivity of powder to ignition by sources such as electrostatic discharges, frictional sparks and heating, and thermal decomposition. Take effective measures to exclude ignition sources. The relevant laboratory tests include Minimum Ignition Energy (MIE), Minimum Ignition Temperature of Dust Cloud, and Thermal Stability (self-heating). Depending on the test results, steps such as electrical bonding and grounding, regular maintenance and cleaning of the ducts and filter units, spark or ember detection, etc., can reduce the probability of an ignition in the dust collector unit.
Inert gas blanketing: The objective is to reduce the oxygen concentration below minimum necessary to support combustion. The relevant laboratory test is Limiting Oxygen Concentration (LOC). Inert gas blanketing may be practical with closed loop systems. It should be noted that where several non-inerted operations are interconnected this measure may not be feasible. Pulse jets can use inert gas instead of air.
Explosion resistant equipment (containment): In this option the dust collector and interconnected ducts and equipment are constructed to withstand the maximum explosion pressure. The relevant laboratory test is Maximum Explosion Pressure (Pmax). It should be noted that explosion containment is generally not feasible due to high costs of ensuring that dust collector, ducts, bins, and upstream equipment are strong enough to withstand the maximum explosion pressure (typically 8-10 bar).
Explosion pressure relief venting: Explosion venting is the most commonly used protection measure for dust collectors. Relief vent should direct the explosion products (fireball, pressure wave, and burning particulates) to a safe area outside the building. Relevant laboratory tests are Explosion Severity (KSt) and Maximum Explosion Pressure (Pmax).
Explosion suppression: Explosion suppression is commonly used when, for example, the location of a dust collector prevents the use of relief venting or if the powder material is environmentally hazardous. This method involves the detection of the explosion Initiation and injection of a suppressant to quench the flame and prevent pressure rise. Relevant laboratory tests are Explosion Severity (KSt) and Maximum Explosion Pressure (Pmax).
Vahid Ebadat Ph.D., M.Inst.P, MIEE, C.Eng., C.Phys., is the CEO of Chilworth North America and a member of Powder/Bulk Solids’ Editorial Advisory Board.