Fundamentals of Dust Explosion: Ignition and Development of Pressure
Basic Concepts
The five ingredients required for a dust explosion are:
Combustible particulates sufficiently small to burn rapidly when ignited
A suspended cloud of these combustible particulates at a concentration above the Minimum Explosible Concentration (MEC)
Confinement of the dust cloud by an enclosure or partial enclosure
Oxygen concentration more significant than the Limiting Oxygen Concentration (LOC) for the suspended dust cloud
The delayed ignition source of adequate energy or temperature to ignite the broken cloud.
The National Fire Protection Association (NFPA) has had several definitions of combustible dust over the years. The current definition in NFPA 654 is “a combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidising medium over a range of concentrations, regardless of particle size or shape.” Previous editions of NFPA 654 and the 2004 edition of the NFPA Glossary of Terms define combustible dust as “any finely divided solid material that is 420 microns or smaller in diameter (material passing a U.S. No. 40 Standard Sieve) and presents a fire or explosion hazard when dispersed in the air.” The reason for the revision is that many combustible fibre segments, flat platelets, and agglomerates do not readily pass through a No. 40 sieves. Still, they can be dispersed to form an explosive dust cloud.
In practice, questions of combustibility and particle size often arise when evaluating the potential explosion hazard of marginally small particles or mixtures of combustible and noncombustible particulates. Many laboratories doing dust explosibility tests have developed dust explosibility screening tests called Go/No Go tests to deal with these questions. Chapter 4 of the CCPS Guidelines for Safe Handling of Powders and Bulk Solids describes some of these tests. The ASTM E27.05 Subcommittee is currently working on revising the ASTM E1226-05 Standard Test Method for Pressure and Rate of Pressure Rise for Combustible Dust to provide a standardised Go/No test for potentially combustible particulates. MEC values are determined in the U.S. per the ASTM E1515 test procedure involving tests with various dust concentrations and a pyrotechnic igniter in a 20-litre sphere. The MEC corresponds to the minor concentration that produces a pressure at least twice as significant as ignition's initial pressure. Eckhoff (2003) reports that MEC values are not very sensitive to particle diameter for diameters less than about 60 μm but increase significantly with an increasing diameter above this approximate threshold. The majority of the materials listed in Eckhoff Table A.1 (2003) have MEC values range of 30 to 125 g/m3. These concentrations are sufficiently high that a 2 m thick cloud can prevent seeing a 25-watt bulb on the other side of the cloud (Eckhoff, 2003, p.9).
The confinement needed for a dust explosion is usually from the process equipment or storage vessel for the powder or dust. In fugitive dust released from equipment and containers, the room or building itself can represent the confinement. Often, the dust cloud occupies only a fraction of the equipment or building volume, and the resulting explosion hazard is called a partial volume deflagration hazard. Pressures produced from partial volume deflagrations and the corresponding deflagration venting design bases are described in NFPA 68. Example applications include dust collectors and spray driers.
LOC values for combustible dust are also determined via tests in a 20-litre vessel, and the ASTM E27 Technical Committee is drafting an ASTM standard for LOC values. LOC values for various combustible powders and dust listed in NFPA 69 Table C.1(b) are primarily in the range 9 v% to 12 v% O2. Paragraph 7.7.2.5 of NFPA 69 requires that the oxygen concentration for an inerted process system should be less than the measured LOC by at least 2 volume per cent for scenarios in which the oxygen concentration is continually monitored and no greater than 60% of the LOC if the oxygen concentration is not monitored.
Ignition Criteria
Hot Temperatures
One hot temperature ignition scenario entails a dust cloud accidentally entering a hot oven or furnace. This occurred in the CTA Acoustics phenolic resin dust explosion incident investigated by the U.S. Chemical Safety Board (CSB, 2005). The cleaning process generated the resin dust cloud during the cleaning of fugitive dust from the area around the oven.
The minimum dust cloud oven ignition temperature is determined by oven tests described in ASTM E1491. These include a vertical oven called the Godbert-Greenwald furnace, and a horizontal oven called the BAM furnace. BAM furnace minimum Auto-Ignition Temperatures (AITs) are usually 20oC to 60oC lower than the corresponding dust cloud ignition temperatures measured in the Godbert-Greenwald furnace. Most of the Godbert-Greenwald dust cloud ignition temperatures listed in Eckhoff’s Table A.1 range from 420oC to 660oC.
When the high temperature is on a limited area's hot surface, the required surface ignition temperature is higher than the standard furnace tests.
Examples of hot surface ignitions in dust explosion incidents include overheated failed bearings and driers. The former would require a surface temperature much higher than the ignition temperature measured in the standard oven tests. Still, the latter might require a lower temperature than the standardised tests because of the possibility of a dust layer remaining in the drier for a long time. Abbot (1990) described an aerated cell test, and the CCPS Guidelines reference (2005) has been developed for drier hot layer ignition scenarios. The aerated cell test produces an exotherm onset temperature at which oxidation reactions leading to layer fires occur. Most of the exotherm onset data reported by Abbot (1990) were in the range of 125oC to 175oC. These temperatures are lower than the dust layer minimum hot surface ignition temperatures measured in the more common tests conducted in ambient air (ASTM E2021). Burning Embers and Agglomerates
Smouldering or flaming particulate embers or agglomerates (also called smouldering nests) are often produced by frictional heating, e.g. during sanding or cutting, by local heating associated with hot work on equipment and ducts containing dust deposits, by powder accumulations on drier walls, and by small heat sources, e.g. a portable lamp, accidentally embedded in a particulate pile. A more significant fire can develop if the hot embers or agglomerates remain stationary in a more enormous accumulation of combustible particulates. On the other hand, if the embers/agglomerates are exposed to an explosive dust cloud in an enclosure (perhaps a silo/hopper being filled), there is a potential for the ignition of a dust explosion.
Tabulations of ignition sources involved in 426 German dust explosions from 1965 to 1985 (Eckhoff, 2003, Tables 1.6 and 1.7) indicate that smouldering nests were the most prevalent cause of those dust explosions in silos (28%) and dryers (29%), and the second most frequent ignition source in dust collector explosions (11%). Zalosh et al. (2005) describe one dust explosion incident in which the hot nest was caused by some bolts falling into a hammermill used for pharmaceuticals production. More recent research (Gummer and Lunn, 2003) has shown that the ignitions in most of these reported incidents were probably due to flaming rather than smouldering nests/agglomerates since the only dust cloud material that could be ignited by smouldering agglomerates banked up in a 10 cm diameter tray was sulfur, which has an exceptionally low AIT (280-370 oC). Previous experiments cited by Gummer and Lunn indicated that dust cloud required a minimum agglomerate burning area of 75 cm2 and a minimum burning temperature of 900 oC to ignite dust clouds with AITs below 600 oC.
The occurrence of agglomerate smouldering versus flaming combustion and self- extinguishment depends on air access and the agglomerate coherence. Burning agglomerate transport experiments reviewed by Gummer and Lunn (2003) showed that glowing agglomerates could be transported large distances through otherwise empty piping with air transport velocities of 10 and 20 m/s. Still, the glowing was extinguished rapidly when non- burning dust was added to the flow. The glowing particles could not ignite the flowing dust even though the dust concentration was above the MEC. Other tests showed that burning nests did not ignite fine sawdust in the transport duct but did ignite the sawdust cloud when it reached the filter media dust collector at the end of the chimney.
Several vendors provide so-called spark/ember detection and extinguishing systems to prevent ignitions by burning agglomerates transported through ducting. Optical detectors sense the radiant energy from the burning embers or agglomerates, and the control module triggers water spray through nozzles situated at an appropriate distance downstream of the sensor. Annexe C of NFPA 654 describes these systems. Self-Heating
Certain particulate materials are prone to self-heating, which can potentially lead to spontaneous ignition. The predominant chemical reaction is low-level oxidation. Examples of materials that can self-heat by oxidation at relatively low temperatures include ABS resin powder, activated carbon, coal (particularly Powder River Basin coal), and various chemical intermediates. Materials such as freshly manufactured/dried wood chips, anhydrous calcium hypochlorite, and hops are subject to self-heating by moisture absorption/condensation. Organic peroxides and other potentially unstable chemicals can self-heat by exothermic decomposition. Various agricultural materials, such as bagasse and soybeans, start self-heating by microbiological processes. In many of these and other materials, multiple self-healing mechanisms overlap, and it is difficult to distinguish the dominant mechanism at a given temperature.
Self-heating is typically manifested as smouldering in the interior of a large storage pile of particulates or an accumulated layer in a dryer. If the smouldering particulates in the pile or dryer are subsequently disturbed and exposed to air, the smouldering can evolve into flaming. When the flaming nest or agglomerate is then transported to a hopper or dust collector, it can ignite the suspended dust cloud, as discussed in the preceding section.
Various laboratory tests have been developed to determine self-heating onset temperatures for different sample sizes and configurations. These include particulate basket tests in an isothermal oven, heated air flow tests with a slow rate of air temperature rise, and material in a package test to determine the Self-Accelerating Decomposition Temperature. Application of laboratory self- heating data to plant conditions requires the use of appropriate volume scaling methods described in handbook references, including Babrauskas (2003), the CCPS Guidelines (2005), and Gray (2002). In addition to showing how the self-heating onset temperature decreases with the increasing size of the particulate pile or layer, the scaling relationships also can be used to assess how the expected time-to-ignition increases with the pile or layer size. Engineers can then use the combination of laboratory data and the scaling equations to establish appropriate plant level precautions to prevent self-heating and spontaneous ignition.
Impact/Friction
Impact and frictional heating during combustible powder processing and maintenance/repairs involving cutting and grinding have been responsible for igniting many dust explosions. Grinders, hammermills, and other size reduction equipment are particularly prone to ignitions during operation. Blenders with rotating element tip speed greater than 1 m/s are also vulnerable to this scenario. Tramp metal stuck in a screw conveyor, or a particle classifier represents another frictional ignition scenario.
The vulnerability of combustible dust to impact/friction ignition is characterised by the material spark Minimum Ignition Energy (MIE) and cloud Auto-Ignition Temperature (AIT). Testing to measure MIE values is described in ASTM E2019. For example, dust with an MIE of 10 J should be immune to steel-steel frictional or impact ignitions as long as its AIT is more significant than 275 oC. Eckhoff (2003) cautions that simple MIE versus AIT correlations cannot apply to steel grinding and impact conditions that may differ from the experiments leading to similar plots. Babrauskas (2003) presented data on the minimum frictional force needed to ignite various dust clouds.
One common friction ignition scenario is a blender with a rotating helical screw impeller. Jaeger (2001) guided how Engineers can use the mixing speed and blender fill level to control frictional ignition hazards. He states a negligible chance of ignition when the fill level is greater than 70%, no matter what the impeller tip speed is. When the tip speed is greater than 10 m/s and the fill level is less than 70%, there is a high probability of dust cloud ignition. Jaeger provides an MIE versus AIT relationship at tip speeds between 1 m/s and 10 m/s and fills levels less than 70% to show which combustible dusts can be blended without any likelihood of ignition.
Single impact spark ignition experiments described by Eckhoff (2003) have shown that the probability of igniting a corn starch dust cloud increased with increasing impact energy and that it also depended on the impact velocity. Lower speed impacts produced a much greater probability of ignition than higher speed impacts for given impact energy. The metal combinations involved in the impact also play an important role in the probability of ignition.
Steel-steel impacts and aluminium-steel impacts did not ignite corn starch dust clouds, whereas titanium impacts against rusty steel did ignite dust with MIE values below roughly 10 mJ. The titanium-rusty steel impacts produced thermite reaction sparks, while the aluminium-rusty steel impacts did not.
Electrical Equipment
Electrical equipment and wiring can potentially ignite dust clouds by sparks, arcs, or heated surfaces. Dust Ignitionproof equipment is enclosed in a manner that excludes dust and does not permit arcs, sparks or heats otherwise generated or liberated inside of the enclosure to cause ignition of exterior accumulations or atmospheric suspensions of specified dust on or in the vicinity of the enclosure. UL 1203 describes the design, fabrication, and testing required to certify electrical equipment as Dust Ignitionproof.
When electrical equipment and wiring are used in locations in which combustible dust can be present, there is a need to establish the area's Class II hazardous location classification. Per NFPA 70, a Class II Division 1 location is one in which combustible dust is in the air under normal operating conditions in quantities to produce explosive or ignitable mixtures, or where mechanical failure or abnormal operation of machinery or equipment might cause such explosive or ignitible mixtures to be produced. It might also provide a source of ignition through a simultaneous electrical equipment failure (NFPA 70 definition). There are three possible conditions for the existence of a Class II Division 2 location. The first condition is a location where combustible dust due to abnormal operations may be present in the air in quantities sufficient to produce explosive or ignitable mixtures. The second and third conditions refer to dust accumulations that could be either suspended or ignited during equipment malfunctions or abnormal operations. Class II locations are further classified as Group E, F, or G depending on the type of dust material. NFPA 499 provides guidance and examples for the assignment of appropriate Class II Division 1 and 2 classifications for combustible powder and dust processing and handling operations.
NFPA 70 Article 500.7 permits Dust Ignitionproof electrical equipment in Class II Division 1 and 2 areas. Similarly, intrinsically safe electrical equipment (in which all circuits cannot produce a spark or thermal effect capable of igniting a dust cloud per UL 913) is also allowed in these areas. Dust-tight equipment is permitted in Class II Division 2 areas. Article 502 of NFPA 70 describes the types of acceptable wiring in Class II Division 1 and 2 locations. Threaded metal conduit together with dust-tight boxes and fittings is one acceptable method commonly used. The use of electrical sealing putty at boundaries of Class II areas is also described in Article 502.
Electrostatic Discharges
Electrostatic discharges occur are preceded by charge accumulation on insulated surfaces, ungrounded conductors (including human bodies), or particulate materials with high resistivities. The subsequent electrostatic discharge is only an ignition threat if it is sufficiently energetic compared to the Minimum Ignition Energy of the pertinent dust cloud.
Pressure Development in Dust Deflagrations
Pressures in Single Enclosures
Deflagration pressures resulting from ignition in process equipment depend on the dust material, particle size distribution, and concentration distribution within the enclosure—the size and location of equipment openings allow the burning and unburned dust to be vented. If there were no openings in the equipment, the deflagration pressure would correspond to the pressures measured in the ASTM E1226 tests. Since these pressures are greater than 2 bar gauge, most process equipment cannot withstand the closed vessel deflagration pressure even at concentrations near the MEC. Pressures at the worst-case dust concentration often range from 7 to 10 bar. Therefore, NFPA 654 paragraph 7.1.2.1 requires process equipment with an explosion hazard to be equipped with one of six specified alternative methods of explosion protection. The most commonly used dust explosion protection method is deflagration venting. The effectiveness of deflagration vents depends on the turbulence level in the process vessel and the vessel size and shape and the vent design, and the dust characteristics cited above. NFPA 68 Chapter 7 provides dust deflagration design requirements. Deflagrations Involving Interconnected Equipment
When process vessels are connected by pipe and ducting, a dust explosion ignited in one vessel can often propagate into the interconnected vessels. Pressures produced in the interconnected vessels can be significantly greater than the pressure experienced in isolated vessels. The reason for the enhanced deflagration pressure in a totally enclosed system is that the initiating explosion pressurizes the interconnected vessels. The deflagration that eventually occurs when the flame reaches the other vessels' dust cloud starts at a higher initial pressure. This effect is called pressure piling. Lunn et al. (1996) conducted interconnected vessel tests with coal dust and toner with Pmax values of 7.7 bar g and 7.1 bar g, respectively, in single closed vessel tests. When the explosions were initiated in a 20 m3 vessel and allowed to propagate via a 25 cm diameter pipe into a 4 m3 vessel with a dust cloud, the measured pressures were 16 to 20 bar g, i.e. more than twice the Pmax values. Inter-vessel deflagration propagation and pressure piling do not always occur. Lunn et al. (1996) did not observe deflagration propagation in tests with a 15 cm diameter pipe. Later vented explosion tests using a pipe with a sharp 90-degree elbow produced pressure enhancement is only one of many tests conducted (Skjold, 2007). However, when the deflagration does propagate into the interconnected vessels, the jet flame ignition of the second vessel's dust cloud produces a much more rapid rate of burning and associated pressure rise. The more rapid burning and pressure rise can render explosion venting or explosion suppression systems ineffective in the second vessel. Hence, there is often a need for explosion isolation systems to supplement an individual vessel's installed explosion protection. NFPA 69 provides the requirements for various types of passive and active explosion isolation systems.
Secondary Dust Explosions
Most of the casualties from dust explosions occur when the initiating explosion within some equipment or enclosure breaches the equipment/enclosure and causes a secondary explosion in the surrounding building. The secondary explosion occurs when dust deposits on exposed surfaces in the building are lifted by the blast wave emanating from the breached equipment/enclosure and then are ignited by the flame vented from the breached equipment/enclosure. Airblast velocities of 12 to 48 m/s lifted 13% to 44% of the deposited cornstarch.
These secondary dust explosions are particularly devastating because they produce large burning dust clouds and pressures beyond most buildings' strength. The two critical prevention measures are installing effective explosion protection for the combustible powder/dust processing and handling equipment (including explosion isolation) and minimizing combustible dust layer accumulations on equipment and building surfaces. NFPA 654 provides requirements for maximum allowable dust layer thicknesses and surface areas with dust accumulations. Some of the other papers at this Symposium offer guidance on how different industrial facilities are attempting to meet these requirements and possibly improve them.
