Dust Cloud Explosion Research Articles

ASTM E2931: A new standard for the limiting oxygen concentration of combustible dusts

The National Fire Protection Agency (NFPA) has a standard on explosion protection systems, NFPA 69, which provides guidelines on effective inerting to prevent explosions. The standard specifies that for inerting to be effective the oxygen concentration must be kept below the Limiting Oxygen Concentration (LOC). It then goes on to specify that the ASTM International standard E2079 be used to establish the LOC. The shortcoming of this approach is that ASTM E2079 only applies to combustible gases and vapors and not combustible dusts. As a result of this deficiency ASTM International has just introduced a new standard, ASTM E2931, Standard Test Method for Limiting Oxygen (Oxidant) Concentration of Combustible Dust Clouds. This paper discusses the nuances of this standard and compares experimental results between the 20-L chamber and the 1-m3 chamber. Differences in the test results between the vessels and between test methods may have safety ramifications to the end user of the data. The large variation present in the repeatability and reproducibility of the LOC means that the current common practice of using a 2% safety margin for particle inerting (the least stringent of the inerting methods) may be insufficient to ensure dust cloud explosion mitigation. It is possible that additional study and improved laboratory proficiency as the test standard matures will bring down these repeatability and reproducibility errors. Additionally, if LOCs are reported in Safety Data Sheets without accompanying information regarding the test method or test vessel size used, the mitigation strategy may not provide adequate protection. © 2015 American Institute of Chemical Engineers Process Saf Prog 35: 159–164, 2016

Dust explosion hazard assessment

The majority of powders that are used in the processing industries are combustible (also referred to as flammable, explosible). An explosion will occur if the concentration of the combustible dust that is suspended in air is sufficient to propagate flame when ignited by a sufficiently energetic ignition source. A systematic approach to identifying dust cloud explosion safety against their consequences generally involves: -Identification of locations where combustible dust cloud atmospheres could be present -Understanding of the explosion characteristics of the dust(s) -Identification of potential ignition sources that could be present under normal and abnormal conditions -Proper process and facility design to eliminate and/or minimize the occurrence of dust explosions and protect people and facilities against their consequences -Adequate maintenance of facilities to prevent ignition sources and minimize dust release This presentation will discuss the conditions that are required for dust cloud explosions to occur and presents a well-tried approach to identify, assess, and eliminate/control dust explosion hazards in facilities.

Dust Explosion Characteristics of Agglomerated 35 nm and 100 nm Aluminum Particles

In the experiment, nanoparticles of 35 nm Al and 100 nm Al powders, respectively, formed particles with average sizes of 161 nm and 167 nm in agglomeration. The characteristics of dust cloud explosions with the two powder sizes, 35 nm and 100 nm, revealed considerable differences, as shown here: (dp/dt)max-35 nm= 1254 bar/s, (dp/dt)max-100 nm= 1105 bar/s; Pmax-35 nm= 7.5 bar, Pmax-100 nm= 12.3 bar, and MEC-35 nm= 40 g/m3, MEC-100 nm= 50 g/m3. The reason of Pmax-35 nmvalue is smaller than Pmax-100 nmmay be due to agglomeration. From an analysis of the explosive residue, the study found that nanoparticles of 35 nm Al powder became filamentous strands after an explosion, where most of 100 nm Al nanoparticles maintained a spherical structure, This may be because the initial melting temperature of 35 nm Al is 435.71°C, while that for 100 nm Al is 523.58°C, higher by 87.87°C. This study discovered that explosive property between the 35 nm Al and 100 nm Al powders after agglomeration were different.

Hazard assessment of complex hydrides as hydrogen storage materials

Hydrogen storage materials containing NaAlH4, LiNH2, Mg(NH2)2, LiH and LiBH4 were subjected to standardized safety tests in order to assess the potential hazards caused by the environmental exposure of these materials. All the materials were judged ‘flammable’, ‘pyrophoric’ and ‘water-reactive’, resulted in being classified as the United Nations Packing Group I, the most stringent category of container regulations in transporting these materials. A small spark energy (1.4 mJ) can trigger an intense dust cloud explosion of the Mg(NH2)2 + LiH system of which the minimum explosive concentration was determined to be 90 mg dm−3. Although this value is lower than those of the hydrogen storage alloys, the minimum explosive concentration of complex hydrides can be comparable to the alloys if expressed in terms of the amount of stored hydrogen in the material. Also examined was the eruption test, a non-standard test, in which the sample powder was pushed out of a container into the atmosphere by pressurized H2. Despite the pyrophoricity, we observed only one explosion of the Ti-doped NaAlH4 in dozens of trials using all the materials. A comparison with other materials points to the inevitability of more cautious measures than metal hydrides when handling these complex hydrides.

Influence of the oxide content on the ignition energies of aluminium powders

Some results of determination of ignition energies for an aluminium powder with various oxide contents are presented. Common use of processes like high-speed cutting produce explosive dust clouds, so that we focused this study on hazard of metallic powders. An industrial aluminium powder has been used for this work. An original process, based on the principle of electrochemical anodisation, has been developed to increase, under control, the oxide coating of particles. The sensitivity study to spark ignition was performed in an Hartmann explosion tube of 1.3 L. The Langlie test method was applied to evaluate the energies leading to a probability of ignition of 50% ( E 50) of the selected samples. The results confirm that the ignition energies increase with the oxide content of the powder.

Measurement of minimum ignition energies of dust clouds in the

The lower energy limit of current standard test apparatus for determining the minimum ignition energy (MIE) of dust clouds is in the range of 1–3 mJ. This is a quite severe limitation because many dusts ignite readily at this energy level. A new spark generator, capable of producing synchronised sparks of very low energies and with an integrated system for measurement of spark energy, has therefore been developed and employed to a number of easily ignitable dusts. Before testing the MIE of dust clouds, it was considered essential to calibrate the new spark generator against a gas of known MIE. For this purpose, a mixture of propane and air was selected. However, a comprehensive literature review revealed that the reported MIEs of this gas mixture vary significantly, depending on the spark discharge characteristics, including discharge duration. When taking these factors into account, it was concluded that the new spark generator yielded reasonable results for propane/air. Applying the new spark generator to explosive dust clouds showed that a number of dusts do in fact have MIEs that are one to two orders of magnitude lower than 1 mJ. The new spark generator may therefore offer a basis for developing a standard test apparatus in the low-energy region. When using a method of triggering the spark by the explosive dust cloud itself, which probably is a more industrially relevant process than synchronisation between the dust dispersion and sparkover, somewhat higher MIEs were found compared to those determined when using synchronised sparks. However, even with this method of spark triggering, MIEs below 1 mJ were found.

Differences and similarities of gas and dust explosions: A critical evaluation of the European ‘ATEX’ directives in relation to dusts

Explosive gas mixtures and explosive dust clouds, once existing, exhibit similar ignition and combustion features. However, there are two basic differences between dusts and gases which are of substantially greater significance in design of safety standards than these similarities. Firstly, the physics of generation and up-keeping of dust clouds and premixed gas/vapour clouds are substantially different. This means that in most situations where accidental explosive gas clouds may be produced quite readily, generation of explosive dust clouds would be highly unlikely. Secondly, contrary to premixed gas flame propagation, the propagation of flames in dust/air mixtures is not limited only to the flammable dust concentration range of dynamic clouds. The state of stagnant layers/deposits offers an additional discrete possibility of flame propagation. The two European Directives 94/9/EC (1994) and 1999/92/EC (1999) primarily address gases/vapours, whereas the particular properties of dusts are not addressed adequately. Some recent IEC and European dust standards resulting from this deficiency are discussed, and the need for revising the two directives accordingly is emphasized.

Modelling of dust lifting using the Lagrangian approach

The subject of this paper is dust lifting behind shock waves, a process that is important for the formation of explosive dust clouds in air. While Eulerian–Eulerian has been the standard numerical technique for such simulations, the Eulerian–Lagrangian technique has been used in this paper, making it possible to take into account more physical phenomena, such as particle–particle and particle–wall collisions. The results of the simulations are shown mainly graphically, as snapshots of particle positions at given times after the passing of the shock wave. The results show that the collisions, and the coefficient of restitution assumed for them, is important in determining the mobility and lifting of dust behind shock waves. The results also show that the idea of a horizontally travelling shock wave is an oversimplification: the strong pressure gradient at the surface results in a series of reflected waves generated at the surface and travelling into the gas phase.

Initiation of dust explosions by electric spark discharges triggered by the explosive dust cloud itself

An investigation of ignition of dust clouds by the use of electric spark discharges triggered by the explosive dust cloud itself has been conducted. This method of triggering capacitive sparks probably represents a realistic mechanism for initiating accidental dust explosions in industrial practice. Unlike the conventional method for determining the minimum ignition energy (MIE) in the laboratory, the delay between dust dispersion and spark discharge is not a degree of freedom. In stead, the transient dust cloud itself is used to initiate spark breakdown between electrodes set at a high voltage lower than breakdown in pure air. In the present study, different kinds of dusts were tested as ‘spark triggers’, and they exhibited quite different abilities to trigger breakdown. Large particles were found to initiate breakdown at lower voltages than smaller ones. In general, conductive particles were not found to initiate breakdown at lower voltages than dielectric ones when using the same dust concentration. Minimum ignition energies (MIE) of three dusts ( Lycopodium clavatum, sulphur and maize starch) were determined using the authors' method of study. The MIEs were somewhat higher than those obtained using conventional methods, but relatively close to the values obtained through conventional methods.

Ignitions of explosive dust clouds by smouldering and flaming agglomerates

Surveys show that burning nests of dusts have been ignition source for dust clouds involved in industrial explosions. The literature indicates, however, that hot nests are a poor ignition source and are difficult to convey through powder handling plant. This paper describes some test in which clouds of dusts with a range of Minimum Ignition Temperatures (MITs) were dispersed around dust agglomerations smouldering and flaming at various temperatures. Smouldering nests of dusts prove to be poor ignition sources for most dust clouds, failing to ignite dusts even when there is a large difference between the nest temperature and the MIT of the dust cloud. Smouldering nests with temperatures above approximately 700–800 °C were, however, able to ignite sulphur clouds. Flaming nests, on the other hand, were able to ignite clouds of dusts up to the maximum MIT used, 600–675 °C.

Minimum ignition energy (MIE) — a basic ignition sensitivity parameter in design of intrinsically safe electrical apparatus for explosive dust clouds

Abstract In general terms, the purpose of any safety standard is to define borderlines between safe and unsafe conditions, with reasonable safety margins. The electrical spark ignition sensitivity of dust clouds (MIE) varies over at least eight orders of magnitude. Therefore, in the case of intrinsically safe electrical apparatus to be used in the presence of explosive dust clouds, substantial differentiation of the minimum requirements to prevent ignition by electrical sparks is needed. The present paper proposes a method by which adequate differentiation of required maximum permissible currents and/or voltages in intrinsically safe electrical circuits to be used in explosive dust clouds can be achieved. In essence, the concept is to use conservative first-order ignition curves, calculated or estimated from the experimental MIE value of clouds in air of the actual dust. Charts to be used for design purposes are given in the paper. Internationally standardised test methods allow MIE for clouds of any dust to be determined, at least down to the range of a few mJ. There is, however, a need for a supplementary method covering the range of lower energies, down to 0.01 mJ.

Catastrophe Dynamics of Dust-Cloud Ignition Phenomena

This paper takes ignition and quenching phenomena of a dust-cloud explosion system as an example to illustrate the application prospect of catastrophe theory in dust explosion science. Through comparative study on catastrophe dynamics of a dust-cloud explosion system under different heat loss models, the significance of catastrophe dynamics of radiation effect in igniting behavior of a dust explosion system is indicated. It is also pointed out that a simplified representation of igniting process model may possibly change catastrophe set’s topological structure and stationary state structure of a dust explosion system, and therefore may have an important influence on realizing igniting process laws. The statement may be useful for understanding many other discontinuity phenomena widely existent in objective world system.

Explosion hazard to buildings and design load parameters

The paper deals with the effect of explosions on building structures. The term hazard is defined as a source of danger. The sources of these effects may include blasting operations in a quarry, a dust explosion close to a pipeline or reservoir crack in a chemical plant or oil refinery, a collision of cistern vehicles in road or rail transport, a terrorist attack or an explosion of explosive or inflammable gas or dust clouds in the vicinity of the assessed structure. The risk of these explosions is explained by the examination of a nuclear reactor structure. First, information on the potential sources of danger is collected from the area from a diameter 10 km [1] area around the assessed structure and the maximum quantities of explosive materials are determined. Then the design load parameters for the concrete nuclear plant are given, related to the quantities and kinds of explosives. The dominant explosion effect on a building structure is defined as the history of (a) propagating loading air pressure wave of maximum intensity caused by dust clouds or (b) seismic effect produced by ground vibrations. The results of the analysis are simplified criteria for the failure of the assess structure and the permissible limit response characteristics determined by a 3D analysis of the structure.

Recent development of standardization of testing methods for dust explosion in Japan

The Dust Explosion Committee of the Association of Powder Process Industry and Engineering (APPIE) of Japan has acted to establish the standard for the member of the Association. Two testing methods have been authorized by the Association. The methods are established to evaluate sensitivity and severity for dust explosion. The ignition sensitivity is evaluated by the minimum explosive dust cloud concentration. Tapping sieve equipment and a strictly-specified Hartmann-type explosion tube have been selected as the testing apparatus. The tapping sieve equipment is applied to measure the minimum explosive concentration by forming a dust cloud supplied from a sieve on the top of the explosion chamber. The Hartmann-type tube is used to investigate roughly whether the test powder is explosive or not. A lot of explosion tests had been carried out to standardize the equipment and evaluate the sensitivity. This APPIE standard is being revised. Along with the ISO standard for explosion severity, Research Institute of Industrial Safety, the Ministry of Labour published a Guide to Test Method for Explosion Pressure and Rate of Pressure Rise for Combustible Dusts in 1994. Based on this Guide, The APPIE Committee has made the draft for the member. These standards and drafts are aimed to be the Japanese Industrial Standard (JIS). No other similar standard exists in Japan. In the present paper, technical data and explosion statistics which are the background of the APPIE standard for the ignition sensitivity are to be described.

Shock waves in the expansion of a burning dust cloud

The destructive power of a dust-cloud explosion is related to the maximum pressure Pmax and the maximum rate of increase in pressure (dp/dt)max. However, those parameters do not define the process completely. Often, the strength of the walls within which the suspension burns is much less than the characteristic Pmax, and consequently, after the walls have failed, the partially burned mixture escapes together with the burning explosion products~ and fragments are formed. A considerable hazard is represented by the shock waves that may result from the nonstationary flow. The mechanical effects are divided into the primary failure due to the internal explosion and the secondary damage to external objects due to the shock waves, fragments, and burning products.

Initiation of grain dust explosions by heat generated during single impact between solid bodies

Several independent investigations of past dust explosions in the grain, feed and flour industries in the U.S.A. and Europe arrive at the conclusion that ‘friction sparks’ may have been the initiator of a substantial fraction, up to 50%, of all the explosions recorded. However, this conclusion has to a large extent been based on indirect evidence, and essential details of the suspected ignition process most often remained unconfirmed. In the present investigation a comprehensive series of single-impact ignition experiments were carried out, by generating explosible dust clouds of dried maize starch, in the region of tangential impact between a moving body and a stagnant horizontal anvil. The net energy of the impact was defined as the loss of kinetic energy of the moving body during impact. The influence of the impact energy on the frequency of ignition was determined. It was found that much higher energies were required for ignition at high velocities of impact than at low ones. The reason is that any moving object colliding with a solid surface, reduces the ignition sensitivity of the dust cloud in the vicinity of the impact point by inducing turbulence. Details of the impact and subsequent ignition of dust clouds were studied using a high-speed movie camera. The temperatures of the metal sparks generated from a range of different metals were measured by a four-wavelength optical pyrometer. Typical spark temperatures were ≈ 1500–2700 °C for mild steel and ≈2000–2900 °C for titanium. The overall conclusion from the present investigation is as follows: steel sparks produced by single impacts of net energies of up to 20 J, between steel and concrete, steel or rusty steel, are unlikely to ignite clouds of dried maize starch. In the case of maize starch of 10% moisture, ignition is unlikely even with titanium sparks. Maize starch is amongst the easiest ignitable dusts encountered in the grain, feed and flour industry. Also, most dusts found in practice will contain some moisture. Therefore, it seems unlikely that dust explosions involving dusts of grain, feed or flour can be initiated by heat from accidental single impacts between tramp metal, and anvils of metal, corroded metal, stone or concrete, unless the net impact energies are much higher than 20 J.