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.     

Flame propagation behaviors of nano- and micro-scale PMMA dust explosions

Flame propagation behaviors of nano- and micro-polymethyl methacrylate (PMMA) dust explosions were experimentally studied in the open-space dust explosion apparatus. High-speed photography with normal and microscopic lenses were used to record the particle combustion behaviors and flame microstructures. Simple physical models were developed to explore the flame propagation mechanisms. High-speed photographs showed two distinct flame propagation behaviors of nano- and micro-PMMA dust explosions. For nano-particles, flame was characterized by a regular spherical shape and spatially continuous combustion structure combined with a number of luminous spot flames. The flame propagation mechanism was similar to that of a premixed gas flame coupled with solid surface combustion of the agglomerates. In comparison, for micro-particles, flame was characterized by clusters of flames and the irregular flame front, which was inferred to be composed of the diffusion flame accompanying the local premixed flame. It was indicated that smaller particles maintained the leading part of the propagating flame and governed the combustion process of PMMA dust clouds. Increasing the mass densities from 105 g/m³ to 217 g/m³ for 100 nm PMMA particles, and from 72 g/m³ to 170 g/m³ for 30 μm PMMA particles, the flame luminous intensity, scale and the average propagation velocity were enhanced. Besides, the flame front became more irregular for 30 μm PMMA dust clouds.     

Does your facility have a dust problem: Methods for evaluating dust explosion hazards

The hazards of dust explosions prevailing in plants are dependent on a large variety of factors that include process parameters, such as pressure, temperature and flow characteristics, as well as equipment properties, such as geometry layout, the presence of moving elements, dust explosion characteristics and mitigating measures. A good dust explosion risk assessment is a thorough method involving the identification of all hazards, their probability of occurrence and the severity of potential consequences. The consequences of dust explosions are described as consequences for personnel and equipment, taking into account consequences of both primary and secondary events.While certain standards cover all the basic elements of explosion prevention and protection, systematic risk assessments and area classifications are obligatory in Europe, as required by EU ATEX and Seveso II directives. In the United States, NFPA 654 requires that the design of the fire and explosion safety provisions shall be based on a process hazard analysis of the facility, process, and the associated fire or explosion hazards. In this paper, we will demonstrate how applying such techniques as SCRAM (short-cut risk analysis method) can help identify potentially hazardous conditions and provide valuable assistance in reducing high-risk areas. The likelihood of a dust explosion is based on the ignition probability and the probability of flammable dust clouds arising. While all possible ignition sources are reviewed, the most important ones include open flames, mechanical sparks, hot surfaces, electric equipment, smoldering combustion (self-ignition) and electrostatic sparks and discharges. The probability of dust clouds arising is closely related to both process and dust dispersion properties.Factors determining the consequences of dust explosions include how frequently personnel are present, the equipment strength, implemented consequence-reducing measures and housekeeping, as risk assessment techniques demonstrate the importance of good housekeeping especially due to the enormous consequences of secondary dust explosions (despite their relatively low probability). The ignitibility and explosibility of the potential dust clouds also play a crucial role in determining the overall risk.Classes describe both the likelihood of dust explosions and their consequences, ranging from low probabilities and limited local damage, to high probability of occurrence and catastrophic damage. Acceptance criteria are determined based on the likelihood and consequence of the events. The risk assessment techniques also allow for choosing adequate risk reducing measures: both preventive and protective. Techniques for mitigating identified explosions risks include the following: bursting disks and quenching tubes, explosion suppression systems, explosion isolating systems, inerting techniques and temperature control. Advanced CFD tools (DESC) can be used to not only assess dust explosion hazards, but also provide valuable insight into protective measures, including suppression and venting.     

Behavior of flames propagating through lycopodium dust clouds in a vertical duct

The structure of flame propagating through lycopodium dust clouds has been investigated experimentally. Upward propagating laminar flames in a vertical duct of 1800 mm height and 150×150 mm square cross-section are observed, and the leading flame front is also visualized using by a high-speed video camera. Although the dust concentration decreases slightly along the height of duct, the leading flame edge propagates upwards at a constant velocity. The maximum upward propagating velocity is 0.50 m/s at a dust concentration of 170 g/m³. Behind the upward propagating flame, some downward propagating flames are also observed. Despite the employment of nearly equal sized particles and its good dispersability and flowability, the reaction zone in lycopodium particles cloud shows the double flame structure in which isolated individual burning particles (0.5–1.0 mm in diameter) and the ball-shaped flames (2–4 mm in diameter; the combustion time of 4–6 ms) surrounding several particles are included. The ball-shaped flame appears as a faint flame in which several luminous spots are distributed, and then it turns into a luminous flame before disappearance. In order to distinguish these ball-shaped flames from others with some exceptions for merged flames, they are defined as independent flames in this study. The flame thickness in a lycopodium dust flame is observed to be 20 mm, about several orders of magnitude higher than that of a premixed gaseous flame. From the microscopic visualization, it was found that the flame front propagating through lycopodium particles is discontinuous and not smooth.     

湍流对粉尘云电火花点火过程的影响

随着现代工业的发展,粉尘爆炸事故发生的频率也逐年增加,因此,对粉尘云点火敏感程度进行测量和计算就变得十分重要。粉尘云最小点火能是粉尘爆炸重要的特性参数之一,是采取粉尘爆炸防护的基础。最小点火能在测量的过程中受到多个敏感条件的影响,其中湍流则是最复杂的影响因素之一。文中对实验过程中粉尘云的湍流进行了定义,并分析了湍流对粉尘云最小点火能影响的内在原因;同时对通过数值模拟计算粉尘云最小点火能过程中的湍流计算给出了数学模型。从实验和数学模型两个方向对湍流进行了全面描述,对粉尘云电火花点火过程中湍流影响的分析结论,可有效的指导实验。     

Dust collector explosions: A quantitative hazard evaluation method

A methodology is presented to evaluate the explosion hazard of typical bag and cartridge dust collectors. The evaluation accounts for the expected development of suspended dust concentrations greater than the dust MEC during the normal pulsing of the bags or cartridges to remove part of the attached dust. Equations are presented to calculate these concentrations and also the associated partial volume explosion pressures resulting from the ignition of these dust clouds. Five quantitative examples are presented. The methodology also includes considerations of potential upset condition full volume explosions associated with the detachment of about half the dust on the bags or cartridges. A flow chart is offered to implement this hazard evaluation method for special situations in which the need for dust explosion protection may not be obvious.     

工业生产粉尘爆炸预防和缓解──近期研究与发展综述

本文包括三个方面，即工业粉爆的基础研究、应用研究以及粉尘点人性和可爆性。基础研究包括粉尘云的形成和点火，火焰的传播以及粉煤产生的冲击波。工业预防措施包括惰化和消除点火源。缓解的方法包括隔爆、泄爆、部分惰化、抑爆和全封闭。基础研究和应用研究相互促进。计算机模拟模型可能成为非常有效的方法，也可用于专家系统。     

A constant pressure dust explosion experiment

A new apparatus has been designed for investigating flame propagation in turbulent dust clouds at near constant pressure conditions. The experimental approach is inspired by the classical soap bubble method for measuring burning velocities in gaseous mixtures. Combustible dust is dispersed with pressurised air to form an explosive mixture inside a transparent latex balloon. After a certain delay time, the turbulent dust cloud is ignited by a 40 J chemical igniter. A digital high-speed video camera records the propagating flame and the expansion of the balloon. Experiments were performed with two types of dust, Lycopódium spores and maize starch, as well as with propane–air mixtures under initially quiescent or turbulent conditions. Although the results are primarily qualitative in nature, they nevertheless demonstrate fundamental differences between premixed combustion of gaseous mixtures, and ‘premixed combustion with non-premixed substructures' in mechanical suspensions of solid particles dispersed in air. The discussion highlights some fundamental challenges for future dust explosion research.     

Estimation of turbulent flame speed during DME/air premixed gaseous explosions

DME is thought to be a good alternative fuel due to its cleanliness and more excellent fuel economy. Although the prediction and loss prevention of flammability hazard is very important for safety of DME installations, the evaluation method with sufficient accuracy has not been established. In this study, a numerical combustion model is constructed and a 3-dimensional computational fluid dynamics (CFD) simulation of a premixed DME/air explosion in a large-scale domain is conducted. The main feature of the numerical model is the solution of a transport equation for the reaction progress variable using a function for turbulent flame velocity which characterizes the turbulent regime of propagation of free flames derived by introducing the fractal theory. The model enables the calculation of premixed gaseous explosion without using fine mesh of the order of micrometer, which would be necessary to resolve the details of all instability mechanisms. The value of the empirical constant contained in the function for turbulent flame velocity is evaluated by analyzing the experimental data of LPG/air and DME/air premixed explosions. The comparison of flame behavior between the experimental result and numerical simulation shows good agreement.     

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.     

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

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.     

Influence of different ignition delay times on the pressure rise rate in hybrid mixture explosions in the 20-L sphere

For the development of a standardized method for measuring the explosion safety characteristics of combustible hybrid dust/vapor mixtures, the influence of the ignition delay time needs to be investigated. The ignition delay time, defined as the time between the injection of dust and the activation of the ignition source, is related to the turbulence of the mixture and thus to the pressure rise rate. The ignition source for pure vapors, however, has to be activated in a quiescent atmosphere according to the standards. Nevertheless, when measuring the explosion safety characteristics of hybrid mixtures, it is important that the dust be in suspension around the igniter. Like pure dust/air mixtures, hybrid dust/vapor/air mixtures need to be ignited in a turbulent atmosphere to keep the dust in suspension.This work will therefore investigate the influence of ignition delay times on the severity of hybrid explosions. It was generally found that at shorter ignition delay times, (dp/dt)_ex increased due to higher turbulence and decreases as the dust sinks to the bottom of the 20 L-sphere. This effect is more pronounced for hybrid mixtures with higher vapor content compared to dust content.     

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 releaseThis 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.     

爆炸灾害的预防和控制乃当务之急

我国爆炸事故近年来屡屡发生，灾害严重，损失巨大。爆炸灾害的预防与控制己是当务之急。当前的主要研究内容，有各种工业粉尘爆炸、气液贮罐爆炸、工业生产中的静电灾害、各种爆炸灾害的实验观测和监控技术、防护与控制技术、模拟与仿真技术、易燃易爆物的危险性评估和系统安全性分析、燃烧转爆炸机理和热爆炸（热自燃）机理等方面。简略介绍了爆炸灾害预防、控制国家重点实验室的筹建工作，希望得到同行专家的指导和帮助，并开展合作研究。     

Flame propagation mechanisms in dust explosions

To reveal the effects of particle characteristics, including particle thermal characteristics and size distributions, on flame propagation mechanisms during dust explosions clearly, the flame structures of dust clouds formed by different materials and particle size distributions were recorded using an approach combining high-speed photography and a band-pass filter. Two obviously different flame propagation mechanisms were observed in the experiments: kinetics-controlled regime and devolatilization-controlled regime. Kinetics-controlled regime was characterized by a regular shape and spatially continuous combustion zone structure, which was similar to the premixed gas explosions. On the contrary, devolatilization-controlled regime was characterized by a complicated structure that exhibited heterogeneous combustion characteristics, discrete blue luminous spots appeared surrounding the yellow luminous zone. It was also demonstrated experimentally that the flame propagation mechanisms transited from kinetics-controlled to devolatilization-controlled while decreasing the volatility of the materials or increasing the size of the particles. Damköhler number was defined as the ratio of the heating and devolatilization characteristic time to the combustion reaction characteristic time, to reflect the transition of flame propagation mechanisms in dust explosions. It was found that the kinetics-controlled regime and devolatilization-controlled regime can be categorized by whether Damköhler number was less than 1 or larger than 1.     

Reconsidering autoignition in the context of modern process safety: Literature review and experimental analysis

Autoignition is regarded as the spontaneous combustion of a fuel without an apparent ignition source. With respect to fires and explosions in the process industries, the autoignition hazard is one that requires management and consideration. Despite the importance of understanding the autoignition hazard, the literature on the topic is often disappointingly sparse and inconsistent. Experimental methods don't adequately represent real-world conditions and are complicated by experimental error, invoking the need for a reconsideration of autoignition as it pertains to process safety. This work utilized the ASTM E659 method to study potential experimental error for the purpose of improving the process safety community's understanding of autoignition phenomena. Of interest to this study were effects of humidity and suspected occurrences of cool flames, two sources of error which have not been fully explained in the literature. For the fuels tested, results show that humidity has only a slight effect on overall autoignition behavior. However, this study's examination of cool flames suggests that they could be a common occurrence in this testing method. Analysis of these experiments show that cool flames can feature significant exothermic effects and are thus of concern from a perspective of risk management. In addition, this work proposes a novel criterion for a more conservative assessment of autoignition experiments, which results in less subjectivity of analysis.     

碳酸盐对密闭空间粉尘爆炸压力影响的试验研究

为了预防和缓解工业粉尘爆炸并研究惰性粉尘对粉尘爆炸的惰化作用,在Siwek 20 L球形爆炸装置内,针对高爆镁粉和高灰分煤粉,选用碳酸钙(CaCO3)、碳酸氢钠(NaHCO3)、碳酸氢钾(KHCO3)等3种碳酸盐作为惰化剂,讨论惰化剂浓度、粒径及点火能量对最大爆炸压力的影响。结果表明,惰化剂粒径越小,浓度越高,对粉尘爆炸的惰化作用越强;粉尘爆炸的净升压与点火能量无关,点火头主要起引燃作用;当惰化剂浓度递增至60%时以上,粉尘爆炸压力急剧下降,直至不爆。此外,CaCO3的抑制效果明显优于NaHCO3、KHCO3,故推荐采用CaCO3来控制粉尘爆炸风险。     

自由空间粉尘云雾爆炸压力场分析

本文旨在研究固体粉尘在自由空间中爆炸压力场的分布。进行了复合粉尘燃料和液体燃料环氧丙烷的爆炸实验,利用压力传感器和高速摄影,获得了在自由空间中的压力实测数据,并对数据进行了详细地分析处理,得到了典型的超压曲线以及高速摄影图片。研究表明：粉尘云雾爆炸的超压峰值较大,在云雾区内压力存在相对低压区,超压持续时间远比液体环氧丙烷长。     

Minimum Ignition Temperature of layer and cloud dust mixtures

The prevention of dust explosions is still a challenge for the process industry. Ignition, in particular, is a phenomenon that is still not completely understood. As a consequence, safety conditions pertaining to ignition suppression are rarely identified to an adequate level. It is well known that, in general, the ignition attitude of a dust depends on several factors, such as the nature of the chemical, the particle size, moisture content, etc., but there is still a lack of knowledge on the effect of the single variables.This paper has the aim of providing data on the Minimum Ignition Temperatures of dust mixtures obtained from a mixing of a combustible dust (flour, lactose, sucrose, sulphur) and an inert dust (limestone, extinguishing powders) as well as from the mixing of two different combustible dusts. Various mixtures with different weight ratios have been tested in a Godbert Greenwald (GG) furnace and on a hot plate in order to measure the effect of mixture composition on the Minimum Ignition Temperature (MIT_L) of the layer and on the Minimum Ignition Temperature (MIT_C) of the cloud. In order to further verify the effects of inert dust particle size, inerts sieved to different size ranges have been tested separately. Generally, both MIT_L and MIT_C increase as the inert content is increased. MIT_C is poorly affected by inert particle size when limestone is used. The MIT_L of pure flour is higher than the MIT_L of mixtures containing up to 40% of 32–75 μm of limestone. This was probably due to the behaviour of pure flour during the test, which demonstrated strong tendency to produce char, cracks in the layer and detachment from the hot plate.     

Experiment-based investigations of magnesium dust explosion characteristics

An experimental investigation was carried out on magnesium dust explosions. Tests of explosion severity, flammability limit and solid inerting were conducted thanks to the Siwek 20 L vessel and influences of dust concentration, particle size, ignition energy, initial pressure and added inertant were taken into account. That magnesium dust is more of an explosion hazard than coal dust is confirmed and quantified by contrastive investigation. The Chinese procedure GB/T 16425 is overly conservative for LEL determination while EN 14034-3 yields realistic LEL data. It is also suggested that 2000-5000 J is the most appropriate ignition energy to use in the LEL determination of magnesium dusts, using the 20 L vessel. It is essential to point out that the overdriving phenomenon usually occurs for carbonaceous and less volatile metal materials is not notable for magnesium dusts. Trends of faster burning velocity and more efficient and adiabatic flame propagation are associated with fuel-rich dust clouds, smaller particles and hyperbaric conditions. Moreover, Inerting effectiveness of CaCO₃ appears to be higher than KCl values on thermodynamics, whereas KCl represents higher effectiveness upon kinetics. Finer inertant shows better inerting effectiveness.