Parametric study of the explosivity of graphite-metals mixtures

The explosivity of dust clouds is greatly influenced by several parameters which depend on the operating conditions, such as the initial turbulence, temperature or ignition energy, but obviously also on the materials composition. In the peculiar case of a mixture of two combustible powders, the physical and chemical properties of both dusts have an impact on the cloud flammability and on its explosivity. Nevertheless, no satisfactory ‘mixing laws’ predicting the mixture behavior are currently available and the composition variable to be considered for such models greatly depend on the safety parameters which have to be determined: from volume ratios for some thermal exchanges and ignition phenomena, to surface proportions for some heterogeneous reactions and molar contents for chemical reactions. This study is mainly focused on graphite/magnesium mixtures as they are encountered during the decommissioning activities of UNGG reactors (Natural Uranium Graphite Gas). Due to the different nature and reactivity of both powders, these mixtures offer a wide range of interests. Firstly, the rate-limiting steps for the combustion of graphite are distinct from those of metals (oxygen diffusion or metal vaporization). Secondly, the flame can be thickened by the presence of radiation during metal combustion, whereas this phenomenon is negligible for pure graphite. Finally, the turbulence of the initial dust cloud is modified by the addition of a second powder. In order to assess the explosivity of graphite/magnesium clouds, a parametric study of the effects of storage humidity, particle size distribution, ignition energy, and initial turbulence has been carried out. In particular, it was clearly demonstrated that the turbulence significantly influences the explosion severity by speeding up the rate of heat release on the one hand and the oxygen diffusion through the boundary layer surrounding particles on the other hand. Moreover, it modifies the mean particle size and the spatial dust distribution in the test vessel, impacting the uniformity of the dust cloud. Thus, the present work demonstrates that the procedures developed for standard tests are not sufficient to assess the dust explosivity in industrial conditions and that an extensive parametric study is relevant to figure out the explosive behavior of solid/solid mixtures subjected to variations of operating conditions.     

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.     

Experimental study on electrostatic charging of polymer powders in mixing processes

Powder mixing is often carried out in the chemical and pharmaceutical industries. Electrostatic charges generated on polymer powders during mixing may lead to electrostatic problems due to the poor conductivity of those powders. In this study, we investigated the electrostatic charges, surface potential, and apparent volume resistivity of sample powders using a simple mixing device utilizing the Faraday cup method. To neutralize the charged powders, we also applied an AC-type ionizer in the mixing study. A commercial polyethylene powder with a mean particle size of 585 μm was tested in this experiment. The charge-to-mass ratios at the end of 600 s of mixing were ?0.075 nC/g at 295 rpm agitation speed, ?0.21 nC/g at 495 rpm, and ?0.31 nC/g at 660 rpm, high enough to cause electrostatic agglomeration and adhesion. The electric fields based on the surface potential on the powders were several hundreds of V/cm, too small to give rise to brush discharge. The apparent volume resistivity of powders estimated by a simple measurement system is 1.0 × 10¹⁶ Ωm, in reasonable agreement with that acquired by the conventional test cell method (5.9 × 10¹⁵ Ωm). The charging level on the polymer powders was reduced with an AC-ionizer.     

Combining product engineering and inherent safety to improve the powder impregnation process

The functionalization of nonwoven textiles can be realized by dry powder impregnation. In order to develop and improve this process, two complementary approaches have been combined: product engineering and inherent safety. It consists in integrating ab-initio consumers' requirements, production constraints as well as safety and environmental considerations. This case study is focused on the proposal, the characterization and the selection of powders mixtures of flame retardants and copolyesters, which will be used to create fire-proofed textiles. The influences of the chemical natures of the flame retardant (e.g. calcium carbonate, aluminium trihydroxide, ammonium polyphosphates), their respective concentrations, particle diameters and the addition of silica to flame retardant/polymer mixtures on their minimum ignition energy has been investigated. It has been determined that ammonium polyphosphates are far more efficient than other flame-retardants and that a minimum of 20%wt. concentration is needed to generate a powder mixture that will be almost insensitive to ignition by an electrostatic source. Modifying the particle size distribution and introducing glidants play also a significant role on flame retardant/polymer interactions, on powder dispersibility and has a strong impact on the minimum ignition energy. Finally, the formulations which have been selected fulfill the requirements for fire resistance, flowability, prevention of dust explosion; they are non-toxic, environmentally friendly and their cost is reduced.     

Applications of dust explosion hazard and disaster prevention technology

Powdered materials are widely used in industrial processes, chemical processing, and nanoscience. Because most flammable powders and chemicals are not pure substances, their flammability and self-heating characteristics cannot be accurately identified using safety data sheets. Therefore, site staff can easily underestimate the risks they pose. Flammable dust accidents are frequent and force industrial process managers to pay attention to the characteristics of flammable powders and create inherently safer designs.This study verified that although the flammable powders used by petrochemical plants have been tested, some powders have different minimum ignition energies (MIEs) before and after drying, whereas some of the powders are released of flammable gases. These hazard characteristics are usually neglected, leading to the neglect of preventive parameters for fires and explosions, such as dust particle size specified by NFPA-654, MIE, the minimum ignition temperature of the dust cloud, the minimum ignition temperature of the dust layer, and limiting oxygen concentration. Unless these parameters are fully integrated into process hazard analysis and process safety management, the risks cannot be fully identified, and the reliability of process hazard analysis cannot be improved to facilitate the development of appropriate countermeasures. Preventing the underestimation of process risk severity due to the fire and explosion parameters of unknown flammable dusts and overestimation of existing safety measures is crucial for effective accident prevention.     

基于PSO-SVM模型的隧道水砂突涌量预测研究

复杂工程地质条件下,隧道水砂混合物突涌的预测防控是隧道安全建设的基础,准确预测水砂混合物突涌量,为工程提供安全保障至关重要。为提高预测准确性,提出一种基于粒子群算法优化的支持向量机(PSO-SVM)的隧道水砂突涌量预测模型。综合考虑地质构造、气象条件、施工影响三类因素,选取七个因子,结合某公路隧道,利用PSO-SVM建立隧道水砂突涌量预测模型,并对该隧道水砂突涌量进行预测,结果与实际突涌量一致。证实综合粒子群算法和支持向量机优势的PSO-SVM方法预测精度高,且易于实现,为类似隧道工程突涌预测提供参考与借鉴。     

Explosibility of micron- and nano-size titanium powders

Explosibility of micron- and nano-titanium was determined and compared according to explosion severity and likelihood using standard dust explosion equipment. ASTM methods were followed using a Siwek 20-L explosion chamber, MIKE 3 apparatus and BAM oven. The explosibility parameters investigated for both size ranges of titanium include explosion severity (maximum explosion pressure (P_max) and size-normalized maximum rate of pressure rise (K_St)) and explosion likelihood (minimum explosible concentration (MEC), minimum ignition energy (MIE) and minimum ignition temperature (MIT)). Titanium particle sizes were ?100 mesh (<150 μm), ?325 mesh (<45 μm), ≤20 μm, 150 nm, 60–80 nm, and 40–60 nm. The results show a significant increase in explosion severity as the particle size decreases from ?100 mesh with an apparent plateau being reached at ?325 mesh and ≤20 μm. Micron-size explosion severity could not be compared with that for nano-titanium due to pre-ignition of the nano-powder in the 20-L chamber. The likelihood of an explosion increases significantly as the particle size decreases into the nano range. Nano-titanium is very sensitive and can self-ignite under the appropriate conditions. The explosive properties of the nano-titanium can be suppressed by adding nano-titanium dioxide to the dust mixture. Safety precautions and procedures for the nano-titanium are also discussed.     

烟花爆竹用氧化剂的研究进展

在调查研究的基础上,分析、总结、研讨用于烟花爆竹的氧化剂的历史、现状及发展方向。分析和讨论氯酸盐类、高氯酸盐类、硝酸盐类以及金属氧化物等氧化剂的物化性质、燃爆性能及各自用于烟火剂的优缺点。研究指出:将不同类型的氧化剂复合使用,当选择的组分及搭配的比例合适时,不仅能使各种氧化剂充分发挥其优点,而且还将产生一些新的实际的性能。同时提出:复合氧化剂是配制燃放效果好、安全性能高、价格低廉的烟花爆竹氧化剂的发展方向。     

A comparison between superfine magnesium hydroxide powders and commercial dry powders on fire suppression effectiveness

The effectiveness of superfine magnesium hydroxide powders and commercial dry powder in fire suppression were compared in a laboratory-scale, immovable fire suppression apparatus. The investigation focused on their suppression effectiveness under various pressures. It likewise studied the relationship between the powders’ surface structures and their effectiveness on fire suppression. The microcosmic structure of the powders was observed by scanning electron microscope (SEM), while the Thermal Gravity Analysis (TGA) technique was used to analyze the thermal behavior of the superfine magnesium hydroxide powders. By analyzing the results, it could be concluded that the pressure, the particle size, and the microcosmic structure of the powders’ surface mainly decide the capability of the powders to extinguish the fire. Comparing this with commercial dry powders, superfine magnesium hydroxide powders extinguished the fire in less than 10 s.     

A simple model for predicting the release of a liquid-vapor mixture from a large break in a pressurized container

This paper presents a simple, accurate model for determining the amount and composition of a liquid-vapor release from a pressurized tank that develops a large break above the level of the liquid. Most models commonly used by the chemical industry assume that there is thermal- and mechanical-equilibrium between the liquid- and the vapor-phase (homogeneous equilibrium models, HEM). While this assumption is valid for releases though long pipes and nozzles, we found that it overestimates the total amount released during rapid discharges through large breaks in a vessel when there is insufficient time for the mixture to become homogeneous. We derived an analytical non-homogeneous, thermal equilibrium model that accurately determines the void fraction of the mixture at the time of the release, and the quantity of a release from a pressurized container. Our model is based on equations describing the transfer of interfacial momentum between the liquid- and the vapor- phases that develop during the quick depressurization of a vessel. The model’s predictions are verified by comparing them with actual measurements of the void fraction, and with the results of the RELAP5 model. Also, our model is used to determine emissions of nitrogen oxides and nitric acid in an actual rupture of a railcar tank. The results agreed with actual observations, whereas a homogeneous equilibrium model gave erroneous predictions.     

湿法粉碎制备纳米RDX安全性分析与性能表征

从热传导理论分析入手,以回收的RDX为原料,选择易获取的水为导热介质,湿法粉碎制备了纳米级RDX样品。利用纳米激光粒度仪、场发射扫描电镜和卡斯特型落锤仪对样品粒径大小、外观特征、感度大小进行了观察和测试。实验结果表明,湿法制备的纳米级RDX感度明显降低,安全性提高。由于本方法成本低廉,使得大规模回收的RDX再次利用成为可能,解决了RDX不方便储存运输使用等难题,扩大了其使用范围。     

基于层次分析法的烟火药制造过程爆炸事故分析

结合烟花爆竹行业安全评价经验,在危险源辨识理论的指导下,运用因果图法对该过程发生的爆炸事故进行定性分析,找出导致事故发生的因素;再运用层次分析法(AHP)进行定量分析,建立层次结构模型,求出各因素的权重,得出各因素的重要程度,从而确定导致事故发生的关键因素,以此作为事故预防的重点。结果表明,人的不安全行为是影响事故的主导因素,此结论符合烟花爆竹行业生产现状,可以为企业制定合理的、有针对性的对策措施提供依据。     

烟花爆竹生产安全问题探析

烟花爆竹生产由于行业的特殊性，危险性很大。近年来，烟花爆竹安全生产形势严峻，事故频发，危害严重。笔者在调查研究的基础上，分析了烟花爆竹生产的危险性以及引发事故的主要原因。着重讨论了烟花爆竹安全生产的可行性途径，强调提高准入门槛，严格安全生产许可证的核发；加大监督管理力度，严格规章制度的落实；发展地方经济，从根本上防止非法生产；并通过对湖南浏阳花炮生产安全管理的分析，探讨安全文化建设在烟花爆竹这一高危行业的作用，认为烟花爆竹行业要发展，要尽可能减少事故的发生，一定要注重安全管理，注重企业安全文化建设，注重提高从业人员的安全文化素质。     

Kinetic free mathematical model for the prediction of Kst values for organic dusts with arbitrary particle size distribution

Risk mitigation in production facilities has been an issue of great interest for decades, especially in activities which represent a serious hazard to human health, environment and industrial plants. Dust explosions are a major hazard in many industrial processes: only in the first part of 2019 (January–June) 34 dust explosions, mainly due to organic powders, occurred worldwide. An explosion may take place whenever there is the presence of combustible dusts, which are frequently generated by activities such as grinding, crushing, conveying and storage. Currently, a relatively expensive experimental test, carried out into a 20-L Siwek apparatus, is used to address the order of magnitude (class) of explosive dust: this piece of information is referred to as the deflagration index, K_st. At the current state, only a few pioneering models have been developed in order to predict the value of the K_st as a function of some relevant properties of the dust: e.g. particle size distribution (PSD), humidity, thermal conductivity, etc‥ Most of these models condense the information about the PSD of a given dust into an average value, referred to as D₅₀. In this work, a kinetic free mathematical model aimed at predicting the deflagration index for organic dusts is presented. This model, unlike the older ones, considers the whole particle size distribution for the computation of the deflagration index. In order to be implemented, only a single experimental K_st value (which works as a reference) and a particle size analysis on the dust are required. The model was validated using the whole granulometric distribution of three different organic powders (fosfomycin, sugar and niacin). In addition, the same estimations were done by considering only the D₅₀ data. It was noticed that, for highly polydispersed dusts, results were less accurate with respect to those obtained using the complete PSD, highlighting the importance of considering a complete granulometric distribution for process safety purposes.     

微波辐照作用对颗粒煤瓦斯解吸特性影响实验研究

为了研究微波场连续-间断辐照作用对颗粒煤瓦斯解吸特性的影响,通过自制的实验装置,分析研究了微波连续-间断作用10 ,20 ,40 s及无微波作用下的构造煤颗粒瓦斯解吸量及解吸速率变化规律,并采用水浴加热装置模拟微波产生的热效应,研究了微波热效应在促进煤粒瓦斯解吸中的影响。实验结果表明:在微波连续作用时间内,瓦斯解吸量和解吸速率均迅速增大,然而随着时间的延长衰减较快,最终瓦斯解吸量趋向于一定值,微波连续-间断辐照作用下的瓦斯解吸量是无微波加载作用下的1.83～3.93倍;微波产生的热效应对瓦斯解吸影响较为显著,权重达82%以上,然而其非热效应的影响也不可忽视。实验方法与结果可望为促进构造瓦斯解吸、降低煤层突出危险性提供参考。     

Ignition temperatures and flame velocities of metallic nanomaterials

The production of materials with dimensions in the nanometre range has continued to increase in recent years. In order to ensure safety when handling these products, the hazard potential of such innovative materials must be known. While several studies have already investigated the effects of explosions (such as maximum explosion pressure and maximum pressure rise) of powders with primary particles in the nanometre range, little is known about the ignition temperatures and flame velocities. Therefore, the minimum ignition temperature (MIT) of metallic nano powders (aluminium, iron, copper and zinc) was determined experimentally in a so called Godbert-Greenwald (GG) oven. Furthermore, the flame velocities were determined in a vertical tube. In order to better classify the test results, the tested samples were characterised in detail and the lower explosion limits of the tested dust samples were determined. Values for the burning velocity of aluminium nano powders are higher compared to values of micrometre powders (from literature). While MIT of nanometre aluminium powders is within the range of micrometre samples, MIT of zinc and copper nano powders is lower than values reported in literature for respective micrometre samples.     

烟花爆竹生产企业应建立远程监控系统

通过对河南省烟花爆竹生产企业一段时间内生产安全事故的分析,认为违章操作是已取得安全生产许可证企业当前安全生产中最主要的危险。进一步分析认为,违章操作产生的原因在于生产企业缺乏有效的监督管理。结合交通行业的成功经验,论述了烟花爆竹企业引入远程监控系统的必要性和可行性,并给出了具体的实施方案:企业端硬件投资约10 000元,再架设宽带网络或专用通信线路,就能实现异地远程监督。安全生产许可证制度加上远程监控系统,对烟花爆竹生产企业实现安全生产将起到积极的推动作用。     

铝粉超细化对KClO3/Al/S配方药剂特性的影响

为研究铝粉超细化后对烟火药剂性能的影响,将普通铝粉和纳米铝粉分别与氯酸钾、硫黄粉按照零氧平衡的同一配比(17%Al+63%KClO3+20%S)配制成烟火药剂,分别用0#样品和1#样品表示。用ARC、WL-1型落锤仪和MGY-1型摆式摩擦感度仪等试验装置从热安全性、撞击感度和摩擦感度等方面进行对比试验。结果表明,与含普通铝粉的0#样品相比,含纳米铝粉的1#烟火药剂热分解的初始反应温度明显降低(118.67℃<123.3℃),反应到达最大温升速率所需的时间明显延长(4.94min>0.13 min),反应所能达到的最高压力明显降低(2.77 MPa/g<3.14 MPa/g),反应动力学因子明显降低(361.85 kJ/mol<409.41 kJ/mol),撞击感度明显下降(12%<100%)。这说明铝粉粒径对药剂的性能有一定的影响。纳米铝粉的加入在加速烟火药剂反应进程的同时,可有效降低其反应的激烈程度、压力危险性和撞击危险性,即铝粉超细化后可以有效改善烟火药剂的性能,提高其安全性。     

基于正交试验法的黄铁矿自燃倾向性影响因素分析

利用X衍射仪、X荧光分析仪对黄铁矿进行了成分分析,运用激光粒度仪对黄铁矿进行了粒径分析。以粒度、空气流量、质量为正交试验的3个因子,运用正交试验法设计实验。利用热分析仪对黄铁矿进行了升温氧化试验,得到5、10、15 K/min升温速率下的TG-DSC曲线。运用FWO等转化率法计算黄铁矿的活化能,并将其作为黄铁矿自燃倾向性评价指标。结果表明,三因素对黄铁矿自燃倾向性影响程度为:粒径空气流量质量,且粒径的大小与活化能的大小成比例关系;当粒径为131.10μm,质量为10 mg,空气流量为30 m L/min时,黄铁矿自燃倾向性最小。     

固体惰性介质对煤粉爆炸压力的影响研究

通过对固体惰性介质在减轻煤粉爆炸作用的实验研究,给出影响固体惰化剂作用效果的主要影响因素。实验分别选用来自加拿大和中国的3种煤粉和石灰石,对每种实验样品的成分、粒度都进行分析。用20L球形容器进行实验,测定煤粉中加入不同含量的石灰石后煤粉爆炸的Pmax和(dp/dt)max值。结果表明,石灰石能够起到减轻煤粉爆炸影响的作用,并且随着煤粉粒度的减小,要达到相同的抑爆效果需要的石灰石的用量将加大。