Monitoring of electrostatic fire and explosion hazards at the inlet to electrostatic precipitators

The paper deals with the continuous monitoring of electrostatic fire and explosion hazards that can occur at the inlet to electrostatic precipitators (ESPs) when highly charged dust particles are transported by a gas carrier that can be the mixtures of both incombustible and combustible flue gases. The risk of ignition and even explosion is especially high in the presence of an explosive mixture of oxygen and, e.g., hydrocarbons, carbon monoxide, etc. To avoid the danger of electrostatic discharges and their consequences for a whole installation including an electrostatic precipitator a method and a specially designed and built system should effectively enable the continuous monitoring of the hazards and should immediately manage any automatic control system or some other control elements. Some theoretical considerations concerning the method proposed, the physical quantities that must be measured, and the derivation of a novel dynamic safety criterion for assessing the risk of hazardous electrostatic discharges are presented. Finally, the author presents and discusses the possible practical application of the microprocessor-based measuring system verified experimentally in the past to the continuous monitoring of the hazards and to the management of an automatic control system to be put into operation. The paper presents a certain idea and proposal of the problem's solution based on the author's many years' experience in the field of pneumatic transport of dusts, powders and granular materials, of the electrostatic measurements of electric and mechanical quantities characteristic of the particulate transport, and of the risk and prevention of discharges of static electricity in transporting pipes and silos, vessels, etc.     

故障树模块化在装甲车辆灭火系统故障诊断中的应用

为了实现装甲装备灭火系统故障的快速诊断,提出了一种故障树模块化分析方法;对灭火系统故障树进行深度优先最左遍历,并记录遍历过程,按照遍历顺序对故障树中的每个事件进行标定,并将灭火系统故障树划分为相互独立的模块,依据划分的模块可以通过故障现象对模块内的故障进行排查及修复;实验结果分析表明,该方法可以快速修复模块故障,恢复系统功能,简化了以往对灭火系统所有子事件遍历查错的繁琐过程。该方法同样可以计算故障模块的失效概率,并可以实现故障模块的整体更换,恢复系统性能;证明了故障树模块化方法在灭火系统故障诊断中具有较高的效率,简化了灭火系统诊断流程,在装甲车辆其他系统故障诊断中具有借鉴作用,符合现代作战对于装备保障的需求。     

Incident case histories on mobile containers

While handling liquids in rigid intermediate bulk containers (RIBC), the assessment of ignition sources due to electrostatic discharges is of especial interest since it is often difficult to rule out all possible conditions which can lead to the generation of an effective discharge. Far from suggesting new criteria for the selection or construction of RIBC or standards for its design, this paper has the aim of describing three incidents where an electrostatic discharge caused a flammable mixture of vapours with air to ignite. Situations presented could usually be rated as highly remote on its probability of occurrence by operators since they are common and never caused an incident, but on the other hand it is rather easy, from a theoretical point of view, to explain the phenomena observed by applying the basic assessment rules for the generation of electrostatic ignition sources.If existing guidelines were to have been followed (mainly CENELEC 50404 and BGR132), those incidents would have not occurred.     

Electrostatic ignition hazards in the process industry

Fires and explosions continue to be triggered by the accumulation of electrostatic charge in different branches of the process industry. This is the case even though nowadays the electrostatic phenomena that give rise to ignition hazards in industrial practice are generally well known. This paper presents various reasons as to why this should be so and draws attention to the conflicts of interest, which arise, when other requirements need to be met. In addition, current trends and new results with respect to the assessment of electrostatic ignition hazards are reviewed. They include (i) incendivity and characterisation of discharges from insulating surfaces with and without contact to a conductive surface, (ii) incidents affecting the present assessment of occurrence and incendivity of propagating brush discharges in practice and (iii) characterisation, measurement method and ignition hazard assessment of modern packaging systems such as flexible and rigid intermediate bulk containers (FIBC and RIBC). Finally, the consequences of Directives issued at the European and national levels are appraised with respect to electrostatic ignition hazards.     

基于故障树的氧碘化学激光器故障分析

为方便快捷地查找出氧碘化学激光器故障发生的原因,以氧碘化学激光器常见故障为例,建立了其功率失常的故障树模型,用最小割集理论按下行法对模型进行了定性分析和定量分析。分析表明:氧碘化学激光器7个子系统中,碘供给子系统的可靠性最低,氯气供给子系统次之,这与实际情况相符。计算出的氧碘化学激光器出光不正常发生概率为3.99%,其可靠度为96.01%。     

Induction charging risk assessment: Charged board alike discharges to metal and human body

Charge can easily be induced on electronics or on other conducting parts if they are exposed to external electrical fields. In production facilities where sensitive electronics are handled, strong electrostatic fields should be avoided due to the risk of causing electrostatic discharges (ESDs) that could damage components. In electronics manufacturing this is usually achieved by grounding all conductors and removing all insulators from an ESD Protected Area (EPA) in the facility. However, it is not always possible to remove all insulators from the EPA as they are sometimes an essential part of the production processes. In this case, a method of risk assessment is necessary to evaluate safe operation. We have studied induction charging of a dummy PWB (Printed Wiring Board) through a grounded MOSFET transistor, by grounding it directly to metal or through the human body, when the PWB is exposed to a static electric field. The experimental setup can easily be turned into an induction charging probe by changing the MOSFET transistor to a low leakage current, high voltage capacitor of suitable size and measuring the voltage over this capacitor.     

动态故障树在主蒸汽管道破裂事故风险评价中的初步应用研究

冗余设计使核电厂系统广泛存在复杂时序失效行为,而基于静态故障树(Static fault tree, SFT)的事故风险评价无法对时序失效行为进行准确模拟。为解决这一问题,本工作提出一种基于事件树+动态故障树(Dynamic Fault Tree, DFT)的事故风险分析框架,并以典型三代压水堆主蒸汽管道破裂事故为例,开展动态事故风险案例分析。首先,建立主蒸汽管道破裂事故的事件树模型以及相关系统的DFT模型;其次,将系统故障树分为DFT模块和SFT模块,并将DFT树模块替换为超级事件参与后续计算;最后,采用割集法计算案例结果,并在相同条件下与传统SFT方法进行对比。案例分析结果表明：(1)相较于SFT方法,所提方法更为贴近系统的真实失效场景;(2)针对文中案例所提方法可以降低相关系统失效概率与部分事故序列的发生频率、有助于释放保守风险。     

Modelling of electrostatic ignition hazards in industry examples of improvements of hazard assessment and incident investigation

Computer modelling and simulation of electrostatic phenomena is nowadays not very often used in industry for an assessment of ignition hazards due to static electricity. With 4 examples (silo filling and assessment of occurrence of cone discharges, calculation of capacitances of screws and flanges for assessment of earthing and bonding requirements, influence of sample geometry on the surface resistance and surface resistivity and assessment of requirements for plastic hoses with a metal spiral for pneumatic transfer of powders) the usefulness and benefit of model calculations of electrostatic phenomena for the assessment of electrostatic ignition hazards is demonstrated.     

Learning lessons from five electrostatic incidents

It is well known that electrostatic discharges can ignite mixtures of flammable gases with air and suspensions of combustible dust in air. For this reason, the prevention of electrostatic discharges is an essential part of measures to prevent explosions and fires. Incendive electrostatic discharges occurred in five cases in various chemical plants.The incidents involved were:

• 1.Spark type electrostatic discharges from a leaking steam pipe.
• 2.Spark type electrostatic discharges inside a charging chute for a solid.
• 3.A spark type electrostatic discharge whilst transferring a solid powder from a bin, which led to ignition of the powder.
• 4.A propagating brush discharge during pneumatic transfer of solid, which caused a severe electric shock to a process operator.
• 5.A brush discharge inside an electrostatic precipitator which caused ignition of an aerosol of hydrocarbon.

To avoid explosions and fires it is important to prevent incendive electrostatic discharges from occurring in industrial facilities. To help managers of chemical plants to do this, the incidents above are presented in the form of learning lessons including:

• •the mechanism of the generation and separation of electrical charges,
• •the mechanism of the electrostatic discharge,
• •the root causes of the incident,
• •the safety measures which are necessary to avoid a repetition.

     

An overview of standards concerning unwanted electrostatic discharges

For users and manufacturers affected by unwanted electrostatic discharges, the publication of standards concerning the control of such discharges is a great improvement and helps to prevent unwanted electrostatic discharges in potentially explosive atmospheres as well as around electrostatically sensitive devices. There are, for instance, handbook-like documents giving recommendations to help set up effective electrostatic suppressing measures and programs. Another group of standards focuses on certain products and processes and is, thus, more specific. A third group deals with test methods and specifications to help manufacturers develop products that are safe. It also helps in comparisons with other products and thus meets the customer's real needs. To assist manufacturers and customers in finding an applicable standard, an annotated list of the most relevant standards and reports on unwanted electrostatic discharges is given. This compilation does not include special electrostatic processes such as electrostatic copying or material separation except for those processes executed in explosive atmosphere.     

二元决策图和故障树相融合的无线传感器网络可靠性分析

为了提高无线传感器分析网络可靠性,提出一种基于二元决策图和故障树相融合的无线传感器网络可靠分析方法;首先根据无线传感器网络拓扑结构建立了故障树模型,然后将无线传感器网络故障树转化为二元决策图结构,最后采用仿真实验分析了不同节点冗余度、不同跳数条件下无线传感器网络的可靠性;仿真结果表明,文章方法的分析结果可以帮助无线传感器网络性能的优化,可以提高无线传感器网络的可靠性。     

压水堆核电厂余热排出系统的可靠性建模与分析

为提高压水堆余热排出系统(RHRS)可靠性,对RHRS的定期检修与维护提供指导与参考,本研究采用故障树对RHRS进行可靠性建模,对系统部件进行重要度指标评价。基于重要度指标评价结果提出了含公用备件的冷备系统的改进方案,引入动态故障树冷备门描述其失效模式,利用Markov模型进行定量分析。研究结果表明,优化后的RHRS可靠度有明显提升。     

基于免疫遗传算法改进的BP神经网络在灭火系统电路板故障诊断中的应用

针对装甲车辆灭火系统电路板规模较大,功能日趋多样与完善的同时,其复杂程度也日益提高,故障层次越来越多,故障现象与故障原因的映射关系更加复杂,组合故障频发,传统的故障诊断方法已不能满足灭火系统电路板故障诊断的要求。设计了基于免疫遗传算法优化的BP神经网络对灭火系统电路板进行故障诊断,并在免疫和遗传过程中保留了部分训练最优解。实现了神经网络收敛速度的提高,使用Matlab编程优化算法并完成了电路板仿真故障的诊断。通过实验验证了该诊断模型的准确性和可靠性,为电气系统通用检测设备的神经网络诊断方法实现提供了理论支撑。     

Technique for calculating the vulnerability of explosive surface objects by lightning

A probabilistic technique for estimating the vulnerability of explosive surface objects by lightning has been proposed. It uses a combined criterion for initiation of upward streamer and leader discharges form the elements of the object and lightning diverters. This criterion takes into account the probabilistic nature of avalanche–streamer and streamer–leader transitions and also the trajectories of the lightning’s downward stepped leader and lightning current. It has been shown that, if the formation of the downward leader of incomplete streamer discharges from the elements of the object in an electric field is neglected (these discharges may set fire on explosive emissions), the probability of lightning-induced failure of the object is underestimated by several times compared with the calculated value.     

Optimal design,robustness, and risk aversion

Highly optimized tolerance is a model of optimization in engineered systems, which gives rise to power-law distributions of failure events in such systems. The archetypal example is the highly optimized forest fire model. Here we give an analytic solution for this model which explains the origin of the power laws. We also generalize the model to incorporate risk aversion, which results in truncation of the tails of the power law so that the probability of catastrophically large events is dramatically lowered, giving the system more robustness.     

Two-stage electrostatic precipitator with co- and counter-flow particle prechargers

Lab-scale, two-stage electrostatic precipitation system comprising of precharging stage, in which PM2.5 particles are electrically charged, and collection stage, in which the charged particles are removed from the flowing gas by electric field, was investigated in this paper. Two types of electrostatic particle prechargers were compared with respect to the collection efficiency of the system: (1) co-flow precharger, in which ionic current was generated co-currently with the gas conveying the particles, and (2) counter-flow precharger, in which ionic current was generated oppositely to the flowing gas. In each case, the electrodes of precharger were supplied with DC or AC high-voltage in order to compare the effect of discharge mode on the collection efficiency of two-stage electrostatic precipitator. The collection stage was formed by two parallel-plate electrodes connected to DC high voltage source. Plate electrodes without discharge points (spikes) are corona-free electrodes, which prevent the collection stage from electrical discharges, and reduce the probability of back discharge ignition. The back discharge decreases collection efficiency of conventional electrostatic precipitators.It was concluded that the co-flow electrode configuration of the precharger, supplied with DC high voltage, has the highest total number collection efficiency for PM2.5 particles, higher than 95% and the mass collection efficiency larger than 99%. The counter-flow precharger provided only about 90% number collection efficiency of two-stage electrostatic precipitator. It was also shown that by AC electrode excitation, the collection efficiency of the system is lower than for DC supply. The two-stage electrostatic precipitators allowed obtaining higher fractional collection efficiency for PM2.5 particles than other conventional systems and can be recommended as highly effective devices for gas cleaning in power plants or cement industry.     

光纤激光器故障模式分析

为满足实际工程、生产应用中专业及非专业人员对故障光纤激光器快速修理、修复的需要,研究了一种能够实现快速故障模式及故障原因分析系统。首先,采集实际使用与实验中损坏激光器的故障模式与故障原因记录为数据样本。接着,利用故障树与故障模式影响及危害性分析模型建立光纤激光器扩展故障树,即构建起故障模式与故障原因的对应关系。然后,利用贝叶斯网络分析并得出各故障模式与故障原因的先验概率与后验概率计算方法,再使用Python语言编写交互式窗口,并完成对数据库数据的调用与相应概率计算。最后,通过交互式界面输出当前故障对应的可能故障原因及发生概率,从而实现对光纤激光器故障的快速分析。程序模拟耗时小于1 s,模拟结果与统计结果相吻合,基本满足指导相关人员对故障快速排查并修理的要求。     

Electrostatic potentials in gas–solid fluidized beds influenced by the injection of charge inducing agents

Accumulation of electrostatic charges in gas–solid fluidized beds can lead to particle adhesions, electrostatic discharges, wall sheeting and even explosion. Therefore, it is of vital importance to reduce the electrostatic level in industrial fluidized beds. Adding chemical static agents was reported as one of the most promising ways to control fluidized bed electrostatic potentials. In this study, the electrostatic potential distributions were measured in a three-dimensional column gas–solid fluidized bed with the LLDPE particles for fluidization and the introduction of chemical static agents. Seven charge inducing agents including TiO₂, Al₂O₃, MgO, H₂O, C₂H₅OH, ZnO, and Fe₂O₃ were employed to investigate their influence on electrostatic potentials distribution respectively. Additionally, Al(Et)₃, which is normally used as the cocatalyst for olefin polymerization, was also studied for comparison. It was found that the adding of negative charge inducing agent such as TiO₂ made the positive electrostatic potentials shift to higher values and the negative electrostatic potentials shift to lower values. The injection of positive charge inducing agent such as Al₂O₃ and MgO caused the positive electrostatic potentials dropping to lower values and the negative electrostatic potentials shifting to the values near zero at low injection amount. Affecting mechanism of charge inducing agents was discussed based on the experimental results and the electronegativity of corresponding compounds. It was found that the electronegativity (χ_i) of the metal ion played an important role in determining the charge polarity on the LLDPE particles and the change in the electrostatic potentials after contact and separation with the charge inducing agent particles.     

Transferred charge of brush discharges in explosive gas atmospheres – A verification of the ignition threshold limits

A wide series of experiments has been performed to check the incentivity of hydrogen/air, ethene/air and propane/air mixtures due to brush discharges. Thereby, the transferred charge as a criterion to judge the ignition potential is determined to verify the thresholds of transferred charge given in the standards IEC 60079-0 and in EN 13463-1. These thresholds have never been examined directly in an experiment before. It is stated that the thresholds for explosion group IIA, IIB and IIC represent different levels of safety. Using adequate thresholds the criterion of transferred charge is suitable for a judgement of potential electrostatic ignition sources.