Large-scale dust explosions in vessel-pipe systems

This study investigates dust explosions in vessel-pipe systems to develop a better understanding of dust flame propagation between interconnected vessels and implications for the proper application of explosion isolation systems. Cornstarch dust explosions were conducted in a large-scale setup consisting of a vented 8-m³ vessel and an attached pipe with a diameter of 0.4 m and a length of 9.8 m. The ignition location and effective dust reactivity were varied between experiments. The experimental results are compared against previous experiments with initially quiescent propane-air mixtures, demonstrating a significantly higher reactivity of the dust explosions due to elevated initial turbulence, leading to higher peak pressures and faster flame propagation. In addition, a physics-based model developed previously to predict gas explosion dynamics in vessel-pipe systems was extended for dust combustion. The model successfully predicts the pressure transients and flame progress recorded in the experiments and captures the effects of ignition location and effective dust reactivity.     

Experimental study on gas explosion and venting process in interconnected vessels

A pilot scale interconnected vessels experiment system was established, and the closed and vented gas explosion characteristics in the system were studied, using 10% methane–air mixture. Regularity of pressure variation in vessels and flame propagation in linked pipes was analyzed. Furthermore, the effects of transmission style, ignition position, pipe length, and initial pressure on explosion severity were discussed. For the closed explosion: explosion in interconnected vessels presents strongly destructive power to secondary vessel, especially transmission from the big vessel to the small one; the worst ignition position is shifting from ignition in the interconnected pipe to the walls of the two vessels; as far as ignition in big vessel is concerned, the peak pressure in secondary vessel increases with the pipe length much faster than that for ignition in small vessel; the peak pressures in two vessels are approximate linear functions of initial pressure. For the vented explosion: the transmission style and interconnected pipe length have significant impacts on the effect of venting on the protection; in order to obtain the better venting effect, the use of a divergent interconnected pipe from the big vessel to the small one in industry is advised and it is necessary to reduce the interconnected pipe length as far as possible or install flame arrester in the interconnected pipe.     

Experimental investigation on explosion flame propagation of wood dust in a semi-closed tube

In order to explore flame propagation characteristics during wood dust explosions in a semi-closed tube, a high-speed camera, a thermal infrared imaging device and a pressure sensor were used in the study. Poplar dusts with different particle size distributions (0–50, 50–96 and 96–180 μm) were respectively placed in a Hartmann tube to mimic dust cloud explosions, and flame propagation behaviors such as flame propagation velocity, flame temperature and explosion pressure were detected and analyzed. According to the changes of flame shapes, flame propagations in wood dust explosions were divided into three stages including ignition, vertical propagation and free diffusion. Flame propagations for the two smaller particles were dominated by homogeneous combustion, while flame propagation for the largest particles was controlled by heterogeneous combustion, which had been confirmed by individual Damköhler number. All flame propagation velocities for different groups of wood particles in dust explosions were increased at first and then decreased with the augmentation of mass concentration. Flame temperatures and explosion pressures were almost similarly changed. Dust explosions in 50–96 μm wood particles were more intense than in the other two particles, of which the most severe explosion appeared at a mass concentration of 750 g/m³. Meanwhile, flame propagation velocity, flame propagation temperature and explosion pressure reached to the maximum values of 10.45 m/s, 1373 °C and 0.41 MPa. In addition, sensitive concentrations corresponding to the three groups of particles from small to large were 500, 750 and 1000 g/m³, separately, indicating that sensitive concentration in dust explosions of wood particles was elevated with the increase of particle size. Taken together, the finding demonstrated that particle size and mass concentration of wood dusts affected the occurrence and severity of dust explosions, which could provide guidance and reference for the identification, assessment and industrial safety management of wood dust explosions.     

Insight into vented explosion mechanism and premixed flame dynamics in linked vessels: Influence of membrane thickness and blocking rate

To effectively prevent and mitigate explosion hazards and casualties, relief venting of flammable gas explosions has been applied in production processes in a broad variety of industries. This work conducted fully vented experiments to investigate the influence of venting membrane thickness, and partially vented experiments to investigate the influence of baffle blocking rate on the explosion characteristics of 9.5 vol% methane-air mixtures in linked vessels with a 0.5 m long vented duct. Results indicate that the membrane thickness and blocking rate for the two types of vented explosions significantly affected the explosion overpressure. The smaller the membrane thickness and blocking rate, the lower the explosion overpressure. Secondary explosions were observed in the vented duct through experiments and a weaker explosion flame appeared at a small blocking rate of 20%. With the further increase in the blocking rate, the flame became extremely weak, and no secondary explosions occurred. The overpressure evolution process at different positions in the explosion duct and secondary explosion phenomenon in the vented duct were investigated. This work could probably serve as an important reference for the selection of technical parameters of explosion venting in the practical industrial processes.     

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.     

连通容器气体密闭爆炸与泄爆过程的实验研究

利用球型容器与管道组合,开展连通容器气体爆炸与泄爆实验,分析连通条件下,火焰在管道中的传播过程及其对起爆容器和传爆容器的压力影响。实验结果表明:连通容器气体爆炸中,火焰从起爆容器到传爆容器传播经历了一段不断加速,但加速度不断减小的过程;泄爆过程中,火焰传播过程与密闭爆炸时基本一致。管道中火焰加速传播,使得传爆容器的爆炸压力和强度相较于作为起爆容器时均明显增加,危险更大,采用与起爆容器相同的泄爆面积,无法满足对连通容器中传爆容器的泄爆。同时,泄爆是一个快速的能量泄放过程应选择合理的泄爆方式,防止二次危害。     

Flame behaviors and pressure characteristics of vented dust explosions at elevated static activation overpressures

Dust explosion venting experiments were performed using a 20-L spherical chamber at elevated static activation overpressures larger than 1 bar. Lycopodium dust samples with mean diameter of 70 μm and electric igniters with 0.5 KJ ignition energy were used in the experiments. Explosion overpressures in the chamber and flame appearances near the vent were recorded simultaneously. The results indicated that the flame appeared as the under-expanded free jet with shock diamonds, when the overpressure in the chamber was larger than the critical pressure during the venting process. The flame appeared as the normal constant-pressure combustion when the pressure venting process finished. Three types of venting processes were concluded in the experiments: no secondary flame and no secondary explosion, secondary flame, secondary explosion. The occurrence of the secondary explosions near the vent was related to the vent diameter and the static activation overpressure. Larger diameters and lower static activation overpressures were beneficial to the occurrence of the secondary explosions. In current experiments, the secondary explosions only occurred at the following combinations of the vent diameter and the static activation overpressure: 40 mm and 1.2 bar, 60 mm and 1.2 bar, 60 mm and 1.8 bar.     

Review of the DESC project

Dust Explosion Simulation Code (DESC) was a project supported by the European Commission under the Fifth Framework Programme. The main purpose of the project was to develop a simulation tool based on computational fluid dynamics (CFD) that could predict the potential consequences of industrial dust explosions in complex geometries. Partners in the DESC consortium performed experimental work on a wide range of topics related to dust explosions, including dust lifting by flow or shock waves, flame propagation in vertical pipes, dispersion-induced turbulence and flame propagation in closed vessels, dust explosions in closed and vented interconnected vessel systems, and measurements in real process plants. The new CFD code DESC is based on the existing CFD code FLame ACceleration Simulator (FLACS) for gas explosions. The modelling approach adopted in the first version entails the extraction of combustion parameters from pressure–time histories measured in standardized 20-l explosion vessels. The present paper summarizes the main experimental results obtained during the DESC project, with a view to their relevance regarding dust explosion modelling, and describes the modelling of flow and combustion in the first version of the DESC code. Capabilities and limitations of the code are discussed, both in light of its ability to reproduce experimental results, and as a practical tool in the field of dust explosion safety.     

Numerical modelling of the effects of vessel length-to-diameter ratio (L/D) on pressure piling

Pressure piling presents a major explosion hazard in interconnected process vessels. Pressure enhancement in the secondary vessel due to the acceleration of the flame through the connecting pipe can generate a disproportionately more violent explosion than would have been expected based on the concentration of dust in the secondary vessel. Pressure piling is a very complex phenomenon that is difficult to investigate through experimentation. Advanced computational fluid dynamics (CFD) modelling is a promising route to accurately account for all the complexities associated with pressure piling.In this paper, the current state of knowledge concerning pressure piling is presented. Further, the effects of varying the length-to-diameter ratio (L/D) of the primary vessel (Vessel 1) on pressure piling was investigated using numerical modelling. The volumes and volume ratio of the interconnected vessels were kept constant while the L/D of Vessel 1 was varied from 0.5 to 15. The simulations of coal dust explosion were performed using the coalChemistryFoam solver from OpenFOAM version 5.0.1. It is hoped that the findings from this study provide insight into the effects of the geometrical design of interconnected vessels, particularly L/D, on pressure piling. Additionally, this work has implications for the optimal placement of explosion isolation devices intended to actuate before the flame front and pressure escape to downstream vessels.     

CFD simulation of pressure piling

The explosion of flammable mixtures in interconnected compartments is commonly defined as “pressure piling”. Peak pressures much higher than the predictable thermodynamic values are likely to be generated in this geometry, yielding the phenomenon of major interest in industrial safety. In this paper, a CFD model was implemented, aiming at understanding the major factors affecting pressure piling in two cylindrical interconnected vessels, by varying the volume ratio between the two interconnected vessels and the ignition position. A combustion model was specifically developed to follow the flame propagation in any combustion regimes as a function of the local conditions: laminar, flamelet and distributed reaction zone.The model was validated by comparison with experimental results. The agreement between the experiments and the simulations has allowed the interpretation of the pressure piling phenomenon and the understanding of the mechanisms involved. More precisely, the results have showed that the pressure peak intensity is mainly affected by the coupling between the pre-compression of the mixture in the secondary vessel and the violence of explosion in the same vessel as related to the venting time, the latest quantified by the turbulent Bradley number, Br_t i.e. by the reaction time to the venting time ratio.     

Dust explosion protection in linked vessels: guidance for containment and venting

Much industrial dust-handling plant consists of vessels connected by pipelines. If a dust explosion propagates through such a system, the overall explosion event can be more violent than if a single vessel only is involved, due to a combination of increased turbulence, pressure piling and a jet flame ignition source in the second vessel. This paper gives guidance, based on an extensive experimental programme reported previously, on two aspects of the protection of interconnected vessels: containment and venting.     

Explosibility of hydrogen–graphite dust hybrid mixtures

To evaluate the hazard of combined hydrogen/dust explosions under severe accident conditions in International Thermonuclear Experimental Reactor (ITER), standard method of 20-L-sphere was used to measure the explosion indices of 4-μm fine graphite dust in lean hydrogen/air mixtures. The mixtures were ignited by a weak electric spark. The tested fuel concentrations were 8–18 vol% H₂ and 25–250 g/m³ dust. If the hydrogen content is higher than 10 vol%, the dust constituent can be induced to explode by the hydrogen explosion initiated by a weak electric spark. Depending on the fuel component concentrations, the explosions proceed in either one or two stages. In two-stage explosions occurring at low hydrogen and dust concentrations, the mixture ignition initiates first a fast hydrogen explosion followed by a slower phase of the dust explosion. With increasing dust concentration, the dust explodes faster and can overlap the hydrogen-explosion stage. At higher hydrogen concentrations, the hybrid mixtures explode in one stage, with hydrogen and dust reacting at the same time scale. Maximum overpressures of hybrid explosions are higher than those observed with hydrogen alone; maximum rates of pressure rise are lower in two-phase explosions and, generally, higher in one-stage explosions, than those characteristic of the corresponding H₂/air mixtures.     

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.     

大型相连容器中火焰传播的研究

为了进一步了解相连装置中粉尘爆炸的火焰传播行为和压力发展,为该结构的安全防护设计提供有价值的信息,采用大型实验装置对相连容器中玉米淀粉/空气混合物爆炸时的火焰传播行为进行了实验研究,同时采用已开发的数值模型对实验进行仿真计算。实验表明:粉尘浓度的变化对粉尘爆炸的火焰传播行为有重要影响;在粉尘浓度很低的情况下,火焰仍然能够在管道中加速传播且爆炸发展的最终结果相当猛烈。数值模型采用欧拉-拉格朗日方法模拟两相流现象,通过求解非稳态的湍流两相反应流守恒方程对实验进行二维仿真,计算结果与实验结果符合性较好,表明该模型可以很好地应用于粉尘爆炸火焰传播的研究。     

Propagation of ethylene–air flames in closed cylindrical vessels with asymmetrical ignition

A study of explosions in several elongated cylindrical vessels with length to diameter L/D = 2.4–20.7 and ignition at vessel's bottom is reported. Ethylene–air mixtures with variable concentration between 3.0 and 10.0 vol% and pressures between 0.30 and 1.80 bara were experimentally investigated at ambient initial temperature. For the whole range of ethylene concentration, several characteristic stages of flame propagation were observed. The height and rate of pressure rise in these stages were found to depend on ethylene concentration, on volume and asymmetry ratio L/D of each vessel. High rates of pressure rise were found in the early stage; in later stages lower rates of pressure rise were observed due to the increase of heat losses. The peak explosion pressures and the maximum rates of pressure rise differ strongly from those measured in centrally ignited explosions, in all examined vessels. In elongated vessels, smooth p(t) records have been obtained for the explosions of lean C₂H₄–air mixtures. In stoichiometric and rich mixtures, pressure oscillations appear even at initial pressures below ambient, resulting in significant overpressures as compared to compact vessels. In the stoichiometric mixture, the frequency of the oscillations was close to the fundamental characteristic frequency of the tube.     

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.     

Experimental study of the venting characteristics of dust explosion through a vent duct

To further understand the dynamic mechanism of dust explosion through a vent duct, we designed a small-scale cylindrical vessel connected with a vent duct and performed a dust explosion venting experiment under different opening pressures using corn starch as the explosive medium in this study. The results show that weakening effect of duct on venting is positively correlated with the opening pressure. The explosion pressure in the duct presents a three-peak-structure with time, successively caused by the membrane breaking shock wave, the secondary explosion in the tube, and the continuous combustion, and decreases gradually with the propagation distance. Meanwhile, the three pressure peaks are positively correlated with the opening pressure, while the time interval between them goes to contrary. The increase of opening pressure leads to the increase of secondary explosion intensity and reverse flow in the vessel, further accelerates the reaction rate in the vessel, and then shortens the duration of combustion in the vessel until the phenomenon of flame reignition in the vessel disappears.     

Effects of ignitors and turbulence on dust explosions

The aim of this work is in an attempt to increase the understanding of the acting behaviour of pyrotechnic ignitors and their effects on confined dust explosions. Flame visualization has shown that pyrotechnic ignitors can initiate an explosion by instantaneous jet-like volumetric and/or multipoint ignition. Hence, the rate of pressure rise and also the apparent burning velocity will be increased to some extent, depending on the ignitor energy and the reactivity of the mixtures. The ignitor effect is more important for the early stages of flame propagation and would be more significant in small explosion chambers. Thus, for dust explosion tests with various purposes, use of pyrotechnic ignitors should be made carefully, and the ignitor effect must be accounted for in the data interpretation. Turbulence induced by dust dispersion is a dominant factor in affecting dust explosions. At different ignition delays, however, the turbulence influence will be coupled with that of ignitors. This complicates further the interpretation of explosion data measured under turbulent conditions.     

Effect of particle morphology on metal dust deflagration sensitivity and severity

Combustible dust explosions continue to present a significant threat toward industries processing, storing, or pneumatically conveying metal dust hazards. Through recent years, investigations have observed the influence of particle size, polydispersity, and chemical composition on dust explosion sensitivity and severity. However, studies characterizing the effect of particle shape (or morphology) on metal dust explosibility are limited and merit further consideration. In this work, high-purity aluminum dust samples of three unique particle morphologies were examined (spherical granular, irregular granular, and dry flake). To maintain consistency in results obtained, all samples were procured with similar particle size distribution and polydispersity, as verified by laser diffraction particle size analysis. Scanning electron microscopy (SEM) imaging and Brunauer-Emmett-Teller (BET) experiments were executed to confirm supplier claims on morphology and to quantify the effective surface area associated with each sample, respectively. Investigations performed in a Kühner MIKE3 minimum ignition energy apparatus and a Siwek 20 L sphere combustion chamber resulted in the direct characterization of explosion sensitivity and severity, respectively, as a function of suspended fuel concentration and variable particle morphology. Recommendations to standard risk/hazard analysis procedures and to existing design guidance for the mitigation of deflagrations that originate from ignition of distinctively processed metal dust fuels have been provided.     

Flame propagation in lycopodium/air mixtures below atmospheric pressure

Industrial processes are often operated at conditions deviating from atmospheric conditions. Safety relevant parameters normally used for hazard evaluation and classification of combustible dusts are only valid within a very narrow range of pressure, temperature and gas composition. The development of dust explosions and flame propagation under reduced pressure conditions is poorly investigated. Standard laboratory equipment like the 20 l Siwek chamber does not allow investigations at very low pressures. Therefore an experimental device was developed for the investigations on flame propagation and ignition under reduced pressure conditions. Flame propagation was analysed by a video analysis system the actual flame speed was measured by optical sensors. Experiments were carried out with lycopodium at dust concentrations of 100 g/m³, 200 g/m³ and 300 g/m³. It was found that both flame shapes and flame speeds were quite different from those obtained at atmospheric pressure. Effects like buoyancy of hot gases during ignition and flame propagation are less strong than at atmospheric conditions. For the investigated dust concentrations the flame reaches speeds that are nearly an order of a magnitude higher than at ambient conditions.