Influence of dustiness on small-scale vented dust explosions

A new safety characteristic the “dustiness” according to VDI 2263 – part 9 (Verein Deutscher Ingenieure, 2008) is investigated. Dustiness means the tendency of a dust to form clouds. The paper deals with the physical reasons for the different behavior of dusts, even if they have similar properties such as particle size and density and the influence of the dustiness on dust explosions. In order to study the effects of the dustiness on dust cloud formation for different dispersion methods experiments in a vertical dust dispersion glass tube apparatus were carried out. Furthermore vented dust explosion experiments were done for two different dispersion methods and two static activation pressures.Experiments show that particle size and density are not the only factors which influence dispersibility. Particle shape, specific surface area, flow and dispersion method have an influence which can outweigh size and density. Preliminary explosion experiments showed that the dustiness has an influence on the reduced explosion pressure and flame speed in a vented 75 L test apparatus. In order to verify the results for applications in the process industries further tests with industrial scale experiments are planned.     

Investigations into the influence of dustiness on dust explosions

A new safety characteristic the “dustiness” according to VDI 2263 – part 9 (Verein Deutscher Ingenieure, 2008) is investigated. Dustiness means the tendency of a dust to form clouds. The paper deals with the influence of the dustiness on vented dust explosions. In order to look into the effects of the dustiness on dust cloud formation and explosion properties experiments and simulations in a vertical dust dispersion glass tube apparatus were carried out.Preliminary explosion experiments showed that the dustiness has an influence on the reduced explosion pressure in a vented 75 L test apparatus. Dusts with comparable p_max and K_St values and different dustiness were tested. Dusts with higher dustiness produced higher overpressures, despite comparable safety characteristics. In order to verify the results for applications in the process industries further tests with different settings are planned as well as industrial scale experiments. Characteristics of the dust such as particle size, density, specific surface area and particle shape, which influence the dispersibility, have been determined experimentally.The Euler/Lagrange and the Euler/Euler approaches are compared for simulating an exemplary dust/air mixture. Especially sedimentation and the ability of the approaches to simulate the tendency of dust to stay airborne were investigated. The Euler/Lagrange approach is better suited for simulating local dust concentrations, particle size distributions and particle forces. It could be used to point out regions of high dust concentrations in a vessel. With the Euler/Euler method it is possible to achieve fast solutions for one specified diameter, but the simulated dust/air mixtures are always more homogenous than in reality. ANSYS CFX version 13 was used in all simulations.     

基于多孔环形喷嘴的可燃爆粉尘分散特征的密度效应研究*

为了研究不同密度的可燃爆粉尘在内置多孔环形喷嘴的20 L爆炸特性测试装置中的分散特征,基于负载粒子流方法、耦合DPM动量平衡方程和时间平均 Navier Stokes控制方程组,实现3种不同密度的煤粉、铝粉和锆粉在20 L爆炸测试装置中粉尘分散全过程的数值模拟。研究结果表明:多孔环形喷嘴的分散较为均匀,但是约束管道末端存在局部粉尘残留区,致使爆炸仓内真实粉尘浓度远低于形式浓度;爆炸仓中心位置的最大湍动能随着粉尘密度的增加而减小,只有显著地变化粉尘密度才能展示区分度较高的浓度峰值和抵达浓度峰值的时间。     

Numerical study of dust lifting using the Eulerian–Eulerian approach

The present paper shows a numerical investigation of dust lifting behind a moving pressure wave. The dispersion of combustible dust has previously been discovered to be a precursor to a potential dust explosion. Consequently, a growing interest on the subject has been observed in recent years. Numerous studies have been performed on dust lifting, however, very few investigations have focused on dust layers with high volume fractions. Therefore, the aim of this investigation was to provide additional data. The simulations were carried out in a three-dimensional duct with a dust layer dispersed along the lower wall. The Eulerian–Eulerian approach was selected as the modelling technique. At first, four simulations varying the initial pressure and volume fraction of the dust were performed. The former parameter was varied between 4 and 8 bar, while the latter varied between 0.4 and 0.6. The combination of high initial pressure and high volume fraction resulted in the greatest dispersion of dust. Subsequently, two different drag force models were compared: the Schiller–Naumann, and the Gidaspow. It was discovered through this research that the choice of model caused significantly different results. The former model was found to underestimate the drag in the diluted parts of the layer. Consequently, this led to a distinctly lower lifting of the dust than in the latter model. Finally, a validation of a particle–particle interaction model was performed. It was observed that in the case where the model was disabled, an unrealistically high maximum volume fraction of the dust layer occurred. Nevertheless, the model did not seem to improve the dispersion results, which indicates that the dust lifting in this research was solely due to fluid–particle interactions.     

Effect of shock strength on dust entrainment behind a moving shock wave

Secondary dust explosion is a serious industrial issue because it occurs under conditions corresponding to an increased quantity and concentration of dispersed, combustible dust when compared with the primary explosion. The problems of lifting and dispersion of a dust layer behind a propagating shock wave must therefore be understood to ensure safety regarding secondary dust explosion hazards. Using a new shock-tube facility for studying shock propagation over dust layers, limestone dust was subjected to Mach numbers ranging from 1.10 to 1.60. A shadowgraph technique was applied by using a high-speed camera (15,000 fps) for visualization of the dust-layer height change behind the moving shock wave. Also, the effect of dust-layer thickness on the entrainment process was observed by performing tests with two different layer depths, namely 3.2- and 12.7-mm thicknesses. New correlations were developed between the shock strength and the dust entrainment height as a function of time for each layer depth. In general, the results herein are in agreement with trends found in previous work, where there is a linear relationship between dust growth rate and shock Mach number at early times after shock passage. Also, new data were collected for image analyses over longer periods, where the longer observation time and higher camera framing rates led to the discovery of trends not previously observed by earlier studies, namely a clear transition time between the early, linear growth regime and a much-slower average growth regime. This second regime is however accompanied by surface instabilities that can lead to a much larger variation in the edge of the dust layer than seen in the early growth regime. In addition, for the linear growth regime, there was no significant difference in the dust-layer height growth between the two layer thicknesses; however, the larger thickness led to higher growth rates and much larger surface instabilities at later times.     

Explosibility and thermal decomposition behavior of nitrile rubber dust in industrial processes

The formation of nitrile rubber (NBR) dust clouds during processing can lead to a potential dust explosion under certain conditions. However, the potential explosion hazard posed by NBR dust is usually overlooked by enterprises. In this paper, the explosive properties of NBR dust are investigated using a Hartmann tube, a G-G furnace, and a 20 L explosion chamber. The results showed that NBR dust could cause explosions severe enough to be classified as St-1. In addition, the thermal decomposition behavior of NBR dust under combustion conditions was investigated using a combination of thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (TGA-FTIR). The results indicated that in the early stage, NBR dust mainly undergoes self-thermal decomposition to produce a large amount of combustible gas, which combines with oxygen to form a mixed gas and cause a gas-phase explosion. In addition, the participation of oxygen could lower the initial temperature of NBR dust thermal decomposition. As a result, decomposition occurred more quickly and a large amount of combustible gas was produced, thus expanding the range of dust explosions. Furthermore, these combustible gases exhibit varying degrees of toxicity, seriously affecting the life and health safety of relevant personnel. This work provides theoretical guidance for the development of safe procedures to prevent and address problems during NBR dust processing in enterprises.     

Dust particle sedimentation in the 20 L standard vessel for dust explosion tests

According to the current international standards, to perform the correct evaluation of the explosion and flammability parameters, a uniform distribution of the dust particles should be achieved inside the 20 L and/or 1 m³ standard vessels.CFD simulations have shown that in both standard test vessels (20 L and 1 m³), the dust particles are not uniformly dispersed, being mostly concentrated at the edge of the macro-vortices generated by the injection of the fluid and particle through the nozzle. In addition, only a partial fed of the particles is obtained, and dust particles sedimentation phenomena can occur.As a result, the dust participating to the reactive process may be much lower than the expected nominal concentration in the vessel due to sedimentation and incomplete feeding. Consequently, misleading values of the flammability/explosion parameters could be measured.Particle sedimentation and incomplete feeding depends both on the Stokes number and on the Reynolds number, whereas the concentration distribution depends on the turbulence level, the fluid flow maps, and the number of particles which enter into the vessel through the nozzle.The aim of this work is to evaluate the key parameters (particle size, particle density, and fluid velocity) affecting sedimentation and incomplete feeding in 20 L vessel. To this end, CFD simulations of dust dispersion are performed at varying the particle density and size. Operating maps, in terms of the key parameters and/or their dimensionless combinations, are developed and a correlation for correction of the data is proposed.     

Safer and stronger together? Effects of the agglomeration on nanopowders explosion

Among the factors influencing dust explosion, the particle size distribution (PSD) is both one of the most important and complex to consider. For instance, it is commonly accepted that the explosion sensitivity increases when the particle size decreases. Such an assertion may be questionable for nano-objects which easily agglomerate. However, agglomerates can be broken during the dispersion process. Correlating the explosion parameters to the actual PSD of a dust cloud at the moment of the ignition becomes then essential. The effects of the moisture content and sieving were investigated on a nanocellulose powder and the impact of a mechanical agglomeration was evaluated using a silicon coated by carbon powder. Each sample was characterized before and after dispersion using in situ laser particle size measurement and a fast mobility particle sizer, and explosion and minimum ignition energy tests were conducted respectively in a 20 L sphere and in a modified Hartmann tube. It was observed that drying and/or sieving the nanocellulose mainly led to variations in terms of ignition sensitivity but only slightly modified the explosion severity. In contrast, the mechanical agglomeration of the silicon coated by carbon led to a great decrease in terms of ignition sensitivity, with a minimum ignition energy varying from 5 mJ for the raw powder to more than 1J for the agglomerated samples. The maximum rate of pressure rise also decreased due to modifications in the reaction kinetics, inducing a transition from St2 class to St1 class when agglomerating the dust.     

New dust explosion venting design requirements for turbulent operating conditions

The 2007 edition of the National Fire Protection Association Standard 68 for Explosion Protection by Deflagration Venting has a new provision to account for the turbulence level in combustible dust or powder processing equipment. This paper explains the development of this new provision for increased deflagration vent area requirements in highly turbulent combustible dust/powder processing equipment. The development includes a review of initial turbulence level effects on vented explosion pressures, and a review of turbulence levels measured in ASTM E1226 and ISO 6184/1 explosion test procedures to determine K_st. A review of operating conditions in some representative spray dryer plant equipment suggests that most equipment of this type probably do not have high enough air flows to require increased explosion vent areas due to turbulence, but some types of equipment with high tangential entrance air flows may well need larger vent areas.     

Experimental study on inert products,moisture, and particle size effect on the minimum ignition energy of combustible dusts

Handling combustible dusts not only continues to pose a risk to industry but can also affect the safety of society. Explosion risk could be avoided or mitigated trying to guarantee inherent safety throughout the product life chain. One way to reduce the risks when dealing with combustible dust is to increase the Minimum Ignition Energy (MIE) in order to decrease combustible dust ignition sensitivity. To achieve this decrease, the inertization technique, also known as moderation, will be used. It consists of adding inert powders or humidity to the combustible dust. As sometimes end-users also must deal with the handling of flammable dusts, this study aims to find the most optimal inert for toner waste from printers and Holi powder (organic coloured dust from Indian parties), taking Lycopodium as a reference. Calcium carbonate, sodium bicarbonate and gypsum are proposed as inert materials. In addition, with the aim of giving a second use to biomass boiler waste or boiler slagging, this waste will be analyzed as inert, as well as how humidity affects the combustible dusts. Then, sodium bicarbonate will be tested at different granulometries to evaluate the effect of particle size on moderation process. The tests were carried out in the modified Hartmann apparatus or MIKE 3.0. Mechanisms such as decomposition of inert dust have been analyzed by thermogravimetric analysis (TGA)). The results show that gypsum and moisture are the best performing inert followed by calcium carbonate. Boiler slagging and solid bicarbonate contribute to a decrease in the MIE in some of the tests. The reasons for this deviation are discussed in the presented article. When sodium bicarbonate is analyzed at different particle sizes, it is found that the optimum particle size does not match the particle size of the combustible dust. According to the tests, there is an optimum point for which the inert powder provides better results.     

Risk assessment method of polyethylene dust explosion based on explosion parameters

The explosion characteristic parameters of polyethylene dust were systematically investigated. The variations in the maximum explosion pressure (P_max), explosion index (K_st), minimum ignition energy (MIE), minimum ignition temperature (MIT), and minimum explosion concentration (MEC) of dust samples with different particle sizes were obtained. Using experimental data, a two-dimensional matrix analysis method was applied to classify the dust explosion severity based on P_max and K_st. Then, a three-dimensional matrix was used to categorize the dust explosion sensitivity based on three factors: MIE, MIT, and MEC. Finally, a two-dimensional matrix model of dust explosion risk assessment was established considering the severity and sensitivity. The model was used to evaluate the explosion risk of polyethylene dust samples with different particle sizes. It was found that the risk level of dust explosion increased with decreasing particle size, which was consistent with the actual results. The risk assessment method can provide a scientific basis for dust explosion prevention in the production of polyethylene.     

Minimum ignition energy of mixtures of combustible dusts

Most industrial powder processes handle mixtures of various flammable powders. Consequently, hazard evaluation leads to a reduction of the disaster damage that arises from dust explosions. Determining the minimum ignition energy (MIE) of flammable mixtures is critical for identifying possibility of accidental hazard in industry. The aim of this work is to measure the critical ignition energy of different kinds of pure dusts with various particle sizes as well as mixtures thereof.The results show that even the addition of a modest amount of a highly flammable powder to a less combustible powder has a significant impact on the MIE. The MIE varies considerably when the fraction of the highly flammable powder exceeds 20%. For dust mixtures consisting of combustible dusts, the relationship between the ignition energy of the mixture and the minimum ignition energy of the components follows the so-called harmonic model based upon the volume fraction of the pure dusts in the mixture. This correlation provides results which show satisfactory agreement with the experimental values.     

Visualization and analysis of dispersion process of combustible dust in a transparent Siwek 20-L chamber

Dust explosion severities are closely associated with dust dispersion behaviors. To characterize the dispersion process of dust cloud, visualization experiments were conducted by using a transparent Siwek 20-L chamber. Dispersion processes of typical carbonaceous dust were recorded by a high-speed camera and, with the image processing technique, the qualitative analysis based on the transmission of dust cloud was carried out. Results have evidenced the three consecutive stages of dust dispersion process: the fast injection stage of dust particles, the stabilization stage and the sedimentation stage of dust cloud. The motion of dust particles and the variations of dust cloud in space and time can be clearly distinguished. In the stabilization stage, the good uniformity of dust dispersion is achieved when the deviation of transmission data at different locations reaches to the minimum value. Under different nominal dust concentrations, the time periods for dust dispersion stabilization are found to be significantly different, suggesting that different dust concentrations should correspond to different ignition delay in order to accurately measure the explosion characteristics in the Siwek 20-L chamber. Moreover, it is found that the decrease trend of transmission with increasing nominal dust concentration will become gradually leveling off, different from the inversely proportional relationship according to the Bouguer's law, and this indicates that the actual dust concentration will be lower than the nominal concentration or the dust cannot be fully dispersed at the case of high dust concentration. According to the experiment, when the nominal dust concentration exceeds to 1000 g/m³, the transmission will no longer vary visibly.     

Aluminium dust explosion risk analysis in metal workings

The aim of this paper is to present a risk analysis method that can be applied to factories where combustible dust is handled, in the form of raw materials, products or by-products, and therefore at risk to explosion. The work was carried out on site: a consistent number of companies that deal with the surface finishing of objects in aluminium through grinding were examined. The aluminium powder produced as a by-product is generally captured by suction plants and then subjected to dry or wet type abatement. In order to provide a rational approach to the risk assessment and frequency estimation, each company was divided into the so-called fields of study; and four risk assessment topics were identified for each field. A brief review of the methods that are available for the consequence magnitude estimation, regarding both the pressure wave and the launching of missiles, is also provided.     

Dustiness in workplace safety and explosion protection – Review and outlook

Behavior of dust/air mixtures is very complex and difficult to predict since it depends on material properties as well as boundary conditions. Without other influences airborne particles deposit due to gravity but the time it takes for total deposition as well as easiness of resurrection depends very much on the specific dust sample and the boundary conditions. It still lacks a complete understanding of all interacting reasons and one approach is using experimentally determined characteristics, one is named dustiness.Dustiness is the tendency of dust to form clouds and to stay airborne. Dustiness is determined with two basic principles, which are light attenuation and ratio of filled-in and measured mass. Assessment of dustiness of industrial powders has been done for a long time regarding work place safety. Dustiness is used there to determine inhalable fraction and to evaluate health risks. Lately it became interesting in dust explosion protection as well. Dustiness could be used to optimize determination of zones, adaption of venting area and/or for positioning of suppression systems.Dustiness can be useful in many ways but is not a physical property of dusts, therefore it depends on material properties such as density, particle size distribution, shape and water content as well as boundary conditions or determination method. This makes it very difficult to compare dustiness for different techniques and apparatuses and determination method as well as results should be considered carefully. This work gives an overview of existing standards, recent research and suggests improvements to the new dustiness as proposed for dust explosion protection.     

Methods for more accurate determination of explosion severity parameters

Accurate determination of explosion severity parameters (p_max, (dp/dt)_max, and K_St) is essential for dust explosion assessment, identification of mitigation strategy, and design of mitigation measure of proper capacity. The explosion severity parameters are determined according to standard methodology however variety of dust handled and operation circumstances may create practical challenge on the optimal test method and subsequent data interpretation. Two methods are presented: a statistical method, which considers all test results in determination of explosion severity parameters and a method that corrects the results for differences of turbulence intensity. The statistical method also calculates experimental error (uncertainty) that characterises the experimental spread, allows comparison to other dust samples and may define quality determination threshold. The correction method allows to reduce discrepancies between results from 1 m³ vessel and 20-l sphere caused by difference in the turbulence intensity level. Additionally new experimental test method for difficult to inject samples together with its analysis is described. Such method is a versatile tool for explosion interpretation in test cases where different dispersion nozzle is used (various turbulence level in the test chamber) because of either specific test requirements or being “difficult dust sample”.     

Process loss prevention of petrochemical process: A case study of the flashing accident of the storage tank on acrylonitrile-butadiene-styrene powder in Taiwan

Qualitative analysis, process hazard analysis, thermal evaluation, and fault tree analysis were applied to a flashing accident involving a storage tank that contained acrylonitrile-butadiene-styrene (ABS) powder in Taiwan. The accident was caused by combustible powder attached to the inner wall of the tank reaching a high temperature and then melting. Thereafter, the molten powder became glue-like and dropped onto the ABS powder, burning at the tank bottom, causing decomposition of the styrene and butadiene derivatives as well as other combustible gases. The high concentration of combustible powder and low ignition temperature triggered the powder, initiating a dust explosion. Finally, we analyzed the findings of each method and examined the properties of ABS powder, realizing that the root cause of the accident included an insufficient understanding of the characteristics of ABS and the failure to comply with the management procedures of hot work. Recommendations and countermeasures were proposed that could proactively ameliorate process safety.     

Analysis on research trends with dust explosions by bibliometric approach

Highly destructive combustible dust explosions, which is prone to cause secondary explosion, has been a concern in industrial processes. To understand the current development and status of research on dust explosions, 1276 publications related to dust explosions from 1998 to 2021 were indexed through the Web of Science Core Collection database. CiteSpace and VOSviewer were used to visualize and analyze the collected literature information. The number of articles related to dust explosions has increased from 12 in 1998 to 191 in 2021. China, the United States, and Canada are the major contributors in this field. Dalhousie University, Beijing Institute of Technology, and Dalian University of Technology are at the core of dust explosion research. Wei Gao, Paul Amyotte, and Chi-Min Shu are the most prolific researchers. Journal of Loss Prevention in the Process Industries, Powder Technology, and Process Safety and Environmental Protection are the major sources of publications related to dust explosions. The research topic of dust explosions mainly evolves into four aspects: explosion characteristics and influencing factors, research media, explosion suppression, and numerical simulation. New research hotspots have appeared related to gas–dust hybrid mixtures, nanomaterials, and powder suppressants. The results can help researchers in the dust explosion field to quickly determine the research frontier and the overall situation.     

Effect of ignition delay on explosion parameters of corn dust/air in confined chamber

This paper presents the explosion parameters of corn dust/air mixtures in confined chamber. The measurements were conducted in a setup which comprises a 5 L explosion chamber, a dust dispersion sub-system, and a transient pressure measurement sub-system. The influences of the ignition delay on the pressure and the rate of pressure rise for the dust/air explosion have been discussed based on the experimental data. It is found that at the lower concentrations, the explosion pressure and the rate of pressure rise of corn dust/air mixtures decrease as the ignition delay increases from 60 ms; But at the higher concentrations, the explosion pressure and the rate of pressure rise increase slightly as the ignition delay increases from 60 ms to 80 ms, and decrease beyond 80 ms. The maximum explosion pressure of corn dust/air mixtures reaches its highest value equal to 0.79 MPa at the concentration of 1000 gm⁻³.     

Numerical study on premixing characteristics and explosion process of starch in a vertical pipe under turbulent flow

Fire and explosion accidents are frequently caused by combustible dust, which has led to increased interest in this area of research. Although scholars have performed some research in this field, they often ignored interesting phenomena in their experiments. In this paper, we established a 2D numerical method to thoroughly investigate the particle motion and distribution before ignition. The optimal time for the corn starch dust cloud to ignite was determined in a semi-closed tube, and the characteristics of the flame propagation and temperature field were investigated after ignition inside and outside the tube. From the simulation, certain unexpected phenomena that occurred in the experiment were explained, and some suggestions were proposed for future experiments. The results from the simulation showed that 60–70 ms was the best time for the dust cloud to ignite. The local high-temperature flame clusters were caused by the agglomeration of high-temperature particles, and there were no flames near the wall of the tube due to particles gathering and attaching to the wall. Vortices formed around the nozzle, where the particle concentration was low and the flame spread slowly. During the explosion venting, particles flew out of the tube before the flame. The venting flame exhibited a “mushroom cloud” shape due to interactions with the vortex, and the flame maintained this shape as it was driven upward by the vortex.