Assessment of explosion risks caused by cone discharges when filling silos and containers with pellets

It is well known that during the filling of silos and containers with bulk material, so-called cone discharges can occur because of electrostatic charges. Whether or not cone discharges occur at all depends on whether the breakdown field strength of air under atmospheric conditions of 3 MV/m is reached at the silo and container inner wall. This in turn depends on the charge to mass ratio of the bulk, the bulk resistivity, the bulk density, the relative permittivity of the bulk material, the silo or container diameter and the filling rate. If cone discharges can't be avoided, the energy of cone discharges can be estimated according to the equation given in the relevant guidelines TRGS 727 (2016) and IEC/TS 60079-32-1 (2013). Therefore, the coarse fraction must be considered. As soon as the energy of the cone discharge is greater than or equal to the minimum ignition energy of the bulk material introduced, there is a risk of dust explosion. Here the fine fraction of the bulk material is relevant.The investigations described are a practical example how computer models can be used to assess the occurrence of cone discharges. It is calculated for which silo and container diameters and filling rates the critical field strength of 3 MV/m is reached. In these calculations the charge relaxation during pneumatic filling with bulk material is taken into account. The results of the computational modelling together with operational boundary conditions serve as a decision basis whether exclusion of incendive ignition sources is an adequate safety measure or whether further explosion protection measures must be considered. Finally, a brief overview of other possible explosion protection measures is given.     

Charge reduction on polypropylene granules and suppression of incendiary electrostatic discharges by using a novel AC electrostatic ionizer

As a method to prevent or mitigate electrostatic charge and/or discharges, we have developed a novel AC electrostatic ionizer. In this study, we evaluated experimentally the practical version of the novel AC electrostatic ionizer with a modeling test system and a pneumatic powder transport facility. In addition, electrostatic discharges generated inside a silo while loading polypropylene (PP, 3 mm) granules were observed visually with/without the novel AC ionizer. The specific charge was clearly decreased with the novel AC electrostatic ionizer. The specific charge obtained with the four arranged AC electrostatic ionizers used was approximately one eighth of that without the AC ionizer. The incendiary bulk surface discharges completely died out inside the silo by using the four arranged ionizers.     

Experimental study on the relationship between the charge amount of polypropylene granules and the frequency of electrostatic discharges while silo loading

This paper is a report on the relationship between the charge amount (charge to mass ratio, q/m) of polypropylene (PP, 2–3 mm) granules and the frequency of electrostatic discharges that occur while loading a metal silo. The feedback control system was used in order to control the q/m of PP granules. The electrostatic discharges inside the silo were also observed using a conventional image-intensifier system. The charging control range for PP granules was from 0 to −12 μC/kg in the q/m. The results obtained from the experiments show that (1) two kinds of electrostatic discharges were clearly observed inside a metal silo while loading PP granules, i.e., brush discharges and incendiary bulk surface discharges; (2) the number of brush discharges and incendiary bulk surface discharges increased with the increase in the q/m of PP granules, almost reaching saturation; and (3) brush discharges and incendiary bulk surface discharges began to occur at the −1.16 μC/kg and −2.33 μC/kg points in the q/m, respectively, in this study.     

Scaling of dust explosion violence from laboratory scale to full industrial scale – A challenging case history from the past

The standardized K_St parameter still seems to be widely used as a universal criterion for ranking explosion violence to be expected from various dusts in given industrial situations. However, this may not be a generally valid approach. In the case of dust explosion venting, the maximum pressure P_max generated in a given vented industrial enclosure is not only influenced by inherent dust parameters (dust chemistry including moisture, and sizes and shapes of individual dust particles). Process-related parameters (degree of dust dispersion, cloud turbulence, and dust concentration) also play key roles. This view seems to be confirmed by some results from a series of large scale vented dust explosion experiments in a 500 m³ silo conducted in Norway by CMI, (now GexCon AS) during 1980–1982. Therefore, these results have been brought forward again in the present paper. The original purpose of the 500 m³ silo experiments was to obtain correlations between P_max in the vented silo and the vent area in the silo top surface, for two different dusts, viz. a wheat grain dust collected in a Norwegian grain import silo facility, and a soya meal used for production of fish farming food. Both dusts were tested in the standard 20-L-sphere in two independent laboratories, and also in the Hartmann bomb in two independent laboratories. P_max and (dP/dt)_max were significantly lower for the soya meal than for the wheat grain dust in all laboratory tests. Because the available amount of wheat grain dust was much larger than the quite limited amount of available soya meal, a complete series of 16 vented silo experiments was first performed with the wheat grain dust, starting with the largest vent area and ending with the smallest one. Then, to avoid unnecessary laborious changes of vent areas, the first experiment with soya dust was performed with the smallest area. The dust cloud in the silo was produced in exactly the same way as with the wheat grain dust. However, contrary to expectations based on the laboratory-scale tests, the soya meal exploded more violently in the large silo than the wheat grain dust, and the silo was blown apart in the very first experiment with this material. The probable reason is that the two dusts responded differently to the dust cloud formation process in the silo on the one hand and in the laboratory-scale apparatuses on the other. This re-confirms that a differentiated philosophy for design of dust explosion vents is indeed needed. Appropriate attention must be paid to the influence of the actual dust cloud generation process on the required vent area. The location and type of the ignition source also play important roles. It may seem that tailored design has to become the future solution for tackling this complex reality, not least for large storage silos. It is the view of the present author that the ongoing development of CFD-based computer codes offers the most promising line of attack. This also applies to design of systems for dust explosion isolation and suppression.     

Testing of dust clouds for the electrostatic-spark ignition hazard in industry. Need for a modified approach?

Current standard test methods for electric-spark minimum ignition energies (MIEs) of dust clouds in air require that a series inductance of at least 1–2 mH be included in the electric-spark discharge circuit. The reason is to prolong the spark discharge duration and thus minimize the spark energy required for ignition. However, when assessing the minimum electrostatic energy ½CU² for dust cloud ignition by accidental electrostatic-spark discharges, current testing standards require that the series inductance of at least 1–2 mH be removed from the spark discharge circuit. No other changes of apparatus and test procedure are required. The present paper questions whether this simple approach is always adequate. The reason is that in practice in industry accidental electrostatic-spark discharge circuits may contain large ohmic resistances due to corrosion, poor electrical grounding connections, poorly electrically conducting construction materials etc. The result is increased spark discharge durations and reduced mechanical disturbance of the dust cloud by the blast wave emitted by the spark. Therefore, testing for minimum ½CU² for ignition by accidental electrostatic spark discharges may not only require removal of the series inductance of 1–2 mH from the standard MIE spark discharge circuit. Additional tests may be needed with one or more quite large series resistances R_s inserted into the spark discharge circuit. The present paper proposes a modified standard test procedure for measurement of the minimum electrostatic-spark ignition energy of dust clouds that accounts for these effects.     

Control of electrostatic charge for powder by using feedback control-type ionizer system

As a method to prevent or mitigate cone discharges in a specific section such as a large silo, we have developed a new feedback control-type ionizer system. The feedback control-type system is composed mainly of an ionizer, an electrostatic field strength meter, and computer control equipment. In this study, we evaluated experimentally the practical version of the feedback control-type ionizer system through several tests in a pneumatic powder transport facility. The specific charges of the falling pellets in the silo were also measured for 10 s using a Faraday cage. Polypropylene (PP) pellets with a mean particle size of 3 mm were used in this experiment. The results of the experiment revealed that the feedback control-type ionizer system had the following characteristics: (1) it is possible to control the performance of the ionizer with a supply current; (2) the electrostatic field strength in the loading pipe from the charged powder is reduced and maintained at near zero by using the feedback control-type system; and (3) the performance of the feedback control-type system is superior to that of others, such as the conventional AC- or DC-type ionizers.     

混合药物粉尘输运过程静电特性实验研

兽药制药粉尘加工工艺过程中,由于粉尘颗粒之间或颗粒与设备、管壁之间的碰撞、摩擦,导致装置内部静电荷量积聚,激发静电放电,粉尘燃烧或爆炸的事故频发。实验主要通过包括粉尘与管材摩擦的漏电电流测试和静电放电火花对粉尘云点燃敏感性测试两部分。结果表明:单一药物药粉的静电漏电电流随着管材管径的增大,管长的增长,静电漏电电流逐渐变大;随着倾斜角的增大,静电漏电电流先增大后降低;镀锌铁管的漏电电流大于PVC管,电荷逸散速度更快。单一兽药粉的粉尘云放电火花最小点燃能量随质量浓度的变化,呈现二次曲线的变化趋势。混合兽药粉与单一兽药粉的漏电电流和粉尘云放电火花最小点燃能量的测试结果的变化趋势是一致的。     

聚烯烃工业粉尘静电爆炸的危险与应对策略

随着我国聚烯烃粉体生产的迅猛发展,粉体生产过程中的静电爆炸事故也相应增多。该文介绍了国外粉尘静电爆炸研究的进展和国内粉尘静电爆炸事故的主要现象。近期国外的研究特点主要是逐渐采用工业规模的实验装置取代实验室的基础研究,着重开展了料仓放电的危险研究和与放电燃爆有关的基础研究,包括安全评价和工业控制条件的推荐研究。国内粉尘静电爆炸事故主要存在的现象有设计缺欠或不合理,装置扩能改造时忽视了脱挥或通风的配套改造,处理不合格料或过渡时应急处理不当,操作失误或不规范,破坏料仓通风控制,料位计选型不当,增加高能放电引燃几率。针对分析出的问题和现象,文章提出了查清运行装置的具体事故隐患,进行防止聚烯烃料仓粉尘静电爆炸的可接受研究和可操作性研究,开展粉尘静电爆炸的危险教育,提高员工的安全意识和应急处理能力等应对策略。     

Analysis of the chosen thermal and flammability parameters of dried sewage dust

The hazardous sludge disposal process in the form of landfills requires the determination inter alia of the flammable and explosion properties of dried sewage sludge dust, which has the ability to ignite and spontaneously combust when stored in silos. At a constant furnace surface temperature, the minimum ignition temperature of the sludge dust layer with a layer thickness of 5 mm is 270 °C, and for a layer thickness of 12.5 mm it is 250 °C. Two selected fire extinguishing powders for Class A, B, C and D fires were used in the study to determine the possibility of reducing the susceptibility of dried wastewater to ignition from heated surface, self-ignition and explosion parameters. The most effective extinguishing powder was ABC Favorit, which increased the value of the minimum ignition temperature of the layer (5 mm thick) to 360 °C and the spontaneous ignition temperature of the sludge with this powder increased by 22 °C at 169.6 cm³ in comparison to the sludge without extinguishing powder, respectively. The lowest self-ignition temperature of 136 °C was recorded for the largest tested volume (169.6 cm³) for dried sewage dust without any fire extinguishing powders. The biggest values of p_max and (dp/dt)_max dried sewage dust were recorded 4.8 bar and 113 bar/s respectively. By analysing the obtained test results, it can be assumed that dried sewage dust is a combustible material with properties similar to biomass.     

Minimum ignition energies of pure magnesium powders due to electrostatic discharges and nitrogen's effect

This paper experimentally investigated the relation between the minimum ignition energy (MIE) of magnesium powders as well as the effect of inert nitrogen (N₂) on the MIE. The modified Hartmann vertical-tube apparatus and four kinds of different-sized pure magnesium powders (median particle size, D50; 28.1 μm–89.8 μm) were used in this study. The MIE of the most sensitive magnesium powder was 4 mJ, which was affected by the powder particle size (D50; 28.1 μm). The MIE of magnesium powder increased with an increase in the N₂ concentration for the inerting technique. The magnesium dust explosion with an electrostatic discharge of 1000 mJ was suppressed completely at an N₂ concentration range of more than 98%. The experimental data presented in this paper will be useful for preventing magnesium dust explosions generated from electrostatic discharges.     

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.     

Methane/coal dust/air explosions and their suppression by solid particle suppressing agents in a large-scale experimental tube

Methane/coal dust/air explosions under strong ignition conditions have been studied in a 199 mm inner diameter and 30.8 m long horizontal tube. A fuel gas/air manifold assembly was used to introduce methane and air into the experimental tube, and an array of 44 equally spaced dust dispersion units was used to disperse coal dust particles into the tube. The methane/coal dust/air mixture was ignited by a 7 m long epoxypropane mist cloud explosion. A deflagration-to-detonation transition (DDT) was observed, and a self-sustained detonation wave characterized by the existence of a transverse wave was propagated in the methane/coal dust/air mixtures.The suppressing effects on methane/coal dust/air mixture explosions of three solid particle suppressing agents have been studied. Coal dust and the suppressing agent were injected into the experimental tube by the dust dispersion units. The length of the suppression was 14 m. The suppression agents examined in this study comprised ABC powder, SiO₂ powder, and rock dust powder (CaCO₃). Methane/coal dust/air explosions can be efficiently suppressed by the suppression agents characterized by the rapid decrease in overpressure and propagating velocity of the explosion waves.     

Effect of scale on freely propagating flames in aluminum dust clouds

The majority of experimental tests done on combustible dusts are performed in constant volume vessels that have limited or no optical access. Over the years, McGill University has been developing alternative experimental techniques based on direct observation of dust flames, yielding reliable fundamental parameters such as flame burning velocity, temperature and structure. The present work describes two new experimental set-ups allowing direct observation of isobaric and freely propagating dust flames at two sufficiently different scales to test the influence of scale on dust flame phenomena. In the laboratory-scale experiments, a few grams of aluminum powder are dispersed in transparent, 30 cm diameter latex balloons that allow for full visualization of the spherical flame propagation. In the field experiments, about 1 kg of aluminum powder is dispersed by a short pulse of air, forming a conical dust cloud with a total volume of about 5 m³. High-speed digital imaging is used to record the particle dispersal and flame propagation in both configurations. In the small-scale laboratory tests, the measured flame speed is found to be about 2.0 ± 0.2 m/s in fuel-rich aluminium clouds. The burning velocity, calculated by dividing the measured flame speed by the expansion factor deduced from thermodynamic equilibrium calculations, correlates well with the previously measured burning velocity of about 22–24 cm/s from Bunsen dust flames. Flame speeds observed in field experiments with large-scale clouds, however, are found to be much higher, in the range of 12 ± 2 m/s. Estimations are presented that show that the presumably greater role of radiative heat transfer in larger-scale aluminium flames is insufficient to explain the six-fold increase in flame speed. The role of residual large-eddy turbulence, as well as the frozen-turbulence effect leading to large-scale dust concentration fluctuations that cause flame folding, are discussed as two possible sources for the greater flame speed.     

Characterization of the ignition by repetitive streamer discharges using laser diagnostics

Repetitive streamer discharges caused by transients, e.g. due to high frequency overvoltages, can ignite combustible mixtures, which has to be taken into account concerning the safety assessment of electrical apparatus for usage in hazardous areas. Hydrogen/air mixtures were ignited inside a closed vessel using a rod/plane electrode configuration. Alternating voltage with a frequency between 600 and 750 kHz and amplitudes of up to 20 kV was used to produce streamer discharges. The ignition process and the subsequent flame front propagation were examined with respect to mixture composition and several electrical parameters using time-resolved measurements of planar laser-induced fluorescence (PLIF) of OH radicals. A multiple pulse laser and detection system was used to assemble four images during one experiment. These measurements have given detailed information about the point of ignition and flame velocities. The experimental results will be used to validate numerical simulations of ignition by streamer discharges, which will yield deep insights into this specific ignition process.     

Application of CFD on the sensitivity analyses of some parameters of the modified Hartmann tube

The aim of this work is to determine the influence of operating parameters such as the dispersion pressure, the ignition delay and height on the dust flammability. A Computational Fluid Dynamics (CFD) simulation, based on an Euler–Lagrange approach, was developed with Ansys Fluent™ and validated experimentally. Such analysis will facilitate the choice of the most conservative conditions for a flammability test. This paper is focused on a case study performed on wheat starch with the modified Hartmann tube. The dispersion process of the powder was studied with granulometric analyses performed in situ and high speed videos. Tests were performed with injections at gas pressure ranging from 3 to 6 bars and the evolution of the particle size distribution (PSD) was recorded at different ignition heights (5, 10 and 15 cm over the dispersion nozzle). The observations highlighted the presence of agglomeration/deagglomeration processes and dust segregation. Besides, a CFD simulation analysis was aimed at evaluating the impact of a set of parameters on the PSD and the local turbulence, which are closely linked to some flammability parameters. For this computational analysis, the CFD simulation was coupled with a collision treatment based on a Discrete Element Method (DEM) in order to consider the cohesive behavior of the combustible dust. Thus the results suggest performing the injection of the gases at approximately 5 bars for the flammability tests of wheat starch in order to obtain the finest PSD at a given ignition height. It is also shown that the finest PSD are obtained at 5 cm over the dispersion nozzle. However, the local instabilities and turbulence levels are so high during the first stages of the dispersion that the flame growth can be disturbed for short ignition delays. Moreover, the stabilization of the bulk of the dust cloud requires longer periods of time when the ignition sources are located at 15 cm. As a result, the recommended height to perform a flammability test is 10 cm in this case. Finally, this study proposes some tools that might improve the procedure of dust flammability testing.     

Experimental investigation and modelling of aluminum dusts explosions in the 20 L sphere

An experimental investigation was carried out on the influences of dust concentration, particle size distribution and humidity on aluminum dust explosion. Tests were mainly conducted thanks to a 20 L explosion sphere. The effect of humidity was studied by storing the aluminum particles at constant relative humidity until the sorption equilibrium or by introducing water vapour in the explosion vessel. The tested particles sizes ranged from a volume median diameter of 7 to 42 μm and the dust concentrations were up to 3000 g m^?3.Among other results, the strong influence of the particle size was pointed out, especially when the Sauter mean diameter is considered. These results stressed the predominance of the specific surface area on the mass median particle diameter.The effect of water on aluminum dust explosion was decoupled: on the one hand, when water adsorption occurs, hydrogen generation leads to an increase of the explosion severity; on the other hand, when the explosion of dried aluminum powder occurs in a humid atmosphere, the inhibiting effect of humidity is put forward.A model based on mass and heat balances, assuming a shrinking core model with chemical reaction limitation, leads to a satisfactory representation of the pressure evolution during the dust explosion.     

Evaluation of the explosibility of malt grain dust based on static electrification during pneumatic transportation

The possibility of dust explosions by static electricity in a malt grain silo was investigated. Two kinds of experimental equipment were applied. One was to supply electrostatic charge in order to investigate the charge build up characteristics. The other was to transport the malt grain pneumatically in order to investigate the frictional charge accumulation in the transportation system.

The particle charge of the pulverized malt grain was in the order of 10⁻¹⁴ C. The particle charge of the malt grain was in the order of 10⁻⁹ C and the pipe charge in the transport system was also in the order of 10⁻⁹ C. The charge accumulated on both the pulverized particle and the grain particle were small in view of the incendiary potentiality. However, attention must be paid when the particles are dumped into isolated space. There might be a charge accumulation that will lead to the ignition of the dust cloud.     

Study of dust cloud behaviour in the modified Hartmann tube using the image subtraction method (ISM)

A dispersion of fine particles in the air is needed for a dust explosion to occur since an explosion is the fast combustion of particles in the air. When particles are poorly dispersed, agglomerated, or their concentration is low, the combustion velocity decreases, and deflagration would not occur. The combustion rate is strictly related to dust concentration. Therefore, the maximum explosion pressure rise occurs at dust concentration close to stoichiometric. Conversely, Minimum Explosion Concentration (MEC) is the lower limit at which self-sustained combustion and a pressure rise are possible. Dust explosion tests are designed to reproduce the dispersion and generation of dust clouds in industrial ambiences by using dispersion devices activated by pressurised air pulses. The resulting dust cloud, which has a marked transient character, is considered representative of real clouds by current standards. Over time, several studies have been carried out to optimise these devices (e.g. to reduce the inhomogeneity of the cloud in the 20 L sphere). The Minimum Ignition Energy (MIE) of dust is measured using the Mike3 modified Hartmann tube, where the ignition attempt is made 60–180 ms after dust dispersion regardless of dust characteristics.This work investigates the dust clouds’ actual behaviour inside the modified Hartmann tube before ignition using high-velocity video movies and a new image post-treatment method called Image Subtraction Method (ISM). Movies are recorded with high-speed cameras at a framerate of 2000 fps and elaborated with an on-purpose developed LabVIEW® code. Concentration (mass per volume) and dispersion pressure are varied to evaluate their effect on dust clouds. Maise starch, iron powder and silica powder are chosen to investigate the effect of particle density and size on the cloud structure and turbulence. This approach will help to investigate the structure of the dust cloud, the shape and size of the particle lumps and the change in dust concentration over time. In addition, information on the actual concentration and cloud turbulence at the ignition location and delay time were obtained, which may help identify the local turbulence scale and widen the characterisation of the cloud generated in the Hartmann tube.     

非平衡式双极性离子风消电技术与应用

为实现风送管道物料静电监测与控制,预防料仓静电燃爆事故,提高聚烯烃装置料仓安全水平,基于非平衡式双极性离子风消电技术,开发了双极性离子风消电器;基于离子风消电器和静电监测器,探讨了石化粉体料仓用离子风消电控制系统组成、电路、气路布局;对非平衡式双极性离子风消电系统进行现场应用测试。结果表明:基于此管道粉体消静电技术,合理调节正、负侧控制电压,可有效控制管道物料荷质比稳定在±0.3 μC/kg以内,保障聚烯烃装置料仓安全、稳定运行。     

Coloured powder potential dust explosions

The use of Coloured powder (Holi powder orcolour dust) has been largely used in India for their festivities. Due to their popularity is extensive around the world since the popularity of the parties and events with this kind of show is increasing considerably. Despite the fact of its extensive use, its highly flammable nature is poorly known. Currently, some serious accidents related to the Coloured powder have been registered. Coloured powder organic nature implies a significant increase in the probability to form an explosive atmosphere as their use includes dust dispersion, leading to explosion hazards as has been previously reported. Moreover, it is important to take into account the effects on the flammability of the additives and the colorings existing in the Coloured powder as they might increase the hazard. To properly understand Coloured powder potential for producing an explosive atmosphere, and the attached risk of dust explosions, several samples were tested. Coloured powder from 6 different manufacturers were gathered. Each manufacturer provided several colours (between 5 and 8) which were characterized through moisture content and particle size determination. Once each sample was characterized, screening tests were performed on each sample determining whether ignition was produced or not. Those screening tests were carried out under certain conditions using the equipment for minimum ignition temperature on cloud determination (0.5 g set at 500 °C and 0.5 bar), and minimum ignition energy determination (using 100 and 300 mJ energies and 900 and 1200 mg). From those test results, important differences were seen between manufacturers, but most important, differences between colours of the same manufacturer were observed. The screening tests allowed the selection of 11 samples that were fully characterized through thermogravimetric analysis, maximum pressure of explosion, K_st, minimum ignition temperature on cloud, and minimum ignition energy. When carrying out thermogravimetric analysis, some samples increased mass at temperatures close to 300 °C and unexpectedly absorbed energy, followed by the expected combustion reaction at higher temperatures. From the obtained results it was noticed that the colour powders that included talcum in its composition did not produce explosion. Flammability and explosion tests, again, showed important differences between manufacturers and colours, and so it was possible to determine the relative flash fire and explosion risks of the various tested powders.