Experimental study on ignitability of pure aluminum powders due to electrostatic discharges and Nitrogen's effect

In order to prevent dust explosions due to electrostatic discharges (ESD), this paper reports the minimum ignition energy (MIE) of aluminum powders in the air and the effective nitrogen (N₂) concentration for the inert technique. The Hartman vertical-tube apparatus and five kinds of different sized pure aluminum powders (median particle size, D50; 8.53 μm–51.2 μm) were used in this study. The statistic minimum ignition energy (MIE_s) of the most sensitive aluminum powder used in this study was 5 mJ, which was affected by the powder particle size (D50; 8.53 μm). In the case of aluminum powder, the inerting effects of N₂ were quite different from the polymer powders. The MIE of aluminum powder barely changed until the N₂ concentration was 89% in comparison with that of the normal air. When the N₂ concentration was 90%, the MIE of aluminum powders suddenly exceeded 1000 mJ, which does not occur easily with ESD in the industrial process.     

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.     

Applications of dust explosion hazard and disaster prevention technology

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

Inhibiting effects by Fe2O3 on combustion and explosion characteristics of ABS resin

The global increase in the use of, and reliance on, plastics has prompted the demand for acrylonitrile-butadiene-styrene (ABS) resin in various fields. With this increased requirement, numerous failures have occurred in the ABS process. Those incidents, resulting from electrostatic discharge, powder accumulation, heat accumulation, construction sparks, and plant fires, have caused dust fire and explosions.In this study, the ABS resin was gleaned from the site and tested for its explosion parameters, including minimum ignition temperature of dust cloud (MIT_C), minimum ignition energy (MIE), and minimum explosion concentration (MEC). To improve loss prevention in the manufacturing process, ferric oxide (Fe₂O₃) as an inert additive was added in the ABS powder. According to the MIE test, Fe₂O₃ has an apparent inhibiting effect on dust explosion for the ABS dust. With the proportion of Fe₂O₃ increased from 25 to 50 mass% in ABS, the MIE increased from 67 to 540 mJ. The explosion tests via 20-L apparatus indicated that Fe₂O₃ mixed with ABS could not increase the MEC significantly. However, the explosion pressure dropped by increasing in the ratio of Fe₂O₃ in ABS. This inerting strategy of ABS was deemed to substantially lessen the probability and severity of fire and explosion.     

Study on safe air transporting velocity of nanograde aluminum,iron, and titanium

The process of delivering nanograde metal powders by a high-speed carrier gas is often subject to high explosion risks. This study adopted initial flow rates of 13.1, 8.5, 6.5, and 3.5 m/s for air transporting 30-nm titanium powder, 35-nm iron powder, and 35-nm aluminum powder to gauge the impact on a pipe bend in a 20-l-apparatus. The test results revealed that the 30-nm titanium powder caused an explosion at all initial flow rates; the 35-nm iron powder also caused an explosion, but dust explosion and sintering were eliminated when the flow rate was adjusted to less than 3.5 m/s; and the 35-nm aluminum powder exhibited no explosion or sintering at all flow rates. When pure nitrogen was used for transporting nanograde metal powders, no explosions occurred for all the three types of metal powders. The minimum ignition energy for these three types of nanograde metal powders was less than 1 mJ.     

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.     

Influence of different ignition delay times on the pressure rise rate in hybrid mixture explosions in the 20-L sphere

For the development of a standardized method for measuring the explosion safety characteristics of combustible hybrid dust/vapor mixtures, the influence of the ignition delay time needs to be investigated. The ignition delay time, defined as the time between the injection of dust and the activation of the ignition source, is related to the turbulence of the mixture and thus to the pressure rise rate. The ignition source for pure vapors, however, has to be activated in a quiescent atmosphere according to the standards. Nevertheless, when measuring the explosion safety characteristics of hybrid mixtures, it is important that the dust be in suspension around the igniter. Like pure dust/air mixtures, hybrid dust/vapor/air mixtures need to be ignited in a turbulent atmosphere to keep the dust in suspension.This work will therefore investigate the influence of ignition delay times on the severity of hybrid explosions. It was generally found that at shorter ignition delay times, (dp/dt)_ex increased due to higher turbulence and decreases as the dust sinks to the bottom of the 20 L-sphere. This effect is more pronounced for hybrid mixtures with higher vapor content compared to dust content.     

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.     

Study on explosion characters of HDPE dust inerted by CaCO3 and NaHCO3

To prevent high density polyethylene (HDPE) dust explosions, this study evaluated HDPE's explosive sensitivity characteristics, and comparatively examined two inert dust types (CaCO₃ and NaHCO₃) to mitigate the explosive sensitivity of HDPE dust. In the serials of experiments, the 1.2 L Hartmann tube and Godbert-Greenwald furnace were used respectively to measure the minimum ignition energy (MIE) and minimum ignition temperature (MIT) of HDPE dust. The findings demonstrated that the MIE and MIT of HDPE are 56.8 mJ and 320 °C under the most sensitive situation. Second, both CaCO₃ and NaHCO₃ can inhibit the explosive sensitivity of HDPE with the variation of several parameters (i.e., quality percentage and particle sizes). Specially, as the quality percentage of 38–48 μm NaHCO₃ come to 70%, the HDPE/NaHCO₃ will not be explosive. Finally, NaHCO₃ had a better inerting effect than CaCO₃ in the reduction of explosive sensitivity of HDPE.     

Effect of concentration and ignition position on vented methane–air explosions

Experiments were conducted in a 1 m³ vessel with a top vent to investigate the effect of methane concentration and ignition position on pressure buildup and flame behavior. Three pressure peaks (p₁, p₂, and P_ext) and two types of pressure oscillations (Helmholtz and acoustic oscillations) were observed. The rupture of vent cover results in p₁ that is insensitive to methane concentration and ignition position. Owing to the interaction between acoustic wave and the flame, p₂ forms in the central and top ignition explosions when the methane–air mixture is near–stoichiometric. When the methane–air mixture is centrally ignited, p₂ first increases and then decreases with an increase in the methane concentration. The external explosion-induced P_ext is observed only in the bottom ignition explosions with an amplitude of several kilopascals. Under the current experimental conditions, flame–acoustic interaction leads to the most serious explosions in central ignition tests. Methane concentration and ignition position have little effect on the frequency of Helmholtz and acoustic oscillations; however, the Helmholtz oscillation lasts longer and first decreases and then increases as the methane concentration increases for top ignition cases. The ignition position significantly affects the Taylor instability of the flame front resulting from the Helmholtz oscillation.     

Ignitability assessment of shredder dusts of refrigerator and the prevention of the dust explosion

Explosibility of polyurethane dusts produced in the recycling process of refrigerator and the ways to prevent the dust explosion were studied. In recent years, cyclopentane is often used as the foaming agent and this produces explosive atmosphere in the shredding process. The minimum explosive concentration of polyurethane dust, influence of coexisting cyclopentane gas on the explosibility, effect of relative humidity on the minimum explosive concentration of polyurethane dusts, the minimum ignition energy, influence of cyclopentane mixture on the explosion severity, etc. were investigated.The minimum explosive dust concentration decreased with the increase of cyclopentane concentration and increased with the increase of relative humidity. The minimum ignition energy was about 11 mJ. The ignition energy decreased with the increase of the cyclopentane gas concentration. The cyclopentane gas concentration up to about 5300 ppm did not influence too much on the explosion index (K_st) and maximum explosion pressure. From these, it would be a good way to increase the relative humidity and to regulate the cyclopentane concentration in the shredding process to prevent the dust explosion hazard.     

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.     

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.     

Experiments on the influence of pre-ignition turbulence on vented gas and dust explosions

Experiments were performed on the influence of pre-ignition turbulence on the course of vented gas and dust explosions. A vertical cylindrical explosion chamber of approximately 100 l volume and a length-to-diameter ratio (l/d) of 4.7 consisting of a steel bottom segment and three glass sections connected by steel flanges was used to perform the experiments. Sixteen small fans evenly distributed within the chamber produced turbulent fluctuations from 0 to 0.45 m/s. A Laser-Doppler-anemometer (LDA) was used to measure the flow and turbulence fields. During the experiments the pressure and in the case of dust explosions the dust concentration were measured. In addition, the flame propagation was observed by a high-speed video camera. A propane/nitrogen/oxygen mixture was used for the gas explosion experiments, while the dust explosions were produced by a cornstarch/air mixture.It turned out that the reduced explosion pressure increased with increasing turbulence intensity. This effect was most pronounced for small vents with low activation pressures, e.g. for bursting disks made from polyethylene foil. In this case, the overpressure at an initial turbulence of 0.45 m/s was twice that for zero initial turbulence.     

Initiation of dust explosions by electric spark discharges triggered by the explosive dust cloud itself

An investigation of ignition of dust clouds by the use of electric spark discharges triggered by the explosive dust cloud itself has been conducted. This method of triggering capacitive sparks probably represents a realistic mechanism for initiating accidental dust explosions in industrial practice. Unlike the conventional method for determining the minimum ignition energy (MIE) in the laboratory, the delay between dust dispersion and spark discharge is not a degree of freedom. In stead, the transient dust cloud itself is used to initiate spark breakdown between electrodes set at a high voltage lower than breakdown in pure air. In the present study, different kinds of dusts were tested as ‘spark triggers’, and they exhibited quite different abilities to trigger breakdown. Large particles were found to initiate breakdown at lower voltages than smaller ones. In general, conductive particles were not found to initiate breakdown at lower voltages than dielectric ones when using the same dust concentration.Minimum ignition energies (MIE) of three dusts (Lycopodium clavatum, sulphur and maize starch) were determined using the authors' method of study. The MIEs were somewhat higher than those obtained using conventional methods, but relatively close to the values obtained through conventional methods.     

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.     

戊唑醇粉尘最小点火能试验研究

采用1.2 L哈特曼管爆炸装置分别对粒径小于54μm、74μm、150μm及大于150μm的戊唑醇粉尘进行测试。针对戊唑醇粉尘浓度及粒径范围对其最小点火能的影响,分别进行单因素试验,并对其危险性进行分级。结果表明,保持粒径小于150μm,环境温度为20℃,喷粉压力为0.7 MPa,在质量浓度100~1 300 g/m~3之间,戊唑醇粉尘的最佳敏感质量浓度ρ_m为983.71 g/m~3,此时的最小点火能为404.74 mJ。保持戊唑醇粉尘质量浓度为900 g/m~3,环境温度为20℃,喷粉压力为0.7 MPa不变,粒径小于54μm、74μm、150μm及大于150μm的戊唑醇粉尘的最小点火能分别为10 mJ、100 mJ、400 mJ和1 000 mJ以上。因此,判定戊唑醇粉尘最小点火能属于M2级,为特别着火敏感性。     

超细三七粉最小引燃温度实验研究

为研究三七粉着火燃烧的参数,用粉尘云引燃温度装置和粉尘层引燃温度装置,对三七粉的最小引燃温度(MIT)进行实验研究。分别研究喷吹压力、质量浓度、粉尘层厚度对MIT的影响。结果表明:三七粉尘云的质量在0.2 g时最小引燃温度随着喷尘压力的增加先减小再增大,在0.3 g到0.6 g时最小引燃温度随着喷尘压力的增加而增大;在压力20 kPa、30 kPa时随着质量浓度的增大,粉尘云引燃温度先减小后增大,在40 kPa到60 kPa时,随着质量浓度的增大,粉尘云引燃温度增大;粉尘云最小引燃温度高于粉尘层最小引燃温度;三七粉尘云的最小引燃温度399℃,粉尘层最小引燃温度240℃。     

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.     

硬脂酸粉着火敏感性影响因素及惰化研究

为预防和减轻硬脂酸粉加工、储存和运输过程中的燃爆危害,采用Godbert-Greenwald恒温炉分别研究质量浓度、分散压力、惰性粉体质量分数对硬脂酸粉尘云最低着火温度的影响规律。研究结果表明：硬脂酸粉尘云的最低着火温度随质量浓度和分散压力的增加先减小后增大,当质量浓度和分散压力分别为485.4 g/m³,15 kPa时,硬脂酸粉尘云最低着火温度达到最小;添加少量惰性粉体增大了硬脂酸粉尘云分散性,对硬脂酸粉尘云最低着火温度的降低起到促进作用;随惰性粉体质量分数的增加,硬脂酸粉尘云最低着火温度先迅速增大后增速变缓;SiO₂通过物理作用抑制硬脂酸粉尘云燃烧,Al(OH)₃除物理作用外还通过化学分解参与自由基碰撞,可有效提升硬脂酸粉尘云的最低着火温度。