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.     

Minimum ignition energy (MIE) — a basic ignition sensitivity parameter in design of intrinsically safe electrical apparatus for explosive dust clouds

In general terms, the purpose of any safety standard is to define borderlines between safe and unsafe conditions, with reasonable safety margins. The electrical spark ignition sensitivity of dust clouds (MIE) varies over at least eight orders of magnitude. Therefore, in the case of intrinsically safe electrical apparatus to be used in the presence of explosive dust clouds, substantial differentiation of the minimum requirements to prevent ignition by electrical sparks is needed. The present paper proposes a method by which adequate differentiation of required maximum permissible currents and/or voltages in intrinsically safe electrical circuits to be used in explosive dust clouds can be achieved. In essence, the concept is to use conservative first-order ignition curves, calculated or estimated from the experimental MIE value of clouds in air of the actual dust. Charts to be used for design purposes are given in the paper. Internationally standardised test methods allow MIE for clouds of any dust to be determined, at least down to the range of a few mJ. There is, however, a need for a supplementary method covering the range of lower energies, down to 0.01 mJ.     

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.     

A theoretical model for the prediction of the minimum ignition energy of dust clouds

In this study, a physical model of the dust cloud ignition process is developed for both cylindrical coordinates with a straight-line shaped ignition source and spherical coordinates with a point shaped ignition source. Using this model, a numerical algorithm for the calculation of the minimum ignition energy (MIE) is established and validated. This algorithm can evaluate MIEs of dusts and their mixtures with different dust concentrations and particle sizes. Although the average calculated cylindrical MIE (MIE_cylindrical) of the studied dusts only amounts to 63.9% of the average experimental MIE value due to reasons including high idealization of the numerical model and possible energy losses in the experimental tests, the algorithm with cylindrical coordinates correctly predicts the experimental MIE variation trends against particle diameter and dust concentration. There is a power function relationship between the MIE and particle diameter of the type MIE ∝ d_p^k with k being approximately 2 for cylindrical coordinates and 3 for spherical coordinates. Moreover, as dust concentration increases MIE(conc) first drops because of the decreasing average distance between particles and, at fuel-lean concentrations the increasing dust cloud combustion heat; however, after the dust concentration rises beyond a certain value, MIE(conc) starts to increase as a result of the increasingly significant heat sink effect from the particles and, at fuel-rich concentrations the no longer increasing dust cloud combustion heat.     

Effects of humidity on minimum ignition energy of gaseous epoxypropane/air mixtures

The minimum ignition energy (MIE) is an important property for designing safety standards and understanding the ignition process of combustible mixtures. The minimum ignition energy (MIE) of gaseous epoxypropane/air mixtures is measured using capacitive spark discharge. The effect of humidity on MIE is studied. It is shown that the MIE is not constant when the relative humidity increases from 40% to 88% at room temperature. The relative humidity has no significant influence on the MIE of gaseous epoxypropane/air mixtures at the lower volume fraction of gaseous epoxypropane in air. But, it has significant influence on that at the higher volume fraction. The MIEs of gaseous epoxypropane/air mixtures vary with the fraction of gaseous epoxypropane in air and the humidity. The lowest value of MIE (0.12 mJ) of gaseous epoxypropane/air mixtures is reached at around 10% in the examined ranges of the concentrations for the humidity 40%. The lowest values of MIE (0.1 mJ) of the mixtures are reached also at around 10% in the examined ranges of the concentrations for the humidity 66% and 88% respectively.     

Minimum ignition energy theoretical model for flammable gas based on flame propagation layer by layer

To achieve the rapid prediction of minimum ignition energy (MIE) for premixed gases with wide-span equivalence ratios, a theoretical model is developed based on the proposed idea of flame propagation layer by layer. The validity and high accuracy of this model in predicting MIE have been corroborated against experimental data (from literature) and traditional models. In comparison, this model is mainly applicable to uniform premixed flammable mixtures, and the ignition source needs to be regarded as a punctiform energy source. Nevertheless, this model can exhibit higher accuracy (up to 90%) than traditional models when applied to premixed gases with wide-span equivalence ratios, such as C₃H₈-air mixtures with 0.7–1.5 equivalence ratios, CH₄-air mixtures with 0.7–1.25 equivalence ratios, H₂-air mixtures with 0.6–3.15 equivalence ratios et al. Further, the model parameters have been pre-determined using a 20 L spherical closed explosion setup with a high-speed camera, and then the MIE of common flammable gases (CH₄, C₂H₆, C₃H₈, C₄H₁₀, C₂H₄, C₃H₆, C₂H₂, C₃H₄, C₂H₆O, CO and H₂) under stoichiometric or wide-span equivalence ratios has been calculated. Eventually, the influences of model parameters on MIE have been discussed. Results show that MIE is the sum of the energy required for flame propagation during ignition. The increase in exothermic and heat transfer efficiency for fuel molecules can reduce MIE, whereas prolonging the flame induction period can increase MIE.     

Explosion characteristics of micron- and nano-size magnesium powders

Explosion characteristics of micron- and nano-size magnesium powders were determined using CSIR-CBRI 20-L Sphere, Hartmann apparatus and Godbert-Greenwald furnace to study influence of particle size reduction to nano-range on these. The explosion parameters investigated are: maximum explosion pressure (P_max), maximum rate of pressure-rise (dP/dt)_max, dust explosibility index (K_St), minimum explosible concentration (MEC), minimum ignition energy (MIE), minimum ignition temperature (MIT), limiting oxygen concentration (LOC) and effect of reduced oxygen level on explosion severity. Magnesium particle sizes are: 125, 74, 38, 22, 10 and 1 μm; and 400, 200, 150, 100, 50 and 30 nm. Experimental results indicate significant increase in explosion severity (P_max: 7–14 bar, K_St: 98–510 bar·m/s) as particle size decreases from 125 to 1 μm, it is maximum for 400 nm (P_max: 14.6 bar, K_St: 528 bar·m/s) and decreases with further decrease of particle size to nano-range 200–30 nm (P_max: 12.4–9.4 bar, K_St: 460–262 bar·m/s) as it is affected by agglomeration of nano-particles. MEC decreases from 160 to 30 g/m³ on decreasing particle size from 125 to 1 μm, its value is 30 g/m³ for 400 and 200 nm and 20 g/m³ for further decrease in nano-range (150–30 nm). MIE reduces from 120 to 2 mJ on decreasing the particle size from 125 to 1 μm, its value is 1 mJ for 400, 200, 150 nm size and <1 mJ for 50 and 30 nm. Minimum ignition temperature is 600 °C for 125 μm magnesium, it varies between 570 and 450 °C for sizes 38–1 μm and 400–350 °C for size range 400–30 nm. Magnesium powders in nano-range (30–200 nm) explode less violently than micron-range powder. However, likelihood of explosion increases significantly for nano-range magnesium. LOC is 5% for magnesium size range 125–38 μm, 4% for 22–1 μm, 3% for 400 nm, 4% for 200, 150 and 100 nm, and 5% for 50 and 30 nm. Reduction in oxygen levels to 9% results in decrease in P_max and K_St by a factor of 2–3 and 4–5, respectively, for micron as well as nano-sizes. The experimental data presented will be useful for industries producing or handling similar size range micron- and nano-magnesium in order to evaluate explosibility of their magnesium powders and propose/design adequate safety measures.     

Combustion characteristics of pure aluminum and aluminum alloys powders

In this work, the explosion and combustion characteristics of aluminum and some aluminum alloys AlSi7Mg0.6, AlSi10Mg, AlMg5 under powders conditioning were studied. The idea was to compare the combustion of pure aluminum and aluminum alloys. The Minimum Ignition Energy (MIE) and explosion severity ΔP_max and (dP/dt)_max which represents the dust explosion parameters were measured for all powders using Hartman tube and 20 L spherical bomb. The particles temperature and flame temperature were determined by using IR pyrometer and spectroscopy respectively. The results showed that pure aluminum was more sensitive and severe than its alloys. MIE were: 4 mJ for pure aluminum, 13–23 mJ for aluminum alloys. For severity parameters, the overpressure ΔP_max were around 7–8 bars with maximum rate of pressure rise at 1170 bar/s for aluminum and 5–7 bars with 250–360 bar/s for alloys. However, it has been observed that flame temperatures were similar for aluminum and alloys and vary around 2800–3300 K as a function of concentration.     

不同条件下6-APA粉体爆炸最小点火能测试研究

6氨基青霉烷酸（6-APA）是生产阿莫西林的重要中间体,在生产过程的离心机分离及干燥等环节存在粉体燃烧爆炸的危险。利用Hartmann管式粉尘最小点火能测试装置,研究6-APA干粉状态及丙酮存在环境粉体最小点火能变化规律。实验结果表明,6-APA粉体在分散质量为0.6g时,最小点火能为14mJ,参照VDI2263的规定,属于一般着火敏感性粉尘。向粉体中加入丙酮溶剂模拟实际生产环境,实验结果显示粉尘云最小点火能下降明显,且混合物着火能力增强。质量为1g的6-APA粉体与0.5mL丙酮溶剂配比条件下,混合物分散质量为0.6g时,最小点火能为6mJ,在此环境中混合粉体属于特别着火敏感性粉尘。实验结果阐明了6-APA在丙酮存在环境条件下混合粉体燃烧的爆炸危险性,为采取相应的爆炸防护措施提供了实验依据。     

MIE determination and thermal degradation study of PA12 polymer powder used for laser sintering

Pulverized materials such as metallic or polymer powders play a considerable role in many industrial processes. Their use requires the introduction of preventive safeguards to control the plant's safety.PA12 polymer powder processing by laser sintering is characteristic of this tendency. The present work concerns PA12 powder (bimodal particle size distribution: 10 μm and 55 μm) and relates to explosion sensitivity and the thermal degradation of this powder, which can occur during laser sintering. Minimum Ignition Energy is determined using a modified Hartmann tube combined with the Langlie method developed in the PRISME Laboratory. This study shows the influence of parameters such as distance between the electrodes, powder concentration and arc power on MIE values. Theses parameters vary in the range of 3–6 A for the current intensity of the spark and the electrode gap in the range of 2.5–4 mm. The MIE is obtained for a spark gap of 3 mm and current intensity of the 4 A spark in our device. It shows that the MIE is less than 40 mJ for concentrations approaching 1000 g/m³. At lower concentrations (under 150 g/m³) the MIE increases but discrepancies in measurements appear, probably because of the static electricity that creates strong irregularities in dust dispersion. The second part of this study concerns the thermal degradation of the PA12 which is performed by thermogravimetric experiments coupled with mass spectrometric (MS) analysis for gas investigation. The mass loss measurement combined with the gas analysis allows the principal stages of degradation to be determined so as to calculate the kinetics parameter PA12. Experiments have been performed for different heating rates between 1 and 30 K min^?1 and the reproducibility of experiments has been verified. The activation energy is determined using two methods: Freidman and KAS. For a reaction rate of between 0.2 and 0.6, the activation energy is nearly constant. The KAS method gives a value of E_a = 250 kJ mol^?1 and the Friedman method gives E_a = 300 kJ mol^?1. The gas analysis by MS shows that oxidation begins at over 350 °C and finishes at under 650 °C with the formation of CO₂ and H₂O. Other major peaks with an m/z ratio of 29, 28 and 30 are noticed in this range of temperature. They show the presence of intermediate species such as C₂H₆, NO or CH₂O. The presence of HCN is also detected (m/z ratio of 27).     

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.     

Statistical method for the determination of the ignition energy of dust cloud-experimental validation

Powdery materials such as metallic or polymer powders play a considerable role in many industrial processes. Their use requires the introduction of preventive safeguard to control the plants safety. The mitigation of an explosion hazard, according to the ATEX 137 Directive (1999/92/EU), requires, among other things, the assessment of the dust ignition sensitivity. PRISME laboratory (University of Orléans) has developed an experimental set-up and methodology, using the Langlie test, for the quick determination of the explosion sensitivity of dusts. This method requires only 20 shots and ignition sensitivity is evaluated through the E₅₀ (energy with an ignition probability of 0.5). A Hartmann tube, with a volume of 1.3 l, was designed and built. Many results on the energy ignition thresholds of partially oxidised aluminium were obtained using this experimental device (Baudry, 2007) and compared to literature. E₅₀ evolution is the same as MIE but their respective values are different and MIE is lower than E₅₀ however the link between E₅₀ and MIE has not been elucidated.In this paper, the Langlie method is explained in detail for the determination of the parameters (mean value E₅₀ and standard deviation σ) of the associated statistic law. The ignition probability versus applied energy is firstly measured for Lycopodium in order to validate the method. A comparison between the normal and the lognormal law was achieved and the best fit was obtained with the lognormal law.In a second part, the Langlie test was performed on different dusts such as aluminium, cornstarch, lycopodium, coal, and PA12 in order to determine E₅₀ and σ for each dust. The energies E₀₅ and E₁₀ corresponding respectively to an ignition probability of 0.05 and 0.1 are determined with the lognormal law and compared to MIE find in literature. E₀₅ and E₁₀ values of ignition energy were found to be very close and were in good agreement with MIE in the literature.     

Predicting work performance in nuclear power plants

Diagnosis and monitoring are the major tasks of an operator in main control room of nuclear power plants (NPPs). The operator’s mental workload influences his/her performance, and furthermore, affects the system safety and operations. This study investigated the operator’s mental workload and work performance of the NPP in Taiwan. An experiment including primary and secondary tasks was designed to simulate the reactor shutdown procedure of the fourth nuclear power plant (FNPP). The performance of the secondary tasks (error rate), subjective mental workload (NASA Task Load Index, NASA-TLX) as well as seven physiological indices were assessed and measured. The group method of data handling (GMDH) was applied to integrate these physiological indices to develop a work performance predictive model. The validity of the proposed model is very well with R² = 0.84 and its prediction capability is high (95% confidence interval). The proposed model is expected to provide control room operators a reference value of their work performance by giving physiological indices. Besides NPPs, the proposed model can be applied to many other fields, e.g. aviation, air transportation control, driving and radar vigilance, etc.     

Electrostatic ignition of sensitive flammable mixtures: Is charge generation due to bubble bursting in aqueous solutions a credible hazard?

Experiments have been conducted to gain insight into the credibility of sparging aqueous solutions as an electrostatic ignition hazard for sensitive hydrogen/air or fuel/oxygen mixtures (Minimum Ignition Energies of ∼0.017 mJ and ∼0.002 mJ, respectively, compared to ∼0.25 mJ for hydrocarbon/air mixtures). Tests performed in a 0.5 m³ ullage produced electric field strengths between 125 and 560 V m⁻¹ for air flows of 5–60 l min⁻¹, respectively, comprised of 2–4 mm diameter bubbles. Field strength can be related to the space charge and fitting to an exponential accumulation curve enabled the charge generation rate from the air flows to be estimated. This was observed to be directly proportional to the air flow and its magnitude was consistent with literature data for bubble bursts. The charge accumulation observed at laboratory scale would not be a cause for concern. On the basis of a simple model, the charge accumulation in a 27 m³ ullage was predicted for a range of air flows. It is apparent from such calculations that ignition of hydrocarbon/air mixtures would not be expected. However, it would seem possible that field strengths might be sufficient to cause a risk of incendive spark or corona discharges in moderately sized vessels with sensitive flammable mixtures.     

Niacin,lycopodium and polyethylene powder explosibility in 20-L and 1-m3 test chambers

An experimental program has been undertaken to investigate the explosibility of selected organic dusts. The work is part of a larger research project aimed at examination of a category of combustible dusts known as marginally explosible. These are materials that appear to explode in laboratory-scale test chambers, but which may not produce appreciable overpressures and rates of pressure rise in intermediate-scale chambers. Recent work by other researchers has also demonstrated that for some materials, the reverse occurs – i.e., values of explosion parameters are higher in a 1-m³ chamber than one with a volume of 20 L. Uncertainties can therefore arise in the design of dust explosion risk reduction measures.The following materials were tested in the current work: niacin, lycopodium and polyethylene, all of which are well-known to be combustible and which cover a relatively wide range of explosion consequence severity. The concept of marginal explosibility was incorporated by testing both fine and coarse fractions of polyethylene. Experiments were conducted at Dalhousie University using the following equipment: (i) Siwek 20-L explosion chamber for determination of maximum explosion pressure (P_max), volume-normalized maximum rate of pressure rise (K_St), and minimum explosible concentration (MEC), (ii) MIKE 3 apparatus for determination of minimum ignition energy (MIE), and (iii) BAM oven for determination of minimum ignition temperature (MIT). Testing was also conducted at Fauske & Associates, LLC using a 1-m³ explosion chamber for determination of P_max, K_St and MEC. All equipment were calibrated against reference dusts, and relevant ASTM methodologies were followed in all tests.The explosion data followed known trends in accordance with relevant physical and chemical phenomena. For example, P_max and K_St values for the fine sample of polyethylene were higher than those for the coarse sample because of the decrease in particle size. MEC values for all samples were comparable in both the 20-L and 1-m³ chambers. P_max and K_St values compared favorably in the different size vessels except for the coarse polyethylene sample. In this case, K_St determined in a volume of 1 m³ was significantly higher than the value from 20-L testing. The fact that the 20-L K_St was low (23 bar m/s) does not indicate marginal explosibility of the coarse polyethylene. This sample is clearly explosible as evidenced by the measured values of MEC, MIE, MIT, and 1-m³ K_St (at both 550 and 600 ms ignition delay times).     

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.