Consequence prediction for dust explosions involving interconnected vessels using computational fluid dynamics modeling

Combustible dust explosions continue to present a significant threat toward operating personnel and pneumatic conveyance equipment in a wide variety of processing industries. Following ignition of suspended fuel within a primary enclosure volume, propagation of flame and pressure fronts toward upstream or downstream interconnected enclosures can result in devastating secondary explosions if not impeded through an appropriate isolation mechanism. In such occurrences, an accelerated flame front may result in flame jet ignition within the secondary vessel, greatly increasing the overall explosion severity. Unlike an isolated deflagration event with quantifiable reduced pressures (vent sizing according to NFPA 68 guidance), oscillation of pressure between primary and secondary process vessels leads to uncertain overpressure effects. Dependent on details of the application such as relative enclosure volumes, relief area, fuel type, suspended concentration, duct size, and duct length, the maximum system pressure in both interconnected vessels can be unpredictable. This study proposes the use of FLame ACceleration Simulator (FLACS) computational fluid dynamics (CFD) modeling to provide reliable consequence predictions for specific case scenarios of dust deflagrations involving interconnected equipment. Required minimum supplement to the originally calculated relief area (A_v) was determined through iterative simulation, allowing for reduced explosion pressures (P_red) to be maintained below theoretical enclosure design strengths (P_es).     

Kinetic free mathematical model for the prediction of Kst values for organic dusts with arbitrary particle size distribution

Risk mitigation in production facilities has been an issue of great interest for decades, especially in activities which represent a serious hazard to human health, environment and industrial plants. Dust explosions are a major hazard in many industrial processes: only in the first part of 2019 (January–June) 34 dust explosions, mainly due to organic powders, occurred worldwide. An explosion may take place whenever there is the presence of combustible dusts, which are frequently generated by activities such as grinding, crushing, conveying and storage. Currently, a relatively expensive experimental test, carried out into a 20-L Siwek apparatus, is used to address the order of magnitude (class) of explosive dust: this piece of information is referred to as the deflagration index, K_st. At the current state, only a few pioneering models have been developed in order to predict the value of the K_st as a function of some relevant properties of the dust: e.g. particle size distribution (PSD), humidity, thermal conductivity, etc‥ Most of these models condense the information about the PSD of a given dust into an average value, referred to as D₅₀. In this work, a kinetic free mathematical model aimed at predicting the deflagration index for organic dusts is presented. This model, unlike the older ones, considers the whole particle size distribution for the computation of the deflagration index. In order to be implemented, only a single experimental K_st value (which works as a reference) and a particle size analysis on the dust are required. The model was validated using the whole granulometric distribution of three different organic powders (fosfomycin, sugar and niacin). In addition, the same estimations were done by considering only the D₅₀ data. It was noticed that, for highly polydispersed dusts, results were less accurate with respect to those obtained using the complete PSD, highlighting the importance of considering a complete granulometric distribution for process safety purposes.     

Numerical analysis of hydrogen deflagration mitigation by venting through a duct

This paper presents a model and simulation results for the mitigation of a hydrogen–air deflagration by venting through a duct. A large eddy simulation (LES) model, applied previously to study both closed-vessel, and open atmosphere hydrogen–air deflagrations, was developed further to model a hydrogen–air explosion vented through a duct. Sub-grid scale (SGS) flame wrinkling factors were introduced to model major phenomena which contribute to the increase of flame surface area in vented deflagrations. Simulations were conducted to validate the model against 20% hydrogen–air mixture deflagrations (vent diameters 25 and 45 cm) and 10% hydrogen–air mixture deflagration (vent diameter 25 cm). There was reasonable correlation between the simulations and the experimental data. The comparative importance of different physical phenomena contributing to the flame wrinkling is discussed.     

Simple method for predicting pressure behavior during gas explosions in confined spaces considering flame instabilities

It is indispensable to predict the pressure behavior caused by gas explosions for the safety management against accidental gas explosions. In this study, a simple method for predicting the pressure behavior during gas deflagrations in confined spaces was examined. Previously the pressure behavior was calculated analytically assuming laminar flame propagation. However, the results of this method often provide underestimation compared with experimental data. It was known the underestimation intensifies as the scale of explosion spaces becomes larger. On the large scale gas deflagration, flame instability (especially hydrodynamic instability) might be more effective and wrinkles appeared on the flame front. Then, the flame surface area was increased and the propagating flame was gradually accelerated. The ordinary prediction methods led to the underestimation because the propagating flame was assumed to be laminar. In this study, we considered the effect of flame wrinkles caused by flame instabilities. By regarding the flame front as a fractal structure, the flame surface area could be modified. Because a flame surface starts to be wrinkled on a certain flame radius, proper determination of the critical flame radius provided accurate prediction of pressure behavior on a large scale deflagration. In addition, correction of the K_G value in a large vessel was discussed.     

Effect of particle morphology on metal dust deflagration sensitivity and severity

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

Study on the preparation of novel NiB/Hβ-Al2O3 micro-mesoporous suppressant and its inhibition mechanism of coal dust explosion flame

Coal dust explosion is one of the serious accidents in the coal industry. It is of great significance to study the flame suppression of coal dust explosions. In this paper, a novel active component NiB with amorphous structure for explosion suppression was synthesized by the chemical reduction method. Furthermore, the novel explosion suppressant NiB/Hβ-Al₂O₃ was prepared through the kneading method by loading novel amorphous NiB nanoparticles on Hβ-Al₂O₃ with the micro-mesoporous structure as the carrier. The morphology and structure of NiB/Hβ-Al₂O₃ were characterized by XRD, BET, SEM, and FTIR, which showed that the NiB/Hβ-Al₂O₃ has proper pore structure and NiB nanoparticles are uniformly distributed as active components for explosion suppression in suppressant. Hartmann tube was used to evaluate the inhibition of coal dust deflagration. The results showed that the flame propagation distance and velocity decreased with the increase of the explosion suppressant. When the addition of explosion suppressant was 30 wt%, the explosion of coal dust was suppressed effectively. Furthermore, combing with the analysis results of the products after coal dust deflagration, the physical and chemical inhibition mechanism of the novel NiB/Hβ-Al₂O₃ explosion suppressant on coal dust deflagration was put forward.     

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

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

Study of the influence of melamine polyphosphate and aluminum hydroxide on the flame propagation and explosion overpressure of aluminum magnesium alloy dust

When aluminum magnesium alloy dust floats in the air, a certain ignition energy can easily cause an accidental explosion. To prevent and control the occurrence of accidental explosions and reduce the severity of accidents, it is necessary to carry out research on the explosion suppression of aluminum magnesium alloy dust. This paper uses a vertical glass tube experimental device and a 20 L spherical explosive experimental device to carry out experimental studies on the suppression of the flame propagation and explosion overpressure of aluminum magnesium alloy dust with melamine polyphosphate (MPP) and Al(OH)₃. With increasing MPP and Al(OH)₃ concentrations, the flame brightness darkened, the flame velocity and propagation distance gradually decreased, and P_max and (dp/dt)_max decreased significantly. When the amount of MPP added reached 60%, the flame propagation distance decreased to 188 mm, which is a decrease of 68%, and the explosion overpressure decreased to 0.014 MPa, effectively suppressing the explosion of aluminum magnesium alloy dust. The experimental results showed that MPP was more effective than Al(OH)₃ in inhibiting the flame propagation and explosion overpressure of the aluminum magnesium alloy dust. Finally, the inhibitory mechanisms of the MPP and Al(OH)₃ were further investigated. The MPP and Al(OH)₃ endothermic decomposition produced an inert gas, diluted the oxygen concentration and trapped active radicals to terminate the combustion chain reaction.     

A review on hybrid mixture explosions: Safety parameters,explosion regimes and criteria,flame characteristics

The hybrid mixture of combustible dusts and flammable gases/vapours widely exist in various industries, including mining, petrochemical, metallurgical, textile and pharmaceutical. It may pose a higher explosion risk than gas/vapor or dust/mist explosions since the hybrid explosions can still be initiated even though both the gas and the dust concentration are lower than their lower explosion limit (LEL) values. Understanding the explosion threat of hybrid mixtures not only contributes to the inherent safety and sustainability of industrial process design, but promotes the efficiency of loss prevention and mitigation. To date, however, there is no test standard with reliable explosion criteria available to determine the safety parameters of all types of hybrid mixture explosions, nor the flame propagation and quenching mechanism or theoretical explanation behind these parameters. This review presents a state-of-the-art overview of the comprehensive understanding of hybrid mixture explosions mainly in an experimental study level; thereby, the main limitations and challenges to be faced are explored. The discussed main contents include the experimental measurement for the safety parameters of hybrid mixtures (i.e., explosion sensitivity and severity parameters) via typical test apparatuses, explosion regime and criterion of hybrid mixtures, the detailed flame propagation/quenching characteristics behind the explosion severities/sensitivities of hybrid mixtures. This work aims to summarize the essential basics of experimental studies, and to provide the perspectives based on the current research gaps to understand the explosion hazards of hybrid mixtures in-depth.     

Experiment-based investigations of magnesium dust explosion characteristics

An experimental investigation was carried out on magnesium dust explosions. Tests of explosion severity, flammability limit and solid inerting were conducted thanks to the Siwek 20 L vessel and influences of dust concentration, particle size, ignition energy, initial pressure and added inertant were taken into account. That magnesium dust is more of an explosion hazard than coal dust is confirmed and quantified by contrastive investigation. The Chinese procedure GB/T 16425 is overly conservative for LEL determination while EN 14034-3 yields realistic LEL data. It is also suggested that 2000-5000 J is the most appropriate ignition energy to use in the LEL determination of magnesium dusts, using the 20 L vessel. It is essential to point out that the overdriving phenomenon usually occurs for carbonaceous and less volatile metal materials is not notable for magnesium dusts. Trends of faster burning velocity and more efficient and adiabatic flame propagation are associated with fuel-rich dust clouds, smaller particles and hyperbaric conditions. Moreover, Inerting effectiveness of CaCO₃ appears to be higher than KCl values on thermodynamics, whereas KCl represents higher effectiveness upon kinetics. Finer inertant shows better inerting effectiveness.     

Detonations and vapor cloud explosions: Why it matters

The methods used to evaluate the consequences of a vapor cloud explosion assume deflagrations within congested process pipework regions and consequently a significant effort has been invested in developing models to estimate the severity of these deflagrations. Models range from the simpler screening approaches to detailed Computational Fluid Dynamics. There is clear evidence from large scale experiments and incidents that transition from deflagration to detonation is credible and has occurred and it is the contention of this paper that deflagration is only the first stage in many major vapor cloud explosions and that detonation is readily foreseeable. Why does this matter? The methods currently used in the design and location of buildings on and around process sites are based on an incomplete picture of vapor cloud explosions. Whilst this might not have a significant effect in some cases, it is shown that there is the potential to significantly underestimate the explosion hazard. This will result in occupied buildings either being placed in the wrong location or under-designed for the explosion threat, increasing the risks to personnel on these sites.     

Venting of deflagrations: hydrocarbon–air and hydrogen–air systems

A set of 34 experiments on vented hydrocarbon–air and hydrogen–air deflagrations in unobstructed enclosures of volume up to 4000 m³ was processed with use of the advanced lumped parameter approach. Reasonable compliance between calculated pressure–time curves and experimental pressure traces is demonstrated for different explosion conditions, including high, moderate, low and extremely low reduced overpressures in enclosures of different shape (L_max:L_min up to 6:1) with different type and position of the ignition source relative to the vent, for near-stoichiometric air mixtures of acetone, methane, natural gas and propane, as well as for lean and stoichiometric hydrogen–air mixtures. New data were obtained on flame stretch for vented deflagrations.The fundamental Le Chatelier–Brown principle analog for vented deflagrations has been considered in detail and its universality has been confirmed. The importance of this principle for explosion safety engineering has been emphasized and proved by examples.A correlation for prediction of the deflagration–outflow interaction number, χ/μ, on enclosure scale, Bradley number and vent release pressure is suggested for unobstructed enclosures and a wide range of explosion conditions. Fractal theory has been employed to verify the universality of the dependence revealed of the deflagration–outflow interaction number on enclosure scale.In spite of differences between the thermodynamic and kinetic parameters of hydrocarbon–air and hydrogen–air systems, they both obey the same general regularities for vented deflagrations, including the Le Chatelier–Brown principle analog and the correlation for deflagration–outflow interaction number.     

Experimental investigation of the complex deflagration phenomena of hybrid mixtures of activated carbon dust/hydrogen/air

This paper aims to develop quantitative insights based on measured deflagration parameters of hybrid mixtures of activated carbon (AC) dust and hydrogen (H₂) gas in air. The generated experimental evidence is used to reject the claim of the null hypothesis (H₀) that severity of deflagrations of H₂/air mixtures always bound the severity of deflagrations of heterogenous combustible mixtures of AC dust/H₂/air containing the same H₂ concentrations as in the H₂/air binaries. The core insights of this investigation show that the maximum deflagration pressure rise (ΔP_MAX) and maximum rate of pressure rise ((dP/dt)_MAX) of this hybrid mixture are greater than those corresponding to deflagrations of H₂/air mixtures for all the dust and H₂ concentrations being examined. The deflagration severity indices (K_St and ES) of the hybrid mixture containing 29 mol% H₂ are found to be greater than those of the H₂/air mixture containing 29 mol% H₂. Also, the minimum explosible concentration (MEC) of the hybrid mixture is lower than that of the AC dust in air only. The insights gained should lead to better realization of the severity of a postulated safety-significant accident scenario associated with on-board cryoadsorption H₂ storage systems for fuel-cell (FC) powered light-duty vehicles. The identified insights could also be relevant to other industrial processes where combustible dusts are generated in the vicinity of solvent vapors. Moreover, these insights should be useful for supporting quantitative risk assessment (QRA) of on-board H₂ storage systems, designing improved safety measures for cryoadsorption H₂ storage tanks, and guiding H₂ safety standards and transportation regulations.     

Post-explosion observations of experimental mine and laboratory coal dust explosions

The Pittsburgh Research Laboratory (PRL) of the National Institute for Occupational Safety and Health (NIOSH) and the Mine Safety and Health Administration (MSHA) conducted joint research on dust explosions by studying post-explosion dust samples. The samples were collected after full-scale explosions at the PRL Lake Lynn Experimental Mine (LLEM), and after laboratory explosions in the PRL 20-L chamber and the Fike 1 m³ chamber. The dusts studied included both high- and low-volatile bituminous coals. Low temperature ashing for 24 h at 515 °C was used to measure the incombustible content of the dust before and after the explosions. The data showed that the post-explosion incombustible content was always as high as, or higher than the initial incombustible content. The MSHA alcohol coking test was used to determine the amount of coked dust in the post-explosion samples. The results showed that almost all coal dust that was suspended within the explosion flame produced significant amounts of coke. Measurements of floor dust concentrations after LLEM explosions were compared with the initial dust loadings to determine the transport distance of dust during an explosion. All these data will be useful in future forensic investigations of accidental dust explosions in coal mines, or elsewhere.     

Some aspects in testing and assessment of metal dust explosions

The dust explosion committee of the Association of Powder Process Industry and Engineering, Japan recently established two testing standards for dust explosions. In the investigations for the standardization, many experimental data have been obtained for the dusts currently used in Japanese industries. Data for zirconium, tantalum and silicone dusts are presented to discuss the use of test methods, which have been accepted internationally. The test methods for dust explosions have to consider a variety of kinds and forms of dusts to be tested.     

Inerting characteristics of ultrafine Mg(OH)2 on starch dust explosion flame propagation

Experiments on the flame propagation of starch dust explosion with the participation of ultrafine Mg(OH)₂ in a vertical duct were conducted to reveal the inerting evolution of explosion processes. Combining the dynamic behaviors of flame propagation, the formation law of gaseous combustion products, and the heat dissipation features of solid inert particles, the inerting mechanism of explosion flame propagation is discussed. Results indicate that the ultrafine of Mg(OH)₂ powders can cause the agglomeration of suspended dust clouds, which makes the flame combustion reaction zone fragmented and forms multiple small flame regions. The flame reaction zone presents non-homogeneous insufficient combustion, which leads to the obstruction of the explosion flame propagation process and the obvious pulsation propagation phenomenon. As the proportion of ultrafine Mg(OH)₂ increases, flame speed, flame luminescence intensity, flame temperature and deflagration pressure all show different degrees of inerting behavior. The addition of ultrafine Mg(OH)₂ not only causes partial inerting on the explosion flame, but also the heat dissipation of solid inert particles affects the acceleration of its propagation. The explosion flame propagation is inhibited by the synergistic effect of inert gas-solid phase, which attenuates the risk of starch explosion. The gas-solid synergistic inerting mechanism of starch explosion flame propagation by ultrafine Mg(OH)₂ is further revealed.     

Experimental determination of dust cloud deflagration parameters of selected hydrogen storage materials: Complex metal hydrides,chemical hydrides,and adsorbents

This paper discusses the results of an experimental program carried out to determine dust cloud deflagration parameters of selected solid-state hydrogen storage materials, including complex metal hydrides (sodium alanate and lithium borohydride/magnesium hydride mixture), chemical hydrides (alane and ammonia borane) and activated carbon (Maxsorb, AX-21). The measured parameters include maximum deflagration pressure rise, maximum rate of pressure rise, minimum ignition temperature, minimum ignition energy and minimum explosible concentration. The calculated explosion indexes include volume-normalized maximum rate of pressure rise (K_St), explosion severity (ES) and ignition sensitivity (IS). The deflagration parameters of Pittsburgh seam coal dust and Lycopodium spores (reference materials) are also measured. The results show that activated carbon is the safest hydrogen storage media among the examined materials. Ammonia borane is unsafe to use because of the high explosibility of its dust. The core insights of this contribution are useful for quantifying the risks associated with use of these materials for on-board systems in light-duty fuel cell-powered vehicles and for supporting the development of hydrogen safety codes and standards. These insights are also critical for designing adequate safety features such as explosion relief venting and isolation devices and for supplementing missing data in materials safety data sheets.     

Observations of microscopic characteristics of post-explosion coal dust samples

To reveal the microscopic characteristics of the post-explosion coal dust samples, coal dust explosion tests were performed in a 20 L spherical vessel. The explosion characteristic parameters, such as the maximum pressure (P_max), the maximum rate of pressure rise ((dP/dt)_max), ignition time (t) and the deflagration index (K_St) were recorded. Meanwhile, the post-explosion dust samples were collected and analyzed. The research efforts include particle size distribution analysis, SEM analysis and FTIR analysis of dust samples before and after the explosion. The particle size range of post-explosion dust samples became wider according to the mass percent analysis. The microscopic appearance of samples in same particle size range showed some similarity. The porous structure of dust samples was observed by improving the SEM magnification. The chemical structure of dust samples before and after explosion was analyzed by FTIR.     

Thermo-kinetic modelling of dust explosions

The guidelines for protection and mitigation against hazard coming from dust explosion require the knowledge and then the evaluation either experimentally or theoretically of the thermo-kinetic parameters (i.e. K_St, P_max). We developed a numerical tool for the evaluation of the thermo-kinetic parameters of dust explosion. This model is based on the simulations of the combustion reaction by means of a detailed reaction mechanism assuming that the pyrolysis/devolatilization step is very fast and then gas combustion is controlling dust explosion. The model allows then the determination of the most conservative values of K_St, S_l, P_max. In the present paper we calculated the deflagration index and the laminar burning velocity for dusts utilized in various process industries (i.e. cornstarch, polyethylene, cellulose) as function of dust concentration. The obtained data were successfully compared with the available experimental results.     

Large-scale dust explosions in vessel-pipe systems

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