Flame behaviors and pressure characteristics of vented dust explosions at elevated static activation overpressures

Dust explosion venting experiments were performed using a 20-L spherical chamber at elevated static activation overpressures larger than 1 bar. Lycopodium dust samples with mean diameter of 70 μm and electric igniters with 0.5 KJ ignition energy were used in the experiments. Explosion overpressures in the chamber and flame appearances near the vent were recorded simultaneously. The results indicated that the flame appeared as the under-expanded free jet with shock diamonds, when the overpressure in the chamber was larger than the critical pressure during the venting process. The flame appeared as the normal constant-pressure combustion when the pressure venting process finished. Three types of venting processes were concluded in the experiments: no secondary flame and no secondary explosion, secondary flame, secondary explosion. The occurrence of the secondary explosions near the vent was related to the vent diameter and the static activation overpressure. Larger diameters and lower static activation overpressures were beneficial to the occurrence of the secondary explosions. In current experiments, the secondary explosions only occurred at the following combinations of the vent diameter and the static activation overpressure: 40 mm and 1.2 bar, 60 mm and 1.2 bar, 60 mm and 1.8 bar.     

Duct-venting of dust explosions in a 20 L sphere at elevated static activation overpressures

Ducts are often recommended in the design of dust explosion venting in order to discharge materials to safe locations. However, the maximum reduced overpressure increases in a duct-vented vessel rather than in a simply vented vessel. This needs to be studied further for understanding the duct-venting mechanism. Numerous duct-vented dust explosion experiments were conducted, using a 20 L spherical chamber at elevated static activation overpressures, ranging from 1.8 bar to 6 bar. Duct diameters of 15 mm and 28 mm, and duct lengths of 0 m (simply venting), 1 m and 2 m, were selected. Explosion pressures both in the vessel and in the duct were recorded by pressure sensors, with a frequency of 5 kHz. Flame signals in the duct were also obtained by phototransistors. Results indicate that the secondary explosion occurring in the duct increases the maximum reduced overpressure in the vessel. The secondary explosion is greatly affected by the duct diameter and static activation overpressure, and hence influences the amplification of the maximum reduced overpressure. Larger static activation overpressure decreases the severity of the secondary explosion, and hence decreases the increment in the maximum reduced overpressure. The secondary pressure peak is more obvious as the pressure accumulation is easier in a duct with a smaller diameter. However, the increment of the maximum reduced overpressure is smaller because blockage effect, flame front distortion, and turbulent mixing due to secondary explosion are weaker in a narrow duct. The influence of duct length on the maximum reduced overpressure is small at elevated static activation overpressures, ranging from 1.8 bar to 6 bar at 15 mm and 28 mm duct diameters.     

Insight into vented explosion mechanism and premixed flame dynamics in linked vessels: Influence of membrane thickness and blocking rate

To effectively prevent and mitigate explosion hazards and casualties, relief venting of flammable gas explosions has been applied in production processes in a broad variety of industries. This work conducted fully vented experiments to investigate the influence of venting membrane thickness, and partially vented experiments to investigate the influence of baffle blocking rate on the explosion characteristics of 9.5 vol% methane-air mixtures in linked vessels with a 0.5 m long vented duct. Results indicate that the membrane thickness and blocking rate for the two types of vented explosions significantly affected the explosion overpressure. The smaller the membrane thickness and blocking rate, the lower the explosion overpressure. Secondary explosions were observed in the vented duct through experiments and a weaker explosion flame appeared at a small blocking rate of 20%. With the further increase in the blocking rate, the flame became extremely weak, and no secondary explosions occurred. The overpressure evolution process at different positions in the explosion duct and secondary explosion phenomenon in the vented duct were investigated. This work could probably serve as an important reference for the selection of technical parameters of explosion venting in the practical industrial processes.     

Flame propagation behaviors of nano- and micro-scale PMMA dust explosions

Flame propagation behaviors of nano- and micro-polymethyl methacrylate (PMMA) dust explosions were experimentally studied in the open-space dust explosion apparatus. High-speed photography with normal and microscopic lenses were used to record the particle combustion behaviors and flame microstructures. Simple physical models were developed to explore the flame propagation mechanisms. High-speed photographs showed two distinct flame propagation behaviors of nano- and micro-PMMA dust explosions. For nano-particles, flame was characterized by a regular spherical shape and spatially continuous combustion structure combined with a number of luminous spot flames. The flame propagation mechanism was similar to that of a premixed gas flame coupled with solid surface combustion of the agglomerates. In comparison, for micro-particles, flame was characterized by clusters of flames and the irregular flame front, which was inferred to be composed of the diffusion flame accompanying the local premixed flame. It was indicated that smaller particles maintained the leading part of the propagating flame and governed the combustion process of PMMA dust clouds. Increasing the mass densities from 105 g/m³ to 217 g/m³ for 100 nm PMMA particles, and from 72 g/m³ to 170 g/m³ for 30 μm PMMA particles, the flame luminous intensity, scale and the average propagation velocity were enhanced. Besides, the flame front became more irregular for 30 μm PMMA dust clouds.     

Overpressure characteristics of aluminium dust explosion vented through a relief pipe

A parametric experimental study of an aluminium dust explosion, initiated in a vessel and vented through a relief pipe, was performed. The aim is to clarify the overpressure characteristics in a vessel and relief pipe, during aluminium dust explosion venting, especially when a burn-up phenomenon occurs. For a vessel of fixed size, the influence of pipe diameter and pipe length on burn-up was discussed. Results demonstrate that burn-up occurs shortly after flame only enters the initial part of the relief pipe when the original dust concentration in the vessel is at relatively high level, which is usually higher than the optimum concentration obtained from the confined vessel. When burn-up occurs, the maximal overpressure continues to increase rather than to decay along the initial part of the relief pipe. If burn-up is vigorous, a second peak on overpressure-time curve in the vessel could appear. By adding 0.1 g aluminium powders on the membrane, the second overpressure peak may even surpass the first peak. Extending pipe lengths can strengthen the overpressures around the position where burn-up occurs in the relief pipe. Reducing the pipe diameter can increase the burn-up severity in the relief pipe owing to the increased dust concentration and the pressure accumulation.     

Experimental study on explosion characteristics of ethanol-gasoline blended fuels

In the present work, a series of experiments have been performed to analyze the explosion characteristics of ethanol-gasoline with various blended ratios (0%, 5%, 10%, 15%, 30%, 50%, 70%, 80%, and 100%). A vented rectangular vessel with a cross-section of 100 mm × 100 mm, 600 mm long and a 40 mm diameter vent on the top is used to carry out the experiments. The flame propagation is recorded by a phantom high-speed camera with 5000 fps, while the histories of the explosion overpressure are measured by two PCB pressure sensors and the explosion sound pressure level is obtained by a CRY sound sensor. The results indicate that the maximum overpressure and flame propagation speed increases linearly as the blended ratio increases when the initial volume of blended fuel is 1.0 mL; While the change of explosion overpressure and flame propagation speed shows a trend of decreasing at first and then increasing as the concentration increases to 1.8 mL. It is also found that the peak of the sound pressure level exceeds 100 dB under all tests, which would damage the human's hearing. What's more, relationships between explosion overpressure and sound pressure level are examined, and the change of the maximum overpressure can be reflected to some extent by the measurement of the maximum sound pressure level. The study is significant to reveal the essential characteristic of the explosion venting process of ethanol-gasoline under different initial blended ratios, and the results would help deepen the understanding of ethanol-gasoline blended fuels explosion and the assessment of the explosion hazardous.     

A simple design method of dust explosion venting size at elevated static activation pressure

To develop the application of explosion venting technology in high-pressure vessels, a new model for the design of dust explosion venting size was presented, which took the physicochemical phenomenon deriving from the elevation of the static activation pressure into account. Firstly, for confined pressure rise, the wall quenching effect originating from the dust flame thickness was considered by adopting the three-zone model. Secondly, for the venting pressure rise, the energy loss due to the discharge of high-energy burnt mixture (quantified as the specific surface area loss of the flame) was taken into account and the induced turbulence factor was introduced. Thirdly, for the venting pressure drop, a dynamic pressure relief capability evaluation model which takes into account the flame morphology evolution (tear-shaped flame) and the proportion of discharged mixture (relative volume ratio) at elevated activation pressure was proposed. The predicted maximum reduced pressure and venting size were checked against the PMMA explosion experiments and a more great performance was obtained compared with standards.     

Effect of flame propagation regime on pressure evolution of nano and micron PMMA dust explosions

Experiments using an open space dust explosion apparatus and a standard 20 L explosion apparatus on nano and micron polymethyl methacrylate dust explosions were conducted to reveal the differences in flame and pressure evolutions. Then the effect of combustion and flame propagation regimes on the explosion overpressure characteristics was discussed. The results showed that the flame propagation behavior, flame temperature distribution and ion current distribution all demonstrated the different flame structures for nano and micron dust explosions. The combustion and flame propagation of 100 nm and 30 μm PMMA dust clouds were mainly controlled by the heat transfer efficiency between the particles and external heat sources. Compared with the cluster diffusion dominant combustion of 30 μm dust flame, the premixed-gas dominant combustion of 100 nm dust flame determined a quicker pyrolysis and combustion reaction rate, a faster flame propagation velocity, a stronger combustion reaction intensity, a quicker heat release rate and a higher amount of released reaction heat, which resulted in an earlier pressure rise, a larger maximum overpressure and a higher explosion hazard class. The complex combustion and propagation regime of agglomerated particles strongly influenced the nano flame propagation and explosion pressure evolution characteristics, and limited the maximum overpressure.     

Experimental study on vented gas explosion in a cylindrical vessel with a vent duct

A study of vented explosions in a length over diameter (L/D) of 2 in cylindrical vessel connecting with a vent duct (L/D = 7) is reported. The influence of vent burst pressure and ignition locations on the maximum overpressure and flame speeds at constant vent coefficient, K of 16.4 were investigated to elucidate how these parameters affect the severity of a vented explosion. Propane and methane/air mixtures were studied with equivalence ratio, Φ ranges from 0.8 to 1.6. It is demonstrated that end ignition exhibited higher maximum overpressures and flame speeds in comparison to central ignition, contrary to what is reported in literature. There was a large acceleration of the flame toward the duct due to the development of cellular flames and end ignition demonstrated to have higher flame speeds prior to entry into the vent due to the larger flame distance. The higher vent flow velocities and subsequent flame speeds were responsible for the higher overpressures obtained. Rich mixtures for propane/air mixtures at Φ = 1.35 had the greatest flame acceleration and the highest overpressures. In addition, the results showed that Bartknecht's gas explosion venting correlation is grossly overestimated the overpressure for K = 16.4 and thus, misleading the impact of the vent burst pressure.     

Flame propagation in lycopodium/air mixtures below atmospheric pressure

Industrial processes are often operated at conditions deviating from atmospheric conditions. Safety relevant parameters normally used for hazard evaluation and classification of combustible dusts are only valid within a very narrow range of pressure, temperature and gas composition. The development of dust explosions and flame propagation under reduced pressure conditions is poorly investigated. Standard laboratory equipment like the 20 l Siwek chamber does not allow investigations at very low pressures. Therefore an experimental device was developed for the investigations on flame propagation and ignition under reduced pressure conditions. Flame propagation was analysed by a video analysis system the actual flame speed was measured by optical sensors. Experiments were carried out with lycopodium at dust concentrations of 100 g/m³, 200 g/m³ and 300 g/m³. It was found that both flame shapes and flame speeds were quite different from those obtained at atmospheric pressure. Effects like buoyancy of hot gases during ignition and flame propagation are less strong than at atmospheric conditions. For the investigated dust concentrations the flame reaches speeds that are nearly an order of a magnitude higher than at ambient conditions.     

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.     

Inerting effect of N2 on explosion of LDPE dust/ethylene hybrid mixtures

Explosion flame propagation characteristics and overpressure distribution of low density polyethylene (LDPE) dust and ethylene hybrid mixture were investigated under N₂ inerting conditions using a custom-designed 12 L cylindrical explosion tank. The results showed that a small amount of ethylene could promote the explosion characteristics of LDPE dust. N₂ inerting had different inhibitory effects on the explosion flame of LDPE dust and its mixture with ethylene. The explosion overpressure strength of the LDPE dust/ethylene hybrid mixture decreased with increasing N₂ concentration. The overall suppression effect of N₂ on the explosion overpressure of the LDPE dust was better than that of the LDPE dust/ethylene hybrid mixture explosion. As the ethylene concentration increased from 0% to 2.5%, the limiting oxygen concentration decreased by 13% oxygen. This small amount of ethylene restricted the traditional inerting process. The study conclusions can provide further scientific basis for the inerting and explosion proofing design of production process equipment involving LDPE dust.     

Wire-mesh inhibition of jet fire induced by explosion venting

Explosion venting is widely applied in industrial explosion-proof designs due to the convenient, economical and practical features of this method. Natural gas is usually stored in storage tanks. If the gas in the vessel is mixed with air and encounters an ignition source, explosion venting might occur, producing jet fire, generating new secondary derivative accidents and causing casualties and property losses. In this paper, a set of test platforms including wire-mesh suppression devices is established to study the inhibition of jet fire induced by explosion venting by wire mesh. The experimental research shows that a wire mesh significantly inhibits the jet fire induced by explosion venting. The flame propagation velocity and pressure clearly decrease with increasing numbers of wire-mesh layers. The wire-mesh structure significantly affects the flame propagation, and the more layers of mesh there are, the better the suppression effect is. The flame temperature gradually decreases with the addition of the wire mesh. The mesh size significantly affects the pressure propagation of explosion venting. The explosion pressure gradually decreases with the addition of the wire mesh. With increasing distance between the wire mesh and the explosion vent, the maximum temperature first increases and then decreases, and the maximum explosion pressure first decreases and then increases. In the case of single gas cloud, the flame suppression effect is the most obvious when the wire mesh is 0.2 m away from the explosion vent. In the case of double gas clouds, the flame suppression effect is the most significant when the distance between the wire mesh and the first gas cloud is 0.4 m.     

Application and effect of negative pressure chambers on pipeline explosion venting

Explosion venting is a frequently-used way to lower explosion pressure and accident loss. Recently, studies of vessel explosion venting have received much attention, while little attention has been paid to pipe explosion venting. This study researched the characteristics of explosion venting for Coal Bed Methane (CBM) transfer pipe, and proposed the way of explosion venting to chamber in order to avoid the influence of explosion venting on external environment, and investigated the effects of explosion venting to atmosphere and chamber. When explosion venting to atmosphere, the average explosion impulse 4.89 kPa s; when explosion venting to 0 MPa (atmospheric pressure) chamber, average explosion impulse is 7.52 kPa s; when explosion venting to −0.01 MPa chamber, explosion flame and pressure obviously drop, and average explosion impulse decreases to 4.08 kPa s; when explosion venting to −0.09 MPa chamber, explosion flame goes out and average explosion impulse is 1.45 kPa s. Thus, the effect of explosion venting to negative chamber is far better than that to atmospheric chamber. Negative chamber can absorb more explosion gas and energy, increase stretch of explosion flame, and eliminate free radical of gas explosion. All these can promote the effect of explosion venting to negative chamber.     

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.     

PAN dust explosion inhibition mechanisms of NaHCO3 and Al(OH)3

We investigate the PAN dust explosion inhibition behaviors of NaHCO₃ and Al(OH)₃ in a 20 L spherical explosion system and a transparent pipe explosion propagation test system. The results show that, in the standard 20 L spherical explosion system, the highest PAN dust explosion concentration is 500 g/m³, the maximum explosion pressure is 0.661 MPa, and the maximum explosion pressure increase rate is 31.64 MPa/s; adding 50% NaHCO₃ and 60% Al(OH)₃ can totally inhibit PAN dust explosion. In the DN0.15 m transparent pipe explosion propagation test system, for 500 g/m³ PAN dust, the initial explosion flame velocity is 102 m/s, the initial pressure is 0.46 MPa, and the initial temperature is 967 °C; adding 60% NaHCO₃ and 70% Al(OH)₃ can totally inhibit PAN dust explosion flames. Through FTIR and TG analyses, we obtain the explosion products and pyrolysis patterns of the explosion products of PAN dust, NaHCO₃, and Al(OH)₃. On this basis, we also summarize the PAN dust explosion inhibition mechanisms of NaHCO₃ and Al(OH)₃.     

CFD simulations of dust dispersion in the 1 m3 explosion vessel

According to standard procedures, flammability and explosion parameters for dusts and dust mixtures are evaluated in 20 L and/or 1 m³ vessels, with equivalent results provided a correct ignition delay time (60 ms in the 20 L vessel; 600 ms in the 1 m³ vessel). In this work, CFD simulations of flow field and dust concentration distribution in the 1 m³ spherical vessel are performed, and the results compared to the data previously obtained for the 20 L. It has been found that in the 1 m³ vessel, the spatial distribution of the turbulent kinetic energy is lower and much more uniform. Concerning the dust distribution, as in the case of the 20 L, dust is mainly concentrated at the outer zones of the vortices generated inside the vessel. Furthermore, an incomplete feeding is attained, with most of the dust trapped in the perforated annular nozzle. Starting from the maps of dust concentration and turbulent kinetic energy, the deflagration index K_St is calculated in both vessels. In the conditions of the present work, the K_St is found to be 2.4 times higher in the 20 L than in the 1 m³ vessel.     

Influence of dustiness on small-scale vented dust explosions

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

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.     

连通容器气体密闭爆炸与泄爆过程的实验研究

利用球型容器与管道组合,开展连通容器气体爆炸与泄爆实验,分析连通条件下,火焰在管道中的传播过程及其对起爆容器和传爆容器的压力影响。实验结果表明:连通容器气体爆炸中,火焰从起爆容器到传爆容器传播经历了一段不断加速,但加速度不断减小的过程;泄爆过程中,火焰传播过程与密闭爆炸时基本一致。管道中火焰加速传播,使得传爆容器的爆炸压力和强度相较于作为起爆容器时均明显增加,危险更大,采用与起爆容器相同的泄爆面积,无法满足对连通容器中传爆容器的泄爆。同时,泄爆是一个快速的能量泄放过程应选择合理的泄爆方式,防止二次危害。