Influence of venting coefficient on disastrous effects of aluminium powder explosions
Introduction
As an important industrial metal raw material, aluminium powder is widely used as aerospace propellant, in the chemical industry, as pyrotechnic powder, and in other fields (Shkolnikov et al., 2011). Aluminium powder is also a dangerous chemical with the risk of fire and explosion, which may pose a serious threat to process safety and environmental protection (Yu et al., 2020). Suspended aluminium powder in a confined space may cause a strong explosion in case of an open fire or electrostatic spark after when the concentration is within the flammability range. At the same time, during storage, the aluminium powder may also be exposed to moisture and react with water to release the combustible gas hydrogen (Dufaud et al., 2010). The reaction is slow at room temperature and intensifies with the increase of temperature. When a large amount of accumulated aluminium powder (Nifuku et al., 2007, Myers, 2008) meets water vapor, an exothermic oxidation reaction may occur, potentially exceeding the maximum ignition temperature, which may cause a series of explosions in the dedusting system and workshop, resulting in environmental damage, casualties and property losses. For example, in 2014, aluminium powder accumulated in a car wheel polishing workshop of China's Kunshan Zhong Rong Metal Products Co., Ltd., leading to an explosion. The accident resulted in a total of 146 deaths and 114 serious injuries (Li et al., 2016a). Given the extreme sensitivity of aluminium powder to ignition sources (Nifuku et al., 2007, Myers, 2008) and the severity of explosion consequences (Dufaud et al., 2010, Castellanos et al., 2014, Li et al., 2016b), it requires attention. This has also become an important topic of industrial process safety management (Sun et al., 2003).
As is known, the explosion characteristics, explosion mechanism, and influencing factors of aluminium powder are of great significance to the safe operation and hazard prevention of aluminium powder, which have been deeply studied by many scholars. Amyotte et al. (2009) elaborated on the suppression methods of dust explosions and analyzed the related explosion mechanisms by evaluating the inhibitory effect of inert substances on the dust explosion process. Li et al. (2016b) used a 20 L sphere to study the explosion characteristics of aluminium powder with different particle sizes. The results show that at the same concentration, the explosion with smaller particle size is mainly affected by oxygen diffusion, while the explosion with a larger particle size is mainly affected by the melting state of the particles. Zhang et al., 2018a, Zhang et al., 2018b studied the explosion characteristics of aluminium powder in a confined space through experiments and numerical simulation. The results show that at a lower concentration, turbulence is the main factor affecting the explosion uniformity of the aluminium powder/air mixture, while at a higher concentration, the suspension uniformity of aluminium powder is mainly affected by the explosion process. Jiang et al. (2019) systematically studied the explosion characteristics of aluminium powder while mixing inert materials. The results show that NaHCO3 and NH4H2PO4 can inhibit the flame propagation.
To reduce the severity of an accidental explosion of aluminium powder, dust explosion venting, as an efficient and inexpensive technical measure, is widely used in the protection process of various process equipment and devices (Rui et al., 2021). In addition, structures such as light-weight walls, doors, and windows of buildings will also induce the phenomenon of venting during an explosion (Tomlin et al., 2015). Explosion venting (Fig. 1) has important research significance whether as a hazard control measure or as a hazardous phenomenon. Bradley and Mitcheson (1978) and Solberg et al. (1981) summarized the explosion venting process with a large amount of experimental results and put forward relevant empirical formulas. May and Berard (1987) analysed the causes of several aluminium powder explosion accidents in the United States and discussed the possibility of using explosion venting technology to prevent and control aluminium powder explosions. May and Berard (1987) point out that when the dust treatment device is located indoors, it is usually necessary to use a conduit to guide the explosion hazard to the outdoor safe area. Lunn et al. (1988) used a spherical device with a volume of 20 L and an explosion vessel of 18.4 m3 to study the characteristic explosion parameters of aluminium powder and the explosion venting process of a pipeline; the effects of the explosion venting area, pipe length–diameter ratio, and turning angle on the explosion overpressure were investigated. The established internal overpressure prediction model could provide a reference for explosion venting design. Yan and Yu (2013) studied the evolution of the internal pressure during aluminium powder explosions using a 1.3 L Hartmann tube with a venting conduit. The results showed that the rapid combustion of dust in the venting pipe induced a multi-peak structure in the overpressure curve, and the pipe length, pipe diameter size, and dust concentration had a significant impact on the combustion state of the dust in the pipe. For dust collection devices such as outdoor dedusting systems, explosion venting plates can usually be installed to achieve rapid explosion venting. However, Taveau et al. (2018) analysed seven serious aluminium powder explosion accidents and found that the size of the explosion vent stipulated in the existing design standards could not ensure safe venting of explosions in the processing of aluminium products; in other words, the size of the designed vent was too conservative, resulting in severe damage to the equipment. In an investigation of an aluminium powder explosion accident in Kunshan, Jiangsu Province, Li et al. (2016a) found that although the vent on the dedusting pipe could reduce the internal pressure of the pipeline to a certain extent, a vent that was smaller than the standard size could induce strong turbulence and lead to detonation of an explosion in the pipeline. The corresponding explosion calculation model was established according to statistical data by Ugarte et al. (2016). Xu et al. (2020) evaluated the explosion venting behaviour of aluminium powder using a 20 L spherical vessel and computational fluid dynamics (CFD); the experimental results were compared with results obtained based on NFPA 68. Because of the scale effect in the process of metal dust explosions, although the explosion pressure relief design models proposed in NFPA, 68 (2007) and VDI 3673 (2002) can provide ideal results for most of the dusts, a large gap remains with regard to the prediction of metal dust explosions (Taveaua et al., 2019).
Because the explosion flame temperature of metal dusts such as aluminium powder and magnesium powder can be as high as 2700–2900 K (Cashdollar and Zlochower, 2007), metal dust explosions usually lead to higher overpressures, faster pressure rise rates, and higher explosion indexes than organic dust explosions. Therefore, even if the designed vent size meets the requirements for safe pressure relief, it is necessary to consider the outdoor disastrous effect caused by the venting process. In particular, the release of high-temperature Al2O3 to the outdoors through explosive venting of aluminium powder can cause secondary accidents (Going and Snoeys, 2002, Cashdollar and Zlochower, 2007). Taveau (2014) also noted that the hazard of outdoor high-temperature thermal radiation caused by explosions should be considered in the explosive venting of aluminium powder. When the combustible dust explodes in a space with venting measures, the flame may also spread along the commonly used pneumatic transport pipelines and dust removal pipes; however, the flame propagates faster at lower dust concentrations and may affect other equipment with no initial dust and cause a secondary explosion in the room (Taveau, 2017). Holbrow et al. (2000) studied the thermal radiation hazard of fireballs during aluminium powder venting using an 18.2 m3 vessel and determined the safe distance required to prevent thermal radiation damage due to a fireball in the most dangerous explosion state. Song et al. (2019) used CFD to study the indoor and outdoor flow field distribution characteristics of aluminium powder venting in the middle of a pipe with a large aspect ratio. The results showed that the explosion overpressure and temperature inside the pipe could be effectively reduced, and the damage range of the overpressure in the external flow field increased with the increase in the explosion outlet; however, this phenomenon was not observed for temperature.
Although achievements have been made in the existing literature on the venting process and effects of aluminium powder explosions, the experimental research is limited by site conditions and experimental schemes because of the danger and transience of an aluminium powder explosion. As a result, there is no clear understanding of the flow field distribution characteristics of aluminium powder explosion venting in a large-scale constrained space. At the same time, the safe distance design of all kinds of industrial buildings in China mainly refers to the standard GB 50016–2014 (2018), and the building fire safe distance given in this code is mainly based on the requirements of a steady-state fire. The thermal radiation range of shock waves caused by an explosion and the transient high-temperature jet flame is not taken into account, resulting in a lack of scientific basis for the safe distance design of industrial buildings handling aluminum powders. It may be that the safe distance does not meet the requirements for explosion protection.
In this study, with the help of CFD and taking the aluminium powder/air cloud as the research object, the flow field characteristics of suspended aluminium dust cloud explosion venting under different venting coefficients in large-scale buildings is explored, the evolution of the aluminium powder explosion is clarified, and the safe distance distribution of outdoor structures and personnel involved in aluminium powder buildings is given based on the explosion damage/injury criterion. The purpose of this study is to provide a scientific basis and theoretical reference for outdoor safe distance design, accident investigation and analysis, and (environmental) protection of aluminium powder industrial buildings.
Section snippets
Numerical method
Fluent, a computational fluid dynamics code, has been widely used to calculate the process of dust explosions (Salamonowicz et al., 2015, Murillo et al., 2013). The numerical simulations in this study were carried out with the Fluent CFD code based on the following equations.
The conservation equations, namely, mass, momentum energy and component conservations of continuous phase are:
Model
The model consisted of a cuboidal explosion vessel with dimensions of 6 m (length) × 3 m (width) × 3 m (height). The adiabatic non-slip boundary condition was uniformly assigned to the walls of the vessel. A square explosion vent was set on the wall with a width of 3 m in the explosion vessel, and the explosion vent surface opened automatically when the indoor pressure reached the set static static activation pressure of 0.01 MPa (Gauge). To consider the influence of the venting area on the
Typical dust explosion flow field
Fig. 9 shows the pressure–time curves for different measuring points in the explosion vessel for a venting coefficient Kv = 0.111. The entire venting process of aluminium powder can be divided into four stages: Stage I is the aluminium powder injection stage after vacuum (from 0 to 40 ms), Stage II is the aluminium powder settling stage (from 40 to 70 ms), Stage III extends from post-ignition stage to opening of the venting port (from 70 to 98 ms), and Stage IV is the venting port opening to
Conclusions
To determine the outdoor safe distance distribution resulting from an aluminium powder explosion under different venting coefficients, the explosion and venting process of aluminium powder in a large-scale confined space was investigated by computational fluid dynamics (CFD). Based on the verification of the reliability of the numerical method, the characteristics of the indoor and outdoor overpressure and temperature distribution of the aluminium powder explosion under the conditions of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this study.
Acknowledgments
The authors thank Elsevier for their English translation service. They also appreciate the financial support from the National Key R&D Program of China (No. 2017YFC0804700), the Beijing Science and Technology Nova Program (No. Z181100006218092), the Shanghai Science and Technology Talents Program Project (No. 19QB1402800).
Conflict of interest
None.
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