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Identify and Assess Process Hazards
Published in James A. Klein, Bruce K. Vaughen, Process Safety, 2017
James A. Klein, Bruce K. Vaughen
Key combustible dust parameters include [44,47–50]: Particle size – Smaller particle powders and dusts have greater total surface area that can support rapid combustion. NFPA has defined dust as “any finely divided solid, 420 µm or 0.017 in, or less in diameter [48–50].” Particle sizes below this level should be assumed to be potentially combustible, and in some cases, larger particle sizes may also be combustible. Coarser powders may also contain a fraction of very fine dusts that may present a hazard. Testing should be done to determine explosivity conclusively if there is an uncertainty. For comparison, the period in this sentence is approximately 600 µm, and the particle size of table salt is approximately 100–150 µm. Particle size is an important factor that can significantly impact the values of other parameters.Minimum explosible concentration (MEC) – When dust particles are dispersed in air, the dust cloud concentration (g/m3) must be above the MEC to support sustained combustion, similar to the flammability range discussed in the previous section. Typical MEC values range from 10 to 100 g/m3, which in most cases, is a high enough concentration to significantly impair visibility although all dense clouds should be assumed to be hazardous. Industrial hygiene professionals generally focus on dust exposure health hazards at concentrations well below the MEC.Dust deflagration index (Kst) – Kst is a measure of the relative explosion severity of dusts, with higher values indicating greater severity, based on the maximum burning rate (bar-m/s) of a dust cloud of ideal concentration under turbulent conditions. Kst hazard levels include ST 1 (weak explosion), ST 2 (strong explosion), and ST 3 (very strong explosion) [46].Limiting oxygen concentration (LOC)* – LOC is the minimum oxygen level (vol %) required for combustion of a dust cloud at any concentration, similar to the minimum oxygen concentration (MOC), which was discussed in the previous section. The typical LOC range of 8%–15% can be used to determine inerting requirements when combustible dusts are being handled.Minimum ignition energy (MIE) – MIE is the minimum spark energy (mJ) required to ignite a dust cloud and support combustion, which is typically in the range of 10–100 mJ. These levels are much higher than the spark energy required to ignite a flammable vapor cloud (<1 mJ). As particle size decreases for a powder, the MIE also decreases.
Assessing ignitions of explosive gas mixtures by low-energetic electrical discharges using OH-LIF and 1D-simulations
Published in Combustion Science and Technology, 2023
Johann-Robert Kummer, Stefan Essmann, Detlev Markus, Holger Grosshans, Ulrich Maas
An important quantity related to the ignition of a combustible mixture is its minimum ignition energy (MIE). Ignition at low energy levels is a strongly stochastic process (Bane 2010), much in contrast to desired ignition processes which are engineered to exhibit a very high repeatability, e.g. in combustion engines. Thus, the MIE is usually defined on a statistical basis, namely as the amount of electrostatic energy which, when released 100 times into identical burnable gas mixtures under specified conditions (ASTM International 2013), causes in average one ignition. However, there is a number of additional parameters which are of utmost importance for the ignition process, such as fluctuations of the initial temperature and pressure, the gas mixture composition, flow effects, the electrode geometry, material, and surface roughness. These contributors are partly challenging to control in an experimental setting and difficult to describe theoretically. Hence, the MIE is not purely a characteristic of a burnable substance, but also depends on the method employed to obtain it. The aforementioned perceived randomness of ignitions was analyzed using the logistic regression approach by Bane et al. (2011) and Coronel et al. (2013) in the context of aviation safety. Other authors then applied this method to fuels relevant for process safety and explosion protection (Eckhoff, Ngo, Olsen 2010; Wähner et al. 2013). However, since the focus of these works was to determine ignition probabilities for certain energy levels, the sub-processes of the ignition process were not analyzed in detail.
Effects of Turbulence on Ignition of Methane–Air and n-Heptane–Air Fully Premixed Mixtures
Published in Combustion Science and Technology, 2018
Naoyuki Saito, Yuki Minamoto, Basmil Yenerdag, Masayasu Shimura, Mamoru Tanahashi
Forced and self-ignition problems have been investigated experimentally and numerically in many previous studies. Kravchik and Sher (1994) investigated the initiation of a flame kernel and the propagation of a self-sustained flame using numerical simulations. The flame kernel growth was described as a two-stage process. In the early short stage, the mass and energy transfer are dominated by the pressure wave and expanding plasma kernel. In the next longer period, they are controlled by diffusion and thermal conduction when the flame becomes self-sustained. Due to the heat release by the chemical reactions, the expansion of the mixture shifts early to the diffusive stage. The non-equilibrium plasma-assisted approach is known as a useful method for igniting combustible mixtures and stabilizing the combustion process. Ignition and combustion using non-equilibrium plasma assist have been reported for various fuels (Leonov et al., 2006, 2007; Starikovskaia, 2006; Starikovskii, 2005). Han and Yamashita (2014) also performed zero- and one-dimensional simulations and investigated the effects of non-equilibrium plasma on the ignition delay of a methane–air mixture. They observed that greater reduced electric fields shortened the ignition delay time and the influence of the reduced electric field on the ignition delay time is weak at high initial temperature and strong at low initial temperature. The minimum ignition energy (MIE) is the ignition energy for which the combustible mixture succeeds in ignition with a probability of 50%. The MIE is necessary for understanding fundamental ignition physics and the design of ignition systems (Lewis and Von Elbe, 1987; Loeb, 1939). Many studies have been performed and considerable literature exist on the MIE . Yuasa et al. (2002) investigated numerically the influence of the energy deposition schedule on the MIE. The effect of the energy deposition schedule was examined using simulations with varying capacitance component energy and inductance spark energy. Han et al. (2010) performed numerical simulations of the spark ignition of methane–air mixtures to investigate the effects on the MIE of the electrode size, electrode gap distance, duration time, and equivalence ratio in laminar conditions. Chen et al. (2011) studied spherical flame initiation using theoretical and numerical methods and determined the critical flame radius for spherical flame initiation and the MIE increase with the Lewis number.