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Dust Explosion
Published in Ko Higashitani, Hisao Makino, Shuji Matsusaka, Powder Technology Handbook, 2019
The limiting oxygen concentration (LOC) is the experimentally determined oxygen concentration which will just not allow an explosion of a dust/air/inert gas mixture. An atmosphere having an oxygen concentration below the limiting oxygen concentration is not capable of supporting combustion and thus cannot support a dust explosion. The limiting oxygen concentration test is used to study explosion prevention or severity reduction involving the use of inert gases and to set oxygen concentration alarms or interlocks in inerted plant and vessels. Limiting oxygen concentration testing can also be measured using the 20 liter-sphere apparatus. The test follows ASTM E2931-13 standard (Standard test method for limiting oxygen (oxidant) concentration of combustible dust clouds).13 Powder or dust samples of various sizes are dispersed in the vessel and attempts are made to ignite the resulting dust cloud with an energetic ignition source. Trials are repeated for decreasing oxygen concentrations until the LOC is determined. Table 7.4.5 summarizes the limiting oxygen concentration of some common dusts.10
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.
Effects of thermal radiation on near-limit flame spread in a low convective microgravity environment
Published in Combustion Theory and Modelling, 2021
To study the extinction characteristics of spreading flames, the ambient oxygen level is reduced in steps till a critical oxygen concentration is reached below which a flame cannot spread and reaches extinction. This critical oxygen level is known as extinction limit or limiting oxygen concentration (LOC). To reduce the oxygen concentration, the data of converged steady solution at a higher oxygen level is utilised. In this sub-section, the effect of radiative heat loss from the flame on the extinction limit is examined. The numerical model contains thermal radiation both from the gas phase and the solid fuel surface. To study the role of gas radiation, a set of simulations is performed in which gas-phase radiation is completely neglected (but surface radiation is included). Figure 2 shows radiative heat flux vectors and divergence of radiation heat flux in the XY and XZ planes at an oxygen level of 30%. In the region downstream of the flame (X < 0 cm) and closer to the fuel surface (Y = 0 cm), the solid fuel is emitting the radiation which is represented by the vectors moving out from the fuel surface. In the region upstream of the flame (X > 0 cm), fuel is absorbing the radiation from the gas-phase and the vectors are seen moving forward, some of them are falling on the fuel surface. It can also be observed that radiative heat loss is higher at the fuel side-edge compared to the centre of the fuel (Z = 0 cm).