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Principle, Classification, and Selection of Flame Arresters 1. Principle of Flame Arresters A flame arrester is a safety device used to prevent the spread of flames from flammable gases and vapors of flammable liquids; it allows gases to pass through while preventing flames from doing so. It was first applied in the oil industry, and later became widely used in mining, coal mining, water transport, and the chemical industry. A flame arrester is mainly composed of a housing and a filter element, with the filter element being the key component that prevents the spread of flames. Depending on the type of filter element, flame arresters can be classified into packed-type flame arresters, plate-type flame arresters, metal mesh flame arresters, corrugated-type flame arresters, and liquid-sealed flame arresters. Taking the common corrugated flame arrester as an example, its filter element is formed by coiling thin stainless steel corrugated strips and flat strips into a disc shape (Figure 1). Its flame-arresting capacity depends solely on the size of the triangular cross-sectional openings created by the corrugations on the filter element, as well as the thickness of the filter element. As the flame passes through the filter element, it is divided into several small flames by these triangular-shaped holes, which increases the contact area between the flame and the channel walls and enhances heat transfer. This results in the flame temperature dropping below the ignition point, thereby preventing the flame from spreading. Furthermore, due to the wall effect, when the burning combustible gas passes through the narrow channels of the flame arrestor, the probability of collisions between free radicals and the channel walls increases, resulting in a decrease in the number of free radicals participating in the reaction. When the passage of the flame arrester becomes narrow enough, collisions between free radicals and the wall of the passage become dominant; as a result, the number of free radicals decreases sharply, thereby preventing the flame from spreading to the unburned gas. Figure 1: Wavy-type flame arrestors. II. Classification of flame arrestors: NFPA 69, the Standard for Explosion Protection Systems, provides a detailed classification of flame arrestors based on their installation location and type of combustion (Figure 2). Figure 2: Classification of flame arrestors according to NFPA 69. III. Selection of flame arrestors: Under certain conditions, an appropriate flame arrestor can effectively prevent the spread of flames. However, each type of flame arrestor has its specific operating range; outside this range, it cannot ensure effective flame suppression. Therefore, it is necessary to select the right flame arrestor for a given application. In the selection process, it is first necessary to determine three basic factors: the location where the flame arrester will be used, the type of medium (explosion level), and the operating conditions (pressure, temperature). Then, the pipe/end flame arresters are classified based on the location where they will be used; the combustion conditions are determined according to the installation location, type of medium, and operating conditions, thereby completing the preliminary selection of flame arresters. Based on the initial selection confirmation, the final choice of flame arrester is made by taking into account other parameters such as the connection method of the flame arrester, its ventilation capacity, the maximum allowable pressure drop, the material used for the flame arrester’s housing/core, the design standards, whether it should be of concentric or eccentric design, and whether a heating jacket is required. Among the parameters involved in the selection of flame arresters mentioned above, those for simple operating conditions can be determined directly based on the process requirements. However, in actual engineering designs, the operating conditions are usually complex; the media involved are often gas mixtures, and the combustion conditions vary widely. Therefore, careful consideration is required when selecting flame arresters. Here, only two influencing factors are introduced: 1. Medium type: Clause 3.4.1 of GB 50058, \"Code for Design of Electrical Installations in Explosive Atmospheres\", stipulates that explosive gas mixtures should be classified based on their maximum test safety gap (MESG) or minimum ignition current ratio (MICR). Typically, when selecting a flame arrester, the type of medium is determined based on its MESG value. In accordance with GB 3836.11 \"Ex electrical equipment for explosive atmospheres – Part 11: Equipment protected by enclosures of type ‘d’\", under the test conditions specified in the standard, when mixtures of the gas or vapor under test at all concentrations in the cavity together with air are ignited, the flame passing through a 25 mm long flame path cannot ignite the maximum gap between the two parts of the outer explosive mixture’s cavity. Different gas media have different MESG values. EN ISO16852 \"Performance requirements, testing methods and application limits for flame arresters\" classifies explosive gas mixtures into 7 explosion categories, namely IIA1, IA, IB1, IB2, IB3, IIB, and IIIC, based on their MESG values1, as shown in Table 1. Table 1: Comparison of explosion levels and MESG values of gas mixtures. Media with different explosion levels present varying degrees of danger, and accordingly, different flame arrestor products are required. The lower the MESG value of a gaseous medium, the more severe the operating conditions for the corresponding flame arrester, and the greater the design complexity and cost of the flame arrester. Therefore, it is particularly important to determine the MESG value of the gas medium before selecting a flame arrester. 2. Combustion conditions: When the pipe is long enough and the combustion occurs fast enough, the flame goes through several combustion stages in sequence, namely deflagration, unstable detonation, and stable detonation (Figure 3). During the low-pressure deflagration phase, the speed is generally around 112 m/s and the pressure is 0.1 MPa; in the medium-pressure deflagration phase, the speed is generally around 200 m/s and the pressure is 0.4 MPa; during the high-pressure deflagration phase, the speed is generally around 300 m/s and the pressure is 2 MPa. In the detonation phase, the speed is generally around 1900 m/s and the pressure is 3.5 MPa. During the superdetonation phase, the speed is generally around 2300 m/s and the pressure is 21 MPa. In the stable detonation phase, the speed is generally around 1830 m/s and the pressure is 35 MPa. This is due to the phenomenon of \"pressure rise\" that occurs during combustion. When one end of a horizontal pipe filled with flammable gas is ignited, the flame first spreads toward the pipe walls and then rapidly moves toward the unignited gas. The heat generated by combustion causes the burning gas to expand rapidly, and this expansion in turn compresses the gas ahead of it, thereby creating a \"pressure rise\". The gas at the front of the flame is compressed, resulting in an increase in density; this accelerates the spread of combustion and increases the heat generated during combustion, leading to a more intense \"pressure rise\" at the front of the combustible gas. Generally, if the flame arrester is located far away from the source of fire, the deflagration of the flame may turn into a detonation; the increase in pressure at the front of the detonation flame leads to an **increase in the risk level within the pipeline, and it also places higher demands on the flame-arresting and pressure-resistant capabilities of the flame arrester. Choosing the wrong flame arrester can pose a serious threat to safe production; therefore, it is essential to select either an explosion suppression or detonation suppression type of flame arrester based strictly on the combustion conditions. However, in practical engineering applications, due to the complexity of the mixed media, it is difficult to determine the pipeline conditions and the location of the flame point; as a result, it is not possible to establish clear guidelines for selecting flame arresters under different conditions. Typically, specific analysis must be carried out by applying relevant standards and leveraging accumulated engineering experience. Figure 3 shows a schematic of the flame combustion process. It should also be noted that the bends in the pipes accelerate the spread of the flame; therefore, this factor must be taken into full consideration when selecting a flame arrester. When the number of elbows exceeds 1, the combustion conditions become more complex; it is necessary to simulate the actual conditions of the pipeline and determine them through experiments. In the absence of test conditions, for safety reasons, detonation-type flame arresters are generally recommended. Therefore, within the limits permitted by the process, the number of elbows between the fire source and the flame arrester should be minimized.