Regarding the working principle of the flame arrester, there are currently two main viewpoints: one is based on heat transfer; the other is based on the wall effect.
1 heat transfer
One of the necessary conditions for combustion is to reach a certain temperature, that is, the point of ignition. Below the point of ignition, the combustion will stop. According to this principle, as long as the temperature of the burning substance is lowered below its ignition point, the spread of the flame can be prevented. When the flame passes
Many of the fine passages of the fire arresting element will then become a number of fine flames. When designing the fire-blocking element inside the flame arrester, the contact area between the small flame and the channel wall is enlarged as much as possible, and the heat transfer is enhanced to lower the flame temperature below the ignition point, thereby preventing the flame from spreading.
2 wall effect
Combustion and explosion are not direct reactions between molecules. Instead, they are excited by external energy. The molecular bonds are destroyed and activated molecules are generated. The activated molecules are split into short-lived but very active free radicals. The free radicals collide with other molecules. , to generate new products, while also generating new free radicals and then continue to react with other molecules. When the combustible combustible gas passes through the narrow passage of the flame arresting element, the collision probability of the radical with the passage wall increases, and the radicals participating in the reaction decrease. When the channel of the flame arrester is narrow to a certain extent, the collision of free radicals with the channel wall predominates. Due to the sharp decrease in the number of free radicals, the reaction cannot continue, that is, the combustion reaction cannot continue to propagate through the flame arrester.
3 Maximum experimental safety clearance - MESG value
The flame passes through the small passage of the flame arresting element and cools down within the passage. When the flame is divided to a certain extent, the heat removed through the passage is sufficient to lower the temperature below the ignition point of the combustible material and extinguish the flame. Or explained by the wall effect, when the channel is narrow to a certain extent, the collision of free radicals with the pipe wall predominates, the free radicals are greatly reduced, and the combustion reaction cannot continue. Therefore, the channel size under which a flame condition is extinguished under certain conditions (0.11 MPa, 20 °C) is defined as "Maximum Experimental Safe Gap" (MESG). The channel size of the flame arresting element is a key factor in determining the performance of the flame arrester. Different gases have different MESG values. Therefore, when selecting a flame arrester, its MESG value should be determined based on the composition of the combustible gas. In the specific selection, the gas is divided into several grades according to the MESG value. Two types of methods are often used internationally. The first is the National Electrical Association (NEC) classification, which classifies gases into four grades (A, B, C, D) based on the MESG value of the gas; the other is the International Electrotechnical Commission (IEC) method. The gas is also divided into four grades (IIC, IIB, IIA and I). The MESG values ​​and test gases for each type of gas classified by the two standards are shown in Table 1.
Table 1 Two MESG classification criteria
NEC IEC MESG/ mm test gas
A IIC 0. 25 acetylene
B IIC 0. 28 hydrogen
C IIB 0. 65 ethylene
D IIA 0. 90 propylene
GMI 1. 12 methane
In this way, when the flame arrester is selected, the grade of the combustible gas used can be first found in the specifications specified in the design, and then the corresponding flame arresting component is selected according to the MESG value corresponding to the set of gases.
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