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Principles of Explosion and Measures for Preventing Explosion
First, the explosion reaction process
Combustible gas, vapour or dust is mixed with air in advance and reaches the explosion limit. This mixture is called an explosive mixture.
According to the chain reaction theory, when the explosive mixture comes into contact with a fire source, active molecules may form or become active centers for continuous reactions. After the explosive mixture catches fire at one point, the heat and the active centers spread outward, prompting the adjacent layer to react chemically. This layer then becomes the source of the heat and the active center and causes the reaction of the other layer. The ground continues until all the explosive mixture has reacted. The flame at the time of the explosion propagates outward in layers. In an explosive mixture surrounded by no boundary, the flame spreads in all directions in the form of layers of concentric spheres.
The speed of the flame is only a few meters per second at a distance of 0.5-1 m from the fire location, but it gradually accelerates afterwards, reaching as much as several hundred meters per second. If there is an obstruction on the extended path of the flame, the shield will be greatly damaged due to the dramatic increase in the temperature and pressure of the mixture. Most of the explosions occur with combustion. Therefore, for a long time, the theory of combustion theory holds that when the combustion is carried out in a certain space, if the heat radiation is unfavorable, the reaction temperature will continuously increase, and the increase in temperature will accelerate the reaction speed. It caused an explosion. That is, the explosion is caused by the thermal effect of the reaction, which is called thermal explosion. In another case, the explosion cannot be explained simply by the thermal effect. For example, when a mixture of hydrogen and bromine is exploded at a relatively low temperature, the reaction formula is: H 2 Br 2 ====2HBr 3.5 kJ/mol, and the reaction heat only 3.5 kJ/mol; and the reaction of sulfur dioxide and hydrogen, which The reaction is:
SO2 H2 = H2S deca 2H2O deca 12.6kJ/mol, the reaction heat is 12.6kJ/mol; it does not explode, and so on. Therefore, some explosions need to be explained from the viewpoint of chemical kinetics. It is believed that the cause of the explosion is not due to a simple thermal effect but rather to the result of a chain reaction.
We know that there are two kinds of chain reaction: unbranched chain reaction (straight chain reaction) and branched chain reaction (branched chain reaction). The chain reaction of hydrogen and oxygen is a branched chain reaction. It is characterized by a free radical (active center) that generates more than one radical in the reaction. For example, H·O2=OH·T O·O·O·H+H2 =OH·HH, then the reaction chain will branch, as shown in Figure 2-3. In the case of chain growth, where the reaction can add value to free radicals, if the rate of simultaneous destruction of free radicals (chain termination) is not high, then the number of free radicals will increase and the number of reaction chains will increase. The speed is also accelerating, so that it will add more free radicals. As the cycle progresses, the reaction speed will increase to the level of explosion.
The chain reaction speed, v, can be represented by the above formula. Where: F(c)—reaction concentration function; fs—chain destruction on the wall;
Fc - the destruction factor of the chain in the gas phase; A - the function related to the reactant concentration; a - the number of branches of the chain, a = 1 in the linear reaction and a > 1 in the chain reaction.
According to the chain reaction theory, increasing the temperature of the gas mixture can increase the speed of the chain reaction and increase the amount of free radicals generated by the thermal motion. At a certain temperature, the number of chained branches exceeds the number of interruptions. At this time, the reaction can be accelerated and the reaction rate of the mixture self-igniting can be reached, so the condition that the gas mixture self-ignites can be considered as the chain reaction number is equal to the number of interruptions. When the number of linkage branches exceeds the number of interruptions, even if the temperature of the mixture remains unchanged, it can still cause self-ignition. Under certain conditions, such as when fs + fc ten A (1-a) a → 0, an explosion occurs.
In summary, the explosion of explosive mixtures has two different mechanisms: thermal reaction and branched chain reaction. As for the circumstances under which thermal reactions occur and under what circumstances branching reactions occur, it depends on the specific circumstances, and even the same explosive mixture may sometimes be different under different conditions; Figure 2-4 shows hydrogen and oxygen The range of temperature and pressure at which the mixture consisting of stoichiometric concentrations (2H2+O2) explodes. It can be seen from the figure that when the pressure is low and the temperature is not high, for example, at a temperature of 500° C. and a pressure of no more than 200 Pa, the free radicals are easily diffused on the walls and destroyed. At this time, the interlocking speed exceeds the speed of the text chain. Therefore, the reaction proceeds slowly and the mixture does not explode. When the temperature is 500°C and the pressure rises to between 200 Pa and 6666 Pa (between points a and b in the figure), the branch speed is greater than the destruction rate. The chain reaction is very violent and it will explode. When the pressure continues to increase beyond point b (greater than 6666 Pa), due to the increase of the concentration of molecules in the mixture, the chain interruption reaction is prone to occur, resulting in the destruction rate of free radicals exceeding the chain production speed. The reaction rate tends to ease and the mixture will not explode.
The pressures at the points a and b in the figure, ie, 200 Pa and 6666 Pa, are the lower explosive limit and explosive upper limit of the mixture at 500°C. As the temperature increases, the explosion limit will widen. This is because the chain reaction requires a certain amount of activation energy, the chain branch reaction rate increases with increasing temperature, and the chain termination reaction decreases with increasing temperature, so raising the temperature favors the chain reaction, resulting in an explosion The limit widens and it appears as a peninsula on the map. When the pressure rises again above c (greater than 666,610 Pa), the following reactions begin to appear: these two reactions are exothermic. As a result, the heat released by the reaction exceeds the heat dissipated from the walls, causing the temperature of the mixture to rise. Further accelerate the response, prompting the release of more heat, which led to a thermal explosion.
Second, the conditions of chemical explosion of combustible materials
The chemical explosion of flammable substances must meet the following three conditions at the same time.
1. There are flammable substances, including flammable gases, vapors or dust.
2. Flammable substances are mixed with air (or oxygen) and reach the explosive limit to form an explosive mixture.
3. The explosive mixture is under the action of a fire source.
For each combustible gas (vapour) explosive mixture, there is a minimum ignition energy that causes the explosion. Below this energy, the mixture does not explode. As can be seen from Figure 2-5, the minimum current intensity of the spark that causes the alkane explosion is: methane 0.57A, ethane 0.45A, propane 0.36A, butane 0,48A, pentane 0.55A.
The unit of minimum ignition energy is usually expressed in millijoules. The minimum ignition energy of combustible gases and vapors in air is shown in Table 3-6.
Third, the relationship between combustion and chemical explosion
Analysing and comparing the conditions of chemical explosion of combustible and combustible materials can be seen that both require three basic factors: combustibles, oxidants and fire. Thus, the combustion and chemical explosion are essentially the same, and are all oxidation reactions of flammable substances, and their main difference lies in the difference in the rate of oxidation reaction. For example, when a 1kg lump coal burns completely, it takes 10 minutes. When a mixture of 1kg gas and air explodes, it takes only 0.2 seconds and the combustion heat value of both is about 2631kJ (the heat released when 1kg of material is burned is the combustion heat value of the material. The combustion heat value of the material is shown in the table 1 - 2).
From the above comparison, it can be clearly seen that the difference between combustion and explosion is not the size of the combustion heat contained in the material, but rather the speed at which the material burns. The faster the combustion rate (ie, oxidation rate), the faster the release of heat of combustion and the greater the destructive power generated. According to the inverse relationship between power and work time, it can be calculated that a piece of coal with a heat content of 2931 kJ emits 47.807 W when combusted, and a piece of coal with a same quantity of heat emits 1471×10 5 w. The ability is great and the destructive power is great.
Since the main difference between combustion and chemical explosion is the burning speed of the material, the development process of fire and explosion is significantly different. The process of fire has its initial stage, development stage, and debilitating extinguishment stage, and the losses caused by the process are aggravated with the passage of time. Therefore, once a fire occurs, if it can be saved as soon as possible, the loss can be reduced. The chemical explosion is essentially It is an instant burn, usually within 1 second; the explosion process is complete. The huge losses caused by the power of the explosion, such as casualties, equipment destruction, and collapse of the factory buildings, all occurred between the moments of encroachment, and they were caught off guard. Therefore, once an explosion occurs, there is no way to reduce the loss.
Combustion and chemical explosions also exist in such a relationship that both can be converted with conditions. The same substance can be burned under one condition and explode under another condition. For example, coal can only be burned slowly. If it is pulverized and mixed with air, it may explode. This also shows that combustion and chemical explosion are essentially the same.
Since combustion and chemical explosions can be transformed with conditions, some of the accidents that occur in the production process are the first to ignite and then catch fire. For example, after an oil tank, a calcium quarry, or an acetylene generator is exploded, it is often a large fire; In some cases, it will start with a fire and then explode. For example, when an evacuated sump catches fire, the flammable vapour will continue to be consumed, and more flammable vapours cannot be added in time. As a result, the concentration will continue to drop. When the vapour concentration falls into the explosion limit range, An explosion occurred. Another example is the occurrence of a fire in dangerous goods stores.
IV. The induction period of combustion and chemical explosion
After the temperature of the flammable substance reaches the point of ignition or ignition, it does not spontaneously ignite or ignite. There is a period of time during which it is known as the induction period (or induction period).
As mentioned earlier, self-ignition of flammable substances cannot occur at the natural point T0 shown in the graph of Figure 1-3. It only occurs at higher temperatures T0'. The interval between T0' and Tc' in the figure, that is, the delay time before the material spontaneously ignites, is expressed as T. This phenomenon during the induction period can be observed when measuring the autoignition point of combustible substances. The container to be measured is heated to the point of spontaneous ignition of a certain substance. However, after the substance is introduced, it does not spontaneously ignite, but only after the time elapses. Flames appear.
When the combustible material comes into direct contact with the fire and catches fire, there is also an induction period. However, due to the high temperature of the flame, the induction period is greatly shortened, so that most people are not aware of the delay before the fire. The explosion of a combustible mixture is essentially instantaneous combustion. Therefore, any such explosions also have time delays.
The reason why the combustible matter is combusted and the flammable mixture explodes in the induction period is because it takes a certain amount of time for the active center of the chemical reaction to develop to a certain number. That is, such combustion and explosion need continuous development. A certain period of time necessary for the process to take place can occur.
The induction period has practical significance in terms of safety. For example, although methane is present in coal mines, it can still be blasted without smoke. This is the induction period using methane. Because the induction period of methane is 8-9 seconds, and the smoke-free firing time is only 2-3 seconds, it can ensure safety. Another example is to select explosion-proof electrical equipment based on the induction period of combustible gas or vapor.
V. Basic theory of explosion-proof technology
The simultaneous existence of the three basic conditions for preventing chemical explosions is the basic theory for preventing the chemical explosion of combustible substances. It can also be said that the essence of all technical measures to prevent chemical explosion of combustible substances is to stop the three basic conditions of chemical explosion. Exist at the same time. We know that modern fuels used in production and life have a wide range of flammable materials, and the number of flammable materials is complex, and the production process is complicated. Therefore, various protective measures must be taken according to different conditions. But overall, technical measures for preventing explosions, They are all taken under the guidance of the basic theory of explosion-proof technology. For example, in order to eliminate inflammables from forming explosive mixtures, inert gas such as nitrogen, carbon dioxide, and water are used to remove flammable materials from the vessel's equipment or pipelines, keeping their concentrations much lower than the lower explosion limit. Anti-leakage is also an important measure for explosion protection. In addition to preventing flammable substances from running, spiking, dripping, and leaking from sliding surfaces, joints, corroded holes, and small cracks on rotating shafts, special care must be taken to prevent disconnection from valves, caps, or pipes. If there is a large amount of leakage, etc., etc., to prevent the formation of explosive mixtures, measures may be taken to strictly control the oxygen content of the system so that it falls below a certain critical value (oxygen limit or limiting oxygen content). In order to ensure the above-mentioned explosion-proof conditions, monitoring measures and alarm devices, as well as various measures to eliminate ignition sources, etc., are taken under the guidance of the basic theory of explosion-proof technology.