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Lecture 4: Fire and Explosion Prevention Knowledge in the Petrochemical Industry 1. Characteristics of combustion and explosion: Both combustion and explosion are essentially oxidation reactions of combustible substances in air. 2. Conditions for combustion: For a combustion phenomenon to occur, three conditions must be present simultaneously (also known as the three elements of combustion): 1. A combustible material. 2. Oxidizers. 3. Ignition source. For combustion to occur, these three conditions must be present; without a combustible material, combustion has no basis ； Without oxygen or an oxidizer, a combustion reaction cannot occur ； However, with combustible materials and oxidizers present, a combustion reaction cannot begin unless there is a heat source to raise the temperature of these materials above their ignition point. 3. Classification of combustion: Based on the characteristics of the moment it occurs, combustion phenomena are divided into three types: ignition, spontaneous combustion, and flash combustion. 1. Ignition. The phenomenon of continuous combustion that begins when a combustible material is directly exposed to an external fire source. 2. Spontaneous combustion. Although combustible materials are not directly exposed to an external fire source, they can catch fire spontaneously when heated to a certain temperature. Spontaneous combustion can be further divided into self-spontaneous combustion and combustion induced by heating. 3. Flash ignition. Flash ignition is one of the characteristics of flammable liquids. The lowest temperature at which the flammable vapor evolved from a flammable liquid is sufficient to form a mixture with air that can ignite upon contact with a source of fire is known as the flash point of that liquid. The flash point is the main criterion for assessing the fire hazard of liquids. The lower the flash point, the greater the fire hazard. 4. Fire extinguishing 1. Basic methods of fire extinguishing a. Cooling method: The cooling method involves reducing the temperature of the burning material; when its temperature drops below the ignition point, the burning stops automatically. b. Asphyxiation method: This method involves preventing air from entering the burning area or diluting the air with inert gases, thereby causing the burning material to go out due to a lack of sufficient oxygen. c. Isolation method: The isolation method involves separating the flammable material from the source of fire. d. Chemical interruption method: cooling, suffocation, and isolation for extinguishing fires. During this process, the fire extinguishing agent does not participate in the combustion reaction; it is a physical method of fire suppression. In contrast, the chemical interruption method involves using a fire extinguishing agent that does participate in the combustion reaction. Agents such as 1211, 1201, and 1301 are used in this approach, but it is essential to apply the extinguishing agent accurately to the area where the fire is burning, otherwise the fire will not be effectively extinguished. 2. Selection of fire extinguishing agents and equipment a. Water (and water vapor): While water is used for extinguishing fires, it also has its disadvantages; for example, it is conductive and therefore not suitable for extinguishing fires involving electrical equipment ； Substances that react chemically with water (such as sodium, potassium, calcium carbide, etc.) cannot be extinguished ； It is also impossible to extinguish flammable liquids that are insoluble in water and have a lower specific gravity than water, such as gasoline and kerosene ； Intense water jets cannot be used to extinguish fires in areas where large quantities of concentrated sulfuric acid or concentrated nitric acid are stored ； It is not appropriate to use water to extinguish fires involving some precision instruments and equipment, valuable cultural relics and archives, or books. b. Foam extinguishants are effective in putting out fires involving liquids such as gasoline, kerosene, and paint, as foam is lighter than oils. They can also be used to extinguish fires involving solids such as wood, fibers, and rubber, but they are not suitable for extinguishing electrical fires or those involving water-sensitive materials. When using a foam fire extinguisher, do not use other liquids at the same time, as this will destroy the foam and render it ineffective. Also, move gradually from the outer edge of the burning material toward the center; otherwise, the pressure exerted in the center may cause burning liquids such as oil to spread in all directions. When carrying a foam fire extinguisher, the cylinder lid should not be tilted too far upward to prevent the two liquids from mixing. When in use, invert the cylinder with the lid facing down, and the liquid medicine will be sprayed out from the nozzle. c. Carbon dioxide extinguishing agent: This agent involves storing carbon dioxide in liquid form under pressure within a fire extinguisher. Since carbon dioxide does not conduct electricity, it can be used to extinguish fires involving electrical equipment. It is also the most suitable choice for extinguishing materials that catch fire when exposed to water and cannot be extinguished using water. d. 1211 fire extinguishing agent: 1211 is a liquid-gas fire extinguishing agent that also has certain cooling and smothering effects. Due to its advantages such as high fire extinguishing efficiency, low corrosivity, and stability over time, it can be used to extinguish fires involving oils, flammable liquids, gases, as well as in computer rooms, libraries, and facilities with precision instruments. However, it is not suitable for extinguishing fires involving chemically reactive metals such as potassium and sodium metals, acetylene, and other substances that can supply their own oxygen, as well as fires involving metal hydrides and chemicals that can decompose spontaneously. (This fire extinguishing agent is about to be phased out due to its environmental pollution.) e. Dry powder fire extinguishers: Dry powder fire extinguishers are dry, flowable fine solid powders; since they leave residues after extinguishing a fire, they are not suitable for use in putting out fires involving precision mechanical equipment or sensitive instruments ； It also has the drawback of poor cooling performance. f. Dry powder (pressure storage) fire extinguishers; g. Portable mechanical foam fire extinguishers are suitable for extinguishing fires involving solid materials and flammable liquids, but not for electrical fires.