Abstract: With the continuous advent of various new foam fire extinguishing agents, accurate research and mastery of their principles and performance can lay the foundation for the firefighting force to effectively fight flammable liquid and oil fires. Based on this, this article analyzes and discusses the five aspects of the definition, classification, properties of foam fire extinguishing agents, the principle, function and application scope of foam fire extinguishing, and the introduction of several other foam fire extinguishing agents.

       1. Definition of foam fire extinguishing agent

       Foam extinguishing agent is a medicament that mixes the aqueous solution of foaming agent with air or carbon dioxide (CO2), and generates foam through chemical reaction or mechanical method to extinguish the fire. Fire extinguishing foam is composed of air-filled water bubbles. The water solution is made by mixing foam concentrate and water in an appropriate ratio (typically 1:3, or 60k concentrate).

       2. Classification of foam fire extinguishing agents

       The classification methods of foam extinguishing agents include: generation mechanism classification, foaming ratio classification, usage classification, and foam base material type classification. Foam extinguishing agents are divided into chemical foam extinguishing agents and air foam extinguishing agents. The chemical foam fire extinguishing agent is composed of foaming agent, foam stabilizer, liquid-resistant additives and other additives. There are two kinds of medicament solutions that produce foam through chemical reactions. Air foam fire extinguishing agent can be mixed with water to produce foam through mechanical methods. In the fire fighting of firefighters, air foam pipe guns are usually used for foaming, and the foam quality obtained when forced air is used is higher. Foam extinguishing agents are divided into low expansion (below 20 times) foam extinguishing agents, medium expansion (21 to 200 times) foam extinguishing agents and high expansion (201 to 11000 times) foam extinguishing agents. It can be divided into ordinary foam fire extinguishing agent, alcohol-resistant foam fire extinguishing agent and general foam fire extinguishing agent.

       3. The nature of foam fire extinguishing agent

       In the foam liquid, the ratio of the volume of the foam produced by the mixed liquid to the volume of the mixed liquid is called the foaming multiple. The foam multiple K bubble is the ratio of the foam volume V bubble to the liquid volume V liquid: V bubble = V bubble/V liquid = (V gas + V liquid)/V liquid, where V gas-gas volume. For low-expansion foam, the foaming ratio is better in the range of 6-8; for "under-liquid jet" fire extinguishing, foam liquid with a foaming ratio of 2 to 4 is used.

       Foam stability refers to the existence time of foam cells, or the existence time of a certain volume of foam. Under standard conditions, the destruction of the foam is caused by the loss of liquid and the rupture of the liquid film in the foam. For the more stable foam, the liquid film will not be destroyed within 10~20imn. The high expansion foam makes it difficult to lose liquid. The damage is mainly due to the rupture of the liquid film. The loss of liquid from the foam is a pure hydraulic phenomenon. The surface layer of the bubble film that is lost by the liquid is elastically deformed-tension and compression can also destroy the bubble film. The rate of liquid loss from the bubble shell and the stability of the bubble are both It is related to the viscosity of the foam solution.

       The burning resistance of foam is also one of the performance indicators of foam fire extinguishing agents. The burning resistance time is an indicator to measure the thermal stability and burning resistance of low-expansion foams. It refers to the heat radiation of a certain amount of foam in a specified area of flame. Under the action, the time to be completely destroyed, the longer the anti-combustion time, the better the stability of the foam.

       4. The principle, function and application scope of foam fire extinguishing

       4.1 Heavy foam (low expansion foam)

       4.1.1 Fire extinguishing effect. The fire extinguishing effect of heavy foam is determined by the cooling effect and isolation effect, and the degree of its effect depends on the conditions of the fire. The cooling effect is produced by the foam itself and the water separated from the foam. The foam is used to fight the smoldering fire of solid materials (such as wood, paper, textiles, etc.) and to fight the oil and liquids that can form a heated layer (liquid suitable for fuel) ) In the event of a fire, the cooling effect of the foam is dominant. When heavy oil and secondary heavy oil burn, the temperature reaches 200~300℃, and the hot oil in the upper layer will move to the lower oil layer at a speed of 5-20cm/h. At this time, the foam fire extinguishing agent is mainly used to cool the heated oil layer. To achieve the purpose of extinguishing fire. Because the foam forms a foam layer, it can prevent oxygen from approaching the flame area and achieve the purpose of isolating air to extinguish the fire.

       4.1.2 Scope of application. Heavy foam is mainly used to extinguish combustible liquid (gasoline, petroleum, benzene, etc.) fires and solid material (wood, rubber, paper, plastic, etc.) fires, and can protect oil tanks, mines, and store combustible liquids and combustible solid materials The warehouse protects the heat radiation from fire to nearby houses and oil tanks, and can prevent re-ignition when extinguishing flammable liquid fires. However, because the aqueous solution of heavy foam has good conductivity, it cannot be used for fire fighting in electrical equipment, and it cannot be contacted with light metals (such as manganese, aluminum, sodium, potassium, files, magnesium, etc.). These metals are in contact with the foam. The water undergoes a violent exothermic reaction, and the heat released is enough to cause the decomposition of hydrogen to burn.

       4.2 Medium and high expansion bubbles

       The main fire extinguishing effect of medium and high expansion foam is inhibition. When the foam is supplied to the fire point, the foam is easily destroyed, so that the moisture starts to vaporize. When the high-expansion foam is supplied with foam, it can cover the entire flame area, and the saturated water vapor forms an oxygen-poor area, thereby reducing the combustion speed and stopping the combustion. Another performance of high-expansion foam is its thermal insulation performance and its ability to prevent fire from spreading to nearby combustibles. Medium and high expansion foam fire extinguishing agents are widely used in limited spaces such as coal mines, tunnels, airports, garages, ships, warehouses, basements, and large-area oil fires on the ground. Medium and high expansion foam extinguishing agents can be used to extinguish the following fires: Class B fires such as gasoline, kerosene, diesel, industrial benzene; Class A fires such as wood, paper, rubber, textiles, etc.; fires in enclosed places with electrified equipment; control of liquefied petroleum gas , The flowing fire of liquefied natural gas.

       5. Introduction to the application of several other foam fire extinguishing agents

       5.1 Protein foam fire extinguishing agent (FP)

       Protein foam extinguishing agent (protein foam extinguishing agent) can be applied to oil field, airport and other oil fires and general solid matter fire fighting, and the general storage time is 2 years.

        5.2 Fluoroprotein foam fire extinguishing agent

       Protein foam extinguishing agents containing fluorocarbon surfactants are called fluor-protein foam extinguishing agents. Fluorocarbon surfactants have good surface activity, high stability, good wettability and fluidity. When the foam passes through the oil layer, the oil cannot diffuse like the foam liquid and is separated into small oil droplets by the foam. The surface of the oil layer forms a non-combustible foam layer with small oil droplets, which will not burn even if the gasoline content in the foam is as high as 25%. Fluoroprotein foam fire extinguishing agent is mainly used for flammable liquid fires such as petroleum and petroleum products (Class B fire) (oil tank liquid injection) and aircraft falling fires. It is also suitable for extinguishing wood, paper, cotton, hemp and synthetic fibers and other solids. Combustible fires (Class A fires) can be combined with various dry powders.

       5.3 Anti-solvent foam fire extinguishing agent (AR)

       Alcohol resistant foam extinguishing agent is used to extinguish water-soluble liquid fuel fires. Adding organic acid metal complex salt to protein hydrolyzate is called protein type anti-solvent foam fire extinguishing agent, which is widely used in alcohol, ether and other flammable polar solvents and oil fires.

       5.4 Aqueous film-forming foam fire extinguishing agent (AFFF)

       Aqueous film forming foam extinguishing agent (aqueous film forming foam extinguishing agent), also known as "light water" foam extinguishing agent, consists of fluorocarbon surfactants, hydrocarbon surfactants and additives, and water. Its characteristic is to form a protective film on the surface of the oil to inhibit the evaporation of the oil. It relies on the dual action of the foam and the protective film. It has high fire extinguishing efficiency, fast speed, anti-re-ignition performance and good sealing performance, long storage period, and can be used with various types. Combination of dry powder fire extinguishing agent can also use "submerged jet" method to extinguish large oil storage tank fires. It is widely used to extinguish polar solvent fires such as alcohols, esters, ethers, ketones, aldehydes, amines, organic acids, etc., and high polar flammable and flammable liquid fires such as propylene oxide.

       Concluding remarks:

       In summary, as a clean "green fire-fighting product", foam fire extinguishing agent has environmental protection performance, fire extinguishing performance, use performance and regeneration performance. The correct use of foam extinguishing agents plays a vital role in fire fighting.