Different foam extinguishing agents, different performance. The correct choice of low-multiple foam extinguishing agents is of great significance.
Analysis of the fire: The three elements of fire: fuel, heat and oxygen. Take any element of it and the fire will go out. If a pile of wood (fuel) is removed from the heat source, the fire triangle will collapse and the fire will go out. If the wood is already on fire, spray water on it, heat is absorbed, the fire triangle collapses, the fire will extinguish. Removing oxygen from a fire triangle requires other media, such as a fire blanket, sand, or foam. Foam extinguishing agents act like blankets, or coat the surface with a thin film of liquid, evaporate into water vapor, and lower the temperature while preventing the flame from contacting oxygen.
Let's start by saying that according to the fire classification of different fire "burning objects":
Class A : paper, wood, plastic, cloth
Category B : Gasoline, Petroleum, Lubricant, Paint
Class C : Electrical fire (not current burning, but fire source)
Category D : Metal fires such as potassium, magnesium, sodium, aluminum
K : Unsaturated Fatty Acid Edible Oil
In general, foam fire extinguishing is mainly used for Class A and B fires, but not for other types of fires.
There are two basic types of foam fire extinguishing agents:
1. Protein foam fire extinguishing agent - Protein accounts for about 17% of human body mass . This also applies to the animal body. It is these animal proteins that are the source of raw materials for making protein foams. These so-called "hoof foot" ingredients are extracted from the crushed animal skeleton. In some naturally occurring items, animal proteins, such as chicken feathers, can also be extracted. In theory, yogurt and eggs are also available. In general, protein foams require the introduction of air and water / foam to be fully functional. In other words, fire monitors or other equipment need to be designed to introduce air so that the protein foam looks like a bubble in the bathroom. It is also for this reason that the protein foam used for extinguishing needs a certain degree of solid suspension to produce a large number of effective fire-fighting foams. Protein foam can be distinguished by its unique dark color and odor.
2. Aqueous film-forming foam fire extinguishing agents ( AFFF ) are a mixture of chemicals that are usually more expensive than protein foams. Aqueous film-forming foam was originally developed during the Second World War, was carried by aircraft carriers, and responded to the special situation of military fires. Aqueous film-forming foam does not require the introduction of air, and can use the same equipment as sprinkler. All it needs is the introduction of water to form a solution (this is also the name of the mixture of foam and water). Aqueous film-forming foams are smaller, lighter, and flow faster than the protein foam layer.
What is the difference in the fire extinguishing performance of different foam extinguishing agents?
These two types of foam extinguishing agents have different characteristics. In order to better distinguish them, it is necessary to analyze these characteristics, including heat resistance, "fire suppression" speed, fuel resistance and anti-evaporation.
Heat resistance
The essence of fire is heat. Therefore, the heat resistance of foam fire extinguishing agents is an important indicator. At this point, the protein foam surpasses the aqueous film-forming foam. Because the percentage of suspended matter in the protein foam is higher, it is more conducive to fighting heat. When the protein foam layer is formed on the surface of the hot metal, a solid particle barrier layer is formed between the protein foam layer and the hot surface and is combined with the foam layer. Aqueous film-forming foam layers tend to disappear from hot surfaces, often leaving exposed flammable materials that re-ignite under certain conditions. The only solution is to spray more foam on the surface until the metal cools, or the amount of foam is sufficient to form a stable foam layer with no edge damage.
"Suppress fire" rate
This feature examines the ability of foam fire extinguishing agents to "suppress fire" or extinguish fire on liquid fuel surfaces. At this point, the aqueous film-forming foam performs better than the protein foam. When applied to a fire, the aqueous film-forming foam ejects a small stream of water and then evolves into a foam layer. The film has a low-viscosity property and can flow quickly and cover the surface of the object, preventing the spread of fire and blocking the surface. The protein foam is more viscous, slower to flow and slower to control the fire. As it progresses, it can eventually form a foam layer on the surface.
Fuel resistance
In general, foam fire extinguishing agents are used as follows:
Small-diameter hose (hand-held hose, no suction device or suction nozzle) fire monitor or monitor
Under the liquid jet, the foam is sprayed toward the bottom of the fuel storage tank and floats upward through the oil layer, covering the fire site.
Liquid spray, suitable for fixed top tanks, the foam is poured down through the space around the floating roof.
Dumping in the closed condition of the edge, suitable for floating roof irrigation, the foam is dumped directly into the sealed area around the tank.
Only in the latter two application methods, the foam slowly pours into the surface area of ​​the combustion. However, as these two dumping methods are often destroyed by explosions, they are usually supplemented by another application method.
Fuel resistance is the ability to prevent fuel from adhering to the foam surface. When using small-diameter water hoses, fire guns, or submerged injection methods, fuel resistance is a crucial indicator because the combustion will continue to burn under the foam extinguishing agent and destroy the foam fire extinguishing layer. The protein foam fire extinguishing agent has better fuel resistance than the water film fire extinguishing agent because it is more suitable to be used in the form of extinguishing the fire with the foam fire extinguishing agent.
Anti-evaporation
In only a few cases, the liquid fuel will remain in a liquid state and, under normal circumstances, it will burn in the fuel.
Becomes steam or gas. The more heat is burned, the more steam will be released. Therefore, an important characteristic of foam fire extinguishing agents is the ability to suppress the divergence of steam from the surface of the combustion products. Aqueous film-forming foam fire extinguishing agent is lighter and can form water-casting film. Compared with protein foam fire extinguishing agent, it has better anti-evaporation performance and is more efficient and excellent.
Protein foam extinguishing agents and aquafoam foam extinguishing agents have three basic ratios of foam liquid: 6% ( 6% foam component and 94% water), 3% ( 3% foam component and 97% water) or 1% ( 1% foam and 99% water). One of the 1% foam liquids is used only for water-based film-forming foam extinguishing agents The petrochemical industry requires 3% foam liquids, although some of them use cheaper 6% foam liquids for training. 1% foam solution is a good idea because of its low demand for foam storage space. However, there is another indicator that determines which percentage of foam liquid is used, ie, fire fighting equipment that mixes foam with water. If a 1% foam solution is selected , the equipment injection rate (foam injected into the hose) is quite accurate (the 1% foam fluid price is more than three times 3% , more than six times 6% ). Pipeline foam proportioners, ring-pumped foam proportioners, self-sensing fire hoses are not as accurate, and they do not change the amount of foam injected into the water stream with the flow of water. Therefore, once the pipeline foam proportioner gives a 3% foam ratio with a certain water flow rate ( l/m ) , even if the water flow rate decreases or increases by 50% , the foam injection ratio will remain consistent, which is obviously not economical. Digital proportional regulators, metered-injection foam proportioners, and balanced-pressure foam proportional mixers are more accurate. They will inject the proportional foam directly into the water stream at a set ratio. In other words, whether the water flow is 5,000 liters / minute or 50,000 liters / minute, the proportion of foam fluid is 3% .
There are different types of protein foam extinguishing agents and aquafoam foam extinguishing agents for special applications and special products. The film-forming fluoroprotein foam extinguishing agent is a protein foam extinguishing agent formed by mixing a film of an aqueous film-forming foam with a firm protein layer. Film-forming fluoroprotein foam fire extinguishing agents are named after fluorinated surfactants, and the fluorination of proteins helps to improve the fire performance of protein foam fire extinguishing agents. Producers of film-forming fluorochemical foam fire extinguishing agents believe that this foam fire extinguishing agent combines the advantages of a protein foam fire extinguishing agent and an aqueous film-forming foam fire extinguishing agent. In addition, there are other foam fire extinguishing agents mixed with special chemical components or liquids. These foam fire extinguishing agents are called film-forming fluoroprotein fire-resistant fire extinguishing agents and water-soluble water-soluble film-forming foam fire extinguishing agents. Normal protein foam and aquafoam extinguishing agents work well in dealing with hydrocarbon fires such as diesel, jet fuel, and gasoline. However, there are some chemicals that can absorb water and are called polar solvents such as alcohol and isopropyl alcohol . These chemical components often appear as a by-product of oil refinery and refinery refinement and require special foam fire extinguishing agents.
It is necessary to harass you to explain why. A simple example explains this phenomenon: if the oil is poured into the water, the oil will float on the surface because water and oil are incompatible, as are hydrocarbons and gasoline. However, if water is poured into polar solutions, such as alcohol, they can easily mix together (Rum and Coca-Cola are a good example). Most of the foam fire extinguishing agent is water (as mentioned above, such as 3% foam and 97% water). Therefore, when ordinary foam fire extinguishing agents and water film forming fire extinguishing agents are applied to a fire caused by a polar solution, the water will be compatible with them, and the foam or a film forming foam will be drained, thereby making the foam fire extinguishing agent ineffective.
Therefore, special foam extinguishing agents (referred to as "anti-alcohol extinguishing agents" such as film-forming fluoroprotein fire-resistant extinguishing agents and water-soluble foam-forming extinguishing agents) must be used for polar solvents (water absorbing or mixing) liquids. In the fire. These foam extinguishing agents contain high-molecular polymers (plastic materials), so when the foam extinguishing agent is destroyed, the high-molecular polymer components will form a protective film on the liquid surface. This layer of protective film will prevent the oxygen needed for fire, prevent water vapor from being generated and evaporate from the liquid surface. At the same time, these foam extinguishing agents can cool the fuel surface and reduce the amount of water vapor emissions.
There are other necessary factors, such as how these foam extinguishing agents are stored. Protein foam fire extinguishing agents can be stored in almost all containers, iron, stainless steel or other materials. Aqueous film-forming foam extinguishing agents cannot be stored in iron containers. The container material must be stainless steel. If it is an iron container, it must be lined with other materials. Unfortunately, the lining material is often damaged over time, and expensive foam fire extinguishing agents can only be eventually abandoned. Therefore, in general, aquafoam foam must be stored in expensive stainless steel containers.
Finally, all of the above is true for low-expansion foam fire extinguishing agents, but there are other foam fire extinguishing agents that are well known as "aqueous film-forming foam fire extinguishing agent" cleaners. They have a special role and use special equipment:
Medium multiple foam extinguishing agents: foam extinguishing agents with a foam expansion ratio of between 20 and 200 . It is often used in defense of liquid fuel storage tanks and aircraft carrying.
High-strength foam fire extinguishing agent: Foam extinguishing agent with a foam expansion factor of 200 or more. Used in large areas such as basements, mines or cargo holds.
As of writing, the author specifically for the big explosion of hazardous chemicals in Tianjin fire, the National Fire Protection Association standards issued to publication - according to the National Fire Protection Association (NFPA) 2005 edition NFPA11 (low, medium and high expansion foam Standard) :
6.3.2 Medium and low magnification foam systems do not apply fires with the following hazards:
1. Chemicals that release enough oxygen or other oxidants to support combustion, such as nitrated products.
2. Energized enclosureless electrical equipment.
3. Metals that react with water such as sodium, potassium and NaK (sodium-potassium alloys).
4. Reaction with water hazardous substances such as triethylaluminum and phosphorus pentoxide.
5. Liquefied flammable gas.
6. Functional assessments (including tests) indicate acceptability, hazards listed in 1-5 .
This article from the "fire front", author: reporter, Chinese rescue equipment network order.
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