2020-05-12
402
1. medium (pl. media) – средство
2. agent – средство, вещество, (ре)агент
3. foam – пена
protein foam – пена на протеиновой основе
fluoroprotein foam – пена на фторпротеиновой основе
aqueous film forming foam – синтетическая пленкообразующая пена
alcohol type AFFF’s – пена для тушения спиртов
low expansion foam – пена низкой кратности
high expansion foam – пена высокой кратности
4. majority – большинство
5. stirrup pump – стационарный насос
6. hose reel – рукавная катушка
7. hose line – гибкий трубопровод
8. installation – установка, устройство, система
9. extinguisher – огнетушитель
10. chemical – химический; химический реагент
11. to secure – защищать, оберегать
12. layer – слой
13. solution – раствор
14. to deliver – поставлять; подавать под давлением,
нагнетать
15. introduction – применение
16. property – свойство
17. halogenated agent – галон (хладон)
18. to suppress – сдерживать, подавлять
19. to interrupt – прерывать
20. advantage – преимущество
21. lack – отсутствие
22. to accomplish – достигать
23. saponification – омыление, сапонификация
24. to drain out – заканчиваться, исчезать, улетучиваться
25. acid – кислота
26. alkali – основание, щелочь
44
TEXT A
Discuss the questions with your partner.
1. What substances are used to put out fires?
2. What method of fire extinguishment are they based on?
3. What extinguishing agent is the most / universal / efficient / expensive / widespread?
Read the text using a dictionary if necessary. Check your answers for Ex. 2.
EXTINGUISHING MEDIA
Classification of extinguishing agents must be flexible and every hazard needs to be considered separately. There are different forms of extinguishing media, e.g. foam, halogenated agents, wet chemical agents, and dry powders.
All of them are widely used in firefighting. But water is considered to be the best extinguishing agent, the most sufficient and the cheapest means of fire extinguishment. Water is used by the Fire Service for the majority of fires and it can be applied to the heart of the fire by means of hand extinguishers, hand or stirrup pumps, hose reels, hose lines or fixed installations.
Firefighting foams have been grouped into two main types: chemical foams and mechanical foams. Chemical foams are rarely found still in use and have been replaced for the most part by different types of mechanical foams. Chemical Foams relied on a chemical reaction between two materials to produce a foam layer that blankets the flammable liquid surface and secures the vapors. Mechanical Foams refer to foam solutions that require a mechanical injection of air to expand and form bubbles. Some examples of mechanical foams are: Protein Foam, Fluoroprotein Foam, Aqueous Film Forming Foam (AFFF), Film Forming Fluoroprotein Foam (FFFP), Alcohol Type AFFF, Alcohol Type FFFP, Low Expansion Foam and High Expansion Foam. All of these foams are sold in concentration, proportioned by different means into a foam concentrate/water solution and delivered to the fire as expanded or finished foam. Foam expansion is usually rated in ratios: low expansion foam (10:1), medium expansion foam (20-100:1), high expansion foam (100-1000:1). All mechanical foams extinguish fires through physical means. The foam blanket or film secures vapors coming off of the fuel
45
surface, the foam concentrate is mixed with water prior to being expanded and therefore has excellent cooling characteristics and mechanical foams will separate the fuel from air.
Fires may also be extinguished by the introduction of inert gases to smother or reduce the oxygen content below the level supporting the combustion. Carbon dioxide and nitrogen are the two gases that are known to be used and involved in this process most readily. They are available in hand extinguishers, fixed installations and so on.
Halogenated agents have been used for fire fighting since the early 1900's. Each halogenated agent has unique physical properties such as vapor pressure, boiling point, specific gravity and the like. Here is the list of them:
– bromotrifluoromethane (BTM-CbrF3);
– bromochlorodifluoromethane (BCF-CBrClF2);
– bromochloromethane (CB-CH2BrCl);
– carbon tetrachloride (CTC – CCl2);
– dibromotetrafluororoethane (DTE – C2Br2F4);
– dibromodifluoroethane (DDE – CBr2F2);
– methyl bromide (MB – CH3Br).
Halogenated agents suppress fire by interrupting the chemical chain reaction in the combustion process, working in the fire chemically instead of physically. It is generally agreed that bromine is released from the agent as it decomposes in the fire carrying away the "free radicals" that cause the combustion and releasing more bromine to continue the "chain breaking" process. The primary advantage of halogenated agents has been the lack of clean up required after using the agent. In some environments (such as electronics, data processing, jet engines and high tech optical equipment), discharging other extinguishing agents such as dry chemical or water could cause more property damage than the fire itself.
Wet Chemical agents are solutions of water mixed with potassium acetate, potassium carbonate, potassium citrate or combinations thereof. They are specifically designed for Class K fires but they have demonstrated superior effectiveness on Class A fires when compared with plain water. Wet chemical extinguishers work on Class K fires through two methods. The solution is alkaline in nature and therefore reacts with the free fatty acids in the cooking medium to form a soapy foam on top of the burning material. This secures the vapors and cools the cooking medium as the foam drains out and converts to steam. This reaction is called saponification. In addition to saponification, the agent is discharged as a fine mist that does not submerge below the surface of the cooking medium but rather it converts to steam on
46
the surface pulling heat out of the material. On Class A fires, the wet chemical works much like water only more efficiently.
Dry chemical agents have unique properties for fire extinguishing applications. On class B fires they demonstrate superior "flame knock-down" over other available agents. For class B fires, all dry chemicals rely on particle size and decomposition to accomplish extinguishment. Theoretically, the smaller the particle size, the more effective the chemical will be as an extinguishing agent. Similar in theory to halogenated agents, the primary mechanism through which dry chemicals extinguish a class B fire is a "chain breaking" action. Application of dry chemical is considered to prevent the "free radicals" from linking up to sustain the reaction. Besides, some "smothering" action occurs when dry chemical is decomposed in a flame front and produces CO2. Cooling also takes place as the dry chemical decomposes. Water vapor is formed as a product of decomposition and the solid particles act as a barrier between the fuel surface and the radiant heat formed by the flames in the vapor space above the liquid fuel. This barrier prevents the radiant heat from returning to the fuel surface and continuing ignition.
