|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
OCCUPATIONAL SAFETY AND HEALTH
Mechanisms of occurrence and development of fires. Occupational Safety and Health
Occupational Safety and Health / Legislative basis for labor protection Fire - this is uncontrolled burning, causing material damage, harm to the life and health of citizens, the interests of society and the state. Burning under human control is not a fire unless it causes damage. unauthorized fire, i.e. the start of combustion under the influence of an ignition source, must be immediately eliminated using primary fire fighting equipment (fire extinguishers or fire water supply). However, the heads of educational institutions need to remember that even trained employees are unsafe to extinguish a fire, and schoolchildren are unacceptable. Combustion - this is an exothermic oxidation reaction of a substance, accompanied by at least one of three factors: glow, flame, the appearance of smoke; smoldering - flameless combustion of the material. Spontaneous combustion - this is ignition as a result of self-initiated exothermic processes; ignition - the beginning of fiery combustion under the influence of an ignition source. Unlike ignition, ignition is accompanied only by fiery combustion. Combustion occurs in the presence of three mandatory components: a combustible substance, an oxidizing agent and an ignition source. Under the term combustible substance refers to a substance that is capable of spontaneous combustion after the external source of ignition is removed. A combustible substance can be in a solid, liquid or gaseous state. Combustible substances are most organic substances, a number of gaseous inorganic compounds and substances, many metals, etc. Gases are the greatest fire and explosion hazard. For ignition flammable liquid above its surface, a vapor-air mixture must first form. The combustion of liquids is possible only in the vapor phase; while the surface of the liquid itself remains relatively cold. Among flammable liquids, the class of the most dangerous is distinguished - flammable liquids (FLL). Flammable liquids include gasoline, acetone, benzene, toluene, some alcohols, ethers, etc. There are a number of substances (gaseous, liquid or solid) that are capable of self-ignition on contact with air without preheating (at room temperature). Such substances are called pyrophoric. These include: white phosphorus, hydrides and organometallic compounds of light metals, etc. There is also a fairly large group of substances, upon contact with water or water vapor in the air, a chemical reaction begins, proceeding with the release of a large amount of heat. Under the action of the released heat, self-ignition of combustible reaction products and starting materials occurs. This group of substances includes alkali and alkaline earth metals (lithium, sodium, potassium, calcium, strontium, uranium, etc.), hydrides, carbides, phosphides of these metals, low molecular weight organometallic compounds (triethylaluminum, triisobutylaluminum, triethylboron), etc. Combustion solid occurs according to a more complex mechanism, in several stages. When exposed to an external source, the surface layer of the solid is heated, and the release of gaseous volatile products begins from it. This process can be accompanied by either the melting of the surface layer of the solid, or its sublimation (formation of gases, bypassing the melting stage). When a certain concentration of combustible gases in the air (the lower concentration limit) is reached, they ignite and, by means of the released heat, begin to act on the surface layer themselves, causing it to melt and new portions of combustible gases and solid vapors enter the combustion zone. Let's take wood as an example. When heated to 110°C, the wood dries out and the resin evaporates slightly. Weak decomposition starts at 130°C. A more noticeable decomposition of wood (discoloration) occurs at temperatures of 150°C and above. The decomposition products formed at 150-200°C are mainly water and carbon dioxide, therefore they cannot burn. At temperatures above 200 ° C, the main component of wood, fiber, begins to decompose. The gases formed at these temperatures are combustible, since they contain significant amounts of carbon monoxide, hydrogen, hydrocarbons, and vapors of other organic substances. When the concentration of these products in the air becomes sufficient, under certain conditions they will ignite. If a combustible substance spreads during melting, it increases the source of combustion (for example, rubber, rubber, metals, etc.). In the event that the substance does not melt, oxygen gradually approaches the surface of the fuel and the process takes the form of heterogeneous combustion (for example, coke burning). The combustion process of solid substances is complex and diverse, it depends on many factors (dispersion of the solid material, its moisture content, the presence of an oxide film on its surface and its strength, the presence of impurities, etc.). More intense (often with an explosion) is the ignition of fine metal powders and dust-like combustible materials (for example, wood dust, powdered sugar). As the oxidizing agent In case of fire, oxygen is most often released, the content of which in the air is about 21%. Strong oxidizing agents are hydrogen peroxide, nitric and sulfuric acids, fluorine, bromine, chlorine and their gaseous compounds, chromic anhydride, potassium permanganate, chlorates and other compounds. When interacting with metals, which in the molten state exhibit a very high activity, water, carbon dioxide and other oxygen-containing compounds, which in common practice are considered inert, act as oxidizing agents. However, only the presence of a mixture of fuel and oxidizer is not enough to start the combustion process. Need more ignition source. In order for a chemical reaction to occur, it is necessary to have a sufficient number of active molecules, their fragments (radicals) or free atoms (which have not yet had time to unite into molecules), which have excess energy equal to the activation energy for a given system or exceeding it. The appearance of active atoms and molecules is possible when the entire system is heated, when gases come into local contact with a heated surface, when exposed to a flame, an electric discharge (spark or arc), local heating of the vessel wall as a result of friction or when a catalyst is introduced, etc. The source of ignition can also be a sudden adiabatic (without heat exchange with the environment) compression of the gas system or the impact of a shock wave on it. It has now been established that the mechanism of occurrence and development of real fires and explosions is characterized by a combined chain-thermal process. Having started in a chain way, the oxidation reaction due to its exothermicity continues to be accelerated by heat. Ultimately, the critical (limiting) conditions for the onset and development of combustion will be determined by heat release and the conditions for heat and mass transfer of the reacting system with the environment. Under the mechanism of cessation of combustion understand the system of factors leading to the end of the process (reaction) of combustion. Flame-out mechanism can be naturally conditioned when it is realized without human participation (self-liquidation of combustion, for example, in nature). At the same time, knowledge of the essence of the mechanism for stopping combustion makes it possible to purposefully use it both in the elimination of small combustion centers and in extinguishing fires. To stop burning, at least one of the following conditions must be met:
In this way, possible principles (methods) of extinguishing a fire can be:
As a rule, the extinguishing process has a combined character. So, foam has an insulating and cooling effect, powder compositions have an inhibitory, fire-retardant and dynamic effect. Fire hazard (OFP) is a factor, the impact of which can lead to human and (or) material damage. OFP are divided into primary and secondary. The primary ones are:
When assessing the primary RPP, it must be remembered that the main of them are toxic products of combustion and thermal decomposition, which are a mixture of highly toxic toxic substances heated to 300-400 ° C, paralyzing the human respiratory organs in one or two breaths. The statistics of deaths in fires for 2003 shows that 77,7% of the dead were affected by this particular OFP, and on average for previous years this figure is at the level of 80%. At the same time, it should be borne in mind that the maximum permissible elevated ambient temperature is also standardized and is 70 ° C for a person. The dynamics of the increase in the temperature of combustion products during a fire in the room at the exit from it at the height of a person has the following example settings:
Consequently, the limiting temperature of the combustion products is reached in the room in about 2 minutes, which must be taken into account when evacuating students. One of the most important OFP is a decrease in the oxygen content in the gaseous environment of a burning room. In clean air, its content reaches 27%. In a burning building, due to intensive combustion, the oxygen content is significantly reduced; its dangerous value is within 17%. This must be taken into account when using filtering respiratory protective equipment intended for use by duty services and other persons. That is, there is a possibility that a person in a fire, protected, for example, by a self-rescuer, may die not from toxic combustion products, but from a lack of oxygen in the gaseous environment of a burning building. Fire extinguishing - a complex professional task. It can only be solved by trained and well-equipped fire departments, who always use insulating respiratory protection. Secondary OFPs include:
The first phase (up to 10 minutes) is the initial stage, including the transition of ignition into a fire in about 1-3 minutes and the growth of the combustion zone within 5-6 minutes. In this case, there is a predominantly linear spread of fire along combustible substances and materials, which is accompanied by abundant smoke emission. In this phase, it is very important to ensure the isolation of the room from the ingress of outside air, since in some cases, self-extinguishing of the fire occurs in the sealed room. The second phase is the stage of volumetric development of the fire, which takes 30-40 minutes in time. It is characterized by a rapid combustion process with a transition to volumetric combustion; the process of flame propagation occurs remotely due to the transfer of combustion energy to other materials. After 15-20 minutes, the glazing breaks down, the oxygen supply increases sharply, the maximum values are reached by the temperature (up to 800-900°C) and the burnout rate. Stabilization of a fire at its maximum values occurs at 20-25 minutes and continues for another 20-30 minutes. In this case, the bulk of combustible materials burn out. The third phase is the stage of fire attenuation, i.e., afterburning in the form of slow smoldering, after which the fire stops. An analysis of the dynamics of the development of a fire makes it possible to make the following findings. Technical fire safety systems (alarms and automatic fire extinguishing) should work before reaching the maximum intensity of combustion, and better - in the initial stage of the fire. This will allow the head of the educational institution to have a margin of time to organize measures to protect people. Fire departments arrive, as a rule, 10-15 minutes after the call, i.e. 15-20 minutes after the fire starts, when it takes on a volumetric shape and maximum intensity. Authors: Volkhin S.N., Petrova S.P., Petrov V.P.
▪ Help with fainting, heat and sunstroke ▪ Factors affecting the outcome of electric shock ▪ Distribution of soap and disinfectants
Machine for thinning flowers in gardens
02.05.2024 Advanced Infrared Microscope
02.05.2024 Air trap for insects
01.05.2024
▪ Electric turbines from Formula 1 for road cars ▪ PNY CS900 960 GB Solid State Drive ▪ Scientists have implanted a chip in the brain ▪ Touch keyboard with Bluetooth ▪ Fifth type of Higgs boson decay into other elementary particles discovered
▪ site section Parameters, analogues, marking of radio components. Article selection ▪ Apollo article. Popular expression ▪ article Manny ash. Legends, cultivation, methods of application ▪ article Invisible ink. Focus Secret
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
We recommend interesting articles Section 
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page