The process of burning depends on the set of conditions, the most important of which are:
· The composition of the combustible mixture;
· Pressure in the burning zone;
· Reaction temperature;
· Geometric system dimensions;
· The aggregate state of the fuel and oxidant, etc.
Depending on the aggregate state of the fuel and oxidant, the following types of combustion distinguish:
· Homogenic;
· Heterogeneous;
· Combustion of explosives.
Homogenic combustion occurs in gas or vapor-shaped combustible systems (Fig. 1.1) (fuel and oxidizing agent are evenly mixed with each other).
Since the partial oxygen pressure in the burning zone (equal) is close to zero - oxygen penetrates quite freely to the burning zone (it is practically in it), so the combustion rate is determined mainly by the speed of the chemical reaction that increases with increasing temperature. Such burning (or combustion of such systems) is called kinetic.
Fig.1.1. Scheme of the process of burning vapor or gases
The total combustion time is generally determined by the formula
t p \u003d t f + t x,
where T f is the time of the physical stage of the process (diffusion of 2 to the hearth through the layer); T x - time of flow of the chemical stage (reaction).
With the combustion of homogeneous systems (mixtures of vapor, gas gas) The time of the physical stage of the process is incommensurable less than the velocity of the flow of chemical reactions, therefore T p »T x - the speed is determined by the kinetics of the chemical reaction and the combustion is called kinetic.
When combustion of chemically inhomogeneous systems, the penetration time of 2 to the combustible substance through the combustion products (diffusion) is incommensurable more than the time of the flow of the chemical reaction, thus determines the overall speed of the process, i.e. T p »T F. such burning is called diffusion.
Examples of diffusion burning (Fig. 1.2) is the combustion of stone coal, coke (combustion products prevent the diffusion of oxygen into the combustion zone)
Fig.1.2. The diagram of the diffusion of oxygen into the combustion zone of the solid
(heterogeneous burning)
The oxygen concentration in the amount of air from 1 is significantly larger than its concentration near the combustion zone from 0. In the absence of a sufficient amount of 2 in the combustion zone, the chemical reaction is inhibited (and is determined by the diffusion rate).
If the duration of the chemical reaction and the physical stage of the process is commensurate, the burning flows in the intermediate region (the combustion rate affect both physical and chemical factors).
At low temperatures, the reaction rate weakly depends on the temperature (the curve slowly rises upwards). For high temperaturesaH The reaction rate is greatly increasing (i.e. the reaction rate in the kinetic region depends mainly on the temperature of the reacting substances).
The rate of oxidation reaction (combustion) in the diffusion region is determined by the diffusion rate and is very little dependent on temperature. Point A is the transition from the kinetic to the diffusion region (Fig.1.3).
The process of burning of all substances and materials, regardless of their aggregate state, occurs, as a rule, in the gas phase (liquid - evaporates, solid combustible substances allocate volatile products). But the combustion of solids has a multistage character. Under the influence of heat - the heating of the solid phase is the decomposition and release of gaseous products (destruction, volatile substances) - combustion - heat heats the surface of the solid - the receipt of the new portion of combustible gases (destruction products) - burning.
Fig. 1.3. The dependence of the speed v kinetic (1)
and diffusion (2) on temperature. Point A - Transition
from the kinetic area in diffusion
Many solid combustible substances (wood, cotton, straw, polymers) in their composition have oxygen. Therefore, for their combustion, a smaller volume of air oxygen is required. And the burning of an explosive (BB) does not need an external oxidizer at all.
Thus, the combustion of explosives is the self-proliferation of the zone of the exothermic reaction of its decomposition or the interaction of its components by transferring heat from the layer to the layer.
More than 90% of the entire energy used by humanity today is produced during the combustion process. The beginning of scientific research on the theory of combustion was made by the Russian scientist Michelson V.A.
Combustion - A complex physico-chemical process of transformation of the initial combustible substances and materials into combustion products, accompanied by intensive heat release, smoke and light radiation of the flame torch.
To occur in such a physicochemical reaction underlying any fire, it is necessary to have three required components: a combustible medium, a ignition source and an oxidizing agent.
Fuel environment - Wednesday capable of self-burn after removing the ignition source.
Ignition source - This is a thermal source with sufficient temperature, energy and duration of action for igniting.
Split combustion kinetic and diffusion.
Kinetic burning It is the combustion of pre-mixed combustible gases and oxidizing agent.
Diffusion burning - This is a combustion at which the oxidizer enters the combustion zone from the outside. Diffusion burning, in turn, is laminar (calm) and turbulent (uneven) in time and in space.
Depending on the aggregate state of the initial combustible substance distinguish homogenic, heterogeneous burning and combustion of condensed systems.
For homogenic burning Oxidizer and fuel are located in the same aggregate state. This type includes combustion of gas mixtures (natural gas, hydrogen, propane, etc. with oxidizing agent - usually oxygen).
For heterogeneous burning Source (for example, a solid or liquid fuel and gas oxidizer) are located in different aggregate states. Solids converted into dust (coal, textile, vegetable, metallic), with air mixing with air form fire-free mixtures.
Combustion condensed systemsrelated to the transition of a substance from a condensed state to gas.
Depending on the speed of the spread of the flame, combustion may be delable - at a speed of several m / s, explosive - speed of about tens and hundreds m / s and detonation - Hundreds and thousands of m / s.
For delable Or the normal propagation of burning is characteristic of heat transfer from the layer to the layer. As a result, the front of the flame moves towards the combustible mixture.
Explosivethe burning is the combustion process with rapid energy release and the formation of overpressure (more than 5 kPa).
For detonation Burning (detonation) The spread of the flame occurs at a speed close to the sound speed or exceeding it.
Detonation There is a process of chemical transformation of the oxidizer system - a reducing agent, which is a totality of a shock wave propagating at a constant speed, and following the front of the chemical transformation zone of the source substances. The chemical energy released in the detonation wave feeds the shock wave without giving it to fond.
The speed of the detonation wave is the characteristic of each specific system. For heterogeneous systems, a low-speed detonation is characterized by the specifics of the gas reaction - a solid. When detonating gas mixtures of the flame propagation rate are (1-3) ∙ 10 3 m / s and more, and the pressure at the front of the shock wave (1-5) MPa and more.
The combustion is characterized by dangerous factors called dangerous fire factors.
Under firemanit is understood as uncontrolled burning, causing material damage, harm of the life and health of citizens, the interests of society and the state.
TO dangerous fire factors (According to GOST 12.1.004-91) include:
Flame and sparks;
Increased ambient temperature;
Reduced oxygen concentration;
Toxic burning products
Thermal decomposition.
Flame - This is the visible part of the space (fiery zone), within which the oxidation, smoke formation and heat generation processes flow, and toxic gaseous products are generated and the oxygen is absorbed from the surrounding space.
The flame in a quantitative relation is mainly characterized by the following values:
Burning area ( F. 0 , m 2), - burnout speed ( Ψ , kg / s), - heat output ( Q. mountains , W) - optical smoke ( Ψd., NEPER ∙ M 2 ∙ kg -1).
Features of burning in a fire, in contrast to other types of burning, are: a tendency to spontaneous spread of fire; A relatively low degree of completeness of combustion and intensive separation of smoke containing full and incomplete oxidation products.
Three zones are formed on fires:
- George Zonei am a part of the space in which the preparation of substances to burning (heating, evaporation, decomposition) and the combustion itself.
- Heat exposure area - A part of the space adjacent to the burning zone in which the thermal impact leads to a noticeable change in the state of materials and structures, and where people are not possible without special thermal protection.
- Zone smoke- Part of the space adjacent to the combustion zone and located both in the heat-exposure zone and outside it and filled with smoke gases in concentrations, threatening life and health of people.
The combustion can be carried out in two modes: self-ignition and distribution Front fame.
Flame spread - the process of propagation of burning on the surface of the substance and materials due to thermal conductivity, heat radiation (radiation) and convection.
Evaluating dynamics of fire development Several of its main phases can be distinguished:
- 1 phase (up to 10 minutes) - the initial stage, including the transition to fire in the fire for about 1-3 minutes and the growth of the combustion zone for 5-6 minutes. At the same time, it takes place mainly linear spread of fire along combustible substances and materials, which is accompanied by plenty of smoke.
- 2 phase - The stage of the volume development of a fire, which occupies 30-40 minutes, is characterized by a rapid combustion process with a transition to volumetric burning. The process of spreading the flame is remotely due to the transfer of combustion energy to other materials. Maximum values \u200b\u200breaches a temperature (up to 800-900 o C) and burnout rates.
The stabilization of the fire at maximum values \u200b\u200boccurs on 20-25 minutes and continues for another 20-30 minutes, while burning the bulk of combustible materials.
- 3 phase- Fire attenuation phases, i.e. Traveling in the form of slow raid. After that, the fire stops.
According to ISO No. 3941-77, fires are divided into the following classes:
- class A. - fires of solids, mainly organic origin, the burning of which is accompanied by a decrease (wood, textiles, paper);
- class B.- fires of combustible fluids or melting solids;
- class S. - Fires gases;
- class D. - fires of metals and their alloys;
- e. class - Fires associated with the burning of electrical installations.
Characteristics The combustible mixture in the indicators of fire hazing are:
Combustion groups
Concentration limits of flame distribution (ignition),
Flash temperature, ignition and self-ignition temperature.
A combustion group - An indicator that is applicable to the following aggregate states of substances:
- gaza - substances, the absolute pressure of the steam of which at a temperature of 50 ° C equals or more than 300 kPa or the critical temperature of which less than 50 ° C;
- liquids - substances with a melting point (dropping) less than 50 o C;
- solids and materials with a melting point (dropping) more than 50 ° C;
- dust - Dispersed substances and materials with particle size of less than 850 μm.
Spray - The ability of a substance or material to burning. In combustion, they are divided into three groups.
Non-combustible (non-regulations) - Substances and materials that are not capable of burning in air. Non-combustible substances can be fire hazardous, (for example, oxidizing agents, as well as substances that allocate combustible products when interacting in water, air oxygen or each other).
Harmony (epoprietia) - Substances and materials that can focus in the air from the ignition source, but unable to burn independently after removing it.
Gorry(burned) - Substances and materials capable of self-turn, as well as to focus in the air from the ignition source and independently burn after it is removed.
From this group allocate easy flammable substances and materials- capable of flammable from short-term (up to 30 seconds) effects of low-energy ignition source (flame match, spark, smoldering cigarette, etc.).
Concentration limits of ignition - The minimum and maximum concentration (mass or volume fraction of fuel in the mixture with an oxidation medium), expressed in%, g / m 3 or l / m 3, below (above) which the mixture becomes incapable of spreading the flame.
Distinguish the lower and upper concentration limits of flame propagation (respectively NKPRP and VKPRP).
NKPRP (VKPRP)- minimal (maximum) fuel content in the mixture (fuel - oxidative medium), in which the flame propagation is possible on the mixture at any distance from the ignition source. For example, for a mixture of natural gas consisting mainly of methane, the concentration limit of ignition (detonation combustion) is 5-16%, and the explosion explosion is possible at a content of 21 liters of gas in 1 m 3, and the ignition is at 95 liters.
Flash temperature (t. pm) - The minimum temperature of the combustible substance, at which gases and pairs are formed on its surface, capable of flashing in the air from the ignition source, but the speed of their education is still insufficient for sustainable burning.
Depending on the numerical value T. pm liquids relate to flammable (LVZ) and furious (Gzh.). In turn LVZH are divided into three discharge in accordance with GOST 12.1.017-80.
Especially dangerous LVZH - these are combustible fluids with t. pm From -18 about C and lower in closed or from -13 ° C in open space. These include acetone, diethyl ether, isopentane, etc.
Constantly dangerous LVZ. - these are combustible fluids with t. pm From -18 ° C to +23 ° C in closed or from -13 ° C to 27 ° C in open space. These include benzene, toluene, ethyl alcohol, ethyl acetate, etc.
Dangerous at elevated damage - these are combustible fluids with t. pm From 23 ° C to 61 ° C in closed or higher than 27 ° C to 66 ° C in open space. These include turbid, White Spirit, Chlorbenzene, etc.
Flash temperature is used to determine the categories of buildings and external plants for explosion and fire hazards according to the NPB 105-03, as well as when developing measures to ensure fire and explosion safety processes.
Self-ignion temperature - SAMIA low temperature Substances at which a sharp increase in energy rate occurs.
The concept of " explosion"Used in all processes that can cause a significant increase in the pressure in the environment.
Based on GOST R 22.08-96 explosion- This is the process of energy release in a short period of time associated with an instantaneous physicochemical change in the state of the substance leading to the occurrence of a pressure jump or shock wave, accompanied by the formation of compressed gases or vapors capable of producing work.
The following types of explosions are possible on explosive objects:
- explosive processes - uncontrolled sharp release of energy in a limited space;
- volume explosion - the formation of clouds of fuel-air or other gaseous, dusty mixtures and their rapid explosive transformations;
- physical explosions - explosions of pipelines, vessels under high pressure or superheated liquid.
Emergency explosion - Emergency situation arising from a potentially dangerous object at any time in a limited space spontaneously, by coincidence or as a result of erroneous actions of personnel working on it
The causes of explosions are mainly:
Violation of technological regulations;
External mechanical impacts;
Aging equipment and installations;
Design errors;
Changing the state of the sealed medium;
Errors of the service personnel;
Fault of control and measuring, regulating and safety devices.
Homogenic and heterogeneous burning.
Based on the considered examples, depending on the aggregate co-standing of a mixture of fuel and oxidant, i.e. From the number of phases in the mixture, they bloom:
1. Homogeneous burning Gas and vapor of combustible substances in the medium of gas - different oxidizing agent. Thus, the combustion reaction proceeds in a system consisting of one phase (aggregate state).
2. Heterogeneous burning solid combustible substances in a gas-shaped oxidant environment. In this case, the reaction proceeds on the surface of the phase partition, while a homogeneous reaction goes throughout the volume.
This is the combustion of metals, graphite, i.e. Almost non-volatile materials. Many gas reactions have a homogeneous heterogeneous nature, when the possibility of leaking a homogeneous reaction is due to the origin of the heterogeneous reaction at the same time.
The combustion of all liquid and many solids from which pairs or gases (volatile substances) are distinguished in the gas phase. The solid and liquid phases play the role of reacting reservoirs.
For example, a heterogeneous coal self-burning reaction passes into the combustion phase of volatile substances. Coke residue is litherogen-but.
According to the degree of preparation of the combustible mixture, diffusion and ki-neutic burning are distinguished.
The considered types of combustion (except explosives) refer to the diffusion-zyon burning. Flame, i.e. The combustion zone of a mixture of fuel with air, to ensure stability should be constantly focused on the combustible and ki-ri society. The flow of fuel gas depends only on the speed of its supply to the burning area. The rate of flow of a flammable fluid depends on the intensity of its evaporation, i.e. From the pressure of vapors above the surface of the liquid, and, therefore, on the temperature of the liquid. Bill-Faming Temperature The smallest temperature of the liquid is called, in which the flame over its surface will not go out.
The combustion of solids differs from the combustion of gases by the presence of a decomposition and gasification stage with subsequent ignition of pyrolysis volatile products.
Pyrolysis- This is heating organic substances up to high temperatures without air access. At the same time, there is decomposition, or splitting, complex compounds to simpler (coal coal, oil cracking, su-hea distillation). Therefore, the combustion of the solid fuel in the pro-duct combustion is not concentrated only in the flame zone, but has a multistage character.
The heating of the solid phase causes the decomposition and separation of gases that ignite and burn. The heat from the torch heats the solid phase, the wave of its gasification and the process is repeated, thus maintaining the go-rhenium.
The combustion model of the solid suggests the presence of the following phases (Fig. 17):
Fig. 17. Model of burning
solid.
Warming up the solid phase. The melting substances in this zone occurs a melting. The thickness of the zone depends on the temperature of the wire-va;
Pyrolysis, or the reaction zone in the solid phase, in which gaseous combustible substances are forming;
Assumed in the gas phase, in which a mixture with oxidized-telem is formed;
Flame, or the reaction zone in the gas phase, in which the transformation of pyrolysis products into gaseous combustion products;
Gore products.
The rate of oxygen supply to the combustion zone depends on its diffusion through the combustion product.
In general, since the chemical reaction rate in the burning zone in the considered types of burning environments from the rate of receipt of the reaction components and the surface of the flame by molecular or kine-thickening diffusion, this type of burning and is called diffusion.
The structure of the diffusion combustion flame consists of three zones (Fig.18):
In the 1st zone there are gases or pairs. The burning in this zone does not occur. The temperature does not exceed 500 0 S. decomposition, pyrolysis of volatile and heating to self-ignition temperature.
Fig. 18. Flame structure.
In the 2 zone, a mixture of vapors (gases) with air oxygen and pro-coming full combustion to CO with partial restoration to carbon (little oxygen):
C n h M + O 2 → CO + CO 2 + H 2 O;
In the 3 outer zone, the second zone products are full combustion and the maximum flame temperature is observed:
2Co + O 2 \u003d 2CO 2;
The height of the flame is proportional to the diffusion ratio and the flow rate of the gases and inversely proportional to the gas density.
All types of diffusion burning are inherent in fire.
Kineticthe burning is the combustion of pre-mixed fuel, steam or dust with an oxidizer. In this case, the combustion rate depends only on the physicochemical properties of a combustible mixture (heat loss, heat capacity, turbulence, concentrations of substances, pressure, etc.). Therefore, the burning rate increases sharply. This type of burning is inherent in the explosions.
In this case, when igniting a combustible mixture at any point, the flame front moves from the combustion products into the fresh mixture. Thus, the flame at kinetic burning is most often non-stationary (Fig. 19).
Fig. 19. Flame distribution scheme in a combustible mixture: - ignition source; - Directions of the movement of the front of the flame.
Although, if you first mix the combustible gas with air and sue the burner, then a stationary flame is formed during the ignition, provided that the feed rate of the mixture will be equal to the rate of flame propagation.
If the gas supply rate is increased, then the flame takes off from the burner and can go out. And if the speed is reduced, then the flame will turn into the burner with a possible explosion.
According to the degree of combustion. completeness of burning reaction to co-neural products, burning happens full and incomplete.
So in Zone 2 (Fig.18), the burning is incomplete, because There is no ki-slier, which is partially consumed in 3 zone, and intermediate products are formed. The latter in 3 zone, where oxygen is larger, until half a combustion. The presence of soot in the smoke speaks of incomplete burning.
Another example: with a lack of oxygen, carbon burns to carbon monoxide:
If add o, the reaction goes to the end:
2 + O 2 \u003d 2SO 2.
The combustion rate depends on the nature of the movement of gases. Therefore, the laminar and turbulent burning is sick.
So, an example of laminar burning can serve as a flame candle in non-moving air. For laminar burning Layers of gases flow parallel, but not swirling.
Turbulent burning - The vortex movement of gases, in which the combustible gases are intense, and the flame front is blurred. Gra-nice between these species serves the Reynolds criterion, which characterizes the ratio between the inertia forces and the friction forces in the stream:
where: u. - gas flow rate;
n. - kinetic viscosity;
l.- Characteristic linear size.
Reynolds Number, in which the transition of a laminar along the boundary layer in a turbulent is called critical Rec, Rec Kr ~ 2320.
Turbulence increases the burning rate due to more intensive heat transfer from burning products in a fresh mixture.
All combustible (combustible) substances contain carbon and hydrogen - the main components gas-air mixtureParticipated in the combustion reaction. The flammability temperature of combustible substances and materials is different and does not exceed for most 300 ° C.
The physico-chemical bases of combustion consist in thermal decomposition of a substance or material to hydrocarbon vapors and gases, which under the influence of high temperatures enter the chemical effect with the oxidizing agent (air oxygen), turning into the combustion process in carbon dioxide (carbon dioxide), carbon monoxide (oxide Carbon), soot (carbon) and water, and this highlights heat and light radiation.
Inflammation is the process of spreading a flame on a gas-air mixture. With the expiration rate of combustible vapors and gases from the surface of a substance equal to the rate of propagation of the flame, there is a steady flame burning. If the rate of flame is greater than the speed of the vapor and gases, the gas-air mixture and flame self-effects occur, i.e. flash.
Depending on the rate of gas expiration and the rate of flame distribution, it is possible to observe:
burning on the surface of the material when the velocity of the combustible mixture from the surface of the material is equal to the speed of propagation of fire on it;
the burning with the separation from the material surface when the fuel mixture is greater than the speed of flame propagation on it.
The combustion of the gas-particle mixture is divided into diffusion or kinetic.
The kinetic combustion is the combustion of pre-mixed combustible gases and oxidizing agent (air oxygen). On the fires, this kind of burning is extremely rare. However, he is often found in technological processes: in gas welding, cutting, etc.
With diffusion burning, the oxidizer enters the combustion zone from the outside. It comes, as a rule, from the bottom of the flame due to the vacuum, which is created at its foundation. In the upper part of the flame, the heat released during the combustion process creates pressure. The main combustion reaction (oxidation) occurs on the border of the flame, since the gas mixtures expire from the substance prevent the oxidant penetration of the flame (the air is displaced). Most of the combustible mixture in the center of the flame that did not enter the oxidation reaction with oxygen, is an incomplete combustion (CO, CH4, carbon, etc.).
Diffusion burning, in turn, is laminar (calm) and turbulent (uneven in time and space). Laminar burning is characteristic of the equal speed of the fuel mixture from the surface of the material and the rate of flame spread over it. Turbulent burning occurs when the rate of
ryach mixture significantly exceeds the rate of flame propagation. In this case, the flame border becomes unstable due to the high diffusion of air into the burning area. Institution at first occurs at the top of the flame, and then moves to the base. Such combustion is found in fires with a volume of its development (see below).
The combustion of substances and materials is possible only with a certain amount of oxygen in the air. The oxygen content at which the possibility of burning various substances and materials is eliminated, is established by experimentally. So, for cardboard and cotton, self-filing occurs at 14% (about.) Oxygen, and polyester wool - at 16% (about.).
The exception of the oxidizing agent (air oxygen) is one of the fire prevention measures. Therefore, the storage of flammable and combustible liquids, calcium carbide, alkali metal, phosphorus should be carried out in a tightly closed container.
7.3.2. Sources of ignition
Prerequisite Flexion of combustible mixture are ignition sources. Sources of ignition are divided into open fire, heat of heating elements and instruments, electrical energy, energy of mechanical sparks, discharges of static electricity and lightning, energy of self-heating processes of substances and materials (self-burning), etc. Identification of ignition sources should be paid to special attention.
The characteristic parameters of ignition sources are accepted by:
The temperature of the zipper channel is 30000 ° C with a current view of 200,000 A and a period of action of about 100 μs. The energy of the spark discharge of the secondary lightning effect exceeds 250 MJ and is sufficient to ignite combustible materials with minimal ignition energy up to 0.25 J. The energy of spark discharges when driving high potential in a metal communications building reaches 100 J and more values, which is enough to ignite all flammable Materials.
Polyviniphloride isolation electric cable (wires) flammable with multiplicity of short circuit current more than 2.5.
The temperature of welding particles and nickel incandescent lamp particles reaches 2100 ° C. Temperature drops with metal cutting 1500 ° C. The arc temperature during welding and cutting reaches 4000 ° C.
The separation zone of particles with a short closure at the height of the wire layout is 10 M ranges from 5 (the probability of incoming 92%) to 9 (probability of 6%) m; at the location of the wire at a height of 3 m - from 4 (96%) to 8 m (1%); At an altitude of 1 m - from 3 (99%) to 6 m (6%).
Maximum temperature, ° C, on the flask of an electric incandescent bulb depends on the power, W: 25 W - 100 ° C; 40 W - 150 ° C; 75 W - 250 ° C; 100 W - 300 ° C; 150 W - 340 ° C; 200 W - 320 ° C; 750 W - 370 ° C.
The sparks of static electricity generated during the operation of people with moving dielectric materials reach values \u200b\u200bfrom 2.5 to 7.5 mJ.
Flame temperature (tensions) and combustion time (dence), ° C (min), some low-calorie heat sources: smoldering cigarette - 320-410 (2-2.5); Glowing cigarette - 420-460 (26-30); Burning match - 620-640 (0.33).
For sparks chimney pipe, boiler houses of steam locomotives and diesel locomotives, as well as
other machines, fires have been established that the spark of a diameter of 2 mm is fireless, if it has a temperature of about 1000 ° C, a diameter of 3 mm - 800 ° C, a diameter of 5 mm - 600 ° C.
1.3.3. Spontaneous combustion
Self-burning is inherent in many flammable substances and materials. it distinctive feature This group of materials.
Self-burning is the following types: thermal, chemical, microbiological.
The thermal self-burning is expressed in the accumulation of heat material, in the process of which the material is self-heating. The self-heating temperature of the substance or material is an indicator of its fire hazard. For most combustible materials, this indicator lies in the range from 80 to 150 ° C: paper - 100 ° C; Felt construction - 80 ° C; Dermatin - 40 ° C; Wood: Pine - 80, oak - 100, firing - 120 ° C; Cotton raw - 60 ° C.
Prolonged depression before the start of fiery burning is distinctive characteristic The processes of thermal self-burning. These processes are detected along the long and stable odor of the glowing material.
All combustible (combustible) substances contain carbon and hydrogen, are the main components of the gas-air mixture involved in the combustion reaction. The flammability temperature of combustible substances and materials is different and does not exceed for most 300 ° C.
The physico-chemical bases of combustion consist in thermal decomposition of a substance or material to hydrocarbon vapors and gases, which under the influence of high temperatures enter the chemical effect with the oxidizing agent (air oxygen), turning into the combustion process in carbon dioxide (carbon dioxide), carbon monoxide (oxide Carbon), soot (carbon) and water, and this highlights heat and light radiation.
Inflammation is the process of spreading a flame on a gas-air mixture. With the expiration rate of combustible vapors and gases from the surface of a substance equal to the rate of propagation of the flame, there is a steady flame burning. If the rate of flame is greater than the speed of the vapor and gases, the gas-air mixture and flame self-effects occur, i.e. flash.
B depending on the rate of gas expiration and the flame propagation rate on them can be observed:
The combustion of the gas-particle mixture is divided into diffusion or kinetic. The main difference is the content or absence of an oxidizing agent (air oxygen) directly in the combustible steam-air mixture.
The kinetic combustion is the combustion of pre-mixed combustible gases and oxidizing agent (air oxygen). On the fires, this kind of burning is extremely rare. However, it is often found in technological processes: in gas welding, cutting, etc.
With diffusion burning, the oxidizer enters the burning zone from the outside . It comes, as a rule, from the bottom of the flame due to the vacuum, which is created at its foundation. In the upper part of the flame, highlighting-I in the process of burning heat, creates pressure. The main reaction of oxidation combustion) occurs on the border of the flame, since the gas mixtures expiring from the substance interfere with the penetration of the oxidizing agent of the flame (oust air). Most of the combustible mixture in the center of the flame, which did not enter the oxidation reaction with oxygen, preserves incomplete combustion products (CO, CH 4, carbon, etc.).
Diffusion burning, in turn, is laminar (controversial) and turbulent (uneven in time and space). Laminar burning is characteristic of equal speeds of the fuel mixture from the surface of the material and the speed of spreading the float on it. Turbulent burning occurs when the rate of gasiness of the mixture significantly exceeds the rate of flame propagation. In this case, the flame border becomes unstable due to the large diffusion of air into the burning area. Institution at first arises the tops of the flame, and then moves to the base. Such combustion is found in fires with a volume of its development (see below).
The combustion of substances and materials is possible only with the quality of oxygen in the air. The oxygen content at which the possibility of burning various substances and materials is eliminated, is established by experimentally. Thus, for cardboard and cotton, self-filing comes ORI 14% (about.) Oxygen, and polyester wool - at 16% (about.).
The exception of the oxidizing agent (air oxygen) is one of the fire prevention measures. Therefore, the storage of flammable and combustible liquids, calcium carbide, alkali metal, phosphorus should be carried out in a tightly closed container.
1.2.2. Ignition sources.
A necessary condition for flammable mixture is ignition sources. Sources of ignition are divided into open fire, heat of heating elements and instruments, electrical energy, energy of mechanical sparks, discharges of static electricity and lightning, energy of self-heating processes of substances and materials (self-burning), etc. Identification of ignition sources should be paid to special attention.
The characteristic parameters of ignition sources are accepted by:
Lightning channel temperature - 30000 ° C at a current of 200,000 A and a period of action of about 100 μs. The energy of the spark discharge of the secondary lightning effect exceeds 250 MJ and is sufficient to ignite combustible materials with minimal ignition energy up to 0.25 J. The energy of spark discharges when driving high potential in a metal communications building reaches 100 J and more values, which is enough to ignite all flammable Materials.
Polyvinyl chloride insulation of the electrical cable (wires) flammives with a short circuit current of more than 2.5.
The temperature of welding particles and nickel incandescent lamp particles reaches 2100 ° C. Temperature drops with metal cutting 1500 ° C. The arc temperature during welding and cutting reaches 4000 ° C.
The separation zone of particles with a short closure at the height of the wire layout is 10 M ranges from 5 (the probability of incoming 92%) to 9 (probability of 6%) m; at the location of the wire at a height of 3 m - from 4 (96%) to 8 m (1%); At an altitude of 1 m - from 3 (99%) to 6 m (6%).
Maximum temperature, ° C, on the flask of an electric incandescent bulb depends on the power, W: 25 W - 100 ° C; 40 W - 150 ° C; 75 W - 250 ° C; 100 W - 300 ° C; 150 W - 340 ° C; 200 W - 320 ° C; 750 W - 370 ° C.
The sparks of static electricity generated during the operation of people with moving dielectric materials reach values \u200b\u200bfrom 2.5 to 7.5 mJ.
Flame temperature (dence) and combustion time (phenomena), "C (min), some low-calorie heat sources: smoldering cigarette - 320-410 (2-2,5); glowing cigarette - 420-460 (26-30); burning Match - 620-640 (0.33).
For sparks of furnace pipes, boiler, steam locomotive pipes and diesel locomotives, as well as other machines, fires have been established that a spark of a diameter of 2 mm is fireless, if there is a temperature of about 1000 ° C, a diameter of 3 mm - 800 ° C, a diameter of 5 mm - 600 ° C .
1.2.3. Spontaneous combustion
Self-burning is inherent in many flammable substances and materials. This is a distinctive feature of this group of materials.
Self-burning is the following types: thermal, chemical, microbiological.
The thermal self-burning is expressed in the accumulation of heat material, in the process of which the material is self-heating. The self-heating temperature of the substance or material is the indicator of its fire hazard ™. For most combustible materials, this indicator lies in the range from 80 to 150 ° C: paper - 100 ° C; Felt construction - 80 ° C; Dermatin - 40 ° C; Wood: Pine - 80, oak - 100, firing - 120 ° C; Cotton raw - 60 ° C.
A long time before the start of fiery burning is a distinctive characteristic of thermal self-burning processes. These processes are detected along the long and stable odor of the glowing material.
Chemical self-burning is immediately manifested in flame burning. For organic substances this species Self-burning occurs when contact with acids (nitric, sulfur), vegetable and technical oils. Oils and fats, in turn, are capable of self-burning in oxygen medium. Inorganic substances are capable of self-turn upon contact with water (for example, sodium hydrosulfite). Alcohols are self-turn at contact with potassium permanganate. Ammonia Selith is self-turn when contact with superphosphate, etc.
Microbiological self-burning is associated with the release of thermal energy by microorganisms in the process of life in nutrient for them, the medium (hay, peat, wood sawdust, etc.).
In practice, the combined comrades of self-burning are most often manifested: thermal and chemical.
2. Firelessness indicators.
The study of fire-hazardous properties of substances and materials applications in the production process is one of the main facilities of fire prophylaxis aimed at eliminating the combustible medium from the fire system.
In accordance with GOST 12.1.044 According to aggative substance and materials are divided into:
Gases - substances, the pressure of saturated vapor of which at a temperature of 25 ° C and a pressure of 101.3 kPa (1 atm) exceeds 101.3 kPa (1 atm).
The liquid is the same, but pressure is less than 101.3 kPa (1 atm). Liquids also include solid melting substances, melting point or ka-places of which are less than 50 ° C.
Solid - individual substances and mixtures thereof with a melting point or dropping point above 50 ° C (for example, Vazilin - 54 ° C), as well as substances that do not have a melting point (for example, wood, fabric, etc.).
Dust - dispersed (crushed) solids and materials with particle size of less than 850 μm (0.85 mm).
The nomenclature of indicators and their use for the characteristics of the fire hazardiness of substances and materials are given in Table 1.
The values \u200b\u200bof these indicators should be included in the standards and technical conditions On substances, as well as indicated in the passports of products.
Table 1
| Indicator | Gaza | Liquids | Solid | Dust |
| A combustion group | + | + | + | + |
| Flash temperature | - | + | - | - |
| Flammation temperature | - | + | + | + |
| Self-ignion temperature | + | + | + | + |
| Concentration limits of ignition | + | + | . - | + |
| Conditions of thermal self-burning | - | - | + | + |
| Oxygen index | - | - | + | - |
| Smoke formation coefficient | - | - | + | - |
| The ability to explode and burn when interacting with water, air oxygen and other substances | + | + | + | + |
| Indicator of the toxicity of products of combustion of polymeric materials and others | + |
(Sign "+" means applying, sign "-" Insusputation of the indicator)
Flash temperature (TVSP,) - Only for liquids - the smallest temperature of the condensed substance, in which under the conditions of special tests over its surface, pairs capable of flashing in the air from the ignition source are formed; Sustainable burning does not occur.
Inflammation temperature (TV,) - In addition to gases - the smallest temperature of the substance, in which the substance highlights combustible pairs and gases at such a speed that when exposed to the ignition source, ignition is observed.
The temperature of self-ignition (T SV) is the smallest ambient temperature in which the self-ignition of the substance is observed.
The conditions of thermal self-burning - only for solid and dust - experimentally identified dependence between the ambient temperature, the amount of substance (material) and the time until its self-burning.
Self-heating temperature is the lowest temperature of the substance at which the spontaneous process of its heating does not lead to a degeneration or fiery burning.
The safe temperature of the long heating of the substance is considered to be the temperature not exceeding 90% of the self-heating temperature.
The ability to explode and burn when interacting with water, air oxygen and other substances (mutual contact of substances) is a qualitative indicator characterizing a special fire danger Some substances.
The smoke coefficient is only for solid - an indicator that characterizes the optical density of smoke generated when flame burning or thermo-oxidative destruction of a certain amount of solid (material) under special tests.
Three groups of materials distinguish:
Materials with a moderate smoke-forming ability the amount of smoke when a person loses the ability to navigate less
or equals the number of combustion products, in which death poisoning is possible. Therefore, the probability of loss of visibility in the smoke is higher than the likelihood of poisoning.
Examples of smoke-forming ability building materials When draining (burning), m 3 / kg ,:::
Wood fiber (birch, aspen) - 62 (20)
Decorative paper-layered plastic - 75 (6)
Plywood grade FSF - 140 (30)
DVP, lined with plastic - 170 (25)
An indicator of toxicity of products of combustion of polymeric materials is the ratio of the amount of material to a unit of volume of closed space, in which gaseous products resulting during combustion causes the death of 50% of experimental animals.
The essence of the method is to burn the material under study in the combustion chamber and detecting the dependence of the lethal effect of gaseous combustion products from the mass of the material (in grams), referred to a unit of volume (1 m 3) of the exposure chamber.
The classification of materials is given in Table:
* For materials extremely dangerous toxicity, the mass does not exceed 25 grams to create a deadly concentration in the amount of 1 m 3 in time 5 minutes. Accordingly, during 15 minutes - to 17; 30 min - up to 13; 60 min-10 grams.
For example: Douglas pine - 21; Vinyl fabric - 19; polyvinyl chloride - 16; Polyurethane foam elastic - 18 (hard - 14) g / m 3 at the exposure time for 15 minutes.
Concentration limits of flame distribution (ignition) - besides solid.
The lower (upper) concentration limits of the flame propagation (ignition) is the minimum (maximum) combustible substance content in a homogeneous mixture with an oxidative medium, in which the flame is possible along the mixture to any distance from the ignition source.
Examples of the lower-upper concentration limits,%: acetylene - 2.2-81; hydrogen - 3.3-81.5; Natural gas - 3.8-24.6; methane - 4.8-16.7; propane - 2-9.5; Bhutan - 1.5-8.5; gasoline pairs - 0.7-6; Couples kerosene - 1-1,3.
The temperature of the tension - for solid and dust - the temperature of the substance at which a sharp increase in the speed of exothermic oxidation reactions, ending with the occurrence of the incidence occurs.
The combustibility group is the classification characteristic of the ability of any substances and materials to burning.
The combustible substances and materials are divided into three groups: non-combustible, difficult and combustible.
Non-combustible (non-aggravated) - substances and materials that are not capable of burning in the air. Non-combustible substances can be fire-free (for example, oxidizing agents or substances that distinguish products when interacting with water, air oxygen or other).
Harbor (difficult) - substances and materials capable of burning in the air when exposed to a ignition source, but not able to burn independently after removing it.
Combustible (combustible) - substances and materials capable of self-turn, as well as ignite when exposed to the source of ignition and on their own after removing it.
Combustible fluids (GZH) with TVSP<61°С в закрытом тигле или 66°С в открытом тигле относят к легковоспламеняющимся (ЛВЖ).
Particularly dangerous GG is called LVZ with TDSP< 28°С.
Gases are considered flammable in the presence of concentration limits of ignition (CPV); difficult-handed - in the absence of CPV and the presence of TSV; non-combustible - in the absence of CPV and TSV.
Liquids are considered combustible in the presence of TV; difficult-handed - in the absence of TV and the presence of TSV; non-flammable - in the absence of TV, TSV, TSP, temperature and concentration limits of flame distribution (ignition).
3. Categories of premises in the explosion and fire hazard.
According to the provisions of the fire safety standards of the NPB 105-03, categories of premises and buildings (or parts of buildings between fire walls - fire compartments) are established in the explosion and fire hazard, depending on the number and fire-free properties of the substances and materials in them, taking into account the characteristics technological processes of production placed in them.
Categorizing the room, compartments, parts of the building, the buildings of classes are subject to them depending on their affiliation to one or another class on functional fire hazard. Buildings and parts of buildings - premises or groups of premises functionally interconnected, according to the functional fire hazard, are divided into classes depending on the method of their use and on what extent the safety of people in them in the event of a fire is under threat, taking into account their age, physical condition, the possibility of staying in a state of sleep, the type of the main functional contingent and its quantity.
The premises, parts of buildings, buildings of classes F3.5 are subject to compulsory categorization of the explosion and fire hazard., F5.1., F5.2., F5.3., Moreover, industrial and storage facilities, including laboratory. And the workshops in the buildings of classes F1, F2, F3 and F4, according to the provisions of P.5.21 * SNiP 21-01-97 * refer to the F5 class.
The methodology given in the NPB 105-03 should be used in the development of departmental technological design standards relating to categorization of premises and buildings.
NPB 105-03 do not apply to premises and buildings for the production and storage of explosives (explosives), funds of initiation of explosives, buildings and structures, designed by special standards and regulations approved in the prescribed manner.
The categories of premises and buildings defined in accordance with the PNB 105-03 should be applied to establish regulatory requirements for the provision of explosive and fire safety of these premises and buildings regarding planning and development, floors, areas, placement of premises, design solutions, engineering equipment. Events to ensure the safety of people should be appointed depending on the fire-hazard properties and quantities of substances and materials in accordance with GOST 12.1.004-91 and GOST 12.3.047-98.
The categories of premises and buildings of enterprises and institutions are determined at the design stages of buildings and structures in accordance with these norms, departmental technological design standards or special lists approved in the prescribed manner.
In the explosion and fire hazardous danger of the room and the buildings are divided into categories A, B, B1-B4, G and D. The category of the explosion and fire danger of the premises and buildings are determined for the most unfavorable in relation to the fire or explosion of the period, based on the species located in the devices and premises combustible substances and materials, their quantities and fire hazard properties, features of technological processes.
The definition of fire-hazard properties of substances and materials is made on the basis of test results or calculations according to standard methods, taking into account the parameters of the state (pressure, temperature, etc.).
The use of reference data published by head research organizations in the field of fire safety or issued by the state-owned standard reference data is allowed. The use of fire hazard indicators for mixtures of substances and materials on the most dangerous component is allowed.
| K-I. | Characteristics of substances and materials, placing located (applying) indoors |
| BUT | Combustible gases (GG), flammable explosion hazardous liquids (LVZ) with a flash point of not more than 28 o C in such a quantity, which can form explosive steam, gas-air mixtures, with the ignition of which the calculated excessive pressure of the explosion explosion is developing exceeding 5 kPa . Substances and materials capable of explode and burn when interacting with water, air oxygen or each other in such a quantity that the calculated excessive explosion pressure in the room exceeds 5 kPa |
| B. | Flawing dust or fiber, explosive damage with a flash point of more than 28 o C, combustible fluids (GZH) in such a quantity that can form explosive dusty or pair-air mixtures, with the ignition of which the estimated excessive pressure of an explosion explosion is developing exceeding 5 kPa |
| B1-B4. | GJ and hard-scale fluids, solid fire hazardous combustible and hard-scale substances and materials (including dust and fibers), substances and materials capable of interacting with water, air oxygen or each other only burn, provided that the rooms in which they are available or appeal, do not belong to categories A or B |
| G. | Non-combustible substances and materials in hot, red or molten state, the processing process of which is accompanied by the release of radiant heat, sparks and flames; Gg, gzh and solids that are burned or disposed of fuel |
| D. | Non-combustible substances and materials in cold condition |
