Businesses that process or use flammable liquids are a major fire hazard. This is explained by the fact that combustible liquids ignite easily, burn more intensively, form explosive vapor-air mixtures and are difficult to extinguish with water.
Burning liquids occurs only in the vapor phase. The rate of evaporation and the amount of vapor of a liquid depend on its nature and temperature. The amount of saturated vapor above the surface of a liquid depends on its temperature and atmospheric pressure. In the saturation state, the number of evaporating molecules is equal to the number of condensing ones, and the vapor concentration remains constant. Combustion of vapor-air mixtures is possible only in a certain range of concentrations, i.e. they are characterized by the concentration limits of flame propagation (NKPRP and VKPRP).
Lower (upper) concentration limits of flame propagation- the minimum (maximum) content of a combustible substance in a homogeneous mixture with an oxidizing environment, at which flame propagation through the mixture is possible at any distance from the ignition source.
Concentration limits can be expressed in terms of temperature (at atmospheric pressure). The liquid temperature values at which the concentration of saturated vapors in the air above the liquid is equal to the concentration limits of flame propagation are called the temperature limits of flame propagation (ignition) (lower and upper, respectively - NTPRP and VTPRP).
Thus, the process of ignition and combustion of liquids can be represented as follows. For ignition, it is necessary that the liquid be heated to a certain temperature (not less than the lower temperature limit of flame propagation). Once ignited, the evaporation rate must be sufficient to maintain constant combustion. These features of the combustion of liquids are characterized by flash and ignition temperatures.
In accordance with GOST 12.1.044 " Fire and explosion hazard of substances and materials", the flash point is the lowest temperature of a condensed substance at which, under the conditions of special tests, vapors are formed above its surface that can flash in air from an ignition source; stable combustion does not occur in this case. The flash point corresponds to the lower temperature limit ignition.
flash point used to assess the flammability of a liquid, as well as in the development of measures to ensure fire and explosion safety technological processes.
Flash point called the lowest value of the liquid temperature at which its evaporation rate is such that after ignition external source self-ignition occurs.
Depending on the numerical value of the flash point, liquids are divided into flammable (FL) and combustible (FL).
Flammable liquids include liquids with a flash point of not more than 61 ° C in a closed crucible or 66 ° C in an open crucible.
For flammable liquids, the ignition temperature is usually 1-5 ° C higher than the flash point, and for combustible liquids, this difference can reach 30-35 ° C.
In accordance with GOST 12.1.017-80, depending on the flash point, flammable liquids are divided into three categories.
Especially dangerous flammable liquids- with a flash point from -18 ° C and below in a closed crucible or from -13 ° C and below in an open crucible. Especially dangerous flammable liquids include acetone, diethyl alcohol, isopentane, etc.
Permanently dangerous flammable liquids- these are flammable liquids with a flash point from -18 o C to +23 o C in a closed crucible or from -13 o C to +27 o C in an open crucible. These include benzyl, toluene, ethyl alcohol, ethyl acetate, etc.
Hazardous at elevated temperatures flammable liquids- these are flammable liquids with a flash point from 23 ° C to 61 ° C in a closed crucible. These include chlorobenzene, turpentine, white spirit, etc.
Flash point of liquids belonging to the same class (liquid hydrocarbons, alcohols, etc.), regularly changes in the homologous series, increasing with increasing molecular weight, boiling point and density. The flash point is determined experimentally and by calculation.
Experimentally, the flash point is determined in devices of a closed and open type:
- in a closed crucible for Martens-Pensky device according to the methodology set out in GOST 12.1.044-89 - for petroleum products;
- in an open crucible on the device TV VNIIPO according to the method given in GOST 12.1.044-89 - for chemical organic products and on the Brenken device according to the method described in the same GOST - for petroleum products and oils.
To create NKPP vapor above the surface of a liquid, it is sufficient to heat to a temperature equal to NTPRP, not the entire mass of the liquid, but only its surface layer.
In the presence of IS, such a mixture will be capable of ignition. In practice, the concepts of flash point and ignition point are most often used.
Under flash point understand the lowest temperature of a liquid at which, under the conditions of special tests, a concentration of liquid vapor is formed above its surface, capable of igniting from IZ, but the rate of their formation is insufficient for subsequent combustion. Thus, both at the flash point and at the lower temperature limit of ignition above the surface of the liquid, a lower concentration limit of ignition is formed, however, in the latter case, HKPRP is created by saturated vapors. Therefore, the flash point is always slightly higher than NTPRP. Although at the flash point there is a short-term ignition of vapors in the air, which is not capable of turning into a stable combustion of a liquid, nevertheless, under certain conditions, an outbreak of liquid vapors can be a source of fire.
The flash point is taken as the basis for the classification of liquids into flammable (flammable liquids) and combustible liquids (FL). Flammable liquids include liquids with a flash point in a closed crucible of 61 0 C or in an open crucible of 65 0 C and below, GZH - with a flash point in a closed crucible of more than 61 0 C or in an open crucible of 65 0 C.
I category - especially dangerous flammable liquids, these include flammable liquids with a flash point of -18 0 C and below in a closed crucible or from -13 0 C and below in an open crucible;
II category - permanently dangerous flammable liquids, these include flammable liquids with a flash point above -18 0 C to 23 0 C in a closed crucible or from -13 to 27 0 C in an open crucible;
Category III - flammable liquids, dangerous at elevated air temperatures, these include flammable liquids with a flash point of 23 to 61 0 C in a closed crucible or from 27 to 66 0 C in an open crucible.
Depending on the flash point, safe methods for storing, transporting and using liquids for various purposes are established. The flash point of liquids belonging to the same class naturally changes with changes in the physical properties of the members of the homologous series (Table 4.1).
Table 4.1.
Physical properties of alcohols
|
Molecular |
Density, |
Temperature, K |
||
|
Methyl CH 3 OH | ||||
|
Ethyl C 2 H 5 OH | ||||
|
n-propyl C 3 H 7 OH | ||||
|
n-Butyl C 4 H 9 OH | ||||
|
n-Amylic C 5 H 11 OH | ||||
The flash point increases with increasing molecular weight, boiling point and density. These patterns in the homologous series suggest that the flash point is related to physical properties substances and is itself a physical parameter. It should be noted that the pattern of changes in the flash point in the homologous series cannot be extended to liquids belonging to different classes of organic compounds.
When mixing flammable liquids with water or carbon tetrachloride, the pressure of flammable vapors at that the same temperature decreases, which leads to an increase in the flash point. Can be diluted with fuel liquid to such an extent that the resulting mixture will not have a flash point (see table. 4.2).
Fire extinguishing practice shows that the combustion of liquids that are highly soluble in water stops when the concentration of the combustible liquid reaches 10-25%.
Table 4.2.
For binary mixtures of combustible liquids that are highly soluble in each other, the flash point is between the flash points of pure liquids and approaches the flash point of one of them, depending on the composition of the mixture.
WITH rise in temperature of the liquid evaporation rate increases and at a certain temperature reaches such a value that, once ignited, the mixture continues to burn after the ignition source is removed. This liquid temperature is called flash point. For flammable liquids, it differs by 1-5 0 С from the flash point, and for GZh - by 30-35 0 С. At the ignition temperature of liquids, a constant (stationary) combustion process is established.
There is a correlation between the flash point in a closed crucible and the lower ignition temperature limit, which is described by the formula:
T sun - T n.p. \u003d 0.125T sun + 2. (4.4)
This relation is valid for T sun< 433 К (160 0 С).
The significant dependence of the flash and ignition temperatures on the experimental conditions causes certain difficulties in creating a calculation method for estimating their values. One of the most common of them is the semi-empirical method proposed by V. I. Blinov:
,
(4.5)
where T sun - flash point, (ignition), K;
p sun - partial pressure of saturated vapor of the liquid at the flash point (ignition), Pa;
D 0 - diffusion coefficient of liquid vapor, m 2 / s;
n is the number of oxygen molecules required for the complete oxidation of one fuel molecule;
Lecture 13
COMBUSTION OF LIQUIDS
The consumption of liquid fuels in the world economy is currently reaching gigantic proportions and continues to grow steadily. This leads to the constant development of the oil and oil refining industry.
Liquid fuel has now become the most important strategic raw material, and this circumstance leads to the need to create huge reserves of it. Ensuring fire safety during the extraction, transportation, processing and storage of liquid fuels is the most important task of the fire department.
Liquid ignition
The most important property of a liquid is its ability to evaporate. As a result of thermal motion, some of the molecules, overcoming the forces of the surface tension of the liquid, pass into the gas zone, forming a vapor-air mixture above the surface of the flammable liquid, liquid liquid. Due to the Brownian motion in the gas zone, the reverse process, condensation, also takes place. If the volume above the liquid is closed, then at any temperature of the liquid, dynamic balance between evaporation and condensation processes.
Thus, above the surface (mirror) of the liquid there is always a vapor-air mixture, which in the state of equilibrium is characterized by the pressure of saturated vapors of the liquid or their concentration. With increasing temperature, the saturated vapor pressure increases according to the Claiperon-Clasius equation:
Where rnp - saturated steam pressure, Pa;
Qsp - heat of vaporization - the amount of heat required to convert a unit mass of liquid into a vapor state, kJ / mol;
T- liquid temperature, K.
From (7.1) it follows that with increasing liquid temperature, the pressure of saturated vapors (or their concentration) increases exponentially (Fig. 7.1). Thus, for any liquid there is always such a temperature range at which the concentration of saturated vapors above the mirror will be in the ignition region, i.e. HKJIB<ф п< ВКПВ
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where Tvs - flash point (ignition), K;
Pvs - partial pressure of saturated vapor of the liquid at the flash point (ignition), Pa;
P- the number of oxygen molecules required for the complete oxidation of one fuel molecule;
IN- constant of the definition method.
Spread of flame over the surface of a liquid.
Analysis of the influence of combustion conditions on the speed of flame propagation
The property of the flame to spontaneous propagation takes place not only in the case of combustion of mixtures of combustible gases With oxidant, but also in the combustion of liquids and solids. Under local exposure to a heat source, for example, an open flame, the liquid will warm up, the evaporation rate will increase, and when the surface of the liquid reaches the ignition temperature, the vapor-air mixture will ignite at the point of impact of the source and a stable flame will be established, which will then spread at a certain speed over the surface of the cold liquid.
What is the driving force behind the propagation of the combustion process and what is its mechanism?
Flame propagation over the liquid surface proceeds as a result of heat transfer by radiation, convection and molecular heat conduction from the flame zone to the surface of the liquid mirror.
The main role in this, according to modern concepts, is played by heat radiation from the flame. The flame, having a high temperature (more than 1000 ° C), is capable, as you know, of radiating thermal energy. According to the Stefan-Boltzmann law, the intensity of the radiant heat flux given off by a heated body is determined by the relation:Where ε - degree of blackness,
σ - Stefan-Boltzmann constant, = 2079 ´ 10-7 kJ/(m2 h K4)
T f, T f- t of the flame and liquid surface, K
This heat is used for evaporation q1) and heating ( q11) of the liquid in depth.
Qf = q1 + q11 = r´ r´ W+r´ U´ (Tf - T0)´ c, Where
r- heat of evaporation, kJ/g
r- density, g/cm3
W- linear burnout rate, mm/h
U- speed of heating in depth, mm/h
T0- initial t-ra liquid, K
With- specific heat capacity of the liquid, j/(g K)
The maximum temperature of a liquid is equal to its boiling point.
In the steady-state combustion process, an equilibrium is observed between the evaporation rate and the burnout rate.
The upper layer of the liquid is heated to a higher temperature than the lower ones. The temperature near the walls is higher than in the middle of the tank.
Thus, the rate of flame propagation through the liquid, i.e., the path traveled by the flame per unit time, is determined by the rate of heating of the liquid surface under the influence of radiant heat flux from the flame, i.e., the rate of formation of a combustible vapor-air mixture above the liquid mirror.
Water sharply lowers the boiling point of oil, fuel oil. During the combustion of oil containing water, water boils up, which leads to the overflow of the burning liquid over the side of the tank (the so-called boiling of the burning liquid.
Above the surface of an open reservoir, the vapor concentration will be different in height: at the surface it will be maximum and correspond to the concentration of saturated vapor at a given temperature, and as it rises above the surface, it gradually decreases due to convective and molecular entrainment (Fig. 7.3).
Thus, above the surface of the liquid mirror in an open tank at any initial temperature of the liquid is higher than Tst, there will be an area in which the concentration of vapors in the air will be stoichiometric. At liquid temperature T2 this concentration will be at a height Well from the surface of the liquid, and at a temperature T3 greater than T2, at a distance H ^ 3st. At a temperature close to the flash point of the liquid TB, the propagation of the flame over the surface of the liquid will be equal to the speed of its propagation through the mixture of vapors in air, at the LECF, i.e., 3-4 cm / s. In this case, the flame front will be located near the surface of the liquid. With a further increase in the initial temperature, the rate of flame propagation through the liquid will increase similarly to the change in the normal rate of flame propagation through the vapor-air mixture with an increase in its concentration.
Lecture 14
Burnout rate of liquids, influencing factors.
At a certain temperature, above tvs, once the ignited liquid continues to burn after the ignition source is removed. This minimum temperature is called the ignition temperature (tair). For flammable liquids, it is higher than tvs by 1-5 °C, for GZH - by 30-35 °C.
Linear burnout rate - the height of the liquid column that burns out per unit time:
Mass burnout rate - the mass of liquid that burns out per unit time per unit surface area:
There is a relationship between the linear and mass burning rates:(You should follow the dimensions of the quantities and, if necessary, enter a correction factor).
Depth heating of the liquid. The heating of the surface of the liquid by the radiant flow from the flame is accompanied by the transfer of heat deep into it. This heat transfer is carried out mainly by thermal conductivity and laminar convection due to the movement of heated and cold fluid layers. Heating of the liquid by thermal conductivity is carried out to a small depth (2-5 cm) and can be described by an equation of the form
Where Tx- temperature of the liquid layer at depth X, TO;
Tk- surface temperature (boiling point), K; To- coefficient of proportionality, m- TO
This type of temperature field is called the temperature distribution of the first kind.
Laminar convection occurs as a result of different liquid temperatures at the walls of the tank and in its center, as well as due to fractional dispersal in the upper layer during combustion of mixtures. Additional heat transfer from the heated walls of the reservoir to the liquid leads to heating of its layers near the walls to a higher temperature than in the center. The liquid that is more heated near the walls (or even vapor bubbles if it is overheated near the walls above the boiling point) rises, which contributes to intensive mixing and rapid heating of the liquid layer to a great depth. A so-called homothermal layer is formed, i.e., a layer with an almost constant temperature, the thickness of which increases with the time of combustion. Such a temperature field is called a temperature distribution of the second kind (Fig. 7.7). The formation of a homothermal layer is also possible as a result of fractional distillation of near-surface layers of mixtures of liquids having different boiling points. As such liquids burn out, the near-surface layer is enriched with denser high-boiling fractions, which sink down, thereby contributing to the convective heating of the liquid.
The decisive influence of liquid overheating near the walls of the tank on the formation of a homothermal layer is confirmed by the following experimental data. During the combustion of gasoline in a tank with a diameter of 2.64 mm without cooling the walls, it led to a fairly rapid formation of a homothermal layer. With intensive cooling of the walls, the heating of the liquid to a depth was carried out mainly by thermal conductivity, and during the entire combustion time, the temperature distribution of the first kind took place. It has been established that the higher the boiling point of a liquid (diesel fuel, transformer oil), the more difficult it is to form a homothermal layer. When they burn, the temperature of the tank walls rarely exceeds the boiling point. However, when burning wet, high-boiling oil products, the probability of the formation of a homothermal layer is also high. When the tank walls are heated to 100°C and higher, water vapor bubbles are formed, which, rushing up, cause intensive mixing of the entire liquid and rapid heating in depth. The possibility of the formation of a sufficiently thick homothermal layer during the combustion of wet petroleum products is fraught with the phenomena of boiling and liquid ejection.
Based on the above concepts of the mechanism of liquid burnout, let us analyze the influence of some factors on the mass velocity.
The rate of burnout depends on: type of liquid, temperature, tank diameter, liquid level, wind speed.
For small diameter burners the combustion rate is relatively high. With an increase in diameter, the velocity first decreases due to heating from the walls, then increases, since laminar combustion turns into turbulent and remains constant at diameters of ³ 2 m.
With turbulent combustion, the completeness of combustion is lower (soot appears), the heat flux from the flame increases, vapors are removed faster, and the rate of evaporation increases.
When the liquid level drops the processes of heat and mass transfer become more difficult (the outflow of combustion products, the inflow of an oxidizer, the flame moves away from the surface of the liquid), so the burning rate decreases and at a certain distance of the liquid from the upper side of the tank (the critical height of self-extinguishing), combustion becomes impossible. The critical height of self-extinguishing at Æ = 23 m is 1 km (real height of the reservoir = 12 m).
Estimating the share of heat from the total heat release during the combustion of the liquid, which is spent on its preparation, it follows that less than 2% of the total heat release during the combustion of the liquid is spent on supplying its vapors to the combustion zone. At the moment the burnout process is established, the temperature of the liquid surface rises sharply from the ignition temperature to the boiling point, which subsequently remains unchanged as the burnout proceeds. However, this is true only for individual liquids. In the process of burning a mixture of liquids with different boiling points (gasoline, oil, etc.), their fractional distillation occurs, as it were. First, the light-boiling fractions are released, then all the higher-boiling fractions. This process is accompanied by a gradual (quasi-stationary) increase in temperature on the surface of the liquid. Wet fuel can be represented as a mixture of two liquids (fuel + water), during the combustion of which their fractional dispersal occurs. If the boiling point of the combustible liquid is less than the boiling point of water (100°C), then the fuel burns out preferentially, the mixture is enriched with water, the burnout rate decreases, and, finally, combustion stops. If the boiling point of the liquid is more than 100 ° C, on the contrary, at first moisture evaporates predominantly, its concentration decreases: the burnout rate of the liquid increases, up to the burning rate of the pure product (Fig. 7.11).
Influence of wind speed. As a rule, with an increase in wind speed, the rate of burnout of the liquid increases. The wind intensifies the process of mixing the fuel with the oxidizer, raising the temperature of the flame and bringing the flame closer to the combustion surface.
All this increases the intensity of the heat flux supplied for heating and evaporation of the liquid, therefore, leads to an increase in the burnout rate. At higher wind speeds, the flame can break off, which will lead to the cessation of combustion. So, for example, during the combustion of tractor kerosene in a tank with a diameter of 3 "M, a flame failure occurred when the wind speed reached 22 m-s-1.
Influence of oxygen concentration in the atmosphere. Most liquids are not capable of burning in an atmosphere with an oxygen content of less than 15%. With an increase in oxygen concentration above this limit, the burnout rate increases (Fig. 7.12). In an atmosphere enriched with oxygen, the combustion of the liquid proceeds with the release of a large amount of soot in the flame, and intense boiling of the liquid phase is observed. For multicomponent liquids (gasoline, kerosene, etc.), the surface temperature increases with an increase in the oxygen content in the environment (Fig. 7.13).
An increase in the burn-out rate and temperature of the liquid surface with an increase in the oxygen concentration in the atmosphere is due to an increase in the emissivity of the flame as a result of an increase in the combustion temperature and a high content of soot in it.
Zones and classes of fires.
Substances
Features of combustion of solid and liquid combustible materials and
Lecture plan
State higher education institution
"NATIONAL MINING UNIVERSITY"
Department of AOT
Lecture #4
Assoc. Alekseenko S.A.
Part 1. Fire safety
Topic №: Fire and explosion hazardous properties of substances and materials.
(for students of specialty 7.0903010 “Development of deposits and extraction of minerals”, specialization: 7.090301.05 “Labor protection in mining”).
Dnepropetrovsk
1. The essence of the combustion process.
1. Demidov P.G. Combustion and properties of combustible substances. M.: Publishing House of the Ministry of Public Utilities of the RSFSR, 1962.-264p.
2. Fundamentals of the funeral practice: Podruchnik./ K.N. Tkachuk, M.O. Khalimovsky, V.V. Zatsarny, D.V. Zerkalov, R.V. Sabarno, O.I. Polukarov, V.S. Koziakov, L.O. Mityuk. For red. K.N. Tkachuk and M.O. Khalimovsky. - K .: Osnova, 2003 - 472 p. (Pozhezhna bezpeka - S. 394-461).
3. Bulgakov Yu.F. Putting out fires in coal mines. - Donetsk: NIIGD, 2001. - 280 p.
4. Aleksandrov S.M., Bulgakov Yu.F., Yaylo V.V. Protection of practice at the coal industry: A textbook for students of primary specialties of all educational institutions / Pid zag. ed. Yu.F. Bulgakov. - Donetsk: RIA DonNTU, 2004. - P.3-17.
5. Rozhkov A.P. Pozhezhna Bezpeka: A Guidebook for Students of the Highest Mortgages of Education in Ukraine. - Kiev: Pozhіnformtehnіka, 1999.- 256 p.: il.
6. Industry standard OST 78.2-73. Combustion and fire hazard of substances. Terminology.
7. GOST 12.1 004-91. SSBT. Fire safety. General requirements.
8. GOST 12.1.010-76. SSBT. Explosion safety. General requirements
9. GOST 12.1.044-89. SSBT. Fire and explosion hazard of substances and materials. Nomenclature of indicators and methods for their determination
1. The essence of the combustion process.
For a better understanding of the conditions for creating a combustible environment, sources of ignition, assessment and prevention of explosion and fire hazard, as well as the choice of effective methods and means of a fire safety system, it is necessary to have an idea of the nature of the combustion process, its forms and types.
One of the first chemical phenomena that mankind met at the dawn of its existence was combustion.
For the first time, the correct idea of the combustion process was expressed by the Russian scientist M.V. Lomonosov (1711-1765), who laid the foundations of science and established a number of the most important laws of modern chemistry and physics.
burning is called an exothermic oxidation reaction of substances, which is accompanied by the release of smoke and the appearance of a flame or the emission of light.
In other words combustion - This is a fast-flowing chemical transformation of substances with the release of a large amount of heat and is accompanied by a bright flame. It may be the result of oxidation, i.e. the combination of a combustible substance with an oxidizing agent (oxygen).
This general definition shows that it can be not only a combination reaction, but also decomposition.
For the occurrence of combustion, the simultaneous presence of three factors is necessary: 1) a combustible substance; 2) oxidizing agent; 3) the initial thermal impulse (ignition source) to communicate the combustible mixture of hot energy. In this case, the combustible substance and the oxidizing agent must be in the required one-to-one ratio and thus create a combustible mixture, and the ignition source must have the appropriate energy and temperature sufficient to start the reaction. A combustible mixture is defined by the term "combustible medium". This is an environment that is able to burn on its own after the ignition source is removed. Combustible mixtures, depending on the ratio of combustible substance and oxidizer, are divided into poor And rich . IN the poor mixtures, there is an excess of an oxidizing agent, and in rich - combustible substance. For complete combustion of substances and materials in the air, the presence of a sufficient amount of oxygen is necessary to ensure the complete conversion of the substance into its saturated oxides. With insufficient air, only part of the combustible substance is oxidized. The residue decomposes with the release of a large amount of smoke. At the same time, toxic substances are also formed, among which the most common product of incomplete combustion is carbon monoxide. (CO), which can lead to poisoning of people. During fires, as a rule, combustion occurs with a lack of oxygen, which seriously complicates fire extinguishing due to reduced visibility or the presence of toxic substances in the air.
It should be noted that the combustion of certain substances (acetylene, ethylene oxide, etc.), which are capable of releasing a large amount of heat during decomposition, is possible even in the absence of air.
2. Types, varieties and forms of combustion.
Burning may be homogeneous And heterogeneous .
At homogeneous combustion, substances that enter into an oxidation reaction have the same state of aggregation. If the initial substances are in different states of aggregation and there is a clear phase separation boundary in the combustible system, then such combustion is called heterogeneous.
Fires are predominantly characterized by heterogeneous combustion.
In all cases, combustion is characterized by three stages: occurrence , spreading And damping flame. The most common properties of combustion is the ability ( middle) flame to move throughout the combustible mixture by transferring heat or diffusion of active parts from the combustion zone into the fresh mixture. Hence the flame propagation mechanism arises, respectively thermal And diffusion . Combustion, as a rule, proceeds by a combined heat-diffusion mechanism.
According to the speed of flame propagation, combustion is divided into:
– deflagration or normal- during this combustion, the flame speed is within the limits of several meters per second (up to 10 m / s);
– explosive - extremely fast chemical transformation, which is accompanied by the release of energy and the formation of compressed gases capable of performing mechanical work (hundreds of m / s);
– detonation – it's burning propagates at supersonic speeds, reaching thousands of meters per second (up to 5000 m/s).
The explosion is also accompanied by the release of heat and the emission of light. At the same time, the explosion of some substances is a decomposition reaction, for example:
2NCl 3 \u003d 3Cl 2 + N 2 (1)
Explosion An extremely rapid chemical (explosive) transformation of a substance is called, which is accompanied by the release of energy and the formation of compressed gases capable of producing mechanical work.
An explosion differs from burning by a high rate of fire propagation. So, for example, the speed of flame propagation in an explosive mixture located in a closed pipe is (2000 - 3000 m / s).
Combustion of a mixture at this rate is called detonation. The occurrence of detonation is explained by compression, heating and movement of the unburned mixture ahead of the flame front, which leads to an acceleration of flame propagation and the appearance of a shock wave in the mixture. The air shock waves formed during the explosion of the gas-air mixture have a large supply of energy and propagate over considerable distances. While moving, they destroy structures and can cause accidents.
Combustion of substances can proceed not only when they are combined with atmospheric oxygen (as is commonly believed), but also when combined with other substances. It is known that the combustion of many substances can occur in an environment of chlorine, sulfur, bromine vapor, etc. The composition, state of aggregation and other properties of combustible substances (HS) are different, however, the main phenomena that occur when combustion occurs are the same.
Combustible substances may be solid, liquid And gaseous .
Solid combustibles, depending on the composition and structure, behave differently when heated. Some of them, such as rubber, sulfur, stearin, melt and evaporate. Others, for example, wood, paper, coal, peat, when heated, decompose with the formation of gaseous products and a solid residue - coal. Third substances do not melt or decompose when heated. These include anthracite, charcoal and coke.
Liquid combustibles when heated, they evaporate, and some may oxidize.
Thus, most combustible substances, regardless of their initial state of aggregation, when heated, turn into gaseous products . In contact with air, they form combustible mixtures. Combustible mixtures can also be formed as a result of spraying solid and liquid substances. When a substance has formed a combustible mixture with air, it is considered prepared for combustion. This state of the substance represents a great fire hazard. It is determined by the fact that a powerful and long-acting source of ignition is not required to ignite the resulting mixture; the mixture quickly ignites even from a spark.
The readiness of the mixture for ignition is determined by the content (concentration) of vapors, dust or gaseous products in it.
Varieties and forms of combustion.
Combustion is characterized by a variety of varieties, forms and features. There are the following varieties and forms of combustion: flash; ignition; fire; self-ignition and spontaneous combustion.
Flash- this is a rapid (instantaneous) ignition of a combustible mixture under the action of a thermal pulse without the formation of compressed gases, which does not turn into stable combustion.
Ignition - this is a relatively calm and prolonged combustion of vapors and gases of combustible liquids, which occurs under the influence of an ignition source. Ignition is an ignition accompanied by the appearance of a flame.
fire- this is combustion that starts without the influence (action) of the ignition source (thermal impulse).
Self-ignition- this is spontaneous combustion, which is accompanied by the appearance of a flame and the process of ignition of solid, liquid and gaseous substances begins, heated by an external source of heat without contact with an open fire to a certain temperature.
Spontaneous combustion- this is self-ignition, which is accompanied by the appearance of a flame. This is the process of spontaneous combustion of solid and bulk materials, which occurs under the influence of their oxidation without heat supply from external sources (coal, sulfide ores, wood, peat). Spontaneous combustion occurs as a result of low-temperature oxidation and self-heating, due to a sufficient flow of air to the combustible substance for oxidation and insufficient air to carry away the resulting heat.
Smoldering- combustion without light emission, which is usually recognized by the appearance of smoke.
Depending on the state of aggregation and the characteristics of combustion of various combustible substances and materials, fires according to GOST 27331-87 are divided into the corresponding classes and subclasses:
class A - combustion of solids, which is accompanied (subclass A1) or not accompanied (subclass A2) by smoldering;
class B - combustion of liquid substances that do not dissolve (subclass B1) and dissolve (subclass B2) in water;
class C - combustion of gases;
class D - combustion of light metals, with the exception of alkaline (subclass D1), alkaline (subclass D2), as well as metal-containing compounds (subclass D3);
class E - burning of electrical installations under voltage.
3. Indicators of fire and explosion hazard of substances and materials. Methods for their determination.
The fire and explosion hazard of substances and materials is a set of properties that characterize their tendency to the occurrence and spread of combustion, the features of combustion and the ability to succumb to combustion. According to these indicators, GOST 12.1.044-89 distinguishes non-combustible, slow-burning and combustible materials and substances.
Non-combustible (non-combustible) - substances and materials that are incapable of burning or charring in the air under the influence of fire or high temperature. These are materials of mineral origin and materials made on their basis - red brick, silicate brick, concrete, asbestos, mineral wool, asbestos cement and other materials, as well as most metals. At the same time, non-combustible substances can be flammable, for example, substances that release combustible products when interacting with water. A sufficient criterion for referring to this group is the inability of the material to burn at an ambient temperature of 900 ° C, this group includes natural and artificial organic materials and metals used in construction.
Slow-burning (slow-burning) substances and materials that are capable of igniting, smoldering or charring in the air from a source of ignition, but not capable of independently burning or charring after its removal. These include materials that contain combustible and non-combustible components, such as wood when deeply impregnated with antipyrogens (beshefit); fibrolite; felt impregnated with clay solution, some polymers and other materials.
Combustible (combustible) - substances and materials that are capable of self-combustion (self-ignition), as well as ignite, smolder or char from a source of ignition or independently burn after its removal.
In turn, in the group of combustible substances and materials, flammable substances and materials are distinguished - these are substances and materials that can ignite from a short-term (up to 30 s) action of a low-energy ignition source. From the point of view of fire safety, indicators of the fire and explosion hazardous properties of combustible substances and materials are of decisive importance. GOST 12.1.044-89 provides for over 20 such indicators. The list of these indicators necessary and sufficient for assessing the fire and explosion hazard of a particular object depends on the state of aggregation of the substance, the type of combustion (homogeneous or heterogeneous) and is determined by specialists.
The lowest temperature at which an explosion of a mixture of air with vapors of a combustible liquid occurs is called flash point (t rev) Degree fire hazard combustible liquids is determined by their flash point. In accordance with this, combustible liquids are divided into the following classes:
1st class: t rev < – 13 о C;
2nd class: t rev= - 13 ... 28 about C
3rd grade: t rev= 29… 61°С;
4th grade: t rev= 62…120°С;
5th grade: t rev> 120°С;
Liquids of the first three classes are conditionally classified as flammable ( LVZH). Characteristic features flammable liquid is that most of them, even at ordinary temperatures in industrial premises, can form vapor-air mixtures with concentrations within the boundaries of flame propagation, i.e. explosive mixtures.
TO LVZH include: gasoline ( t rev from -44 to -17°С); benzene ( t rev-12 about C); methyl alcohol ( t rev=8 about C); ethanol ( t rev=13 about C); tractor kerosene ( t rev\u003d 4-8 about C), etc.
Liquids of the 4th and 5th classes are flammable liquids ( GJ)
To GZh include: lighting kerosene (t vsp = 48-50 about C); vaseline oil (t vsp =135 about C); transformer oil (t rev =160 o C); engine oil (t vsp = 170 o C), etc.
When ignited, a sufficient amount of heat is released to form vapors and gases of a combustible liquid, which ensure continuous flame combustion even after exposure to a thermal pulse. The lowest temperature at which, under the conditions of special tests, a substance releases vapors or gases at such a rate that, after their ignition from an external source, a flash is observed - the beginning of stable combustion, is called flash point (t resurface).
The flash and ignition temperatures of liquids are very close, which determines their great fire hazard.
The flash point and ignition point of liquids differ by 5-25 ° C. The lower the flash point of a liquid, the smaller this difference is, and, accordingly, the more flammable liquid. The ignition temperature is used in determining the flammability group of substances, in assessing the fire hazard of equipment and technological processes associated with the processing of combustible substances, in developing measures to ensure fire safety.
Auto ignition temperature (t svpl) is the lowest temperature of substances at which, under special test conditions, a sharp increase in the rate of exothermic volumetric reactions occurs, which leads to the occurrence of fiery combustion or explosion in the absence of an external source of flame. The self-ignition temperature of substances depends on a number of factors and varies over a wide range. The most significant is the dependence of the self-ignition temperature of a particular substance on the volume and geometric shape of the combustible mixture. With an increase in the volume of the combustible mixture, with its shape unchanged, the self-ignition temperature decreases, because more favorable conditions to accumulate heat in the combustible mixture. With a decrease in the volume of a combustible mixture, its auto-ignition temperature rises.
For each combustible mixture, there is a critical volume in which self-ignition does not occur due to the fact that the heat transfer area per unit volume of the combustible mixture is so large that the rate of heat generation due to the oxidation reaction, even at very high temperatures cannot exceed the heat dissipation rate. This property of combustible mixtures is used to create barriers to the spread of flame. The value of the autoignition temperature is used to select the type of explosion-proof electrical equipment, in the development of measures to ensure the fire and explosion hazard of technological processes, as well as in the development of standards or specifications for substances and materials.
Autoignition temperature ( t svpl) combustible mixture significantly exceeds the flash point ( t rev) and the ignition temperature (t resurfacing) - by hundreds of degrees.
According to GOST 12.1.004-91 “SSBT. Fire safety. General requirements”, depending on the flash point, liquids are divided into flammable (flammable) and combustible (FG). Flammable liquids have a flash point of no more than 61 ° C (in a closed crucible) or 66 ° C (in an open crucible), and GZh have a flash point of more than 61 ° C.
Flammable liquids are combustible substances (materials, mixtures) that can be ignited by short-term exposure to a match flame, spark, incandescent electrical wire, and similar low-energy ignition sources. These include almost all combustible gases (for example, hydrogen, methane, carbon monoxide, etc.), combustible liquids with a flash point of not more than 61 ° C in a closed crucible or 66 ° C in an open crucible (for example, acetone, gasoline, benzene, toluene, ethyl alcohol, kerosene, turpentine, etc.), as well as all solid substances (materials) that ignite from the flame of a match or burner, the combustion spreads over the surface of a horizontally located test sample (for example, dry wood shavings, polystyrene and etc.).
Flammable - these are combustible substances (materials, mixtures) that can ignite only under the influence of a powerful source of ignition (for example, a PVC conveyor belt, carbamide foam for sealing the surface of a rock mass in underground workings, flexible electrical cables with PVC insulation, ventilation pipes vinyl leather, etc.).
The fire-hazardous properties of solids and materials are characterized by a tendency to burn (ignite), combustion features, and the ability to be extinguished by one or another method.
Various in chemical composition hard materials and substances burn differently. Combustion of solids has a multi-stage character. Simple solids (anthracite, coke, soot, etc.), which are chemically pure carbon, are heated or smolder without the formation of sparks, flames and smoke, since there is no need to decompose before reacting with atmospheric oxygen.
Burning complex by chemical composition solid combustible substances (wood, rubber, plastics, etc.) occur in two stages: decomposition, which is not accompanied by flame and light emission; combustion, which is characterized by the presence of a flame or smoldering.
The explosion and fire hazard of substances depends on their state of aggregation (gaseous, liquid, solid), physical and chemical properties, storage and application conditions.
The main indicators characterizing the fire hazard combustible gases are the concentration limits of ignition, ignition energy, combustion temperature, normal flame propagation speed, etc.
Combustion of a mixture of gas with air is possible within certain limits, called the concentration limits of ignition. The minimum and maximum concentrations of combustible gases in the air that can ignite are called respectively, the lower and upper concentration limits of ignition.
The ignition energy is determined by the minimum energy of an electric discharge spark that ignites a given gas-air mixture. The amount of ignition energy depends on the nature of the gas and the concentration. Ignition energy is one of the main characteristics of explosive environments when solving the issues of ensuring the explosion safety of electrical equipment and developing measures to prevent the formation of static electricity.
combustion temperature is the temperature of the product of a chemical reaction during combustion of the mixture without heat loss. It depends on the nature of the combustible gas and the concentration of its mixture. The highest combustion temperature for most combustible gases is 1600-2000 °C.
The normal speed of flame propagation is the speed at which the boundary surface between the burnt and unburned parts of the mixture moves relative to the unburned one. Numerically, the normal flame speed is equal to the amount (volume) of the combustible mixture that burns out per unit area of the flame per unit time. The normal flame speed depends on the nature of the gas and the concentration of its mixture. For most combustible gases, the normal flame speed is in the range of 0.3-0.8 m/s.
The normal flame speed is one of the main physical and chemical characteristics that determine the properties of the mixture, and determine the combustion rate and, accordingly, the explosion time. The higher the normal flame speed, the less time explosion and the more stringent its parameters.
Combustion of flammable and combustible liquids occurs only in vapor phase. Combustion of vapors in air, as well as gases, is possible in a certain range of concentrations. Since the maximum possible content of vapor in the air cannot be greater than in the saturation state, the concentration limits of ignition can be expressed in terms of temperature. The liquid temperature values at which the concentration of saturated vapors in the air above the liquid is equal to the flammable concentration limits are called the flammable temperature limits (lower and upper, respectively).
Thus, for the ignition and combustion of a liquid, it is necessary that the liquid be heated to a temperature not less than the lower temperature limit of ignition. Once ignited, the evaporation rate must be sufficient to maintain constant combustion. These features of the combustion of liquids are characterized by flash and ignition temperatures.
flash point called the lowest value of the liquid temperature at which a vapor-air mixture is formed above its surface, capable of flashing from an external source of ignition. In this case, stable combustion of the liquid does not occur.
According to the flash point, liquids are divided into flammable (flammable) liquids. the flash point of which does not exceed 45 ° C (alcohols, acetone, gasoline, etc.) and combustibles (FG), the flash point of which is more than 45 ° C (oils, fuel oils, glycerin, etc.).
Flash point called the lowest value of the temperature of the liquid, at which the intensity of its evaporation is such that after ignition by an external source, independent flame combustion occurs. For flammable liquids, the ignition temperature is usually 1–5 °C higher than the flash point, and for FL, this difference can reach 30–35 °C.
Steam-air mixtures, as well as gas-air mixtures, are explosive. Their explosiveness is characterized by parameters that determine the explosiveness of gas-air mixtures - ignition energy, combustion temperature, normal flame propagation speed, etc.
fire hazard solid combustibles substances and materials are characterized by the calorific value of 1 kg of the substance, the combustion, self-ignition and ignition temperatures, the rate of burnout and the spread of combustion over the surface of the materials.
The fire and explosive properties of dusts are determined by the concentrations of the dust-air mixture, the presence of an ignition source with sufficient thermal energy, the size of dust particles, etc.
Small particles of solid combustible substances with dimensions of 10~5-10~7 cm can stay in the air for a long time in a suspended state, forming a dispersed system - an air suspension. To ignite the air suspension, it is necessary that the concentration of dust in the air be not less than the lower concentration limit of ignition. The upper concentration limit of ignition of the dust-air mixture in most cases is very high and difficult to achieve (for peat dust - 2200 g/m3, for powdered sugar - 1350 g/m3).
The thermal energy of the ignition source for igniting the dust-air mixture should be of the order of several MJ or more.
Depending on the value of the lower concentration limit of ignition, dusts are divided into explosive and flammable. Explosive dusts include dusts with a lower concentration flammability limit of up to 65 g/m3 (dust of sulfur, sugar, flour), and flammable dusts with a lower flammability limit above 65 g/m3 (tobacco and wood dust).
The fire hazard of substances and materials is characterized; and such properties as the tendency of certain substances and materials to electrify and ignite spontaneously when in contact with air (phosphorus, sulfurous metals, etc.). water (sodium, potassium, calcium carbide, etc.) and with each other (methane + chlorine, nitric acid + sawdust etc.).
The fire hazard of non-combustible substances and materials is determined by the temperature at which they are processed, the possibility of sparks, flames, radiant heat, as well as loss of bearing capacity and destruction.
