Kinetic and diffusion combustion of gases. Diffusion combustion of liquid Fundamentals of the theory of diffusion and kinetic combustion

If the duration of the chemical reaction and the physical stage of the combustion process are commensurate, then combustion proceeds in the so-called intermediate area, in which the burning rate is influenced by both chemical and physical factors.

Combustion of any material occurs in the gas or vapor phase. Liquid and solid combustible materials, when heated, transform into another state - gas or steam, after which they ignite. During steady combustion, the reaction zone acts as an ignition source for the rest of the combustible material.

The region of a gaseous medium in which an intense chemical reaction causes luminescence and heat is called flame. A flame is an external manifestation of intense oxidation reactions of a substance. When burning solids, the presence of a flame is not necessary. One of the types of combustion of solids is smoldering(flameless combustion), in which chemical reactions occur at a low speed, a red glow and weak heat generation predominate. Flame combustion of all types of flammable materials and substances in the air is possible when the oxygen content in the fire zone is at least 14% by volume, and smoldering of flammable solid materials continues until the oxygen content is ~ 6%.

Thus, combustion is a complex physical and chemical process.

Modern combustion theory is based on the following principles. The essence of combustion is the transfer of valence electrons to the oxidizing substance by the oxidizing substance. As a result of the transfer of electrons, the structure of the outer (valence) electronic level of the atom changes. Each atom then passes into the state that is most stable under the given conditions. In chemical processes, electrons can completely transfer from the electron shell of atoms of one type to the shell of atoms of another type. To get an idea of ​​this process, let's look at a few examples.

Thus, when sodium burns in chlorine, the sodium atoms give up one electron to the chlorine atoms. In this case, the outer electronic level of the sodium atom has eight electrons (stable structure), and the atom that has lost one electron turns into a positive ion. A chlorine atom that gains one electron fills its outer level with eight electrons and the atom becomes a negative ion. As a result of the action of electrostatic forces, oppositely charged ions come together and a sodium chloride molecule is formed (ionic bond)

Na + + C1 - → Na + C1 -

In other processes, electrons from the outer shells of two different atoms seem to come into “common use,” thereby pulling the atoms together into molecules (covalent bond)

H ∙ + · C1: → H: C1:

Atoms can donate one or more electrons for “common use.”

As an example, Fig. 2 shows a diagram of the formation of a methane molecule from one carbon atom and four hydrogen atoms. Four electrons of the hydrogen atoms and four electrons of the outer electron level of the carbon atom are shared, and the atoms are “pulled together” into a molecule.

Fig.2. Scheme of formation of a methane molecule

The doctrine of combustion has its own history. Among the scientists who studied combustion processes, it is necessary to note A.N. Bach and K.O. Engler, who developed the peroxide theory of oxidation, according to which, when a combustible system is heated, an oxygen molecule is activated by breaking one bond between atoms.

molecule active molecule

The active oxygen molecule easily combines with a flammable substance and forms a compound of the type R-O-O-R (peroxide) and R-O-O-H (hydroperoxide); here R is the radical symbol. Radicals are particles (atoms or atomic groups) that have unpaired electrons, for example, , , etc. An example of such a reaction:

CH 4 + -O-O- → -O-O-

methyl hydroperoxide

The energy of breaking the -О-О- bond in peroxides and hydroperoxides is much lower than in the O2 oxygen molecule, so they are very reactive. When heated, they easily decompose to form new substances or radicals. This generates heat.

The further development of the theory of combustion is associated with the works of N.N. Semenov, who created theory of combustion chain reactions, which made it possible to penetrate deeper into the physics of the phenomenon and explain various combustion modes, including self-ignition, deflagration combustion and combustion leading to an explosion. In addition, the difference between the modern understanding of the combustion mechanism and the peroxide theory is that the initial phase of the process is not the activation of oxygen molecules, but the activation of molecules of the oxidizing substance.

The structure of a diffusion flame above the surface of a flammable liquid, the mechanism and speed of its propagation.

The structure of the diffusion flame above the flammable liquid mirror is approximately the same. The only difference is that the flammable vapors coming from the surface of the liquid do not have such an initial reserve of kinetic energy as a gas stream, and before ignition they mix with the surrounding gaseous medium not due to the kinetic energy of the incoming gas flow, but more slowly through the mechanism of convective and molecular diffusion . But if an ignition source is connected to the resulting steam-air mixture, a flame torch will appear, which will change the ratio of gas and heat flows above the liquid surface: hot combustion products, as lighter ones, will rush upward, and in their place fresh cold air will come from the surrounding space, which will lead to dilution of flammable liquid vapors. A radiant flow of thermal energy will flow from the flame to the liquid mirror, which will go to heat the surface layers of the liquid and, as they warm up, intensify the process of its evaporation.

If the liquid before ignition had a temperature significantly higher than the ignition temperature, then the combustion of the liquid above the tank or spilled liquid will intensify and progress, and the size of the flame will increase. Accordingly, the intensity of the radiant heat flow to the surface of the liquid increases, the evaporation process intensifies, the intensity of the convective gas flow around the flame increases, it will be more strongly pressed from the sides, taking the shape of a cone, increasing in size. With further combustion, the flame enters a turbulent combustion mode and will grow until a thermal and gas-dynamic equilibrium regime is established. The maximum temperature of the turbulent diffusion flame of most flammable liquids does not exceed 1250-1350°C.

The propagation of combustion over the surface of the liquid surface depends on the rate of formation of the combustible mixture through the mechanisms of molecular and convective diffusion. Therefore, for liquids with a temperature below the ignition temperature, this speed is less than 0.05 m/s, and for liquids heated above the ignition temperature it reaches 0.5 m/s or more.

Thus, the speed of flame propagation over the surface of a flammable liquid depends mainly on its temperature.

If the liquid temperature is equal to or higher than the ignition temperature, combustion may occur. Initially, a small flame is established above the surface of the liquid, which then quickly increases in height and after a short period of time reaches its maximum value. This suggests that a certain heat and mass transfer has established between the combustion zone and the surface of the liquid. Heat is transferred from the combustion zone to the surface layer of liquid by radiation and thermal conduction through the walls of the container. There is no convective flow, since the vapor flow in the plume is directed upward, i.e. from a less heated surface to a more heated surface. The amount of heat transferred to the liquid from the combustion zone is not constant and depends on the temperature of the torch, the transparency of the flame, its shape, etc.

The liquid receives some of the heat from the tank wall. This portion of the heat can be significant when the liquid level in the tank is low and also when flames are flowing around the outer wall of the tank. The heat perceived by the liquid is mostly spent on evaporation and heating it, and some heat is lost by the liquid to the environment:

Q = q 1 + q 2 + q 3

where Q is the amount of heat received by the liquid from the flame, kJ/ (m 2 -s);

q 1 - the amount of heat lost by the liquid into the environment, kJ/ (m 2 -s);

q 2 - amount of heat spent on liquid vaporization, kJ/ (m 2 s);

qз - amount of heat spent on heating the liquid, kJ/ (m 2 -s).

If the diameter of the tank is large enough, then the value of q1 compared to q 2 and q 3 can be neglected:

Q = q 2 + q 3 = rlс + cpс (T-T 0) u.

Where r is the heat of evaporation of liquid, kJ/kg;

Ср - heat capacity of liquid, kJ/ (kg K);

p - liquid density, mg/m3;

T is the temperature at the surface of the liquid, K;

T 0 - initial liquid temperature K;

u is the growth rate of the heated liquid layer, m/s;

l - linear speed of liquid burnout, m/s.

If an individual liquid burns, then the composition of its vapor phase does not differ from the composition of the liquid phase. If a liquid of complex composition (mixture) burns, then fractional distillation occurs in its upper layer and the composition of the spherical phase differs from the composition of the liquid phase. Such mixtures include oil and all petroleum products. When they burn, mostly low-boiling fractions evaporate, as a result of which the liquid phase changes its composition, and at the same time vapor pressure, specific gravity, viscosity and other properties. Table 3.1 shows the change in the properties of Karachukhur oil in the surface layer when it burns in a reservoir with a diameter of 1.4 m.

Table 1.11.1

Changes in the properties of Karachukhur oil during combustion

According to Table 1.11.1, due to the burnout of low-boiling fractions, the density of the remaining product increases. The same thing happens with viscosity, flash point, resin content and boiling point. Only the moisture content decreases as the oil burns out. The intensity of changes in these properties during combustion in tanks of different diameters is not the same. In large-diameter tanks, due to an increase in convection and the thickness of the liquid layer involved in mixing, the rate of change in these properties decreases. The change in the fractional composition of petroleum products that occurs in the upper layer gradually leads to a change in the layer in the thickness of the heated petroleum product.

If you use the first law of D.P. Konovalov, the conclusion about the combustion of mixtures can be formulated as follows: a mixture of two liquids is enriched during combustion with those components, the addition of which to the liquid lowers the vapor pressure above it (or increases the boiling point). This conclusion is also valid for mixtures in which the number of components is more than two.

When burning mixtures of flammable and some flammable liquids with water as a result of fractional distillation, the percentage of water in the liquid phase increases all the time, which leads to an increase in the specific gravity of the burning mixture. This phenomenon is typical for mixtures in which the flammable component has a boiling point lower than the boiling point of water (methyl, ethyl alcohol, diethyl ether, acetone, etc.). When such liquid mixtures burn for a long time, due to the increase in water in them, a moment comes when the combustion stops, although not all of the mixture has yet burned out.

A mixture of flammable liquids with water, when the boiling point of the liquid is higher than the boiling point of water, behaves somewhat differently during the combustion process. The percentage of water in the liquid phase does not increase, but decreases. As a result, the mixture burns out completely. This is how a mixture of acetic acid and water burns.

When petroleum products burn, their boiling point (see Table 1.11.1) gradually increases due to the fractional distillation that occurs, and therefore the temperature of the upper layer also increases. Figure 1.11.1 shows the change in temperature on the surface

Fig.1.11.1

At low liquid temperatures, heat transfer from the flame to the liquid plays a significant role in flame propagation. The flame heats the surface of the liquid adjacent to it, the vapor pressure above it increases, a flammable mixture is formed, which burns when ignited.

The moving flame heats the next section of the liquid surface, and so on.

The dependence of the speed of flame movement on the surface of the liquid on temperature is shown in Fig. 1.11.2.

When the liquid temperature is below the flash point, the flame travel speed is low.

It increases as the temperature of the liquid increases and becomes the same as the speed of flame propagation through the steam-air mixture at a liquid temperature above the flash point.

Fig.1.11.2 Change in the speed of flame movement along the surface of liquids depending on temperature: 1-isoamyl alcohol, 2 - butyl alcohol, 3 - ethyl alcohol, 4 - toluene

Kinetic combustion is the combustion of a pre-mixed mixture of fuel and oxidizer.

In this case, the flame through the combustible mixture will spread in all directions. The volume engulfed in flames will increase. The flame always spreads towards the unburnt mixture.

Rice. 7.1. Scheme of flame propagation through a pre-mixed homogeneous mixture: 1 – initial combustible mixture; 2 – flame front; 3 - combustion products; d f.p. – flame front thickness

The narrow strip between the initial mixture (1) and combustion products (PG) (3) is the flame (2). For most hydrocarbon mixtures with air, the thickness of this strip is 0.1-1.0 mm. This is the combustion zone or flame front. A chemical reaction takes place in it and all the heat is released. The glow is the result of the presence of CH, HCO, C2, etc. radicals in it.

Thus, the flame front is a narrow luminous zone separating the GHG and the original combustible mixture.

In the flame front, as a result of the chemical combustion reaction, the concentration of the initial components sharply decreases to zero, and the temperature reaches its maximum value. Due to molecular thermal conductivity, the temperature in front of the reaction zone monotonically increases from the initial temperature of the combustible mixture to a temperature close to the combustion temperature, forming a physical heating zone.

Since the thickness of the flame zone does not, as a rule, exceed fractions of a mm, the flame front is conventionally considered a plane.

If the flame front moves, then the flame is called non-stationary, if it doesn’t move – stationary.

The main characteristics are:

Normal flame propagation speed is the speed at which the flame front moves relative to the unburned gas in a direction perpendicular to its surface. The normal speed is a function of a number of physicochemical properties of the mixture and the rate of the chemical reaction at the combustion temperature.

This is one of the fire hazard characteristics of gaseous substances. Since it is determined by the physicochemical properties of the combustible mixture, it is also called fundamental.

Mass burnout rate. This is the mass of a substance burned per unit time from a unit surface area of ​​the flame front.

There are two theories that explain the nature of flame propagation through a combustible mixture.

According to the diffusion theory, the movement of the flame front occurs due to the diffusion of active particles formed in the combustion zone - radicals - into a fresh mixture, where they initiate a chemical reaction.

According to thermal theory, the movement of the flame front is carried out due to the transfer of heat by thermal conduction into the fresh mixture, due to which the latter is heated to the auto-ignition temperature, followed by a chemical reaction.

In fact, there are elements of both theories, because the process is very complex.

Factors affecting normal speed:

Concentration and composition of the combustible mixture.

Theoretically, u n should be maximum at j st. Almost the maximum occurs in a mixture containing more than the stoichiometric ratio of fuel (a in< 1 – богатая смесь). u н для различных газов составляет ~ 0,3 – 1,6 м/с. Она редко превышает значение 2,5 м/с, а для углеводородно-воздушных смесей находится в пределах 0,4 – 0,8 м/с. Смеси, имеющие u н < 0,04 м/с, не способны к распространению пламени.

The presence of phlegmatizers (N 2 , CO 2 , H 2 O (steam), Ar, etc.).

A dilution effect is observed, which entails a decrease in the reaction rate, heat generation and u n. The effectiveness of phlegmatizing gases is determined by their thermophysical properties.

Temperature (initial) of the combustible mixture. As T o increases, the temperature of the combustible mixture increases: T g = T o + Q n /(ås p i V PG i)

All flammable (combustible) substances contain carbon and hydrogen, the main components of the gas-air mixture involved in the combustion reaction. The ignition temperature of flammable substances and materials varies and does not exceed 300°C for most.

The physicochemical basis of combustion lies in the thermal decomposition of a substance or material to hydrocarbon vapors and gases, which, under the influence of high temperatures, enter into a chemical reaction with an oxidizing agent (air oxygen), turning during the combustion process into carbon dioxide (carbon dioxide), carbon monoxide (carbon monoxide). carbon), soot (carbon) and water, and this produces heat and light radiation.

Ignition is the process of flame propagation through a gas-vapor-air mixture. When the rate of flow of flammable vapors and gases from the surface of a substance is equal to the speed of flame propagation along them, stable flaming combustion is observed. If the flame speed is greater than the flow rate of vapors and gases, then the gas-vapor-air mixture burns out and the flame self-extinguishes, i.e. flash.

Depending on the speed of gas flow and the speed of flame propagation through them, one can observe:

• combustion on the surface of a material, when the rate of release of the flammable mixture from the surface of the material is equal to the rate of fire propagation along it;
• combustion with separation from the surface of the material, when the rate of release of the flammable mixture is greater than the speed of flame propagation along it.

Combustion of a gas-vapor-air mixture is divided into diffusion or kinetic. The main difference is the content or absence of an oxidizer (air oxygen) directly in the combustible steam-air mixture.

Kinetic combustion is the combustion of pre-mixed combustible gases and an oxidizer (air oxygen). This type of combustion is extremely rare in fires. However, it is often found in technological processes: gas welding, cutting, etc.

During diffusion combustion, the oxidizer enters the combustion zone from the outside . It comes, as a rule, from below the flame due to the vacuum that is created at its base. In the upper part of the flame, the heat released during the combustion process creates pressure. The main combustion reaction (oxidation) occurs at the flame boundary, since gas mixtures flowing from the surface of the substance prevent the oxidizer from penetrating deep into the flame (displace air). Most of the combustible mixture in the center of the flame, which has not entered into an oxidation reaction with oxygen, is the products of incomplete combustion (CO, CH 4, carbon, etc.).

Diffusion combustion, in turn, can be laminar (disputable) and turbulent (uneven in time and space). Laminar combustion is characteristic when the rate of flow of the combustible mixture from the surface of the material is equal to the rate of spread of the flux along it. Turbulent combustion occurs when the rate of release of the combustible mixture significantly exceeds the speed of flame propagation. In this case, the flame boundary becomes unstable due to the large diffusion of air into the combustion zone. Instability first occurs at the top of the flame and then moves to the base. Such combustion occurs in fires with their volumetric development (see below).

Combustion of substances and materials is possible only with a certain quality of oxygen in the air. The oxygen content, at which the possibility of combustion of various substances and materials is excluded, is established experimentally. So, for cardboard and cotton, self-extinguishing occurs at 14% (vol.) oxygen, and for polyester wool - at 16% (vol.).

Elimination of the oxidizing agent (air oxygen) is one of the fire prevention measures. Therefore, storage of flammable and combustible liquids, calcium carbide, alkali metals, phosphorus should be carried out in tightly closed containers.

1.2.2. Ignition sources.

A necessary condition for ignition of a combustible mixture is ignition sources. Ignition sources are divided into open fire, heat from heating elements and devices, electrical energy, energy of mechanical sparks, discharges of static electricity and lightning, energy of self-heating processes of substances and materials (spontaneous combustion), etc. Particular attention should be paid to identifying ignition sources available in production.

The characteristic parameters of ignition sources are taken according to:

The lightning channel temperature is 30,000°C with a current of 200,000 A and an action time of about 100 μs. The energy of a spark discharge from the secondary impact of lightning exceeds 250 mJ and is sufficient to ignite combustible materials with a minimum ignition energy of up to 0.25 J. The energy of spark discharges when high potential is carried into a building through metal communications reaches values ​​of 100 J or more, which is sufficient to ignite all combustible materials materials.

Polyvinyl chloride insulation of an electrical cable (wire) ignites when the short circuit current ratio is more than 2.5.

The temperature of welding particles and nickel particles of incandescent lamps reaches 2100°C. The droplet temperature when cutting metal is 1500°C. The arc temperature during welding and cutting reaches 4000°C.

The scattering zone of particles during a short circuit at a wire height of 10 m ranges from 5 (probability of hitting 92%) to 9 (probability of hitting 6%) m; when the wire is located at a height of 3 m - from 4 (96%) to 8 m (1%); when located at a height of 1 m - from 3 (99%) to 6 m (6%).

The maximum temperature, °C, on the bulb of an incandescent light 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.

Sparks of static electricity generated when people work with moving dielectric materials reach values ​​from 2.5 to 7.5 mJ.

Temperature of the flame (smoldering) and burning time (smoldering), "C (min), of some low-calorie heat sources: smoldering cigarette - 320-410 (2-2.5); smoldering cigarette - 420-460 (26-30); burning match - 620-640 (0.33).

For sparks from stove pipes, boiler rooms, pipes of steam locomotives and diesel locomotives, as well as other machines, fires, it has been established that a spark with a diameter of 2 mm is fire hazardous if it has a temperature of about 1000 ° C, with a diameter of 3 mm - 800 ° C, with a diameter of 5 mm - 600 ° C .

1.2.3. Spontaneous combustion

Spontaneous combustion is inherent in many flammable substances and materials. This is a distinctive feature of this group of materials.

Spontaneous combustion can be of the following types: thermal, chemical, microbiological.

Thermal spontaneous combustion is expressed in the accumulation of heat by the material, during which self-heating of the material occurs. The self-heating temperature of a substance or material is an indicator of its fire hazard™. For most flammable materials, this indicator ranges from 80 to 150°C: paper - 100°C; construction felt - 80°C; leatherette - 40°C; wood: pine - 80, oak - 100, spruce - 120°C; raw cotton - 60°C.

Prolonged smoldering before the onset of flaming combustion is a distinctive characteristic of thermal spontaneous combustion processes. These processes are detected by the long-lasting and persistent smell of smoldering material.

Chemical spontaneous combustion immediately manifests itself in flaming combustion. For organic substances, this type of spontaneous combustion occurs upon contact with acids (nitric, sulfuric), vegetable and industrial oils. Oils and fats, in turn, are capable of spontaneous combustion in an oxygen environment. Inorganic substances can spontaneously ignite upon contact with water (for example, sodium hydrosulfite). Alcohols ignite spontaneously upon contact with potassium permanganate. Ammonium nitrate spontaneously ignites upon contact with superphosphate, etc.

Microbiological spontaneous combustion is associated with the release of thermal energy by microorganisms during their life activity in a nutrient medium for them (hay, peat, sawdust, etc.).

In practice, combined processes of spontaneous combustion most often occur: thermal and chemical.

2. Fire and explosion hazard indicators.

The study of fire and explosion hazard properties of substances and materials circulating during the production process is one of the main tasks of fire prevention, aimed at eliminating the flammable environment from the fire system.

In accordance with GOST 12.1.044 According to their aggregate state, substances and materials are divided into:

GASES - substances whose saturated vapor pressure at a temperature of 25°C and a pressure of 101.3 kPa (1 atm) exceeds 101.3 kPa (1 atm).

LIQUIDS - the same, but at a pressure less than 101.3 kPa (1 atm). Liquids also include solid melting substances whose melting or dropping point is less than 50°C.

SOLID - individual substances and their mixtures with a melting or dropping point above 50°C (for example, vasiline - 54°C), as well as substances that do not have a melting point (for example, wood, fabrics, etc.).
DUSTS - dispersed (crushed) solids and materials with a particle size of less than 850 microns (0.85 mm).

The nomenclature of indicators and their applicability to characterize the fire and explosion hazard of substances and materials are given in Table 1.
The values ​​of these indicators must be included in the standards and technical specifications for substances, and also indicated in product passports.

Table 1

(The “+” sign indicates applicability, the “-” sign indicates the non-applicability of the indicator)

FLASH POINT (Tfsp) - only for liquids - the lowest temperature of a condensed substance at which, under special test conditions, vapors are formed above its surface that can flash in the air from an ignition source; In this case, stable combustion does not occur.

IGNITION TEMPERATURE (Tv,) - except for gases - the lowest temperature of a substance at which the substance emits flammable vapors and gases at such a speed that when exposed to an ignition source, ignition is observed.

Self-ignition temperature (T sv) is the lowest ambient temperature at which self-ignition of a substance is observed.

CONDITIONS OF THERMAL SPONTANEOUS COMBUSTION - only for solids and dusts - an experimentally identified relationship between the ambient temperature, the amount of substance (material) and the time until its spontaneous combustion.

SELF-HEATING temperature is the lowest temperature of a substance at which the spontaneous process of its heating does not lead to smoldering or flaming combustion.

A safe temperature for prolonged heating of a substance is considered to be a temperature not exceeding 90% of the self-heating temperature.

THE ABILITY TO EXPLODE AND BURN WHEN INTERACTING WITH WATER, OXYGEN IN THE AIR AND OTHER SUBSTANCES (mutual contact of substances) is a qualitative indicator characterizing the special fire hazard of some substances.

SMOKE FORMATION COEFFICIENT - only for solids - an indicator characterizing the optical density of smoke formed during flaming combustion or thermal-oxidative destruction (smoldering) of a certain amount of solid substance (material) under special test conditions.

There are 3 groups of materials:

For materials with moderate smoke-generating ability, the amount of smoke when a person loses the ability to navigate is less

or equal to the amount of combustion products that can cause fatal poisoning. Therefore, the likelihood of loss of visibility in smoke is higher than the likelihood of poisoning.

Examples of smoke-generating ability of building materials during smoldering (burning), m 3 /kg:

Wood fiber (birch, aspen) - 62 (20)

Decorative laminate - 75 (6)

FSF grade plywood - 140 (30)

Fibreboard, lined with plastic - 170 (25)

TOXICITY INDICATOR OF COMBUSTION PRODUCTS OF POLYMER MATERIALS - the ratio of the amount of material per unit volume of a closed space in which gaseous products formed during the combustion of the material cause the death of 50% of experimental animals.

The essence of the method is to burn the material under study in a combustion chamber and identify the dependence of the lethal effect of gaseous combustion products on the mass of the material (in grams) per unit volume (1 m 3) of the exposure chamber.

The classification of materials is given in the table:

* For extremely toxic materials, the mass does not exceed 25 grams in order to create a lethal concentration in a volume of 1 m 3 in 5 minutes. Accordingly, in a time of 15 minutes - up to 17; 30 min - up to 13; 60 minutes - up to 10 grams.

For example: Douglas pine - 21; vinyl fabric - 19; polyvinyl chloride - 16; elastic polyurethane foam - 18 (hard - 14) g/m 3 with an exposure time of 15 minutes.

CONCENTRATION LIMITS OF FLAME PROPAGATION (IGNITION) - except for solids.

Lower (upper) concentration limits of flame propagation (ignition) - the minimum (maximum) content of a flammable substance in a homogeneous mixture with an oxidizing environment, at which it is possible for a flame to spread through the mixture to any distance from the ignition source.

Examples of 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; butane - 1.5-8.5; gasoline vapors - 0.7-6; kerosene vapor - 1-1.3.

Smoldering temperature - for solids and dusts - the temperature of a substance at which a sharp increase in the rate of exothermic oxidation reactions occurs, ending in smoldering.

FLAMMABILITY GROUP - a classification characteristic of the ability of any substances and materials to burn.

Based on flammability, substances and materials are divided into three groups: non-flammable, slow-burning and flammable.

NON-COMBUSTABLE (non-combustible) - substances and materials that are not capable of burning in air. Non-flammable substances can be fire-explosive (for example, oxidizing agents or substances that release products when interacting with water, atmospheric oxygen, or with each other).

HIGHLY flammable (hard to burn) - substances and materials that can burn in air when exposed to an ignition source, but are not capable of burning independently after it is removed.

COMBUSTIBLE (combustible) - substances and materials capable of spontaneous combustion, as well as ignite when exposed to an ignition source and burn independently after its removal.

Flammable liquids (FL) with Tvsp<61°С в закрытом тигле или 66°С в откры­том тигле относят к легковоспламеняющимся (ЛВЖ).

Particularly dangerous flammable liquids are called flammable liquids with TVSP< 28°С.

GASES are considered flammable if they have flammable concentration limits (FLCL); low-flammability - in the absence of CPV and the presence of TSV; non-flammable - in the absence of CPV and TSV.

LIQUIDS are considered flammable in the presence of TV; low-flammability - in the absence of TV and the presence of TV; non-flammable - in the absence of Тв, Тсв, Твсп, temperature and concentration limits of flame propagation (ignition).

3. Categories of premises according to explosion and fire hazard.

According to the provisions of the fire safety standards NPB 105-03, categories of premises and buildings (or parts of buildings between fire walls - fire compartments) are established according to explosion and fire hazards, depending on the quantity and fire-explosive properties of substances and materials located (circulating) in them, taking into account the characteristics technological processes of production facilities located in them.

Rooms, compartments, parts of a building, class buildings are subject to categorization depending on their belonging to a particular class in terms of functional fire hazard. Buildings and parts of buildings - premises or groups of premises functionally interconnected are divided into classes according to functional fire hazard depending on the method of their use and the extent to which the safety of people in them in the event of a fire is at risk, taking into account their age, physical condition, ability to stay asleep, type of main functional contingent and its quantity.

Premises, parts of buildings, buildings of classes F3.5., F4.3., F5.1., F5.2., F5.3., including production and warehouse premises, including laboratories, are subject to mandatory categorization according to explosion and fire hazard. and workshops in buildings of classes F1, F2, F3 and F4, according to the provisions of clause 5.21* of SNiP 21-01-97* belong to class F5.

The methodology given in NPB 105-03 should be used in the development of departmental technological design standards related to the categorization of premises and buildings.

NPB 105-03 does not apply to premises and buildings for the production and storage of explosives, means of initiating explosives, buildings and structures designed according to special norms and rules approved in the prescribed manner.

The categories of premises and buildings, defined in accordance with PNB 105-03, should be used to establish regulatory requirements for ensuring explosion and fire safety of these premises and buildings in relation to planning and construction, number of floors, areas, placement of premises, design solutions, engineering equipment. Measures to ensure the safety of people should be prescribed depending on the fire hazardous properties and quantities of substances and materials in accordance with GOST 12.1.004-91 and GOST 12.3.047-98.

Categories of premises and buildings of enterprises and institutions are determined at the design stage of buildings and structures in accordance with these standards, departmental technological design standards or special lists approved in the prescribed manner.

According to the explosion and fire hazard, premises and buildings are divided into categories A, B, B1-B4, D and D. Categories of explosion and fire hazard of premises and buildings are determined for the most unfavorable period in relation to a fire or explosion, based on the type of equipment and premises located in flammable substances and materials, their quantity and fire hazard properties, features of technological processes.

The fire hazardous properties of substances and materials are determined based on test results or calculations using standard methods, taking into account state parameters (pressure, temperature, etc.).

It is permitted to use reference data published by leading research organizations in the field of fire safety or issued by the State Standard Reference Data Service. It is allowed to use fire hazard indicators for mixtures of substances and materials based on the most dangerous component.