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A fire in a tank begins, in most cases, with an explosion of the steam-air mixture located under its roof. As a result of the explosion, the tank roof is completely torn off or partially destroyed and the liquid ignites on the entire free surface. The force of the explosion is usually greater in those tanks where there is a large gas space filled with a mixture of oil product vapors and air (low liquid level). Depending on the force of the explosion, the following situation may be observed in a vertical metal tank: --- - - the roof is completely torn off and thrown to the side at a distance of 20-30 m; the liquid burns over the entire area of ​​the tank.

The roof is raised slightly, opened fully or partially, and then plunged into the burning liquid.

The roof is deformed and forms small gaps at the points of attachment to the tank wall, as well as in welds the roof itself.

Fire situation as a result of depressurization of the tank roof.

In case of fire in reinforced concrete buried (underground) tanks from

explosion causes destruction of the roof, in which holes are formed large sizes, then during the fire the coating may collapse.

Collapse of the roof of a reinforced concrete buried (underground) tank.

In cylindrical horizontal tanks, during an explosion, one of the end walls most often ruptures, which often leads to the tank being torn off the foundation, overturning and liquid spilling.

Consequences of an explosion in a horizontal cylindrical tank.

When oil products burn over the entire area of ​​the tank mirror, the height of the luminous part of the flame is 1.5-2 times the diameter of the tank and is more than 40 m. In windy conditions, the flame tilts at an angle to the horizon, sometimes touching the surface of the earth, and has approximately the same dimensions.

Standout thermal energy transmitted to the walls of the tank,

the upper layer of petroleum product into the environment and causes heating of neighboring tanks and communications. As a result of this, it is possible: the formation of explosive concentrations in adjacent tanks, which can lead to an explosion and fire; flare combustion of oil vapors near breathing valves or leaks in the roofs of adjacent tanks; heating of communications, their deformation, leakage and burning of liquid from them

12. Stationary fire extinguishing systems using air-mechanical foam. In oil and petroleum product warehouses, it is necessary to provide fire extinguishing with air-mechanical foam of medium and low expansion. The following installations are provided: stationary automatic fire extinguishing, stationary non-automatic fire extinguishing and mobile. SNS buildings and premises to be equipped with permanent installations automatic fire extinguishing, are given in the table.

A stationary automatic fire extinguishing installation consists of pumping station, tanks for water, foaming agent or its solution, installed on tanks and in buildings of foam generators, pipelines for supplying foaming agent solution (mortar lines) to foam generators and automation equipment.

A stationary non-automatic fire extinguishing installation consists of the same elements as a stationary automatic one, with the exception of permanently installed foam generators and automation equipment; Fire hydrants or risers with connecting heads are provided on the mortar lines for connecting fire hoses and fire foam generators.

13. AUTOMATION OF FIRE EXTINGUISHING SYSTEMS WITH AIR MECHANICAL FOAM

As part of an automatic fire extinguishing system includes a fire pumping station, the automation of which should provide: automatic start of the working pump;

automatic start of the backup pump in case of failure of the working pump within a set time;

automatic activation of shut-off valves with electric drive; automatic switching of control circuits from working to backup power source electrical energy(when voltage disappears at the working input);

automatic start of the working dosing pump;

automatic start of a backup metering pump in the event of a failure of the working pump within a set time;

generation of a command impulse to automatically turn off ventilation technological equipment;

generation of a command impulse for automatic shutdown of energy receivers of the 3rd and 2nd categories.

A light and sound alarm system must be provided in the pumping station premises:

about the presence of voltage at the main and backup power supply inputs and the grounding of phases to the ground (on call);

about disabling the automatic start of pumps and the dosing pump; about the emergency level in the water reservoir and in the drainage pit.

At the same time, signals are sent to the room fire station or other premises with round-the-clock presence of personnel on duty:

about the occurrence of a fire; about starting pumps;

about the start of operation of sprinkler and deluge installations, indicating the direction in which water (foaming agent solution) is supplied;

about turning off the audible fire alarm;

about a malfunction of the installation (loss of voltage at the main power supply input);

about a drop in pressure in the hydropneumatic tank or in the pulse device;

about the emergency water level in the reservoir and drainage pit;

about the position of the valves;

Continued 13 AUTOMATION OF FIRE EXTINGUISHING SYSTEMS WITH AIR MECHANICAL FOAM

about damage to the control lines of shut-off devices installed on the incentive pipelines of the control units of deluge units and metering pumps.

Sound signals fire signals differ in tone (howlers, sirens) from sound signals about a malfunction (bell).

Automatic switching on The system is duplicated by remote activation from the control panel of the system control station, as well as from the site of a possible fire.

Operating principle of the KPA fire column is based on the opening and closing of the fire hydrant valve to supply water from the water supply. The KPA column is installed on the fire hydrant so that the square key at the bottom of the column fits into the square end of the hydrant rod. The fire hose is screwed onto the hydrant by rotating its body clockwise (the socket wrench does not turn). After this, the hydrant valve opens (at closed valves columns), by rotating the socket wrench counterclockwise (the hydrant valve fully opens at 10-14 turns of the socket wrench) and water from water supply network enters the cavity of the fire column. After connecting the hoses to the nozzles of the fire column, the valves open and water from the fire column enters the hose line.

14. Fire detectors

Fire detectors are classified according to their activation parameter and physical detection principle. The following activation parameters are used to detect fire:

Concentration of smoke particles in the air;

Temperature environment;

Radiation from an open flame.

There are five main types of fire detectors:

thermal fire detectors

smoke detectors

flame detectors

manual fire detectors

combined fire detectors

Thermal fire detectors respond to changes in ambient temperature. They are installed in the following cases:

When, in a controlled volume, the structure of the materials used is such that when burned it produces more heat than smoke.

When the spread of smoke is difficult due to either close quarters [for example, behind suspended ceilings], or external conditions[low temperature, high humidity, etc.]

When there is a high concentration of any aerosol particles in the air that are not related to combustion processes [for example, soot from running cars in a garage or flour in flour mills]

The simplest maximum thermal fire detectors consist of a soldered contact of two conductors. Typically the maximum temperature set in them is 75 °C.

More complex maximum heat fire detectors are equipped with a temperature-sensitive semiconductor element

In all these cases, it is necessary to use thermal linear fire detectors.

An open flame contains characteristic radiation in both the ultraviolet and infrared parts of the spectrum. Accordingly, there are two types of these devices: ultraviolet and infrared detectors flame.

The infrared flame detector, using an IR sensitive element and an optical focusing system, registers characteristic

Fire zones and classes.

Substance

Features of combustion of solid and liquid combustible materials and

Lecture outline

State higher educational institution

"NATIONAL MINING UNIVERSITY"

Department of AOT

Lecture No. 4

Assoc. Alekseenko S.A.

Part 1. Fire safety

Topic No.: Fire and explosion hazard properties of substances and materials.

(for students of specialty 7.0903010 “Reserve development and mining”, specialization: 7.090301.05 “Labor safety 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 Communal Services of the RSFSR, 1962.-264 p.

2. Fundamentals of defense practice: Pidruchnik./ K.N. Tkachuk, M.O. Khalimovsky, V.V. Zatsarniy, D.V. Zerkalov, R.V. Sabarno, O.I. Polukarov, V.S. Kozyakov, L.O. Mityuk. Per ed. K.N. Tkachuk and M.O. Khalimovsky. – K.: Osnova, 2003 – 472 p. (Pozhezhna bezpeka – pp. 394-461).

3. Bulgakov Yu.F. Extinguishing fires in coal mines. – Donetsk: NIIGD, 2001.- 280 p.

4. Aleksandrov S.M., Bulgakov Yu.F., Yaylo V.V. Protection of work in the agricultural industry: Study guide for students of agricultural specialties of higher academic degrees / Under the title. ed. Yu.F. Bulgakov. – Donetsk: RIA DonNTU, 2004. – P.3-17.

5. Rozhkov A.P. Fire safety: A basic textbook for students of advanced knowledge of Ukraine. – Kiev: Pozhіnformtekhnika, 1999.- 256 pp.: ill.

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 flammable environment, sources of ignition, assessment and prevention of explosion hazards, as well as the selection effective ways and fire safety systems, it is necessary to have an understanding of the nature of the combustion process, its forms and types.

One of the first chemical phenomena that humanity became acquainted with 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 called an exothermic oxidation reaction of substances, which is accompanied by the release of smoke and the appearance of a flame or emission of light.

In other words combustion is a rapid chemical transformation of substances that releases a large amount of heat and is accompanied by a bright flame. It may result from oxidation, i.e. combining a flammable substance with an oxidizing agent (oxygen).

This general definition shows that it can be not only a reaction of connection, but also decomposition.

For combustion to occur, the simultaneous presence of three factors is necessary: ​​1) a flammable substance; 2) oxidizing agent; 3) the initial thermal impulse (ignition source) to impart hot energy to the combustible mixture. In this case, the combustible substance and the oxidizer must be in the required ratio of one to one and thus create a combustible mixture, and the ignition source must have the appropriate energy and temperature sufficient to start the reaction. A flammable mixture is defined by the term “flammable medium”. This is a medium that is capable of burning on its own after the source of ignition is removed. Combustible mixtures, depending on the ratio of combustible substance and oxidizer, are divided into poor And rich . IN poor mixtures there is an excess of oxidizing agent, and in rich – flammable substance. For complete combustion of substances and materials in air environment sufficient oxygen must be present to provide complete transformation substances into its saturated oxides. If there is insufficient air, only part of the combustible substance is oxidized. The residue decomposes, releasing large amounts of smoke. This also produces toxic substances, among which the most common product of incomplete combustion is carbon monoxide. (CO), which can lead to poisoning of people. In fires, as a rule, combustion occurs with a lack of oxygen, which seriously complicates fire extinguishing due to poor 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 number of heat, possibly in the absence of air.

2. Types, varieties and forms of combustion.

Combustion may be homogeneous And heterogeneous .

At homogeneous When burning, 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: emergence , spreading And attenuation flame. Most general properties combustion is the ability ( middle) the flame moves throughout the combustible mixture by transferring heat or diffusion of active parts from the combustion zone into the fresh mixture. This is where the mechanism of flame propagation arises, respectively thermal And diffusion . Combustion, as a rule, occurs through 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 several meters per second (up to 10 m/s);

– explosive – extremely rapid 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 – this is burning propagates at supersonic speeds that reach thousands of meters per second (up to 5000 m/s).

The explosion is also accompanied by the release of heat and emission of light. At the same time, the explosion of some substances is a decomposition reaction, for example:

2NCl 3 = 3Cl 2 + N 2 (1)

Explosion is an extremely rapid chemical (explosive) transformation of a substance, which is accompanied by the release of energy and the formation of compressed gases capable of performing mechanical work.

An explosion differs from combustion by the high speed of fire propagation. For example, the speed of flame propagation in an explosive mixture located in a closed pipe is (2000 - 3000 m/s).

The combustion of the mixture at this rate is called detonation. The occurrence of detonation is explained by compression, heating and movement of the unburned mixture in front of the flame front, leading to acceleration of flame propagation and the appearance of a shock wave in the mixture. The air shock waves formed during the explosion of a gas-air mixture have a large supply of energy and spread over considerable distances. While moving, they destroy structures and can cause accidents.

Combustion of substances can occur not only when they are combined with oxygen in the air (as is commonly believed), but also when combined with other substances. It is known that 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.

Flammable substances may be solid, liquid And gaseous .

Solid flammable substances, depending on their composition and structure, behave differently when heated. Some of them, for example, 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 flammable substances when heated, they evaporate, and some may oxidize.

Thus, most flammable substances, regardless of their initial state of aggregation when heated they transform into gaseous products . In contact with air, they form flammable mixtures. Combustible mixtures can also be formed as a result of the spraying of solid and liquid substances. When a substance has formed a flammable mixture with air, it is considered ready for combustion. This state of the substance poses a great fire hazard. It is determined by the fact that to ignite the resulting mixture, a powerful and long-lasting ignition source is not required; the mixture quickly ignites even from a spark.

The readiness of the mixture to ignite is determined by the content (concentration) of vapors, dust or gaseous products in it.

Types and forms of combustion.

Combustion is characterized by a variety of varieties, forms and features. The following types and forms of combustion are distinguished: flash; ignition; fire; spontaneous combustion and spontaneous combustion.

Flash– this is the rapid (instantaneous) ignition of a combustible mixture under the influence of a thermal impulse without the formation of compressed gases, which does not transform into stable combustion.

Ignition – this is a relatively calm and prolonged combustion of vapors and gases of flammable liquids, which occurs under the influence of an ignition source. Ignition is a fire accompanied by the appearance of a flame.

Fire– this is combustion that begins 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, heated external source heat without contact with 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 solids and bulk materials, arising 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, caused by a sufficient flow of air to the combustible substance for oxidation and insufficient air flow to carry away the heat generated.

Smoldering– combustion without emission of light, which is usually recognized by the appearance of smoke.

Depending on the state of aggregation and combustion characteristics of various flammable substances and materials, fires according to GOST 27331-87 are divided into corresponding classes and subclasses:

class A – combustion of solid substances, 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 alkali (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 susceptibility to the occurrence and spread of combustion, the characteristics of combustion and the ability to succumb to combustion. Based on these indicators, GOST 12.1.044-89 distinguishes non-flammable, low-combustible and combustible materials and substances.

Non-flammable (non-combustible) - substances and materials that are incapable of burning or charring in air under the influence of fire or high temperature. These are materials of mineral origin and materials made on their basis - red brick, sand-lime brick, concrete, asbestos, mineral wool, asbestos cement and other materials, as well as most metals. In this case, non-flammable substances can be fire hazardous, for example, substances that release flammable products when interacting with water. A sufficient criterion for inclusion in 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.

Low-flammability (hard-to-burn) 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, for example wood when deeply impregnated with antipyrogens (bechefit); fiberboard; felt impregnated with clay solution, some polymers and other materials.

Combustible (combustible) - substances and materials that are capable of burning (spontaneously), as well as igniting, smoldering or charring from a source of ignition or burning independently after its removal.

In turn, the group of flammable substances and materials includes flammable substances and materials - 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, the indicators of fire and explosion hazard properties of flammable substances and materials are crucial. 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 aggregate state of the substance, the type of combustion (homogeneous or heterogeneous) and is determined by specialists.

The lowest temperature value at which a mixture of air and vapor flashes flammable liquid, called flash point (t ref) Degree fire danger combustible liquids is determined by their flash point. In accordance with this, combustible liquids are divided into the following classes:

1st class: t ref < – 13 о C;

2nd class: t ref= – 13…28 o C

3rd grade: t ref= 29... 61°C;

4th grade: t ref= 62…120°С;

5th grade: t ref> 120°C;

Liquids of the first three classes are conventionally classified as flammable ( LVZH). The characteristic features of flammable liquids are that most of them, even at ordinary temperatures in production premises, can form steam-air mixtures with concentrations within the boundaries of flame propagation, i.e. explosive mixtures.

TO LVZH include: gasoline ( t ref from -44 to -17°C); benzene ( t ref-12 o C); methyl alcohol ( t ref=8 o C); ethanol ( t ref=13 o C); tractor kerosene ( t ref=4-8 o C), etc.

Liquids of classes 4 and 5 are flammable liquids ( GJ)

GJ includes: lighting kerosene (tf = 48-50 o C); Vaseline oil (t vsp =135 o C); transformer oil (tvsp =160 o C); machine oil (tvsp =170 o C), etc.

Upon ignition, a sufficient amount of heat is released to form vapors and gases of a flammable liquid, ensuring continuous flaming combustion even after exposure to a thermal impulse. The lowest temperature value at which, under special test conditions, a substance emits vapors or gases at such a rate that, after they ignite from an external source, a flash is observed - the beginning of stable combustion is called ignition temperature (t float).

The flash and ignition temperatures of liquids are very close, which determines their high fire hazard.

The flash point and ignition point of liquids differ by 5-25 o C. The lower the flash point of the liquid, the smaller this difference is, and, accordingly, the more fire hazardous the liquid is. The ignition temperature is used to determine the flammability group of substances, to assess the fire hazard of equipment and technological processes related to the processing of flammable substances when developing measures to ensure fire safety.

Auto-ignition temperature (t svpl) is lowest temperature substances in which, under special test conditions, a sharp increase in the rate of exothermic volumetric reactions occurs, which leads to the occurrence of flaming combustion or explosion in the absence of an external flame source. 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, while its form remains unchanged, the self-ignition temperature decreases, because more favorable conditions are created for the accumulation of heat in the combustible mixture. As the volume of the combustible mixture decreases, its auto-ignition temperature increases.

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 rate of heat removal. This property of flammable mixtures is used to create barriers to the spread of flame. The value of the self-ignition temperature is used to select the type of explosion-proof electrical equipment, when developing measures to ensure the fire and explosion hazard of technological processes, as well as when developing standards or technical specifications on substances and materials.

Auto-ignition temperature ( t SVPL) of the combustible mixture significantly exceeds the flash point ( t ref) and ignition temperature (tflash) – 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 liquids) and flammable liquids (CG). flammable liquids have a flash point of no more than 61°C (in a closed crucible) or 66°C (in an open crucible), and gas liquids have a flash point above 61°C.

Flammable liquids are flammable substances (materials, mixtures) that can ignite from short-term exposure to a match flame, spark, hot electrical wire and similar low-energy ignition sources. These include almost all flammable gases (for example, hydrogen, methane, carbon monoxide, etc.), flammable liquids with a flash point of no 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; combustion spreads over the surface of a horizontally located test sample (for example, dry wood shavings, polystyrene and etc.).

Relatively flammable are flammable substances (materials, mixtures) that can ignite only under the influence of a powerful ignition source (for example, a polyvinyl chloride conveyor belt, urea foam for sealing the surface of a rock mass in underground mines, flexible electrical cables with PVC insulation, ventilation pipes from vinyl leather, etc.).

The fire hazardous properties of solids and materials are characterized by their tendency to burn (ignition), combustion characteristics, and ability to be extinguished by one or another method.

Solid materials and substances of different chemical compositions burn differently. The combustion of solids has a multi-stage character. Simple solids (anthracite, coke, soot, etc.), which are chemically pure carbon, heat up or smolder without producing sparks, flames or smoke, since there is no need to decompose before reacting with oxygen in the air.

Combustion of complex chemical composition solid flammable 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 flame or smoldering.

Flammable and combustible liquids differ in characteristics such as flash point. Flash point is the temperature of a liquid at which vapors above the surface of the liquid can ignite when exposed to an open flame source. Flammable liquids have a flash point not higher than 61°C, flammable liquids – higher than 61°C.

Types of flammable liquids and flammable liquids

Flammable liquids are of three categories: especially dangerous (first category), constantly dangerous (second category), dangerous at elevated air temperatures (third category). The flash point of especially dangerous flammable liquids is -13°C. A characteristic feature of especially dangerous flammable liquids is the need for certain conditions for their transportation, because If the seal of a storage vessel is broken, liquid vapor can quickly spread and ignite at a distance from the container. Such liquids include acetone, some types of gasoline, ether, petroleum ether, diethyl ether, hexane, isopentane, cyclohexane.

Second-class flammable liquids have a flash point from -13 to +23°C. Such liquids have the ability to ignite when room temperature when their vapors combine with air. These are liquids such as ethyl alcohol, benzene, methyl acetate, ethyl acetate, ethylbenzene, octane, toluene, isooctane, lower alcohols, dioxolane and dioxane

Third-class flammable liquids are flammable liquids with a flash point from +23 to +60°C. Such liquids will only ignite if there is a source of fire in the immediate vicinity. These include the following liquids: turpentine, solvent, white alcohol, xylene, cyclohexanone, amyl acetate, butyl acetate, chlorobenzene.

Flammable liquids have the property of spontaneous combustion at a flash point above 61°C. Flammable liquids include fuel oil, oils (vaseline, castor), diesel fuel, glycerin, ethylene glycol, hexyl alcohol, hexadecane, aniline. Such liquids may be stored in open containers and containers (for example, drums), including outdoors. When working with flammable and combustible liquids, remember to comply with fire safety regulations for storage, transportation and use.

Various in chemical composition solid materials and substances burn differently. Simple ones (soot, charcoal, coke, anthracite), which are chemically pure carbon, glow or smolder without the formation of sparks, flames or smoke. This is because they do not need to decompose before combining with atmospheric oxygen. This (flameless) combustion usually proceeds slowly and is called heterogeneous(or surface) combustion. The combustion of solid combustible materials with a complex chemical composition (wood, cotton, rubber, rubber, plastic, etc.) occurs in two stages: 1) decomposition, the processes of which are not accompanied by flame and light emission; 2) combustion itself, characterized by the presence of flame or smoldering. Thus, complex substances themselves do not burn, but the products of their decomposition burn. If they burn in the gaseous phase, then such combustion is called homogeneous.

A characteristic feature of chemical combustion complex materials and substances is the formation of flame and smoke. The flame is formed by luminous gases, vapors and solids in which both stages of combustion occur.

Smoke is a complex mixture of combustion products containing solid particles. Depending on the composition of combustible substances, their complete or incomplete combustion, the smoke has specific color and smell.

Most plastics and man-made fibers are flammable. They burn to form liquefied resins and release significant quantities of carbon monoxide, hydrogen chloride, ammonia, hydrocyanic acid and other toxic substances.

Combustible liquids are more fire hazardous than solid flammable substances, since they ignite more easily, burn more intensely, and form explosive air-steam mixtures. Combustible liquids do not burn on their own. Their vapors above the surface of the liquid burn. The amount of vapor and the rate of its formation depend on the composition and temperature of the liquid. Combustion of vapors in air is possible only at certain concentrations, depending on the temperature of the liquid.

To characterize the degree of fire hazard of combustible liquids, it is customary to use the flash point. The lower the flash point, the more dangerous the liquid is in terms of fire. The flash point is determined using a special technique and is used to classify combustible liquids according to the degree of their fire hazard.

Flammable liquid (FL) is a liquid that can burn independently after removing the ignition source and has a flash point of more than 61 ° C. Highly flammable liquid (flammable liquid) is a liquid with a flash point of up to 61 °C. Most low temperature flashes (-50? C) has carbon disulfide, the highest – linseed oil(300? C). Acetone has a flash point of minus 18, ethyl alcohol - plus 13? C.

For flammable liquids, the ignition temperature is usually several degrees higher than the flash point, and for gas liquids it is higher than the flash point. - 30…35? C.

The auto-ignition temperature is significantly higher than the ignition temperature. For example, acetone can ignite spontaneously at temperatures above 500°C, gasoline - about 300°C.

Other important properties (in fire terms) of combustible liquids include high density vapors (heavier than air); the low density of liquids (lighter than water) and the insolubility of most of them in water, which does not allow the use of water for extinguishing; the ability to accumulate static electricity when moving; greater heat and combustion rate.

Flammable gases (GG) present great danger not only because they burn, but also because they are capable of forming explosive mixtures with air or other gases. Thus, all flammable gases are explosive. However, flammable gas is capable of forming explosive mixtures with air only at a certain concentration. The lowest concentration of flammable gas in the air at which ignition (explosion) is already possible is called lower flammable concentration limit (LECL). The highest concentration of flammable gas in the air at which ignition is still possible is called upper flammable concentration limit (UCFL). The concentration region lying inside these boundaries is called ignition area. LKPV and VKPV are measured as a percentage of the volume of the combustible mixture. When the concentration of flammable gas is less than the LVPV and greater than the VCPV, the mixture of flammable gas with air does not ignite. A flammable gas is more dangerous in terms of explosion and fire, the larger the ignition area and the lower the LEL. For example, the ignition range of ammonia is 16...27%, hydrogen 4...76%, methane 5...16%, acetylene 2.8...93%, carbon monoxide 12.8...75%. Thus, acetylene has the greatest explosion hazard, having the largest ignition area and the lowest LEL. To others dangerous properties flammable gases include a large destructive force of explosion and the ability to generate static electricity when moving through pipes.

Combustible dust are formed during the manufacturing process when processing certain hard and fibrous materials and pose a significant fire hazard. Solids in a highly crushed and suspended state in a gaseous medium create a dispersed system. When the dispersed medium is air, such a system is called aerosol. Dust that settles from the air is called airgel. Aerosols can form explosive mixtures, and aerogels can smolder and burn.

Dusts have a fire hazard many times greater than the product from which they are obtained, since dust has a large specific surface area. The smaller the dust particles, the more developed their surface is and the more dangerous the dust is in terms of ignition and explosion, since the chemical reaction between gas and solid matter, as a rule, occurs on the surface of the latter and the reaction rate increases as the surface increases. For example, 1 kg of coal dust can burn in a fraction of a second. Aluminum, magnesium, and zinc in a monolithic state are usually not capable of burning, but in the form of dust they can explode in the air. Aluminum powder can spontaneously ignite in its airgel state.

The presence of a large surface area of ​​dust determines its high adsorption capacity. In addition, dust has the ability to acquire charges of static electricity as it moves, due to friction and impacts of particles against each other. When transporting dust through pipelines, the charge accumulated by it can increase and depends on the substance, concentration, particle size, speed of movement, environmental humidity and other factors. The presence of electrostatic charges can lead to the formation of sparks and ignition of dust-air mixtures.

However, the fire and explosive properties of dust are determined mainly by its self-ignition temperature and lower explosive concentration limit.

Depending on the state, any dust has two auto-ignition temperatures: for airgel and for aerosol. Auto-ignition temperature airgel is significantly lower than aerosol, because the high concentration of flammable substance in the airgel favors heat accumulation, and the presence of a distance between dust particles in the aerosol increases heat loss during the oxidation process during self-ignition. The auto-ignition temperature also depends on the degree of particle size of the substance.

Lower concentration limit of explosion(LKPV) is the smallest amount of dust (g/m3) in the air at which an explosion occurs in the presence of an ignition source. All dusts are divided into two groups. TO group A include explosive dusts with LEL up to 65 g/m3. IN group B includes flammable dusts with LEL above 65 g/m3.

In production areas, dust concentrations are usually well below the lower explosive limits. The upper explosive limits of dust are so high that they are practically unattainable. Thus, the concentration of the upper explosion limit of sugar dust is 13500, and peat - 2200 g/m3.

Ignited fine dust in the aerosol state can burn at the rate of combustion of the gas-air mixture. In this case, the pressure may increase due to the formation of gaseous combustion products, the volume of which in most cases exceeds the volume of the mixture, and due to their heating to a high temperature, which also causes an increase in their volume. The ability of dust to explode and the magnitude of pressure during an explosion largely depend on the temperature of the ignition source, the humidity of the dust and air, ash content, dust dispersion, air composition and the temperature of the dust-air mixture. The higher the temperature of the ignition source, the lower the concentration of dust that can explode. An increase in the moisture content of air and dust reduces the intensity of the explosion.

The fire hazard properties of gases, liquids and solids can be judged by flammability coefficient TO, which is determined by the formula (if the substance has a chemical formula or it can be derived from its elemental composition)

K = 4C + 1H + 4S - 2O - 2CI - 3F - 5 Br,

where C, H, S, O, Cl, F, Br are the number of atoms of carbon, hydrogen, sulfur, oxygen, chlorine, fluorine and bromine, respectively chemical formula substances.

At K? 0 the substance is non-flammable, at K > 0 it is flammable. For example, the flammability coefficient of a substance with the formula C5HO4 will be equal to: K = 4·5+1·1-2·4=13.

Using the flammability coefficient, it is possible to quite accurately determine the lower concentration limits of ignition of flammable gases of a number of hydrocarbons using the formula NKPV = 44 / K.

Life safety summary

The explosion and fire hazard of substances depends on their state of aggregation (gaseous, liquid, solid), physical and chemical properties, storage and use conditions.

The main indicators characterizing fire danger flammable gases are the concentration limits of ignition, ignition energy, combustion temperature, normal flame propagation speed, etc.

Combustion of a mixture of gas and air is possible within certain limits, called ignition concentration limits. The minimum and maximum concentrations of flammable gases in the air that can ignite are called the lower and upper flammability limits, respectively.

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 its concentration. Ignition energy is one of the main characteristics of explosive environments when addressing issues of ensuring the explosion safety of electrical equipment and developing measures to prevent the formation of static electricity.

Combustion temperature- this 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 unburnt parts of the mixture moves relative to the unburned part. Numerically, the normal flame speed is equal to the amount (volume) of the combustible mixture burned per unit flame area per unit time. The normal flame speed depends on the nature of the gas and the concentration of its mixture. For most flammable gases, normal flame speed is in the range of 0.3-0.8 m/s.

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 even 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 concentration range. Since the maximum possible vapor content in the air cannot be greater than in the saturated 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 concentration limits of ignition are called temperature limits of ignition (lower and upper, respectively).

Thus, for a liquid to ignite and burn, it is necessary that the liquid be heated to a temperature not less than the lower ignition temperature limit. Once ignited, the rate of evaporation must be sufficient to maintain continuous combustion. These features of the combustion of liquids are characterized by flash and ignition temperatures.

Flash point is the lowest value of liquid temperature at which a steam-air mixture is formed above its surface, capable of igniting from an external ignition source. In this case, stable combustion of the liquid does not occur.

Based on their flash point, liquids are divided into flammable liquids (flammable liquids). the flash point of which does not exceed 45 °C (alcohols, acetone, gasoline, etc.) and flammable fuels (GL), the flash point of which is more than 45 °C (oils, fuel oils, glycerin, etc.).

Ignition temperature is the lowest value of the temperature of a liquid at which the intensity of its evaporation is such that, after ignition by an external source, independent flaming combustion occurs. For flammable liquids, the ignition temperature is usually 1-5 °C higher than the flash point, and for flammable liquids 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 danger solid combustibles substances and materials are characterized by the calorific value of 1 kg of the substance, combustion, self-ignition and ignition temperatures, burnout rate and combustion propagation over the surface of 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 flammable substances measuring 10~5-10~7 cm can remain suspended in the air for a long time, forming a dispersed system - an air suspension. To ignite an air suspension, it is necessary that the concentration of dust in the air is 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, powdered sugar - 1350 g/m3).

The thermal energy of the ignition source to ignite the dust-air mixture must be on the order of several MJ or more.

Depending on the value of the lower concentration limit of ignition, dusts are divided into explosive and fire hazardous. Explosive dusts include dusts with a lower flammable concentration limit of up to 65 g/m3 (dust of sulfur, sugar, flour), and fire hazardous dusts include dusts with a lower flammable limit above 65 g/m3 (tobacco and wood dust).

The fire hazard of substances and materials is characterized by; and such properties as the tendency of some substances and materials to electrify and spontaneously ignite when in contact with air (phosphorus, sulfur 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 generating sparks, flames, radiant heat, as well as loss of load-bearing capacity and destruction.