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Combustion of liquids.

All flammable liquids are capable of evaporation, and their combustion occurs only in the vapor phase located above the surface of the liquid. The amount of vapor depends on the composition and temperature of the liquid. Burning of vapors in air is possible only at a certain concentration.

The lowest temperature of a liquid at which the concentration of its vapors in a mixture with air ensures ignition of the mixture from open source ignition without subsequent stable combustion is called the flash point. At the flash point, stable combustion does not occur, since at this temperature the concentration of the mixture of liquid vapor and air is not stable, which is necessary for such combustion. The amount of heat released during the flash is not enough to continue combustion, and the substance is not yet heated enough. In order to ignite a liquid, you need not a short-term, but a long-lasting ignition source, the temperature of which would be higher than the self-ignition temperature of the mixture of vapors of this liquid with air.

In accordance with GOST 12.1.004-76, a flammable liquid (FL) is understood as a liquid that can burn independently after removing the ignition source and has a flash point above +61 ° C (in a closed crucible) or +66 ° C (in an open crucible).

A flammable liquid (FLL) is a liquid that can burn independently after removing the ignition source and has a flash point not higher than +61 ° C (in a closed crucible) or +66 ° C (in an open crucible).

The flash point is the lowest temperature at which a liquid becomes particularly dangerous in terms of fire, therefore its value is taken as the basis for the classification of flammable liquids according to the degree of their fire hazard. The fire and explosion hazard of liquids can also be characterized by the temperature limits of ignition of its vapors.

The temperature of the liquid at which the concentration saturated vapors in air in a closed volume is capable of igniting when exposed to an ignition source, called the lower temperature limit of ignition. The temperature of the liquid at which the concentration of saturated vapors in air in a closed volume can still ignite when exposed to an ignition source is called the upper ignition temperature limit.

The temperature limits of ignition of some liquids are given in table. 29.

Table 29 Temperature limits of ignition for some liquids: acetone, A-76 gasoline, benzene, tractor kerosene, ethyl alcohol.

Temperature limits show in what temperature range liquid vapors will form flammable mixtures with air.

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According to the “Rules for the Construction of Electrical Installations”, the definition of a flammable liquid sounds quite succinct - it is a liquid that flares up at a temperature above 61℃, and then continues to burn independently without external initiation or influence. A flammable liquid according to the PUE is a gas liquid with a flash temperature of no more than 61℃, and those that have an evaporation pressure of at least 100 kPa at T = 20℃ are explosive.

GCs are classified as flammable materials, but they are explosive if they are heated to flash temperatures during the technological process.

Such a preliminary categorization of protection objects makes it possible to adopt organizational, technical solutions by choice, installation, suitable for requirements regulatory documents, for example, such as types, types, incl. explosion-proof flame detectors, smoke detectors for alarm systems, stationary fire extinguishing systems; to eliminate primary sources of fire in premises with the presence of flammable liquids and gases.

Additional information in the table:

Name of material	Analogue or original material	Net calorific value	GJ density	Specific burnout rate	Smoke generating ability	Oxygen consumption	CO2 release	CO release	HCL isolation
		Q n	R	Ψ beat	Dm	L O 2	L CO 2	L CO	LHCl
		MJ/kg	kg/m 3	kg/m 2 s	Np m 2 /kg	kg/kg	kg/kg	kg/kg	kg/kg
Acetone	Chemical substance; acetone	29,0	790	0,044	80,0	-2,220	2,293	0,269	0
Gasoline A-76	Gasoline A-76	43,2	745	0,059	256,0	-3,405	2,920	0,175	0
Diesel fuel; solarium	Diesel fuel; solarium	45,4	853	0,042	620,1	-3,368	3,163	0,122	0
Industrial oil	Industrial oil	42,7	920	0,043	480,0	-1,589	1,070	0,122	0
Kerosene	Kerosene	43,3	794	0,041	438,1	-3,341	2,920	0,148	0
Xylene	Chemical substance; xylene	41,2	860	0,090	402,0	-3,623	3,657	0,148	0
Medicines containing ethyl alcohol and glycerin	Medicines a drug; ethyl. alcohol + glycerin (0.95+0.05)	26,6	813	0,033	88,1	-2,304	1,912	0,262	0
Oil	Raw materials for petrochemicals; oil	44,2	885	0,024	438,0	-3,240	3,104	0,161	0
Toluene	Chemical substance; toluene	40,9	860	0,043	562,0	-3,098	3,677	0,148	0
Turbine oil	Coolant; turbine oil TP-22	41,9	883	0,030	243,0	-0,282	0,700	0,122	0
Ethanol	Chemical substance; ethanol	27,5	789	0,031	80,0	-2,362	1,937	0,269	0

Source: Koshmarov Yu.A. Predicting indoor fire hazards: A tutorial

Fire class of flammable liquids

Due to their parameters, flammable and combustible liquids when burning both in enclosed spaces of production, warehouse buildings, technological structures, and in open industrial areas; where external oil processing plants are located, gas condensate, chemical organic synthesis apparatus, storage facilities for raw materials, finished commercial products in the event of fire outbreaks or fire spread, classify it as class B.

The fire class symbol is applied to containers with flammable liquids, flammable liquids, and their storage facilities, which allows you to quickly right choice, reducing the time for reconnaissance, localization and elimination of fires of such substances and their mixtures; minimize material damage.

Classification of flammable liquids

The flash point of a flammable liquid is one of the main parameters for classifying and assigning flammable liquids to one type or another.

GOST 12.1.044-89 defines it as the lowest temperature of a condensed substance that has vapor above the surface that can flare up in air environment premises, or open space when applying a low-calorie source of open flame; but a stable combustion process does not occur.

And the flash itself is considered to be the instantaneous burning out of an air mixture of vapors and gases over the surface of a flammable liquid, which is visually accompanied by a short period of visible glow.

The value of T℃, obtained as a result of tests, for example, in a closed laboratory vessel, at which the gas fluid flares up, characterizes its explosion fire danger.

Important parameters for GZh, LVZh specified in this state standard, also the following parameters:

• The ignition temperature is the lowest temperature of flammable liquids that emit flammable gases/vapors with such intensity that when a source of open fire is brought in, they ignite and continue to burn when it is removed.
• This indicator is important when classifying flammability groups of substances, materials, hazards technological processes, equipment in which gas fluids are involved.
• T of self-ignition is minimum temperature GZ, in which self-ignition occurs, which, depending on the prevailing conditions in the protected room, storage facility, building technological equipment– the apparatus or installation may be accompanied by open flame combustion and/or explosion.
• The data obtained for each type of gas liquid capable of self-ignition allows you to select suitable types explosion-proof electrical equipment, incl. for installations of buildings, structures, structures; for the development of explosion and fire safety measures.

For information: “PUE” defines a flash by the rapid burnout of a flammable air mixture without the formation of compressed gas; and an explosion is an instantaneous combustion with the formation of compressed gases, accompanied by the appearance of a large amount of energy.

The speed and intensity of evaporation of flammable liquids and flammable liquids from the free surface with open tanks, containers, and process plant housings are also important.

Fires of gas liquids are also dangerous for the following reasons:

• These are spreading fires, which are associated with the spilling, free spreading of flammable liquids across the premises or territory of enterprises; if measures for isolation are not taken - diking of storage tanks and external technological installations; the presence of construction barriers with walls installed in the openings.
• Fires of gas liquids can be both local and volumetric, depending on the type, storage conditions, and volume. Since volumetric combustion intensively affects the load-bearing elements of buildings and structures, it is necessary.

You should also:

• Install on air ducts ventilation systems premises where there are gas liquids to limit the spread of fire through them.
• Conduct for shift, operational/duty personnel, organize those responsible for the fire safety condition of storage, processing, transportation, transit of flammable liquids, gas, leading specialists, engineering staff; conducting regular practical training with members of the DPD of enterprises and organizations; tighten the process, carry out strict control over the place where they are held, incl. after finishing.
• Install on smoke and exhaust pipes of heating, power units, furnaces, install on pipelines of the technological chain for transporting flammable liquids and gases across the territory of production enterprises.

The list, of course, is far from complete, but everything necessary measures can be easily found in the regulatory and technical database of documents on industrial safety.

How to properly store flammable and liquid liquids is probably the question most people ask. The answer can be found in the “Technical Regulations on Fire Safety Requirements” dated July 22, 2008 No. 123-FZ, in Table 14 Categories of warehouses for storing oil and petroleum products. More detailed information on storage and distance to objects is presented in. (SP 110.13330.2011)

Class B fires are extinguished, according to the standards, as follows:

• Air-mechanical foam obtained from aqueous solutions of a foaming agent. They are especially effective for extinguishing industrial and warehouse buildings.
• Fire extinguishing powder, what is it used for.
• Used for small premises and compartments, for example, fuel and lubricants warehouses, engine rooms.

The use of sprayed water to extinguish the flames of gasoline and other liquids with low temperature flash is difficult, since water drops cannot cool the heated surface layer below the flash point. The decisive factor in the mechanism of the fire extinguishing action of VMP is the insulating ability of the foam.

When the liquid combustion mirror is covered with foam, the flow of liquid vapor into the combustion zone stops, and combustion stops. In addition, the foam cools the heated layer of liquid with the released liquid phase - the compartment. The smaller the foam bubbles and the higher the surface tension of the foam solution, the higher the insulating ability of the foam. Inhomogeneity of the structure and large bubbles reduce the effectiveness of the foam.

Elimination of fires of flammable liquids and gases is also carried out for particularly important protection objects; as well as for premises with different types of fire loads, the fire of which is difficult or impossible to eliminate with one fire extinguishing agent.

Table of supply intensity of a 6 percent solution when extinguishing flammable liquids with air-mechanical foam based on foaming agent PO-1

According to . V.P. Ivannikov, P.P. Clews,

Substances	Solution supply rate l/(s*m2)
	Medium expansion foam	Low expansion foam
Spilled petroleum product from apparatus technological installation, in rooms, trenches, technological trays	0,1	0,26
Containerized storage facilities for fuels and lubricants	1	–
Flammable liquid on concrete	0,08	0,15
Flammable liquid on the ground	0,25	0,16
Petroleum products of the first category (flash point below 28 °C)	0,15	–
Petroleum products of the second and third categories (flash point 28 °C and above)	0,1	–
Gasoline, naphtha, tractor kerosene and others with a flash point below 28 0C;	0,08	0,12*
Kerosene for lighting and others with a flash point of 28 °C and above	0,05	0,15
Fuel oils and oils	0,05	0,1
Oil in tanks	0,05	0,12*
Oil and condensate around the fountain well	0,06	0,15
Spilled flammable liquid on the territory, in trenches and technological trays (at the normal temperature of the leaking liquid)	0,05	0,15
Ethyl alcohol in tanks, pre-diluted with water to 70% (supply 10% solution based on PO-1C)	0,35	–

Notes:

An asterisk indicates that extinguishing with low expansion foam oil and petroleum products with a flash point below 280 C is allowed in tanks up to 1000 m 3, excluding low levels(more than 2 m from the top edge of the tank side).

When extinguishing oil products using the foaming agent PO-1D, the intensity of supply of the foaming solution increases by 1.5 times.

Class “B” fires are the combustion of liquid substances that can be soluble in water (alcohols, acetone, glycerin) and insoluble (gasoline, oil, fuel oil).

As well as solids, flammable liquids release vapors when burned. The process of vaporization differs only in speed - in liquids it happens much faster.

The level of danger of flammable liquids depends on the flash point - the lowest temperature of a condensed substance at which vapors above it are able to ignite under the influence of an ignition source, but combustion does not occur after it is eliminated. Also, the degree of danger of flammable liquids is influenced by the ignition temperature, flammability range, evaporation rate, chemical reactivity under the influence of heat, density and vapor diffusion rate.

Flammable liquids are considered to be liquids with a flash point of up to 61°C (gasoline, kerosene), flammable liquids are those with a flash point above 61°C (acids, vegetable and lubricating oils).

Class B fires

The following materials can cause a class B fire:

• paints and varnishes;
• flammable and combustible liquids;
• liquefiable solids (paraffins, stearins).

1. Varnishes, paints, enamels. Liquids on water based less dangerous than oil ones. The flash point of oils contained in paints, varnishes and enamels is quite high (about 200°C), but the flammable solvents contained in them flare up much earlier - at a temperature of 32°C.

Paints burn well, producing large amounts of thick black smoke and toxic gases. When paints or varnishes catch fire, explosions often occur in the containers in which they are located.

It is impossible to extinguish paints, varnishes and enamels with water due to the low flash point. Water can only be used to cool surrounding objects or extinguish dry paint.

The burning of paints and varnishes is suppressed with foam, in some cases with carbon dioxide or powder fire extinguishers.

1. Flammable and combustible liquids. Their combustion is accompanied by the release of non-standard combustion products characteristic of such liquids.

Alcohols burn with a clear blue fire with a small amount of smoke.

The combustion of liquid hydrocarbons is characterized by an orange flame and the formation of thick, dark smoke.

Esters and terpenes burn accompanied by boiling on their surface.

During the combustion of petroleum products, oils and fats, a toxic, irritating gas, acrolein, is released.

Extinguishing flammable and combustible liquids is not an easy task, and each fire has its own characteristics and the sequence of its suppression. First, you need to block the flow of liquid into the fire.

Surrounding objects and containers with burning liquids should be cooled with water. There are different ways to extinguish a Class B fire:

• a foam or powder fire extinguisher or a spray of water can handle a small fire;
• in case of large spreading of flammable liquid, it is better to use powder fire extinguishers in conjunction with fire hoses to supply foam;
• if a liquid burns on the surface of water, then you must first limit its spreading, and then cover the flame with foam or a powerful water jet;
• When extinguishing equipment operating on liquid fuel, it is necessary to use sprayed water or foam.

Paraffins and others similar products oil refining. Extinguishing them with water is strictly prohibited and dangerous. Small fires can be suppressed carbon dioxide fire extinguishers. Large fires - with the help of foam.

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: Educational allowance 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 p.: 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 the air, a sufficient amount of oxygen must be present to ensure complete conversion of the substance 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 – an 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 closed pipe– (2000 – 3000 m/s).

The 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 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 decomposes when heated to form 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, 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 at which a mixture of air and flammable liquid vapor flashes is called flash point (t ref) The degree of fire hazard of 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). Characteristic Features flammable liquid is 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(tvsp =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 in determining the flammability group of substances, in assessing the fire hazard of equipment and technological processes associated with the processing of flammable substances, and 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 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 auto-ignition temperature of a particular substance on the volume and geometric shape flammable 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 high temperatures cannot exceed the heat removal rate. 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.

Facilities that process or use flammable liquids pose a major fire hazard. This is explained by the fact that flammable liquids are easily flammable, burn more intensely, form explosive vapor-air mixtures and are difficult to extinguish with water.
Combustion of liquids occurs only in the vapor phase. The rate of evaporation and the amount of liquid vapor 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 a state of saturation, the number of evaporating molecules is equal to the number of condensing ones, and the vapor concentration remains constant. Combustion of steam-air mixtures is possible only in a certain concentration range, i.e. they are characterized by concentration limits of flame propagation (NKPRP and VKPRP).
Lower (upper) concentration limits of flame propagation– 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.
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, the liquid must be heated to a certain temperature (not less than the lower temperature limit of flame propagation). 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.
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 special test conditions, vapors are formed above its surface that can ignite in the air from an ignition source; stable combustion does not occur. The flash point corresponds to the lower temperature limit of ignition.
Flash point used to assess the flammability of a liquid, as well as when developing measures to ensure fire and explosion safety of technological processes.
Ignition temperature is the lowest value of liquid temperature at which the intensity of its evaporation is such that, after ignition by an external source, independent flaming combustion occurs.
Depending on the numerical value of the flash point, liquids are divided into flammable (flammable) and combustible (GC).
Flammable liquids include liquids with a flash point of no more than 61 o C in a closed crucible or 66 o C in an open crucible.
For flammable liquids, the ignition temperature is usually 1-5 o C higher than the flash point, and for flammable 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.
Particularly dangerous flammable liquids– with a flash point of -18 o C and below in a closed crucible or from -13 o C and below in an open crucible. Particularly dangerous flammable liquids include acetone, diethyl alcohol, isopentane, etc.
Constantly 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.
Dangerous at elevated temperatures flammable liquids– these are flammable liquids with a flash point from 23 o C to 61 o 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.), naturally changes in the homologous series, increasing with increasing molecular weight, boiling point and density. The flash point is determined experimentally and by calculation.
The flash point is determined experimentally in closed and open type:
- in a closed crucible Martens-Pensky device according to the methodology set out in GOST 12.1.044-89 - for petroleum products;
– in an open crucible on the VNIIPO TV device according to the method given in GOST 12.1.044-89 - for chemical organic products and on the Brenken device according to the method set out in the same GOST - for petroleum products and oils.