Fire extinguishing with flammable and combustible liquids is based on the analysis of all variants of their development. The fires occurring in the tanks are more prolonged, therefore they require a large amount of funds and forces to eliminate.

Storage tanks for flammable and combustible liquids

For storage of flammable and combustible liquids, containers made of metal, reinforced concrete, ice ground and synthetic material are used. The most popular are steel tanks. They are classified by design and capacity into:

• vertical in the form of a cylinder, with a conical or spherical roof, with a volume of 20 thousand cubic meters for storing flammable liquids and 50 thousand cubic meters for storing combustible liquids;
• vertical in the form of a cylinder, with a fixed roof and a floating pontoon, with a volume of 50 thousand cubic meters;
• vertical in the form of a cylinder, with a floating roof, with a volume of 120 thousand cubic meters.

The development of a fire in a tank

Extinguishing the fires of storage tanks for flammable and combustible liquids depends on the complexity of the ignition development process. Combustion starts due to the explosion of the gas-air mixture in the presence of an ignition source. The formation of a gas-polluted environment occurs due to the properties of GZH and HFL, as well as operating modes and climatic conditions around the reservoir. Exploding, the gas-air mixture rushes upward at high speed, often tearing off the roof of the container, after which ignition begins over the entire surface of the stored combustible liquid.

The further fate of the flame will depend on the site where it began, its dimensions, the fire resistance of the tank structure, weather conditions, the actions of workers and fire-fighting systems.

When storing combustible liquids and flammable liquids, for example, in tanks made of reinforced concrete during an explosion, part of it is destroyed, and combustion begins precisely in this area, which within the next 30 minutes leads to the complete destruction of the container and the spread of fire. Other types of containers, in the absence of cooling from the side, are deformed within 15 minutes, provoking a spill of flammable liquids and the spread of fire.

Foam fire extinguishing

Extinguishing flammable and combustible liquids with low and medium expansion foam is the most demanded way to fight the flame. The advantage of foam is that it insulates the surface of the flammable liquid from the flame, which leads to a decrease in its evaporation and, accordingly, the volume of flammable gases in the air. This forms a foaming agent solution with cooling properties. Thus, convective heat and mass transfer is achieved, and the temperature level becomes the same throughout the depth of the container within 15 minutes from the beginning of the application of the foam.

Foam extinguishing

Extinguishing flammable liquids with a foam solution of various multiplicity depends on where the combustion takes place:

• low expansion rate for the lower part of the tank, used for the "under-layer" extinguishing method, the extinguishing agent contains a fluorine-containing film-forming foaming agent, due to which, when the foam rises through the layer of combustible contents, it is not saturated with hydrocarbon vapors, retains its extinguishing ability; obtained with the help of low expansion foam barrels;
• medium expansion rate for surface extinguishing, the foam is also inert, does not interact with flammable liquids vapors, cools the liquid, helps to reduce the formation of an explosive air mixture; are obtained with the help of specialized foam generators of the GPS type.

After the extinguishing of flammable and combustible liquids is completed, a thick foam layer forms on the surface of the liquid, which protects it from the resumption of combustion.

When supplying fire extinguishing foam, the flame should be at an intensity of 0.15 l / s.

Foam fire extinguishing is allowed by three methods:

• delivery of foam concentrate using a foam lifter and other similar equipment;
• delivery of foam to the surface of burning flammable and combustible liquids using monitors;
• delivery of foam by means of sub-layer quenching.

Water fire extinguishing

If it is not possible to extinguish flammable liquids fires with the help of foam, it is allowed to use sprayed water, which helps to cool the combustible contents to a temperature at which its flashing is impossible.

In this case, the flow rate of the aqueous solution must be at least 0.2 l / s.

Powder quenching

Fighting fires in storage tanks for flammable liquids using powder is suitable for situations when combustion occurs in the area of ​​valves, flange joints or gaps between the roof and the wall of the tank. The flow rate must exceed 0.3 kg / s. The powder is not able to cool the liquid, therefore, it may be necessary to extinguish the flammable liquid again.

Powder extinguishing - only for minor fires and quick extinguishing

To avoid such situations, powder fire extinguishing is combined with foam in the following ways:

• maximum extinguishing of the flame with a foam solution, after which individual foci of flame are localized with the help of powder;
• elimination of the flame with the help of the powder component, followed by the supply of a foaming agent to cool the damaged surface and prevent the resumption of combustion.

In this case, it is prohibited to reduce the volume of supplied fire extinguishing agents.

Tank fire fighting plan

It is advisable to start extinguishing flammable liquids and combustible liquids in tanks with an assessment of the current situation, as well as with a calculation of the required means and forces. In the event of such an emergency, a voluntary fire brigade should be organized, the head of which will be the responsible person for managing the process of eliminating the flame and distributing tasks among the participants in the firefighting.

The person in charge must determine the volume of the territory on which the extinguishing work will be carried out, organize the elimination of unauthorized persons into the danger zone.

Upon arrival at the place of fire, the leader conducts reconnaissance and indicates to other participants in the firefighting areas where maximum forces should be thrown.

Throughout the entire work, the tasks of the manager include the provision of all available forces and means of cooling flammable and combustible liquids in tanks, as well as the choice of the optimal method of combating fire.

When the main forces are thrown into work with a burning container, it is important to protect neighboring tanks in case the damaged one collapses, or the formed gas-air mixture explodes. It is for this purpose that all fire trucks are installed at a safe distance, and hose lines are laid to the place of work.

Extinguishing tank farms with flammable and combustible liquids directly depends on the duration of combustion, the nature of the resulting destruction of tanks, the volume of stored liquids in damaged and adjacent containers, the likelihood of an explosion and subsequent accidental spillage of the contents.

During the design and construction of tank farms, a sewerage system must be envisaged, into which water can be drained for the fire extinguishing process, and devices for emergency pumping of the contents into a safe tank are designed.

How tanks are cooled during firefighting

Extinguishing fires with flammable and combustible liquids in tanks must be accompanied by cooling the contents of the damaged container. The latter needs to be cooled throughout its entire circumference. For adjacent tanks, there is also a requirement for mandatory cooling, but only along the entire length of the semicircle of the tank on the side facing the combustion zone. In some cases, it is allowed not to carry out the cooling procedure for adjacent containers if there is no threat of flame spread. The water supply for cooling purposes must be at least 1.2 l / s.

For extinguishing reservoirs with combustible liquids and flammable liquids with a volume of 5 thousand cubic meters, it is recommended to use fire monitors, which not only provide the required water return power, but also have a mode of irrigation of a burning object.

The sequence of work with adjacent undamaged containers is such that the first to be protected and cooled are those located on the leeward side of the fire site.

The duration of operation is determined until the flame is completely extinguished and the temperature level inside the container is normalized.

Dangerous areas in combustion in tank farms

Extinguishing fires with flammable and combustible liquids should also be carried out taking into account hazardous factors and zones that can reduce the effectiveness of fire extinguishing measures:

1. Formation of zones where it is impossible to deliver the extinguishing agent.
2. Warming up the combustible contents of the tank to a depth of 1 m or more.
3. Low air temperature around the fire site.
4. Lighting of several containers at the same time.

Extinguishing a real fire of flammable liquids bottling of a large area Angarsk 2014:

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

Just like solids, flammable liquids give off vapors when they burn. The vaporization process differs only in speed - for liquids it happens much faster.

The hazard level of flammable liquids depends on the flash point - the lowest temperature of a condensed substance at which vapors above it can flare up under the influence of an ignition source, but combustion does not occur after its elimination. Also, the hazard of flammable liquids is influenced by the flash point, flammability range, evaporation rate, reactivity under the influence of heat, density and diffusion rate of vapors.

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

Class B fires

A class B fire can result from the combustion of the following materials:

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

1. Varnishes, paints, enamels. Water-based fluids are less hazardous than oil-based fluids. The flash point of the 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, emitting large amounts of thick black smoke and toxic gases. When paints or varnishes ignite, the containers in which they are located often explode.

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

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

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

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

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

Esters and terpenes burn while boiling on their surface.

In the process of burning petroleum products, oils and fats, a poisonous irritating gas, acrolein, is released.

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

The surrounding objects and containers with burning liquids should be cooled with water. There are several ways to extinguish a class B fire:

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

Paraffins and other similar refined products. Putting them out with water is strictly prohibited and dangerous. Small fires can be suppressed with carbon dioxide fire extinguishers. Large fires - with foam.

Combustion is a complex physical and chemical process of interaction of a combustible substance and an oxidizer, characterized by a self-accelerating chemical excess and accompanied by the release of a large amount of heat and radiant energy.

For the onset and development of the combustion process, a combustible substance, an oxidizing agent and an ignition source are required, initiating the reaction between the fuel and the oxidizing agent. Combustion is distinguished by a variety of types and characteristics. Depending on the state of aggregation of combustible substances, combustion can be homogeneous and heterogeneous. With homogeneous combustion, the components of the combustible mixture are in the same state of aggregation (more often in gaseous form). Moreover, if the reacting components are mixed, then the combustion of the premixed mixture occurs, which is sometimes called kinetic (since the combustion rate in this case depends only on the kinetics of chemical transformations). If the gaseous components are not mixed, then diffuse combustion occurs (for example, when a flow of combustible vapors enters the air). The combustion process is limited by the diffusion of the oxidant. Combustion characterized by the presence of phase separation in a combustible system (for example, combustion of liquid and solid materials) is heterogeneous. Combustion is also differentiated by the speed of flame propagation, and depending on this factor, it can be deflagration (within a few m / s), explosive (tens and hundreds of m / s) and detonation (thousands of m / s). In addition, combustion is laminar (layer-by-layer propagation of the flame front through a fresh combustible mixture) and turbulent (mixing of the flow layers with an increased burnout rate).

As a rule, fires are characterized by heterogeneous diffuse combustion, and the rate of combustion depends on the diffusion of atmospheric oxygen in the environment. The emergence and development of fires significantly depends on the degree of fire hazard of substances. One of the criteria for the fire hazard of solid, liquid and gaseous substances is the autoignition temperature, i.e. the ability of a substance to ignite spontaneously.

For the initiation of an endogenous fire, a substance must be present that can quickly oxidize at low temperatures, as a result of which spontaneous combustion can occur. This property of the substance is called chemical activity to spontaneous combustion. As a result of oxidation and heat accumulation, self-heating turns into ignition.

Ignition is a qualitatively new process different from self-heating, characterized by high oxidation rates, heat release and light emission. Self-heating and self-ignition are generated by separate small nests, and therefore, it is very difficult to detect it.

Spontaneous combustion occurs due to the accumulation of heat inside the substance and does not depend on the influence of an external heat source.

All substances, according to their danger in relation to spontaneous combustion, can be divided into four groups:

* Substances that can ignite spontaneously when in contact with air at normal temperatures (vegetable oils, drying oil, oil paints, primers, brown and black coals, white phosphorus, aluminum and magnesium powder, soot, etc.);

* substances that can ignite spontaneously at elevated ambient temperatures (50 ° C and above) and as a result of external heating to temperatures close to the temperatures of their ignition and spontaneous combustion (films of nitro-lacquers, pyroxylin and nitroglycerin powders, vegetable semi-drying oils and drying oils made from them, turpentine etc.);

* substances, the contact of which with water causes the combustion process (alkali metals, carbides of alkali metals, calcium carbide, aluminum, etc.);

* substances that cause spontaneous combustion of combustible substances in contact with them (nitric, magnesium, hypochlorous, chloride and other acids, their anhydrides and salts; sodium, potassium, hydrogen peroxides, etc.; gases - oxidizing agents - oxygen, chlorine, etc.).

The most important characteristic of solid bulk materials is the degree of their flammability.

All materials, regardless of the field of application, are divided into three groups:

* Fireproof materials, which, under the influence of fire or high temperature, do not ignite, do not smolder or char.

* Flame retardant materials, which, under the influence of fire or high temperature, ignite, smolder or char and continue to burn or smolder in the presence of a fire source, and after removing the fire source, combustion and smoldering cease.

* Combustible materials, which, when exposed to fire or high temperature, ignite or smolder and continue to burn or smolder after removal of the fire source.

Certain chemicals, fuels and lubricants in certain concentrations and conditions are capable of not only igniting from heat sources, but also exploding.

The fire hazard of substances (gaseous, liquid, solid) is determined by a number of indicators, the characteristics and quantity of which depend on the state of aggregation of the given substance.

Fire hazard criteria for solid, liquid and gaseous substances are: flash point, ignition and self-ignition temperature, flame spread index, oxygen index, smoke production coefficient, indicator of toxicity of combustion products, etc.

One of the criteria for the fire hazard of flammable liquids is the flash point.

Flash point of vapor A combustible liquid is the minimum temperature of the liquid at which, under normal pressure, the liquid releases vapor over its free surface in an amount sufficient to form a mixture with the surrounding air that flares up when an open flame is brought to it.

For flammable liquids(Flammable liquids) include liquids that can burn independently after removing the ignition source and have a flash point of no higher than 61 °? in a closed crucible and 66 ° C in an open crucible.

For flammable liquids(ГЖ) include liquids that can burn independently after removing the ignition source and have a flash point above 61 °? in a closed crucible and 66 ° C in an open crucible.

Ignition temperature is called the minimum temperature at which a liquid heated under certain conditions ignites when a flame is brought to it and burns for (at least) 5 s. The flash point is more dangerous than the flash point, since the vapors and liquid, if ignited, continue to burn after the flame has been removed.

During construction work, especially when preparing mastics, painting works, it is necessary to clearly know the degree of flammability of nearby materials and structures, properly organize control to prevent fires and provide the necessary amount of extinguishing agents.

Depending on the type of combustible material, fires are divided into classes: A, B, C and D (Fig. 4.2.1.).

Fires are accompanied by dangerous and harmful phenomena that must be taken into account in the design and construction of buildings and structures, in the conduct of work. From the point of view of fire safety, it is very important to make the right planning decision, to offer protection of building structures, to provide for the necessary escape routes.

Explosion is a kind of burning and is characterized by extremely fast processes of physical and chemical transformations of combustible substances with the formation of huge amounts of thermal energy, practically without heat dissipation into the environment.

There are two concentration limits for the explosiveness of substances.

The minimum concentration of gas, vapor or dust in a mixture with air that can ignite or explode is calledlower flammable limit (LP).

The highest concentration of gases or vapors in the air, at which ignition or explosion is still possible (further, with an increase in concentration, ignition or explosion is considered impossible)n calledupper flammable limit (VP).

Explosion differs from combustion by an even greater speed of fire propagation. So, the speed of flame propagation in an explosive mixture in a closed tube is 2000 - 3000 m / s. Combustion of a mixture at such a rate is called detonation... The occurrence of detonation is explained by the compression, heating and movement of the unburned mixture ahead of the flame front, which leads to acceleration of flame propagation and the appearance of a shock wave in the mixture. The air shock waves formed during the explosion of the gas-air mixture have a large reserve of energy and propagate over considerable distances. While driving, they destroy structures and can cause accidents. The assessment of the danger of air shock waves for people and various structures is carried out according to two main parameters - pressure in the front of the shock wave? P and compression f. The compression phase is understood as the time of action of excess pressure in the wave. When f? 11 ms, a pressure of 0.9-113 Pa is considered safe for people. Calculations of safe distances for people with a potential explosion threat are carried out only by the pressure in the shock front, since in explosions, f is always many times greater than 11 ms.

Lecture 13

COMBUSTION OF LIQUIDS

The consumption of liquid fuels in the world economy is currently reaching gigantic proportions and continues to grow steadily. This leads to the constant development of the oil and oil refining industries.

Liquid fuel has now become the most important strategic raw material, and this circumstance leads to the need to create huge reserves. Ensuring fire safety in the production, transportation, processing and storage of liquid fuel is the most important task of the fire protection authorities.

Fluid ignition

The most important property of a liquid is its ability to evaporate. As a result of thermal motion, part of the molecules, overcoming the forces of surface tension of the liquid, passes into the gas zone, forming a vapor-air mixture above the surface of the flammable liquid, combustible liquid. Due to the Brownian motion in the gas zone, the opposite process also takes place - condensation. If the volume above the liquid is closed, then at any temperature of the liquid a dynamic equilibrium is established between the processes of evaporation and condensation.

Thus, above the surface (mirror) of the liquid there is always a vapor-air mixture, which in equilibrium is characterized by the pressure of saturated vapors of the liquid or their concentration. As the temperature rises, the saturated vapor pressure increases according to the Cliperon-Clausius equation:

where rnp - saturated steam pressure, Pa;

Qevap - heat of vaporization - the amount of heat required to convert a unit mass of liquid into a vapor state, kJ / mol;

T- liquid temperature, K.

From (7.1) it follows that with an increase in the temperature of the liquid, the pressure of saturated vapors (or their concentration) increases exponentially (Fig. 7.1). Thus, for any liquid there is always such a temperature range at which the concentration of saturated vapors above the mirror will be in the ignition region, i.e., HKJIB<ф п< ВКПВ

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where Tvs is the flash point (ignition), K;

Рвс - partial pressure of saturated vapor of liquid at flash (ignition) temperature, Pa;

NS- the number of oxygen molecules required for the complete oxidation of one fuel molecule;

V- constant of the determination method.

Flame spread over the surface of the liquid.

Analysis of the effect of combustion conditions on the speed of flame propagation

The property of a flame to spontaneous propagation occurs not only in the case of combustion of mixtures of combustible gases with oxidizing agent, but also when burning liquids and solids. With local exposure to a heat source, for example, an open flame, the liquid will warm up, the evaporation rate will increase, and when the surface of the liquid reaches the ignition temperature at the point of impact of the source, the vapor-air mixture will ignite and a stable flame will be established, which will then propagate at a certain speed over the surface of the cold liquid.

What is the driving force behind the propagation of the combustion process and what is its mechanism?

Flame propagation over the liquid surface occurs as a result of heat transfer by radiation, convection and molecular heat conduction from the flame zone to the surface of the liquid mirror.

According to modern concepts, the main role in this is played by heat radiation from a flame. The flame, having a high temperature (more than 1000 ° C), is capable, as is known, of emitting thermal energy. According to the Stefan-Boltzmann law, the intensity of the radiant heat flux given off by a heated body is determined by the ratio:

where ε - degree of blackness,

σ - Stefan - Boltzmann constant, = 2079 ´ 10-7 kJ / (m2 h K4)

T f, T w- t of the flame and liquid surface, K

This heat is spent on evaporation ( q1) and warming up ( q11) liquid in depth.

Qf = q1 + q11 = r´ r´ W +r´ U´ (Tzh - T0)´ c, where

r- heat of vaporization, kJ / g

r- density, g / cm3

W- linear burnout rate, mm / h

U- heating rate in depth, mm / h

T0- initial t-ra of liquid, K

with- specific heat capacity of liquid, J / (g K)

The maximum temperature of the liquid is equal to its boiling point.

In a steady-state combustion process, an equilibrium is observed between the rate of evaporation and the rate of burnout.

The upper layer of the liquid is heated to a higher temperature than the lower ones. The temperature at the walls is higher than in the middle of the tank.

Thus, the speed of flame propagation through the liquid, i.e., the path traversed by the flame per unit time, is determined by the rate of heating of the liquid surface under the influence of the radiant heat flux from the flame, i.e., by the rate of formation of a combustible vapor-air mixture above the liquid mirror.

Water sharply reduces the boiling point of oil and fuel oil. When burning oil containing water, water boils up, which leads to the overflow of the burning liquid over the side of the tank (the so-called boiling of the burning liquid.

Above the surface of an open reservoir, the concentration of vapors will be different along the height: at the surface it will be maximum and correspond to the concentration of saturated vapor at a given temperature, and as it rises above the surface, it gradually decreases due to convective and molecular entrainment (Fig, 7.3).

Thus, above the surface of the liquid mirror in an open reservoir at any initial temperature of the liquid is higher than Tst, there will be an area in which the concentration of vapors in the air will be stoichiometric. At liquid temperature T2 this concentration will be at a height Well from the surface of the liquid, and at a temperature T3 greater than T2, at a distance H ^ Zst. At a temperature close to the flash point of the liquid TV, the propagation of the flame over the surface of the liquid will be equal to the speed of its propagation through the mixture of vapors in the air, at the NKPV, i.e. 3-4 cm / s. In this case, the flame front will be located at the surface of the liquid. With a further increase in the initial temperature, the speed of flame propagation through the liquid will increase similarly to the change in the normal speed of flame propagation through the vapor-air mixture with an increase in its concentration.

Lecture 14

Burnout rate of liquids, influencing factors.

At a certain temperature, above tвс, once the ignited liquid continues to burn after the ignition source is removed. This minimum temperature is called the flash point (tbos). For flammable liquids it is higher than tvs by 1-5 ° C, for flammable liquids - by 30-35 ° C.

Linear burn-up rate is the height of the liquid column, which burns out per unit time:

The mass burnup rate is the mass of liquid that burns out per unit time from a unit of surface area:

There is a relationship between the linear and mass combustion rates:

(you should follow the dimensions of the quantities and, if necessary, enter the correction factor).

Warming up the liquid in depth. Heating the surface of a liquid by a radiant flow from a flame is accompanied by the transfer of heat deep into it. This heat transfer is carried out mainly by thermal conductivity and laminar convection due to the movement of heated and cold layers of liquid. The heating of the liquid by thermal conductivity is carried out to a shallow depth (2-5 cm) and can be described by an equation of the form

where Th- temperature of the liquid layer at a depth NS, TO;

TC- surface temperature (boiling point), K; To- coefficient of proportionality, m - TO

This type of temperature field is called the first kind temperature distribution.

Laminar convection occurs as a result of different temperatures of the liquid at the walls of the tank and in its center, as well as as a result of fractional distillation in the upper layer during the combustion of mixtures. Additional heat transfer from the heated walls of the reservoir to the liquid leads to heating of its layers near the walls to a higher temperature than in the center. A liquid that is more heated near the walls (or even vapor bubbles if it is overheated near the walls above the boiling point) rises, which contributes to intensive mixing and rapid heating of the liquid layer to a greater depth. A so-called homothermal layer is formed, that is, a layer with an almost constant temperature, the thickness of which increases with the time of combustion. Such a temperature field is called a temperature distribution of the second kind (Figure 7.7). The formation of a homothermal layer is also possible as a result of fractional distillation of the surface layers of mixtures of liquids with different boiling points. As such liquids burn out, the near-surface layer is enriched with denser high-boiling fractions, which descend, thereby facilitating convective heating of the liquid.

The decisive influence of liquid overheating at the walls of the reservoir on the formation of a homothermal layer is confirmed by the following experimental data. When gasoline burned in a tank with a diameter of 2.64 mm without cooling the walls, it led to a fairly rapid formation of a homothermal layer. With intensive cooling of the walls, the heating of the liquid to depth was carried out mainly by thermal conductivity, and during the entire combustion time, a temperature distribution of the first kind took place. It was found that the higher the boiling point of the liquid (diesel fuel, transformer oil), the more difficult it is to form a homothermal layer. When they burn, the temperature of the walls of the tank rarely exceeds the boiling point. However, when humid high-boiling petroleum products are burned, the likelihood of the formation of a homothermal layer is also high. When the walls of the tank are heated to 100 ° C and above, water vapor bubbles are formed, which, rushing upward, cause intensive mixing of the entire liquid and rapid heating in depth. The possibility of the formation of a sufficiently thick homothermal layer during the combustion of wet oil products is fraught with the phenomena of boiling and liquid ejection.

Based on the concepts considered above about the mechanism of liquid burnout, let us analyze the influence of some factors on the mass velocity.

Burnout rate depends on: type of liquid, temperature, diameter of the tank, liquid level, wind speed.

For small burners the combustion rate is comparatively high. With an increase in diameter, the speed first decreases due to heating from the walls, then increases, since laminar combustion turns into turbulent and remains constant at diameters ³ 2 m.

With turbulent combustion, the completeness of combustion is lower (soot appears), the heat flux from the flame increases, vapors are removed faster, and the rate of evaporation increases.

When the liquid level drops the processes of heat and mass transfer are hindered (the outflow of combustion products, the inflow of the oxidizer, the flame moves away from the surface of the liquid), therefore the combustion rate decreases and at a certain distance of the liquid from the upper side of the tank (critical height of self-extinguishing) combustion becomes impossible. The critical self-extinguishing height at Æ = 23 m is equal to 1 km (the actual height of the reservoir = 12 m).

Estimating the share of heat from the total heat release during the combustion of a liquid, which is spent on its preparation, it follows that less than 2% of the total heat release during the burning of a liquid is spent on the supply of its vapors to the combustion zone. At the moment of the establishment of the burnout process, the temperature of the liquid surface rises sharply from the ignition temperature to the boiling point, which subsequently remains unchanged as the burnout progresses. However, this is only true for individual fluids. In the process of combustion of a mixture of liquids with different boiling points (gasoline, oil, etc.), their fractional distillation occurs, as it were. First, there is a release of low-boiling fractions, then all higher-boiling ones. This process is accompanied by a gradual (quasi-stationary) increase in temperature on the surface of the liquid. Wet fuel can be represented as a mixture of two liquids (fuel + water), during the combustion of which their fractional distillation occurs. If the boiling point of the combustible liquid is less than the boiling point of water (100 ° C), then the fuel burns out predominantly, the mixture is enriched with water, the burnout rate decreases and, finally, combustion stops. If the boiling point of the liquid is more than 100 ° C, on the contrary, initially, moisture predominantly evaporates, its concentration decreases: the rate of burnout of the liquid increases, up to the rate of combustion of a pure product (Fig. 7.11).

Influence of wind speed. As a rule, as the wind speed increases, the burn-out rate of the liquid increases. The wind intensifies the process of mixing the fuel with the oxidizer, increasing the flame temperature and bringing the flame closer to the burning surface.

All this increases the intensity of the heat flux entering the heating and evaporation of the liquid, therefore, leads to an increase in the burnout rate. At higher wind speeds, the flame can break off, which will lead to the cessation of combustion. So, for example, when a tractor kerosene burned in a tank with a diameter of 3 "M, the flame was blown out when the wind speed reached 22 m-s-1.

Influence of oxygen concentration in the atmosphere. Most fluids are incapable of burning in an atmosphere with an oxygen content of less than 15%. With an increase in oxygen concentration above this limit, the burnup rate increases (Fig. 7.12). In an atmosphere enriched with oxygen, the combustion of the liquid proceeds with the release of a large amount of soot in the flame and intense boiling of the liquid phase is observed. For multicomponent liquids (gasoline, kerosene, etc.), the surface temperature increases with an increase in the oxygen content in the environment (Fig. 7.13).

An increase in the burnup rate and temperature of the liquid surface with an increase in the oxygen concentration in the atmosphere is due to an increase in the emissivity of the flame as a result of an increase in the combustion temperature and a high content of soot in it.

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Combustion of liquids.

All flammable liquids are capable of evaporating, 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. Combustion 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 the ignition of the mixture from an open ignition source 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 vapors with air is not stable, which is necessary for such combustion. The amount of heat released during the flash is not enough to continue burning, and the substance is not yet sufficiently heated. In order to ignite a liquid, you need not a short-term, but a long-term ignition source, the temperature of which would be higher than the autoignition temperature of a mixture of vapors of this liquid with air.

In accordance with GOST 12.1.004-76, a combustible liquid (GZH) 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 highly flammable liquid (FL) 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).

Flash point is the lowest temperature at which a liquid becomes especially dangerous in terms of fire, therefore, its value is adopted as the basis for classifying flammable liquids according to their degree of 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 of saturated vapors in the air in a closed volume is capable of igniting when exposed to an ignition source is called the lower temperature limit of ignition. The temperature of the liquid at which the concentration of saturated vapors in the air in a closed volume can still ignite when exposed to an ignition source is called the upper temperature limit of ignition.

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

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

The temperature limits indicate in which temperature range the liquid vapors will form combustible mixtures with air.