OOO CIB Controls. Flash, ignition and self-ignition temperatures. The flash point is the temperature at which an oil product heated under standard conditions. The concept of flash point

Flash point of petroleum products called the temperature at which the vapors of the sample, heating up, flare up when a source of fire is brought up, mixing with air. The flash point is measured in open And closed crucible, and for the first this value is always several degrees higher.

Determining the flash point is important for reliable information about the properties of an oil product and assessing its quality. Also, this parameter is used to divide industrial premises and equipment into fire hazard classes.

Methods of determination

GOST offers 2 main methods for determining the flash point:

- in a closed crucible,
- in an open crucible.

crucibles - chemical vessels designed for heating, melting, burning and other operations with experimental materials, including various fuels.

An open cup test is less accurate because sample vapors mix freely with air and take longer to build up to the required volume. IN oil product quality certificate the flash point in a closed crucible (TVZ) is indicated as the most reliable.

To measure it, the vessel is filled with fuel to the specified mark and heated with continuous stirring. When the lid of the vessel is opened, an open fire automatically appears above the surface of the mixture. The measurement is carried out every degree of heating, and during the opening of the lid, the stirring stops. The flash point is the value at which a bluish flame occurs with the appearance of a fire source.

There are also special devices to determine the flash point. Such a device includes the following elements:

600 W electric heater,
standard vessel with an inner diameter of 50.8 mm and a capacity of about 70 ml,
brass stirrer,
igniter (electric or gas),
thermometers with graduation in 1⁰С.

Flash point of various petroleum products

According to the flash point, liquid petroleum products are classified into flammable liquids (flammable liquids) And combustible liquids (GZH) . The flash point of flammable liquids is above 61⁰С for a closed crucible and above 65⁰С for an open one. Liquids flashing at temperatures below these values are classified as flammable. LVZH are divided into 3 categories:

1. Especially dangerous (TVZ from -18⁰С and below).
2. Constantly dangerous (TVZ from -18⁰С to 23⁰С).
3. Dangerous when the air temperature rises (TVZ from 23⁰С to 61⁰С).

Flash point of diesel fuel is one of the important indicators of its quality. It directly depends on the type of fuel. For example, a modern diesel fuel EURO flashes when it reaches a value of 55⁰С and above.

The flash point of fuel for diesel locomotives and marine engines is higher than for diesel fuel for general use. And summer fuel, when heated, flares up 10-15⁰С earlier than winter and arctic fuel.

Light oil fractions have a low TTR and vice versa. For example:

flash point of engine oil (heavy oil fractions) - 130-325⁰С,
flash point of kerosene (medium kerosene and gas oil fractions) - 28-60⁰С,
the flash point of gasoline (light gasoline fractions) is up to -40⁰С, that is, gasoline flashes at sub-zero temperatures.

The flash point of oil is determined fractional composition, but basically its values are negative (as for gasolines) and range from -35⁰С to 0⁰С. And the flash point of gases, as a rule, is not determined at all. Instead, upper and lower flammability limits are used, which depend on the amount of gas vapor in the air.

FLASH AND FLASH POINT. Combustible substances, especially liquid ones, are found, depending on the conditions in which they are located, three types of combustion that are separate from each other: flash, ignition and ignition; an explosion can be considered as a special case of a flash. A flash is a rapid, but relatively calm and short-term combustion of a mixture of vapors of a combustible substance with oxygen or air, resulting from a local increase in temperature, which can be. caused by an electrical spark or by touching a mixture of a hot body (solid, liquid, flame). The phenomenon of a flash is like an explosion, but, unlike the latter, it occurs without a strong sound and does not have a destructive effect. Flash is distinguished from ignition by its short duration. Ignition, arising, like an outbreak, from a local increase in temperature, can then last until the entire supply of combustible substance is exhausted, and vaporization occurs due to the heat released during combustion. In turn, ignition is different from ignition, since this latter does not require an additional local increase in temperature.

All types of combustion are associated with the spread of heat from the area where combustion has occurred to the adjacent areas of the combustible mixture. During a flash, heat release in each section is sufficient to ignite an adjacent section of an already prepared combustible mixture, but not enough to replenish it by evaporating new quantities of fuel; therefore, having exhausted the supply of combustible vapors, the flame goes out, and the flash ends there, until combustible vapors accumulate again and receive local overheating. When ignited, the vapor-forming substance is brought to such a temperature that the heat from the combustion of the accumulated vapors is sufficient to restore the stock of the combustible mixture. The ignition that has begun, having reached the surface of the combustible substance, becomes stationary until the combustible substance burns out completely; but, however, once stopped, the ignition is no longer renewed without a local overheating applied from the outside. Finally, when ignited, the combustible substance is at a temperature sufficient not only for vaporization, but also for the flash of a continuously formed combustible mixture, without additional local heating. In this latter case, combustion, if it were stopped, for example, by cutting off the free access of oxygen, occurs spontaneously after the elimination of the obstructing cause: a spontaneous outbreak will go further into ignition.

The possibility of burning one type or another depends primarily on the chemical composition of the combustible mixture, i.e., the chemical nature of combustible vapors, the oxygen content in the mixture, on the content of extraneous indifferent impurities, such as: nitrogen, water vapor, carbon dioxide, and on the content of impurities, actively opposing combustion reactions, for example, negative catalysts, silencers, etc. And since all types of combustion process begin with a flash, consideration of a flash in its dependence on the chemical composition of the mixture is of general importance for all cases. It is obvious in advance that under given conditions of pressure and temperature, a mixture of combustible vapor or gas with oxygen (or air) may not flare in any proportion, and that a very small or, conversely, too high fuel content in the mixture excludes a flare. In addition, different combustible vapors require different amounts of oxygen for their combustion, and therefore the "flash limits" of mixtures of oxygen and combustible vapors always depend on the type of combustible vapor. The method of calculating these limits for chemically individual substances was indicated by Thornton. If we denote by N the number of oxygen atoms necessary for the complete combustion of M molecules of a combustible substance in a gaseous or vaporous form, then, according to Thornton, the limits of mixtures that retain the ability to flash can be expressed:

If the mixture contains not pure oxygen, but air, then it must be taken into account that 1 volume of oxygen is contained in 5 (more precisely, 4.85) volumes of air. So, for example, the combustion of methane can be expressed by the equation:

so for this case, M = 1 and N = 4. Hence, the composition of the upper limit for a mixture of methane with oxygen is given by:

from here it is easy to calculate that the upper flash limit for a mixture of methane with air is determined by a ratio of 1:5, i.e., with a content of 1/6 methane in the mixture, or 16.7% (experiment gives 14.8%). For the lower limit, we similarly have the composition of the mixture CH 4 (1 volume) + 6 O (3 volumes), which corresponds to the content of methane in the mixture with air 1/16, or 6.25% (experiment gives 5.6%). Similarly, for pentane, C 6 H 12, we get M \u003d 1 and N \u003d 16, from which 1/21, or 4.75%, of pentane mixed with air is calculated for the upper limit (experiment gives 4.5%), for the lower 1/76, or 1.35% (experience gives 1.35%). Since the values of M and N in Thornton's formulas are proportional to the partial vapor pressures of the combustible substance and oxygen, it is obvious that a flash is possible only within certain limits of the partial vapor pressure, and its limits change with temperature. It is also obvious that a flash becomes possible when the saturated vapor pressure reaches a known value. Knowing this value and the dependence of vapor pressure on temperature, it is possible to calculate the temperature at which a flash is possible. Studies by E. Mack, C. E. Burd and G. N. Borgem showed that for most substances, at the lower limit of the flash, a fairly good agreement between the calculated temperature and the directly observed temperature is observed.

Vapor mixtures are also in some cases subject to the specified method of determining the temperature at which a flash is possible. If this is a mixture of naphthenes C n H 2 n, then in all homologues the ratio of the content of C to H is the same, so that the average molecular weight of the mixture makes it possible to determine the number of CH 2 groups and, consequently, the amount of their O required for combustion. In addition In addition, the flash point here is an almost linear function of the molecular weight and the associated boiling point. For a mixture of methane hydrocarbons C n H 2 n+2 (for example, gasoline), the number N is also calculated from the average molecular weight. After subtracting 2 from it (for two hydrogen atoms at the end of the chain) and dividing the residue by 14 (the sum of the atomic weights of the CH 2 group), the number of these groups is obtained, which corresponds to the average molecular weight of the mixture. If this number is multiplied by 3 and 1 is added, for two previously neglected hydrogen atoms, then N is obtained. So, for gasoline, the average molecular weight is 107 and therefore:

With an increase in the pressure of the mixture, the partial elasticity of the combustible vapor increases, and therefore the flash point also increases. An increase in pressure by 1 mm increases the flash point of Mexican oil cuts by 0.033°, as shown by Loman, who studied the flash at different heights (according to Golde, who worked with other materials, this change is 0.036°). Especially for kerosene, there is a correction table that allows you to bring the flash point found at any barometric pressure to normal. In addition to atmospheric pressure, the flash point also changes the humidity of the air, since the partial elasticity of water vapor lowers the pressure of the combustible component of the mixture.

Flash evaporating liquid. The flash of a ready mixture of gases or vapors is the simplest case. The flash phenomenon is more complicated, when the flashing mixture arises continuously from the evaporation of the immediately located liquid. The flash of a gas mixture also depends on many experimental conditions: increasing the width of the explosive burette, transferring the explosive spark from top to bottom, increasing the capacity of the vessel, lengthening the spark gap, etc. - all this expands the limits of a possible flash. In addition, some, as yet insufficiently studied, impurities can significantly change these limits. The question of a flash of fog from an atomized combustible liquid was investigated by Gider and Wolf. The lower limit of the flash turned out to be the same here as for the mixture with the corresponding vapor; but the speed of propagation of the explosion in the fog is less, and the consumption of oxygen is greater than in the case of vapors. The state of the surface of the liquid, its volume, the distance to the ignition flame, the rate of exchange of outside air and the resulting vapors, the rate of evaporation, and, consequently, the power of the heat source heating the liquid, the thermal conductivity of the walls of the vessel, the thermal conductivity and viscosity of the liquid itself, the loss of heat by the vessel through radiation, etc. d. - all this can significantly change the observed flash point and in addition to the factors indicated in the discussion of the flash of a gas mixture. Therefore, one can speak about the flash as a constant only conditionally, conducting the experiment only under precisely defined conditions. For chemically individual substances, Ormandy and Crevin established the proportionality of the flash and boiling points (in absolute degrees):

where the coefficient k for the lower flare limit is 0.736, and for the upper 0.800; T° b.p. should be determined by the initial reading of the thermometer. The formula of Ormandy and Crevin also extends to a certain extent to very narrow fractions of various kinds of mixtures. However, for those combustible liquids that in most cases have to be dealt with in practice, i.e., for complex mixtures, simple relationships that determine the flash point have not yet been found. Even binary mixtures do not follow the mixing rule with respect to the flare, and the low-flashing component significantly reduces the flare of the other, which is highly flaring, while this latter slightly increases the flare of the first. So, for example, a mixture of equal amounts of fractions (gasoline and kerosene components) of a specific gravity of 0.774 with a flash at 6.5 ° and a specific gravity of 0.861 with a flash at 130 ° does not have a flash point at 68.2 °, as one would expect from the mixing rule , and at 12°. At 68.2°, a mixture containing only about 5% of the lighter component flashes, so that this small admixture lowers the flash point of the heavier component by 61.8°. However, the result of testing such mixtures in an open crucible, where vapors of the volatile component cannot accumulate, is not so distorted by impurities, especially if the difference in flashes in both components is significant. In some cases, such mixtures can give a double flash at different temperatures.

Ignition. The ignition temperature exceeds the flash point the more significantly, the higher the flash point itself. As shown by Kunkler and M. V. Borodulin, when oil products are heated from flash to ignition, the test substance loses about 3% of its weight, and this loss relates to lighter cuts. Therefore, the presence of small amounts (not more than 3%) of light distillates, which significantly distorts the flash point of a substance, does not interfere with accurate measurement of the ignition temperature. Conversely, the presence of more than 10% gasoline in the oil makes the ignition point undetermined.

Spontaneous combustion, or self-ignition, of a mixture of combustible vapors occurs when the heat release of an oxidizing system is equalized with heat loss, and therefore even an insignificant acceleration of the reaction leads to a violent process. Obviously, the temperature equilibrium boundary changes with the same composition of the mixture depending on its mass, thermal conductivity and heat-emitting ability of the shell containing the combustible mixture, on the ambient temperature, the presence of catalysts in the mixture and a number of other conditions, so that the spontaneous combustion temperature has a certain value only under strictly defined conditions. The dependence of the autoignition temperature on the presence or absence of catalyzing platinum is proved, for example, by the data of E. Constant and Schlönfer (Table 1).

The dependence of the autoignition temperature on the presence of oxygen or air in the mixture is shown by the data of the same researchers (Table 2).

S. Gvozdev's study of spontaneous combustion of various substances in quartz and iron tubes in an atmosphere of oxygen and air gave results that are compared in Table. 3.

In relation to spontaneous combustion, experience has established some general provisions, namely: 1) pressure lowers the temperature of spontaneous combustion; 2) the presence of moisture also lowers the spontaneous combustion temperature; 3) in air, the spontaneous combustion temperature is higher than in oxygen; 4) the temperature of spontaneous combustion in an open tube is higher than in a closed space; 5) the auto-ignition temperature of cyclohexane hydrocarbons is lower than that of aromatic hydrocarbons and is close to the auto-ignition temperature of saturated hydrocarbons; 6) for aromatic hydrocarbons, the spontaneous combustion temperatures in air and oxygen are close to each other; 7) some substances (turpentine, alcohols) give very fluctuating self-ignition temperatures during a successive series of tests (especially turpentine). A special case of spontaneous combustion is fibrous materials (cotton, fleece, wool, rags) impregnated with oils; the ease of self-ignition in such cases is related to the self-ignition temperature of the respective oils. Phenomena of this kind are of such significant practical importance that special methods and instruments have been developed for testing the ability of oils to ignite spontaneously in the presence of cotton.

Measurement of flash and fire points. Being closely related to molecular weight and boiling point, flash and ignition are indirectly related to these constants and therefore characterize a given substance. They are even more important in practice, when judging the degree of flammability of a substance under given conditions of use and, consequently, for establishing preventive measures, a circumstance that is especially important in industry (petroleum, wood processing, alcohol, varnish, oil) and in general in all when dealing with volatile solvents.

The need to measure flash and ignition temperatures led to the construction of numerous, often expensive, special devices and to the development of instructions for working with them, and in individual industries, in relation to certain classes of substances, even related to each other, various devices with different instructions were built and standardized. . Having no rational basis, varying from country to country, from one industrial organization to another and from one class of substances to another, methods of measuring flash and ignition give results that are consistent with each other only very approximately. The main types of devices for measuring the flash point are: a) with an open vessel, b) with a closed vessel.

but) Open Vessel Appliances. The flash point measurement was originally made by pouring the test liquid onto the water contained in the cup; this latter was then heated. Later flash in an open vessel began to be made by hl. arr. in relation to substances that are difficult to flash, for example, lubricating oils, gas coal tars, various mastics, etc. These are the devices of Marcusson, Brenken, Cleveland, Moore, de Graaff, Krupp, which differ mainly in size, shape and material of the crucible, the design of the heating parts and the method of conducting heating. Details on the handling of these devices can be found in the dedicated manuals. It should be noted that the protrusion of the mercury column of the thermometer outside the crucible and its presence in an environment with different temperatures in different places leads to the need for a significant correction, which increases with an increase in the flash or ignition temperature, for example, up to 10-14 °, when the flash point is 300 °. The true flash point is calculated using the formula:

where θ is the directly observed flash (or ignition) temperature, n is the number of degrees of the part of the mercury column outside the test liquid, and t" is the temperature corresponding to the middle of the protruding part of the mercury column; although t "m. b. calculated, but usually it is measured directly, using an additional thermometer. To quickly find this correction, a special table is used. A special table also serves for corrections for barometric pressure, which are especially important when determining the flash point of flammable liquids (kerosene); for the latter, devices with a closed vessel are usually used.

b) Closed Vessel Appliances. Of the various instruments of this kind, the best known are those of Abel and Martens (both improved by Pensky), Elliot (New York), Tag. In the USSR and some other countries (Germany, Austria), the Abel-Pensky device for low-boiling liquids (kerosene) and the Martens-Pensky device for high-boiling liquids (oils) are used almost exclusively. The working part of these devices consists of a strictly standardized crucible, tightly covered with a lid, in which, at certain intervals, a window is opened to introduce a small flame into the crucible. The crucible contains a thermometer and a stirrer. The heating of the crucible, and in some cases, on the contrary, cooling, is carried out under strictly defined conditions, using special baths. The devices adopted in different countries for testing kerosene, and the normal flash points for the corresponding tests, are compared in Table. 4.

The readings of various devices in determining the flash point always diverge from each other, and the determination of a flash in an open vessel always gives a temperature higher than in a closed device. This is due to the fact that in closed devices, vapors gradually accumulate in the device, while in an open vessel they constantly diffuse into the surrounding atmosphere. The size of these discrepancies can be judged on the basis of the data in Table. five.

This table also shows that the difference between the flash point in closed and open devices increases with increasing flash point, and also, as the last two examples show, with increasing heterogeneity of the product. In this regard, the presence of a large difference in the flash point for the same substance when determining its flash in open and closed devices indicates either an admixture to a heavy substance, for example, oil, some light substance (gasoline, kerosene) or some distillation defects (decomposition with the formation of easily volatile products). Thus, comparing the flash point of the same substance in open and closed devices can serve to control the correctness of both the use and production of lubricating oils.

To create NKPP vapor above the surface of a liquid, it is sufficient to heat to a temperature equal to NTPRP, not the entire mass of the liquid, but only its surface layer.

In the presence of IS, such a mixture will be capable of ignition. In practice, the concepts of flash point and ignition point are most often used.

Under flash point understand the lowest temperature of a liquid at which, under the conditions of special tests, a concentration of liquid vapor is formed above its surface, capable of igniting from IZ, but the rate of their formation is insufficient for subsequent combustion. Thus, both at the flash point and at the lower temperature limit of ignition above the surface of the liquid, a lower concentration limit of ignition is formed, however, in the latter case, HKPRP is created by saturated vapors. Therefore, the flash point is always slightly higher than NTPRP. Although at the flash point there is a short-term ignition of vapors in the air, which is not capable of turning into a stable combustion of a liquid, nevertheless, under certain conditions, an outbreak of liquid vapors can be a source of fire.

The flash point is taken as the basis for the classification of liquids into flammable (flammable liquids) and combustible liquids (FL). Flammable liquids include liquids with a flash point in a closed crucible of 61 0 C or in an open crucible of 65 0 C and below, GZH - with a flash point in a closed crucible of more than 61 0 C or in an open crucible of 65 0 C.

I category - especially dangerous flammable liquids, these include flammable liquids with a flash point of -18 0 C and below in a closed crucible or from -13 0 C and below in an open crucible;

II category - permanently dangerous flammable liquids, these include flammable liquids with a flash point above -18 0 C to 23 0 C in a closed crucible or from -13 to 27 0 C in an open crucible;

Category III - flammable liquids, dangerous at elevated air temperatures, these include flammable liquids with a flash point of 23 to 61 0 C in a closed crucible or from 27 to 66 0 C in an open crucible.

Depending on the flash point, safe methods for storing, transporting and using liquids for various purposes are established. The flash point of liquids belonging to the same class naturally changes with changes in the physical properties of the members of the homologous series (Table 4.1).

Table 4.1.

Physical properties of alcohols

	Molecular	Density,	Temperature, K
	Molecular	Density,
Methyl CH 3 OH
Ethyl C 2 H 5 OH
n-propyl C 3 H 7 OH
n-Butyl C 4 H 9 OH
n-Amylic C 5 H 11 OH

The flash point increases with increasing molecular weight, boiling point and density. These patterns in the homological series indicate that the flash point is related to the physical properties of substances and is itself a physical parameter. It should be noted that the pattern of changes in the flash point in the homologous series cannot be extended to liquids belonging to different classes of organic compounds.

When mixing flammable liquids with water or carbon tetrachloride, the pressure of flammable vapors at that the same temperature decreases, which leads to an increase in the flash point. Can be diluted with fuel liquid to such an extent that the resulting mixture will not have a flash point (see table. 4.2).

Fire extinguishing practice shows that the combustion of liquids that are highly soluble in water stops when the concentration of the combustible liquid reaches 10-25%.

Table 4.2.

For binary mixtures of combustible liquids that are highly soluble in each other, the flash point is between the flash points of pure liquids and approaches the flash point of one of them, depending on the composition of the mixture.

FROM rise in temperature of the liquid evaporation rate increases and at a certain temperature reaches such a value that, once ignited, the mixture continues to burn after the ignition source is removed. This liquid temperature is called flash point. For flammable liquids, it differs by 1-5 0 С from the flash point, and for GZh - by 30-35 0 С. At the ignition temperature of liquids, a constant (stationary) combustion process is established.

There is a correlation between the flash point in a closed crucible and the lower ignition temperature limit, which is described by the formula:

T sun - T n.p. \u003d 0.125T sun + 2. (4.4)

This relation is valid for T sun< 433 К (160 0 С).

The significant dependence of the flash and ignition temperatures on the experimental conditions causes certain difficulties in creating a calculation method for estimating their values. One of the most common of them is the semi-empirical method proposed by V. I. Blinov:

, (4.5)

where T sun - flash point, (ignition), K;

p sun - partial pressure of saturated vapor of the liquid at the flash point (ignition), Pa;

D 0 - diffusion coefficient of liquid vapor, m 2 / s;

n is the number of oxygen molecules required for the complete oxidation of one fuel molecule;

Temperature limits of ignition. The temperature of a liquid at which a saturated vapor concentration equal to the lower concentration limit of ignition is created above the surface is called lower temperature limit of ignition (NTPV).

The temperature of a liquid at which a saturated vapor concentration equal to the upper concentration limit of ignition is created above the surface is called upper temperature limit of ignition (VTPV).

For example, for acetone, the temperature limits are: LTPV 253 K, VTPV 279 K. At these temperatures, vapor concentrations are formed, respectively, 2.6 and 12.6% (vol.).

Flammable temperature limits are used to assess the fire hazard of liquids, when calculating the safe operation of closed technological apparatus and storage containers with liquids and volatile solids. For the fire safety of a technological process associated with the use of liquids, the latter is carried out at temperatures below the LTLW by 10 K or above the LTLW by 15 K. For many liquids, temperature limits have been determined and the results are summarized in reference tables.

Temperature limits can be calculated. The calculation method is used for the approximate determination of the temperature limits of ignition in order to find the expected temperature limits before the start of their experimental determination, as well as for the approximate calculation of the safe modes of operation of technological equipment at the stage of pre-design development of the technological process in the absence of experimental data. The flammable temperature limits can be calculated using saturation vapor pressure data at various temperatures, using the formula

where R 1, R 2- closest to R p lower and higher table values of vapor pressure corresponding to temperatures T 1 And T 2.

Temperature limits of ignition can be calculated from experimentally determined concentration limits. If the calculated value does not match the experimental value, then the lower value for LTPV and the higher value for LTPV are taken as the actual value. The temperature limits are calculated as follows.

Determine the vapor pressure R n And R in substance corresponding to the lower and upper concentration limits of vapors in air

If P total= 101080 Pa, then R in=1010 From to And R n = 1010 C n, where R n And R in- experimental values of the lower and upper concentration limits of ignition of vapors in air, % (vol.).

According to the found values R n And R in calculate the temperature flammable limits using the formulas above and the vapor pressure versus temperature tabular data.

Flash point. The flash point is the lowest temperature (under special test conditions) of a substance at which vapors and gases are formed above its surface that can flare up in air from an ignition source, but the formation rate is still insufficient for subsequent combustion.

This term is used to characterize flammable liquids and is included in many standards. According to GOST 12.1.004-90 (Fire safety. General requirements), liquids capable of burning are divided into flammable (FL) and combustible (FL). Flammable liquids are liquids with a flash point not higher than 61 0 C (in a closed crucible) or 65 0 C (in an open crucible). GZh are liquids with a flash point above 61 0 С (in a closed crucible) or 66 0 С (in an open crucible).

I category - especially dangerous flammable liquids, these include flammable liquids with a flash point of -18 0 C and below in a closed crucible or from -13 0 C and below in an open crucible;

Category II - permanently hazardous flammable liquids, these include flammable liquids with a flash point above -18 0 C to 23 0 C in a closed crucible or above -13 0 C to 27 0 C in an open crucible;

III category - flammable liquids, dangerous at elevated air temperatures, these include flammable liquids with a flash point above 23 0 C to 61 0 C in a closed crucible or above 27 0 C to 66 0 C in an open crucible.

Depending on the flash point, safe methods for storing, transporting and using liquids for various purposes are established. the flash point of liquids belonging to the same class naturally changes with a change in the physical properties of the members of the homologous series (Table 5.2).

From the data in Table. 5.2 shows that the flash point increases with increasing molecular weight, boiling point and density. These patterns in the homological series indicate that the flash point is related to the physical properties of substances and is itself a physical parameter. It should be noted that the pattern of changes in the flash point in the homologous series cannot be extended to liquids belonging to different classes of organic compounds.

Table 5.2

Physical properties of alcohols

Molecular mass

Density, kg / m 3

Temperature, K

Methyl CH 3 OH

Ethyl C 2 H 5 OH

n-Propyl C 3 H 7 OH

n-Butyl C 4 H 9 OH

n- Amyl C 5 H 11 OH

When combustible liquids are mixed with water or carbon tetrachloride, the pressure of combustible vapors at the same temperature decreases, which leads to an increase in the flash point. It is possible to dilute a flammable liquid to such an extent that the resulting mixture does not have a flash point:

solution, % …………………

Flash point, 0 С

methyl alcohol …………

ethyl alcohol …………..

Fire extinguishing practice shows that the combustion of liquids that are highly soluble in water stops when the concentration of the combustible liquid reaches 10-25%.

With an increase in the temperature of the liquid, the evaporation rate increases and at a certain temperature reaches such a value that, once the mixture has been ignited, it continues to burn after the ignition source has been removed.

This liquid temperature is commonly called the ignition temperature. For flammable liquids, it differs by 1 - 5 0 С from the flash point, and for GZH - by 30 - 35 0 С. At the ignition temperature of liquids, a constant (stationary) combustion process is established.

5.3. The process of burning liquids. Burnout rate

The combustion of liquids is accompanied not only by a chemical reaction (the interaction of a combustible substance with atmospheric oxygen), but also by physical phenomena, without which combustion is impossible. The interaction of combustible vapors with air oxygen occurs in the combustion zone, into which combustible vapors and air must continuously flow. This is possible if the liquid receives a certain amount of heat necessary for evaporation. Heat in the combustion process comes only from the combustion zone (flame), where it is continuously released. Heat from the combustion zone to the surface of the liquid is transferred by radiation. Heat transfer by thermal conduction is impossible, since the rate of vapor movement from the liquid surface to the combustion zone is greater than the rate of heat transfer through them from the combustion zone to the liquid. Heat transfer by convection is also impossible, since the vapor flow in the flame volume is directed from the less heated surface (liquid) to the more heated surface.

The amount of heat emitted by a flame depends on its degree of blackness and temperature. The degree of blackness of the flame is determined by the concentration of carbon released in the flame of the liquid when the liquid burns. For example, the degree of blackness of the flame during the combustion of oil and oil products in large tanks is close to unity.

The amount of heat coming from the torch QR per unit time per unit surface of the liquid, can be determined by the formula

where e is the degree of emissivity; s is the Stefan-Boltzmann constant, equal to 2079 × 10 -7 kJ / (m 2 × h × K 4); Т f is the temperature of the torch flame, K; Tf is the liquid surface temperature, K.

This heat is spent on the evaporation of the liquid, its heating from the initial temperature to the surface temperature, i.e. heating the liquid in depth:

where r is the heat of evaporation, kJ/h; r– density, g/cm3; v– linear burning rate, mm/h; u is the rate of liquid heating in depth, mm/h; T p is the liquid surface temperature, K; T 0 is the initial temperature of the liquid, K; from is the specific heat capacity of the liquid, J/(g×K).

In this way,

In a steady-state combustion process (i.e., at a constant flame temperature), an equilibrium is observed between the amount of substance burned in the combustion zone (flame) and the mass of steam entering the flame. This determines the constant rate of evaporation and, consequently, the burnout of the liquid during the entire combustion process.

Burning rate of liquids. There are two burning rates of liquids - mass and linear. Bulk speed G called the mass of liquid (kg), burning out per unit time (h, min) from a unit surface. Under linear speed v combustion of a liquid understand the height of its layer (mm, cm), which burns out per unit time:

where r- density of the liquid, kg / m 3; h is the height of the burnt liquid layer, mm; t- burning time.

Knowing or determining the linear burnout rate, it is possible to calculate the mass burnout rate and vice versa.

The burning rate of liquids is not constant and varies depending on the initial temperature, tank diameter, liquid level in the tank, wind speed and other factors. For burners of small diameters, the combustion rate is relatively high. As the diameter increases, the combustion rate first decreases and then increases until it reaches a certain constant value for a given liquid. This dependence is due to various reasons. The combustion rate in small burners is significantly affected by the walls, since the flame, in contact with them, heats the upper edge to a high temperature. From the upper edge, heat conduction spreads over the entire wall and is transferred to the liquid. This additional influx of heat from the side of the wall increases the rate of evaporation of the liquid. An increase in the burning rate with an increase in diameter is associated with the transition from laminar to turbulent combustion. This transition is accompanied by a decrease in the completeness of combustion, and a large amount of soot released contributes to an increase in the blackness of the flame, which leads to an increase in the heat flux from the flame. With turbulent combustion, the fastest removal of vapors from the surface of the liquid is ensured, and the rate of evaporation increases.

The burning rate in large tanks increases slightly with increasing diameter. It is believed that the burning rate in tanks with a diameter of more than 2 m is almost the same.

A strong wind contributes to the mixing of vapors with air, an increase in the temperature of the flame, as a result of which the intensity of combustion increases.

As the level of the liquid in the tank decreases, the distance from the flame to the surface of the liquid increases, therefore, the flow of heat to the liquid decreases. The combustion rate gradually decreases, and at a certain critical distance of the liquid surface from the edge of the bead, self-extinguishing may occur. This distance is called critical height ; it increases with increasing tank diameter. For large tanks, the dependence of the burning rate on the height of the free side is of no practical importance, since the height of standard tanks is always much less than the critical height. Thus, the calculation shows that self-extinguishing in a tank with a diameter of 23 m can occur at a height of more than 1 km. The actual height of the tank is 12 m.

The flash point is that at which its vapors briefly flash above the surface of a liquid combustible substance heated in a crucible. Usually, the flash does not go into combustion, since the rate of formation of combustible vapors at this temperature is less than the rate of their combustion. Burning with a flame occurs later, at a higher temperature, called the ignition (or ignition) temperature.

This parameter is of key importance in the technique of using all types of flammable liquids, since it allows you to set the rules and limits for their safe handling, determine the purity of the fuel, the presence of hazardous additives, detect counterfeit products, and reliably calculate the operating modes of engines and power plants.

The flash point of liquid fuels is measured by two methods - open and closed crucible. They differ in that in the latter method, vapors cannot escape into the surrounding space, and the flash occurs at a lower temperature. The flash point in an open cup is always higher, and this temperature difference increases with the absolute value of the parameter.

In our country, two methods for determining the flash point in an open crucible are standardized in GOST 4333-87 - Cleveland and Brenken. Another standard - GOST 6356-75 - establishes a similar technique for a closed crucible.

Measuring principle

The study is carried out on a domestic device such as TVO.

Both GOSTs establish the following procedure for measuring flash points.
Petroleum products are poured into an open (or closed) metal cup-shaped crucible up to the marked mark on the inner wall. The crucible is installed in the device on the asbestos surface of the heating device, with the help of a tripod, the thermometer is fixed so that the mercury head is inside the liquid at a height of at least 8 mm from the bottom of the crucible in the center of the circle. Turn on the heating, set the desired temperature rise rate.

Every 2 ºС above the liquid surface, it is carried out in the horizontal direction with the tip of a gas burner with a flame no longer than 4 mm. When a brief blue flash of vapor occurs, the temperature is recorded. This is the desired value. When the liquid is further heated, it ignites with a red flame. Record the ignition temperature.

When studying a flash in a closed crucible, a gas igniter with constant burning is placed under the lid. Vapors in such a crucible accumulate faster, the flash occurs earlier.

Some data on the measurement of flash temperatures

Today, there are more advanced than TVO devices for determining flash points. They are characterized by high measurement accuracy, automation of operations, friendly interfaces, high productivity, and therefore greatly facilitate the work of operators in busy laboratories.

The open crucible technique is used to study substances with low volatile vapor pressure - mineral oils, residual petroleum products. Closed cup analyzes are more applicable to liquids with highly volatile vapors. The results of studies using both methods can have significant differences (up to two tens of ºС).

Substances with flash points in a closed cup below 61 ºС are classified as flammable. They, in turn, are divided into especially dangerous (T flare ≤ -18 ºС), dangerous (T flare from -18 ºС to +23 ºС) and dangerous at elevated temperature (T flare from 23 ºС to 61 ºС) .

For diesel fuel, the flash point in an open crucible ranges from 52 to 96 ºС, for gasoline - -43 ºС. Self-ignition temperature for gasoline - 246 ºС, for diesel fuel - 210 ºС. Since the latter is not ignited in the combustion chamber of the internal combustion engine, but self-ignites, it becomes clear why it is characterized by such a high flash point compared to gasoline and a lower auto-ignition temperature.

The flash point of a fuel in an open cup is an important informative parameter of a liquid fuel used to determine the quality of a product.

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