Liquids with a flash point greater than 61. Flash point, ignition point and self-ignition point. Spontaneous combustion. combustion of solids

Flash point concept

Flash point is the temperature at which a petroleum product, heated under standard conditions, releases such an amount of vapor that it forms a flammable mixture with the surrounding air that flares up when a flame is brought to it.

For individual hydrocarbons there is a certain quantitative relationship between the flash point and boiling point, expressed by the ratio:

For petroleum products that boil over a wide temperature range, such a dependence cannot be established. In this case, the flash point of petroleum products is related to their average temperature boiling, i.e. with volatility. The lighter the oil fraction, the lower its flash point. Thus, gasoline fractions have negative (up to minus 40°C) flash points, kerosene fractions 28-60°C, oil fractions 130-325°C. The presence of moisture and decomposition products in the petroleum product significantly affects the value of its flash point. This is used in production conditions to make a conclusion about the purity of the kerosene and diesel fractions obtained during distillation. For oil fractions, the flash point indicates the presence of easily evaporating hydrocarbons. Of the oil fractions of various hydrocarbon compositions, the most high temperature oils from paraffinic low-sulfur petroleums have flashes. Oils of the same viscosity from resinous naphthenic aromatic oils are characterized by a lower flash point.

Methods for determining flash point

Two methods have been standardized for determining the flash point of petroleum products in open (GOST 4333-87) and closed (GOST 6356-75) crucibles. The difference in flash points of the same petroleum products when determined in open and closed crucibles is very large. In the latter case, the required amount of oil vapor accumulates earlier than in devices open type. In addition, in an open crucible, the resulting vapors diffuse freely into the air. The higher the flash point of the petroleum product, the greater the indicated difference. The admixture of gasoline or other low-boiling fractions in heavier fractions (with fuzzy rectification) sharply increases the difference in their flash points in open and closed crucibles.

When determining the flash point in an open crucible, the petroleum product is first dehydrated using sodium chloride, sulfate or calcium chloride, then poured into the crucible to a certain level, depending on the type of petroleum product. The crucible is heated at a certain speed, and at a temperature 10°C below the expected flash point, the flame of a burner or other ignition device is slowly passed along the edge of the crucible above the surface of the oil product. This operation is repeated every 2°C. The flash point is the temperature at which a blue flame appears above the surface of the oil product. When determining the flash point in a closed crucible, the oil product is poured to a certain mark and, in contrast to the method described above, it is heated with continuous stirring. When you open the crucible lid in this device, a flame is automatically brought to the surface of the oil product.

Determination of the flash point begins 10°C before the expected flash point - if it is below 50°C, and 17°C - if it is above 50°C. The determination is carried out every degree, and at the moment of determination, stirring is stopped.

All substances having a flash point in a closed crucible below 61°C are classified as flammable liquids(LVZH), which, in turn, are divided into:

• especially dangerous ( T ref below minus 18°C);
• constantly dangerous ( T ref from minus 18°C ​​to 23°C);
• dangerous at elevated temperatures ( T ref from 23°C to 61°C).

Explosion limits

The flash point of a petroleum product characterizes the ability of this petroleum product to form an explosive mixture with air. A mixture of vapors and air becomes explosive when the concentration of fuel vapors in it reaches certain values. In accordance with this, they distinguish lower And upper explosive limits mixtures of petroleum product vapors and air. If the concentration of petroleum product vapor is less than the lower explosive limit, an explosion does not occur, since the available excess air absorbs the heat released at the initial point of the explosion and thus prevents the ignition of the remaining parts of the fuel. When the concentration of fuel vapor in the air is above the upper limit, an explosion does not occur due to a lack of oxygen in the mixture. The lower and upper explosive limits of hydrocarbons can be determined, respectively, using the formulas:

In the homologous series of paraffin hydrocarbons with increasing molecular weight both the lower and upper explosive limits are reduced, and the explosive range is narrowed from 5-15% (vol.) for methane to 1.2-7.5% (vol.) for hexane. Acetylene, carbon monoxide and hydrogen have the widest explosion ranges and are therefore the most explosive.

As the temperature of the mixture increases, its explosive range narrows slightly. Thus, at 17°C the explosion range of pentane is 1.4-7.8% (vol.), and at 100°C it is 1.44-4.75% (volume). The presence of inert gases (nitrogen, carbon dioxide, etc.) in the mixture also narrows the explosion range. An increase in pressure leads to an increase in the upper explosive limit.

The explosive limits of vapors of binary and more complex mixtures of hydrocarbons can be determined by the formula:

Temperatureflashes called minimum temperature, in which petroleum product vapors form a mixture with air capable of short-term formation of a flame when an external source of ignition (flame, electric spark, etc.) is introduced into it.

A flash is a weak explosion that is possible within strictly defined concentration limits in a mixture of hydrocarbons and air.

Distinguish upper And lower concentration limit flame propagation. The upper limit is characterized by the maximum concentration of organic vapor in a mixture with air, above which ignition and combustion with the introduction of an external ignition source is impossible due to lack of oxygen. The lower limit is found at the minimum concentration of organic matter in the air, below which the amount of heat released at the site of local ignition is insufficient for the reaction to occur throughout the entire volume.

Temperatureignition is the minimum temperature at which the vapors of the test product, when introducing an external ignition source, form a stable, undying flame. The ignition temperature is always higher than the flash point, often quite significantly - by several tens of degrees.

Temperaturespontaneous combustion name the minimum temperature at which vapors of petroleum products mixed with air ignite without an external ignition source. The performance of diesel internal combustion engines is based on this property of petroleum products. The auto-ignition temperature is several hundred degrees higher than the flash point. The flash point of kerosene, diesel fuel, lubricating oils, fuel oil and other heavy petroleum products characterizes the lower explosive limit. The flash point of gasolines, the vapor pressure of which is significant at room temperatures, usually characterizes the upper explosive limit. In the first case, the determination is carried out during heating; in the second, during cooling.

Like any conditional characteristic, flash point depends on the design of the device and the conditions of determination. In addition, its value is influenced by external conditions - atmospheric pressure and air humidity. The flash point increases with increasing atmospheric pressure.

The flash point is related to the boiling point of the substance being tested. For individual hydrocarbons, this dependence according to Ormandy and Crewin is expressed by the equality:

Tsp = K T kip, (4.23)

where Tfsp is the flash point, K; K - coefficient equal to 0.736; T boil - boiling point, K.

Flash point is a non-additive value. Its experimental value is always lower than the arithmetic mean value of the flash temperatures of the components included in the mixture, calculated according to the rules of additivity. This is because the flash point depends mainly on the vapor pressure of the low-boiling component, while the high-boiling component serves as a heat transfer agent. As an example, we can point out that even 1% gasoline in the lubricating oil reduces the flash point from 200 to 170 ° C, and 6% gasoline reduces it by almost half. .

There are two methods for determining flash point - in closed and open type devices. Flash point values ​​of the same petroleum product determined in instruments various types, differ noticeably. For highly viscous products this difference reaches 50, for less viscous products it is 3-8°C. Depending on the composition of the fuel, the conditions for its self-ignition change significantly. These conditions, in turn, are associated with the motor properties of fuels, in particular, detonation resistance.

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

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

Under flash point understand the lowest temperature of a liquid at which, under special test conditions, a concentration of liquid vapor is formed above its surface that is capable of ignition from ignition, 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, but in the latter case, HKPRP is created by saturated vapor. 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 developing into a stable combustion of a liquid, nevertheless, under certain conditions, a flash of liquid vapors can be a source of fire.

The flash point is taken as the basis for classifying liquids into flammable liquids (FLL) and flammable liquids (CL). flammable liquids include liquids with a flash point in a closed crucible of 61 0 C or in an open crucible 65 0 C and below, GL - with a flash point in a closed crucible of more than 61 0 C or in an open crucible 65 0 C.

Category I – especially dangerous flammable liquids, these include highly 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 dangerous flammable liquids, these include highly 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;

III category – flammable liquids, dangerous at elevated air temperatures, these include highly flammable liquids with a flash point from 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 of storing, transporting and using liquids for various purposes are established. The flash point of liquids belonging to the same class changes naturally with changes in the physical properties of the members of the homologous series (Table 4.1).

Table 4.1.

Physical properties of alcohols

Flash point increases with increasing molecular weight, boiling point and density. These patterns in the homologous series indicate that the flash point is related to physical properties substances is itself a physical parameter. It should be noted that the pattern of changes in flash point in 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 flash point. You can dilute the 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 flammable liquid reaches 10-25%.

Table 4.2.

For binary mixtures of flammable 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.

WITH by increasing the temperature of the liquid, the rate of evaporation increases and at a certain temperature reaches such a value that once the ignited mixture continues to burn after the ignition source is removed. This temperature of the liquid is usually called ignition temperature. For flammable liquids, it differs by 1-5 0 C from the flash point, and for flammable liquids - by 30-35 0 C. 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 temperature limit of ignition, described by the formula:

T sun – T n.p. = 0.125T sun + 2. (4.4)

This relation is valid at 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 a liquid at the flash (ignition) temperature, Pa;

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

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

Flash point is the temperature at which a petroleum product, heated under standard conditions, releases such an amount of vapor that it forms a flammable mixture with the surrounding air, which flares up when a flame is applied and goes out due to a lack of combustible mass in this mixture.

This temperature is a characteristic of the fire hazardous properties of petroleum products, and on its basis, oil production and oil refining facilities are classified into fire hazard categories.

The flash point of NPs is related to their average boiling point, i.e. with evaporation. The lighter the oil fraction, the lower its flash point. Thus, gasoline fractions have negative (up to -40 °C) flash points, kerosene and diesel fractions 35-60 °C, oil fractions 130-325 °C. For oil fractions, the flash point indicates the presence of easily evaporating hydrocarbons.

The presence of moisture and decomposition products in the NP significantly affects the value of its flash point.

Two methods for determining flash point have been standardized: in open and closed crucibles. The difference in flash temperatures of the same NPs in open and closed crucibles is very large. In the latter case, the required amount of oil vapor accumulates earlier than in open-type devices.

All substances that have a flash point in a closed crucible below 61 ° C are classified as flammable liquids (FLL), which, in turn, are divided into especially dangerous (flash point below minus 18 ° C), constantly dangerous (flash point from minus 18 °C to 23 °C) and hazardous at elevated temperatures (flash point from 23 °C to 61 °C).

The flash point of a petroleum product characterizes the ability of this petroleum product to form an explosive mixture with air. A mixture of vapors and air becomes explosive when the concentration of fuel vapors in it reaches certain values. In accordance with this, the lower and upper limits of explosion of a mixture of petroleum product vapors and air are distinguished.

If the concentration of petroleum product vapor is less than the lower explosive limit, an explosion does not occur, since the available excess air absorbs the heat released at the initial point of the explosion and thus prevents the ignition of the remaining parts of the fuel. When the concentration of fuel vapor in the air is above the upper limit, an explosion does not occur due to a lack of oxygen in the mixture.

Acetylene, carbon monoxide and hydrogen have the widest explosion ranges and are therefore the most explosive.

Ignition temperature called minimal permissible temperature, in which the mixture of NP vapors with air above its surface, when a flame is applied, flares up and does not go out for a certain time, i.e. the concentration of flammable vapors is such that even with excess air, combustion is maintained.

The ignition temperature is determined using a device with an open crucible, and its value is tens of degrees higher than the flash point in an open crucible.

Self-ignition temperature is the temperature at which contact of a petroleum product with air causes it to ignite and burn steadily without the presence of a fire source.

The auto-ignition temperature is determined in an open flask by heating until a flame appears in the flask. The self-ignition temperature is hundreds of degrees higher than the flash and ignition temperatures (gasoline 400-450 °C, kerosene 360-380 °C, diesel fuels 320-380°C, fuel oil 280-300°C).

The self-ignition temperature of petroleum products depends not on evaporation, but on their chemical composition. Aromatic hydrocarbons, as well as petroleum products rich in them, have the highest auto-ignition temperature, while paraffin products have the lowest. The higher the molecular weight of hydrocarbons, the lower the auto-ignition temperature, since it depends on the oxidizing ability. As the molecular weight of hydrocarbons increases, their oxidizing power increases and they undergo oxidation (causing combustion) at a lower temperature.

FLASH AND FLASH POINT. Combustible substances, especially liquid ones, exhibit, depending on the conditions in which they are located, three distinct types of combustion: flash, ignition and combustion; How special case flares can be considered an explosion. A flash is a rapid, but relatively calm and short-term combustion of a mixture of vapors of a flammable substance with oxygen or air, resulting from a local increase in temperature, which may. caused by an electric spark or touching the mixture with a hot body ( solid, liquid, flame). The phenomenon of a flash is similar to an explosion, but, unlike the latter, it occurs without a strong sound and does not have a destructive effect. A flash differs from ignition in its short duration. Ignition, arising, like a flash, 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 combustion, since the latter does not require an additional local increase in temperature.

All types of combustion involve the spread of heat from the area where combustion occurred to adjacent areas of the combustible mixture. During an outbreak, the heat release in each section is sufficient to ignite the adjacent section of the already prepared combustible mixture, but not enough to replenish it by evaporating new quantities of fuel; therefore, having spent the supply of flammable vapors, the flame goes out, and the outbreak ends there, until flammable 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 vapor is sufficient to restore the supply of the combustible mixture. The ignition that has begun, having reached the surface of the flammable substance, becomes stationary until the combustible substance burns out completely; but, however, having been stopped, ignition no longer resumes without local overheating applied from outside. Finally, when ignited, the flammable 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 stopping the free access of oxygen, occurs spontaneously after the interfering cause is eliminated: the spontaneous outbreak will further develop into ignition.

The possibility of combustion of one type or another depends primarily on the chemical composition of the combustible mixture, i.e. the chemical nature of the combustible vapors, the oxygen content in the mixture, the content of foreign indifferent impurities, such as nitrogen, water vapor, carbon dioxide, and the content of impurities, active counteracting the combustion reaction, for example, negative catalysts, mufflers, etc. And since all types of combustion process begin with a flash, then consideration of the flash in its dependence on the chemical composition of the mixture has general meaning for all cases. It is obvious in advance that under given conditions of pressure and temperature, a mixture of flammable vapor or gas with oxygen (or air) may not flare up in any proportion and that a very small or, conversely, too high content of flammable material in the mixture excludes a flare-up. In addition, various flammable vapors require various quantities oxygen, and therefore the “flash limits” of mixtures of oxygen and flammable vapors always depend on the type of flammable vapor. The method for 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 gas or vapor form, then, according to Thornton, the limits of mixtures that retain the ability to flash can be expressed:

If the mixture does not contain pure oxygen, but air, then it is necessary to take into account that 1 volume of oxygen is contained in 5 (more precisely, 4.85) volumes of air. For example, methane combustion 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 determined by the formula:

from here it is easy to calculate that the upper flash limit for a mixture of methane with air is determined by the ratio 1:5, i.e., when the mixture contains 1/6 methane, or 16.7% (experience gives 14.8%). For the lower limit, similarly, we have the composition of the mixture CH 4 (1 volume) + 6 O (3 volumes), which corresponds to the methane content in the mixture with air 1/16, or 6.25% (experience gives 5.6%). Similarly for pentane, C 6 H 12, we obtain M = 1 and N = 16, from which for the upper limit we calculate 1/21, or 4.75%, of pentane in a mixture with air (experience 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, then, obviously, 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, we can calculate the temperature at which a flash is possible. Research by E. Mack, C.E. Burd and G.N. Borgem showed that for most substances, at the lower limit of the flare, a fairly good agreement between the calculated temperature and the directly observed temperature is observed.

Vapor mixtures are also in some cases subject to a specified method for determining the temperature at which flashover is possible. If this is a mixture of naphthenes C n H 2 n, then in all homologues the ratio of C to H content is the same, so the average molecular weight of the mixture makes it possible to determine the number of CH 2 groups and, therefore, the amount of O required for their combustion. Moreover, the flash point here is almost linear function molecular weight and 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, corresponding to the average molecular weight of the mixture. If this number is multiplied by 3 and added 1, for two previously ignored hydrogen atoms, we get N. So, for gasoline, the average molecular weight is 107 and therefore:

As the pressure of the mixture increases, 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 the shoulder straps Mexican oil by 0.033°, as shown by Lohmann, who studied the flash at different altitudes (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. Except atmospheric pressure, the flash point also changes air humidity, since the partial elasticity of water vapor lowers the pressure of the combustible component of the mixture.

Flash evaporating liquid. Flash ready mixture gases or vapors represents the simplest case. The flash phenomenon occurs more complexly, when a flashing mixture arises continuously from the evaporation of an immediately present liquid. The flash of a gas mixture also depends on many experimental conditions: increasing the width of the explosive burette, transferring the exploding 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 impurities that have not yet been sufficiently studied can significantly change these limits. The issue of a flash of fog from a sprayed flammable liquid was studied by Guider and Wolf. The lower limit of the flash turned out to be the same as for the mixture with the corresponding steam; but the speed of explosion propagation in fog is less, and the oxygen consumption is greater than in the case of vapor. The state of the surface of the liquid, its volume, the distance to the igniting flame, the rate of exchange of external 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. etc. - all of this can significantly change the observed flash point and in addition to the factors indicated when discussing the flash of a gas mixture. Therefore, one can only talk about a flash as a constant conditionally, conducting experiments only under precisely defined conditions. For chemically individual substances, Ormandy and Creven established the proportionality of flash and boiling points (in absolute degrees):

where the coefficient k for the lower flash limit is 0.736, and for the upper one 0.800; Boil temperature should be determined by the initial thermometer reading. The Ormandy and Creven formula also applies to a certain extent to very narrow fractions of various types of mixtures. However, for those flammable liquids with which in most cases we have to deal in practice, i.e. for complex mixtures, simple dependencies that determine the flash point have not yet been found. Even double mixtures do not obey the rule of mixing with respect to flash, and a low-flashing component significantly reduces the flash of another, high-flashing one, while the latter slightly increases the flash of the first. For example, a mixture of equal amounts of fractions (gasoline and kerosene components) specific gravity 0.774 with a flash at 6.5° and a specific gravity of 0.861 with a flash at 130° have a flash point not at 68.2°, as would be expected by the mixing rule, but 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 by more heavy component at 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 may produce a double flash at different temperatures.

Ignition. The ignition temperature exceeds the flash point, the higher the flash point itself. As Kunkler and M.V. Borodulin showed, when petroleum products are heated from flash to ignition, the test substance loses about 3% of its weight, and this loss applies to lighter shoulder straps. Therefore, the presence of small quantities (no more than 3%) of light shoulder straps, which significantly distorts the flash point of the substance, does not interfere with the accurate measurement of the ignition temperature. On the contrary, the presence of more than 10% gasoline in the oil makes the ignition temperature uncertain.

Spontaneous combustion, or self-ignition, of a mixture of flammable vapors occurs when the heat release of the oxidizing system is equalized with the heat loss, and therefore even a slight 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, and on temperature environment, 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 spontaneous combustion temperature on the presence or absence of catalytic platinum is proven, for example, by the data of E. Constant and Schlönfer (Table 1).

The dependence of the spontaneous combustion 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 research on the 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 spontaneous combustion temperature; 2) the presence of moisture also lowers the spontaneous combustion temperature; 3) the spontaneous combustion temperature in air is higher than in oxygen; 4) the spontaneous combustion temperature in an open tube is higher than in a closed space; 5) the spontaneous combustion temperature of cyclohexane hydrocarbons is lower than that of aromatic hydrocarbons and is close to the spontaneous combustion temperature of saturated hydrocarbons; 6) for aromatic hydrocarbons spontaneous combustion temperatures in air and oxygen are close to each other; 7) some substances (turpentine, alcohols) give very fluctuating values ​​of spontaneous combustion temperature in a successive series of tests (especially turpentine). A special case of spontaneous combustion is represented by fibrous materials (cotton, fleeces, wool, rags) soaked in oils; the ease of spontaneous combustion in such cases is related to the spontaneous combustion temperature of the corresponding oils. Phenomena of this kind have such an essential practical significance that are developed special methods and instruments for testing the ability of oils to spontaneously ignite in the presence of cotton.

Measuring flash and ignition temperatures. Being closely related to molecular weight and boiling point, flash and ignition are indirectly related to these constants and therefore characterize this substance. They also own higher value in practice, when judging the degree of flammability of a substance under given conditions of use and, therefore, for establishing preventive measures, this is a circumstance that is especially important in industry (petroleum, wood processing, alcohol, varnish, oil mills) and in general in all cases where with volatile solvents.

The need to measure flash and ignition temperatures led to the design of numerous, often expensive, special devices and to the development of instructions for working with them, and in certain industries, in relation to certain classes of substances, even related to each other, various devices with different instructions have been 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 only very approximately consistent with each other. The main types of devices for measuring flash point are: a) with an open vessel, b) with a closed vessel.

A) Open vessel devices. Flash point measurements were initially made by pouring the test liquid onto water contained in a cup; this latter was then heated. Later, they began to produce a flash in an open vessel. arr. in relation to substances that do not flare up easily, for example, lubricating oils, gaseous coal tars, various mastics, etc. These are the instruments of Markusson, Brenken, Cleveland, Moore, de Graaf, Krupp, which differ mainly in the size, shape and material of the crucible, design of heating parts and method of heating. Details regarding 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 beyond the boundaries of the crucible and its presence in an environment with different temperatures in different places lead to the need for a significant correction, which increases with increasing 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 located outside the test liquid, and t" is the temperature corresponding to the middle of the protruding part of the mercury column; although t" may. calculated, but is usually measured directly, using an additional thermometer. To quickly find this amendment, use a special table. A special table also serves for barometric pressure corrections, 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 devices. Of the various instruments of this kind, the most famous are the instruments of Abel and Martens (both improved by Pensky), Elliott (New York), Tag. In the USSR and some other countries (Germany, Austria), the Abel-Pensky device is used almost exclusively for low-boiling liquids (kerosene) and the Martens-Pensky device for high-boiling liquids (oil). The working part of these devices consists of a strictly standardized crucible, tightly covered with a lid, in which a window is opened at certain intervals to introduce a small flame into the crucible. The crucible contains a thermometer and a stirrer. Heating of the crucible, and in some cases, on the contrary, cooling, is carried out under strictly defined conditions, using special baths. Devices accepted in different countries for testing kerosene, and normal temperatures flashes in the corresponding tests are compared in table. 4.

The readings of different instruments in determining the flash point always diverge from each other, and determining the 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 based on the data in Table. 5.

It is also clear from this table 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 product heterogeneity. 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 of a heavy substance, for example, oil, of some light substance (gasoline, kerosene) or some distillation defects (decomposition with the formation of easily volatile products). Thus, comparison of the flash point of the same substance in open and closed devices can serve to monitor the correctness of both the use and production of lubricating oils.