What is flash point?

The flash point of a flammable liquid is minimum temperature at which the flammable liquid gives off enough vapor to form a flammable mixture with air above the surface of the flammable liquid (at normal atmospheric pressure). If the flash point of a flammable liquid is above the maximum environment, then an explosive atmosphere cannot form.

Note: The flash point of a mixture of different flammable liquids may be lower than the flash point of its individual components.

Flash point examples for typical fuels:

Gasoline is used for engines internal combustion which are powered by spark ignition. The fuel must be pre-mixed with air in accordance with its explosive limits and heated above the flash point, then ignited by the spark plugs. The fuel must not ignite before the ignition point when the engine is warm. Therefore, gasoline has a low flash point and a high self-ignition temperature.

The flash point of diesel fuel can range from 52°C to 96°C depending on the type. Diesel fuel is used in the engine with a high degree compression. The air is compressed until it is heated above the auto-ignition temperature of diesel fuel, after which the fuel is injected in the form of a jet under high pressure, maintaining the air-fuel mixture within the flammability limit of diesel fuel. There is no source of ignition in this type of engine. Therefore, ignition of diesel fuel requires a high flash point and a low autoignition temperature.

flash point is the temperature at which an oil product heated under standard conditions emits such an amount of vapor that forms a combustible mixture with the surrounding air, which flares up when the flame is brought up and goes out due to the lack of combustible mass in this mixture.

This temperature is a characteristic of the fire hazard 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 temperature boiling, i.e. with evaporation. The lighter the oil fraction, the lower its flash point. So, 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 volatile hydrocarbons.

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

Two methods for determining the flash point are standardized: open and closed crucible. The difference between the flash points 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 with a flash point in a closed crucible below 61 °C are classified as flammable liquids (flammable liquids), which, in turn, are divided into especially dangerous (flash point below minus 18 °C), permanently hazardous (flash point from minus 18 °С to 23 °С) and dangerous at elevated temperatures (flash point from 23°С to 61°С).

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

If the concentration of oil vapors is less than the lower explosive limit, no explosion occurs, since the existing excess air absorbs the heat released at the starting point of the explosion and thus prevents the remaining parts of the fuel from igniting. When the concentration of fuel vapor in the air is above the upper limit of the explosion does not occur due to the lack of oxygen in the mixture.

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

Flash point called minimal allowable temperature, at which the mixture of NP vapors with air above its surface, when the flame is brought up, flares up and does not go out for a certain time, i.e. the concentration of combustible vapors is such that even with an excess of air, combustion is maintained.

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

Self-ignition temperature called the temperature at which the contact of an oil product with air causes its ignition and stable combustion without bringing a source of fire.

The autoignition 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 fuel 320-380 ° C, fuel oil 280-300 ° C).

The self-ignition temperature of petroleum products does not depend on volatility, but on their chemical composition. Aromatic hydrocarbons, as well as petroleum products rich in them, have the highest autoignition temperature, and paraffinic hydrocarbons have the lowest. The higher the molecular weight of hydrocarbons, the lower the autoignition temperature, since it depends on the oxidizing ability. With an increase in the molecular weight of hydrocarbons, their oxidizing ability increases, and they enter into an oxidation reaction (causing combustion) at a lower temperature.

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

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 homologous series suggest that the flash point is related to physical properties substances and is itself a physical parameter. It should be noted that the pattern of changes in the flash point in homologous series cannot be applied 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.

WITH 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 saturated steam liquids at 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;

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

This indicator is closely related to the boiling point, i.e. with evaporation. The lighter the oil product, the better it evaporates, the lower its flash point. For example, gasoline fractions have negative flash points (up to -40°C), kerosene fractions have flash points in the range of 28-60°C, diesel fuel fractions - 50-80°C, heavier oil fractions - 130-325°C . The flash points of various oils can be either positive or negative.

The presence of moisture in oil products leads to a decrease in the flash point. Therefore, when determining it in laboratory conditions, the oil product must be freed from water. There are two standard methods for determining the flash point: in an open (GOST 4333-87) and closed (GOST 6356-75) crucible. The difference in determining the flash point between them is 20-30°C. When determining a flash in an open crucible, part of the formed vapors flies into the air, and the required amount of them, necessary for a flash, accumulates later than in a closed crucible.

Therefore, the flash point of the same oil product, determined in an open crucible, will be higher than in a closed crucible. As a rule, the flash point in an open crucible is determined for high-boiling oil fractions (oils, fuel oils). The flash point is taken to be the temperature at which the first blue flame appears on the surface of the oil product and immediately goes out. The flash point is used to judge the explosive properties of an oil product, i.e. about the possibility of the formation of explosive mixtures of its vapors with air. There are lower and upper explosive limits.

If the concentration of oil vapor in a mixture with air is below the lower limit, an explosion will not occur, since the existing excess air absorbs the heat released at the point of explosion and thus prevents the ignition of other parts of the fuel.

When the concentration of oil vapors in a mixture with air above the upper limit of the explosion does not occur due to lack of oxygen in the mixture.

Ignition temperature. When determining the flash point, a phenomenon is observed when an oil product flares up and immediately goes out. If the oil product is heated even higher (by 30-50°C) and the source of fire is again brought to the surface of the oil product, then it will not only flare up, but will also burn quietly. The minimum temperature at which an oil product flares up and begins to burn is called the ignition temperature.


Auto ignition temperature. If the oil product is heated to a high temperature without contact with air, and then such contact is ensured, then the oil product may ignite spontaneously.

The minimum temperature corresponding to this phenomenon is called the autoignition temperature. It depends on the chemical composition. Most high temperatures Aromatic hydrocarbons and oil products rich in them have self-ignition, followed by naphthenes and paraffins.

The lighter the oil product, the higher its autoignition temperature. So, for gasoline it is in the range of 400-450°C, for gas oils - 320-360°C.

Self-ignition of petroleum products is often the cause of fires in factories. Any depressurization of flange connections in columns, heat exchangers, pipelines, etc. may cause a fire.

The insulating material doused with oil must be removed, since its catalytic effect can cause self-ignition of the oil product at much more low temperatures Oh.

pour point. When transporting petroleum products through pipelines and using them in the low-temperature region in aviation, their mobility and good pumpability under these conditions are of great importance. The temperature at which the oil loses its mobility under standard test conditions is called the pour point.

The loss of mobility of the oil product can occur due to two factors: either an increase in the viscosity of the oil product, or due to the formation of paraffin crystals and thickening of the entire mass of the oil product.

Accompanied by a bright short-term glow. There is no sustainable combustion. Flash point - the minimum temperature of condensed substances at which vapors are formed above their surface, flashing when a spark, flame or hot body appears.

The ability to flare up at relatively low temperatures is possessed by liquids classified as flammable. The maximum flash point of such substances in closed crucibles is + 61 °С, in open - + 66 °С. Some substances are capable of spontaneous combustion, reaching their characteristic ignition temperature.

Determination of pressure is possible for any combustible liquid. It increases in proportion to the increase in the temperature of the substance. As soon as the flash point reaches a critical (maximum) value, it becomes possible to maintain combustion.

However, the onset of vapor-liquid equilibrium will require some time, which is proportional to the rate of vapor formation. Stable combustion can be achieved by reaching a certain (for each substance individual) ignition temperature, since the combustion temperature is always higher than the flash point.

Directly changing the temperatures at which substances flare up has certain difficulties. Therefore, the flash point is considered to be the temperature of the walls of the reaction vessels in which this flash is observed. The temperature depends directly on the conditions of the ongoing heat exchange inside the vessel itself, on its catalytic activity, on the environment, and on the volume of liquid in the vessel.

Particularly dangerous are liquids that can flare up at temperatures below -18 ° C in closed crucibles, below -13 ° C - in open ones. Liquids are considered to be permanently dangerous, the flash of which is possible at a temperature of + 23 ° C in closed crucibles and up to + 27 ° C in open ones. The temperature indicators of hazardous liquids are up to + 60 °С inclusive with closed crucibles, up to + 66 °С inclusive - with open ones.

The difference in combustion varies significantly, and it is individual for each substance. The flash point, for example, is not more than + 70 ° С. Its combustion temperature is + 1100 °С. Ignition temperature - from + 100 °С to + 119 °С. But the flash point of gasoline, due to its very high volatility, is + 40 ° C, and sometimes less. Its ignition temperature is + 300 °C. The figures for gasoline are somewhat generalized. They must be considered average, since there are different kinds gasoline (automobile (summer, winter), aviation) with significantly different characteristics and, accordingly, different temperatures flashes, ignition, burning.

Combustion is a process accompanied by the release a large number heat with a characteristic emission of light (glow), which is possible when a certain temperature is reached for each substance and access to oxygen or other substances (sulfur, bromine vapor, etc.).

Explosions characterized by instantaneous chemical reaction with the release of enormous energy and carrying mechanical work. An explosion fire can spread 3,000 meters in one second. The combustion of the mixture at this speed is called detonation. The resulting shock waves often cause significant damage and accidents.