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Flash point- this 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 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, and 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 both positive and negative.
The presence of moisture in petroleum products leads to a decrease in flash point. Therefore, when determining it in laboratory conditions, the oil product must be freed from water. There are two standard methods for determining 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 resulting vapor flies into the air, and the required amount required for a flash accumulates later than in a closed crucible.
Therefore, the flash point of the same petroleum 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 explosive properties of the petroleum product are judged by the flash point, 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 petroleum product vapor in a mixture with air is below the lower limit, an explosion will not occur, since the available 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 petroleum product vapor in a mixture with air is above the upper limit, an explosion does not occur due to a lack of oxygen in the mixture.
Ignition temperature. When determining the flash point, a phenomenon is observed when the petroleum product flares up and immediately goes out. If the oil product is heated even higher (by 30-50°C) and the fire source is again brought to the surface of the oil product, it will not only flare up, but will also burn quietly. The minimum temperature at which a petroleum product flashes and begins to burn is called the ignition temperature.
Auto-ignition temperature. If an oil product is heated to a high temperature without contact with air, and then such contact is provided, the oil product may ignite spontaneously.
The minimum temperature corresponding to this phenomenon is called the auto-ignition temperature. It depends on chemical composition. Aromatic hydrocarbons and petroleum products rich in them have the highest auto-ignition temperatures, followed by naphthenes and paraffins.
The lighter the petroleum product, the higher its auto-ignition temperature. So, for gasoline it is in the range of 400-450°C, for gas oils - 320-360°C.
Spontaneous combustion 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.
Insulating material doused with petroleum product must be removed, since its catalytic effect can cause spontaneous combustion of the petroleum product at much lower temperatures.
Pour point. When transporting petroleum products through pipelines and using them in low temperatures in aviation, their mobility and good pumpability under these conditions are of great importance. The temperature at which a petroleum product loses its mobility under standard test conditions is called the pour point.
Loss of mobility of an 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.
The flash point is the one at which vapors briefly flash above the surface of a liquid flammable substance heated in a crucible. Typically, a flash does not turn into combustion, since the rate of formation of flammable vapors at this temperature is less than the rate of their combustion. Flame combustion occurs later, at a higher temperature, called the ignition (or ignition) temperature.
This parameter is of key importance in the technology of using all types of flammable liquids, since it allows you to establish rules and boundaries for their safe handling, determine the purity of the fuel, the presence of dangerous additives, identify counterfeits, and reliably calculate the operating modes of engines and power plants.
The flash point of liquid fuel is measured by two methods - in open and closed crucibles. They differ in that in the latter method the vapors cannot escape into the surrounding space, and the outbreak occurs at a lower temperature. The flash point in an open crucible is always higher, and this temperature difference increases with increasing 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 temperatures.
Petroleum products are poured into an open (or closed) metal bowl-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, the thermometer is secured using a tripod 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 and set the desired rate of temperature increase.
Every 2 ºС above the surface of the liquid, conduct in a horizontal direction with the tip of a gas burner with a flame no more than 4 mm long. When a short blue flash of vapor occurs, the temperature is recorded. This is the desired value. When the liquid is heated further, it ignites with a red flame. The ignition temperature is recorded.
When studying a flash in a closed crucible, a gas igniter with constant burning is placed under the lid. Vapors accumulate faster in such a crucible, and the outbreak occurs earlier.
Some data on measuring flash temperatures
Today there are more advanced devices than TVO for determining flash points. They are distinguished by high measurement accuracy, automation of operations, user-friendly interfaces, and high productivity, therefore significantly facilitating 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 assays are more suitable for liquids with highly volatile vapors. The results of studies using both methods may have significant differences (up to two tens of ºС).
Substances with flash points in a closed crucible below 61 ºС are classified as flammable. They, in turn, are divided into especially dangerous (T acc. ≤ -18 ºC), dangerous (T acc. from -18 ºC to +23 ºC) and dangerous at elevated temperatures (T acc. from 23 ºC to 61 ºC) .
For diesel fuel, the flash point in an open crucible ranges from 52 to 96 ºС, for gasoline - -43 ºС. The auto-ignition temperature for gasoline is 246 ºС, for diesel fuel - 210 ºС. Since the latter is not ignited in the combustion chamber of the internal combustion engine, but ignites spontaneously, it becomes clear why it is characterized by such a high flash point compared to gasoline and a lower self-ignition temperature.
The flash point of fuel in an open crucible is an important informative parameter of liquid fuel used to determine the quality of the product.
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