The flash point of a volatile liquid is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Measuring a liquid's flash point requires an ignition source. At the flash point, the vapor may cease to burn when the source of ignition is removed.
The flash point is not to be confused with the autoignition temperature, which does not require an ignition source.
The fire point, a slightly higher temperature, is defined as the temperature at which the vapor continues to burn after being ignited. Neither the flash point nor the fire point is related to the temperature of the ignition source or of the burning liquid, which are much higher.
The flash point is often used as a descriptive characteristic of liquid fuel, and it is also used to describe liquids that are not normally used as fuels. “Flash point” refers to both flammable liquids and combustible liquids. There are various international standards for defining each, but most agree that liquids with a flash point less than 43°C (109°F) are flammable, while those having a flash point above this temperature are combustible.
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Every liquid has a vapour pressure, which is a function of that liquid's temperature. As the temperature increases, the vapour pressure increases. As the vapour pressure increases, the concentration of evaporated flammable liquid in the air increases. Hence, temperature determines the concentration of evaporated flammable liquid in the air.
Each flammable liquid requires a different concentration of its vapour in air to sustain combustion. The flash point of a flammable liquid is the lowest temperature at which there will be enough flammable vapour to ignite when an ignition source is applied
There are two basic types of flash point measurement: open cup and closed cup.
In open cup devices the sample is contained in an open cup which is heated, and at intervals a flame is brought over the surface. The measured flash point will actually vary with the height of the flame above the liquid surface, and at sufficient height the measured flash point temperature will coincide with the fire point. The best known example is the Cleveland Open Cup (COC).
There are two types of Closed cup testers: non-equilibrium, such as Pensky-Martens where the vapours above the liquid are not in temperature equilibrium with the liquid, and equilibrium, such as Small Scale (commonly known as Setaflash) where the vapours are deemed to be in temperature equilibrium with the liquid. In both these types the cups are sealed with a lid through which the ignition source can be introduced. Closed cup testers normally give lower values for the flash point than Open cup (typically 5-10 °C) and are a better approximation to the temperature at which the vapour pressure reaches the lower flammable limit (LFL).
The flash point is an empirical measurement rather than a fundamental physical parameter. The measured value will vary with equipment and test protocol variations, including temperature ramp rate (in automated testers), time allowed for the sample to equilibrate, sample volume and whether the sample is stirred.
Methods for determining the flash point of a liquid are specified in many standards. For example, testing by the Pensky-Martens closed cup method is detailed in ASTM D93, IP34, ISO 2719, DIN 51758, JIS K2265 and AFNOR M07-019. Determination of flash point by the Small Scale closed cup method is detailed in ASTM D3828 and D3278, EN ISO 3679 and 3680, and IP 523 and 524.
| Fuel | Flash point | Autoignition temperature |
|---|---|---|
| Ethanol (70%) | 16.6 °C (61.88 °F) [1] | 363 °C (685.40 °F) [2] |
| Gasoline (petrol) | -43 °C (-45 °F) | 246 °C (495 °F) |
| Diesel | >62 °C (143 °F) | 210 °C (410 °F) |
| Jet fuel | >60 °C (140 °F) | 210 °C (410 °F) |
| Kerosene (paraffin oil) | >38°–72 °C (100°–162 °F) | 220 °C (428 °F) |
| Vegetable oil (canola) | 327 °C (620 °F) | |
| Biodiesel | >130 °C (266 °F) |
Gasoline (petrol) is designed for use in an engine which is driven by a spark. The fuel should be premixed with air within its flammable limits and heated above its flash point, then ignited by the spark plug. The fuel should not preignite in the hot engine. Therefore, gasoline is required to have a low flash point and a high autoignition temperature.
Diesel fuel flash points vary between 52 °C and 96 °C (126 °F to 204 °F). Diesel is designed for use in a high-compression engine. Air is compressed until it has been heated above the autoignition temperature of diesel; then the fuel is injected as a high-pressure spray, keeping the fuel-air mix within the flammable limits of diesel. There is no ignition source. Therefore, diesel is required to have a high flash point and a low autoignition temperature.
Jet fuel flash points also vary greatly. Both Jet A and Jet A-1 have flash points between 38 °C and 66 °C (100 °F to 150 °F), close to that of off-the-shelf kerosene. Yet both Jet B and FP-4 have flash points between -23 °C and -1 °C (-10 °F to +30 °F ).
Flash points of substances are measured according to standard test methods. These test methods define the apparatus required to carry out the measurement, key test parameters, the procedure for the operator or automated apparatus to follow, and the precision of the test method. Standard test methods are written and controlled by a number of national and international committees and organizations. The three main bodies are the CEN / ISO Joint Working Group on Flash Point (JWG-FP), ASTM D02.8B Flammability Section and the Energy Institute's TMS SC-B-4 Flammability Panel.