The flash point of a flammable liquid is the lowest temperature at which it can form an ignitable mixture in air. At this temperature the vapor may cease to burn when the source of ignition is removed. A slightly higher temperature, the fire point, is defined as the temperature at which the vapor continues to burn after being ignited. Neither of these parameters are related to the temperatures of the ignition source or of the burning liquid, which are much higher. The flash point is often used as one descriptive characteristic of liquid fuel, but it is also used to describe liquids that are not used intentionally as fuels.
Every flammable liquid has a vapor pressure, which is a function of that liquid's temperature. As the temperature increases, the vapor pressure increases. As the vapor pressure increases, the concentration of evaporated flammable liquid in the air increases. Hence, temperature determines the concentration of evaporated flammable liquid in the air under equilibrium conditions. Different flammable liquids require different concentrations of the fuel in air to sustain combustion. The flash point is that minimum temperature at which there is a sufficient concentration of evaporated fuel in the air for combustion to propagate after an ignition source has been introduced.
Measuring flash points
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 (hence the name) 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.
Closed cup testers, of which the Pensky-Martens closed cup is one example, are sealed with a lid through which the ignition source can be introduced periodically. The vapour above the liquid is assumed to be in reasonable equilibrium with the liquid. Closed cup testers give lower values for the flash point (typically 5-10 K) and are a better approximation to the temperature at which the vapour pressure reaches the Lower Flammable Limit (LFL).
The flash point is an experimental 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 equilibriate, sample volume and whether the sample is stirred. The testers and protocols are specified in standards such as DIN 51758, ASTM 93, and Determination of flash point: Closed cup equilibrium method (ISO 1523:2002).
Examples of fuel flash points
|Gasoline (petrol)||<-40°C (-45°F)||246°C (475°F)|
|Diesel||>62°C (143°F)||210°C (410°F)|
|Jet fuel||>38°C (100°F)||210°C (410°F)|
|Kerosene (paraffin oil)||>38-72°C (100-162°F)||220°C (428°F)|
Petrol (gasoline) 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 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.
Diesel varies between 126°F and 204°F (52°C-96°C/WJ). Jet fuels also vary a lot. Jet A and jet A-1 have a FP between 100°F and 150°F (38°C-66°C/WJ), close to that of off the shelf kerosene. However, both Jet B and FP-4 have a FP between -10°F and +30°F (-23°C - -1°C/WJ)
Sources of data
Flash point data are found in many physical property data collections as well as Material Safety Data Sheets (MSDS) however not all data sources are equal, and the usefulness of the data depends on the method (specifically open or closed cup) which is not always specified.
Two of the most reliable data collections:
Sicherheitstechnische Kenngrößen. Band 1: Brennbare Flüssigkeiten und Gase. Elisabeth Brandes and Wolfgang Möller, Wirtschaftsverlag NW (2003), ISBN 3-89701-745-8
NFPA 325, Fire Hazard Properties of Flammable Liquids, Gases and volatile solids, 1994; contained within NFPA Guide to Hazardous Materials (13th Ed), ISBN 0-87765-473-5
Another useful compilation of data can be found on the CONCAWE website under Product Dossiers (http://www.concawe.be)