The flash point of a substance is the lowest temperature at which can form an explosive vapor -air mixture to a substance. The combustion process stops again usually a short time after the ignition, because at this temperature is not sufficient flammable vapors to the combustion to maintain. If the volume of the mixture large enough, it can explode. Below the flash point of the flame front can not propagate away from the ignition source, since the heat from the oxidation is not sufficient to heat the mixture to the necessary temperature for combustion. Literature values for flash points apply generally to an air pressure of 1013 mbar. At higher pressure, although the vapor pressure increases slightly, but the flash point increases because of the flammable vapor is diluted by more air molecules, so to speak.
- The source of ignition (eg static spark or flame) must have a minimum ignition produce (eg: for methane 0.2 mJ ),
- The atmosphere must have a minimum oxygen content have (eg: for bisphenol A 2.0 vol - %).
To maintain the combustion additionally has at least the heat of vaporization are applied ( Many fabrics are on their flash point is already in the liquid state, while others are fixed and sublime, some are at the flash point is not stable, so that the steam decomposition products contains ). The required higher vapor concentration occurs at a higher temperature by a few degrees, the focal point. Indistinguishable from the flash and fire point is the ignition temperature, an ignition source in is no longer necessary.
Diesel fuel or fuel oil with a flash point of about 55 ° C can not ignite at room temperature with a match. However, the match is retained in the liquid long enough for locally increasing the temperature on the surface of the liquid, whereby the flash point is reached and so that the fluid is ignited locally. From here, the flame propagates circularly from then on the surface.
Flash point determination
The flash point is decisive in the classification and classification as a hazardous substance or by the Self-assessment.
There are various standardized equipment for determining the flash point of a liquid:
- Pensky -Martens (> 50 ° C; DIN 51758, EN 22719, currently standard equipment )
- Method according to Abel - Pensky (<50 ° C; DIN 51755, closed cup = cc closed cup )
- Method to Cleveland ( DIN 51376, open cup = open cup )
- Method of Marcusson (DIN 51584, open cup, outdated method of 1959)
In general, provide closed- cup methods lower flash points than the outdated open- cup methods. The latter served in variations to determine the rarely used focus.
Mixtures of combustible substances
Mixtures in the vapor pressure of the lowest boiling compound determines the flash point of the mixture.
- The petrol (gasoline) are newly -boiling ether ( methyl tert- butyl ether, ethyl tert-butyl ether ) is added, which not only reduce the flash point, but also the ignition temperature of the hydrocarbon mixture.
- Hefeweizen ( = 5 vol - % ethanol in water ) gave when measuring a flash point of 81 ° C; i.e. 5 -percent ethanol at 81 ° C until development is necessary for the ignition of flammable vapor concentration of 3.5 % (= LEL).
This can recalculate the Raoult's law on the partial vapor pressures of water and ethanol.
1.0 vol - % = 10,000 ppm
The data of rapeseed oil are representative of all edible fats and oils. The flash point of rapeseed oil can be quite reliably estimated at approximately 230 ° C by the examples. Fires on the stove caused by exceeding the ignition temperature of edible fats or oils ( 300 ° C).
Pressure and concentration dependence of the flash point
The data in the table were obtained under standardized conditions with pure substances. On dilution with inert gases and / or under pressure, it is likely to reduce the values of the lower explosion limit to 20 % (by 100 ° C) and increase the upper explosive limit to 10 % (by 100 ° C). The lowering of the lower explosive limit of 20% corresponds approximately to a 5 ° C lower flash point (see saturation vapor pressure curve).