The flash point of a substance is the lowest temperature according to DIN V 14011, in which an ignitable vapor-air mixture capable of forming a substance.
Due to the vapor pressure of liquids, part of the liquid evaporates even below the normal boiling point . The vapor pressure increases with the liquid temperature, i. H. the higher the temperature of the liquid, the more the liquid evaporates into the gas state. The gaseous parts of the liquid accumulate in the space above the liquid surface and form a vapor / air mixture there. If the vapor concentration exceeds the lower ignition limit, this mixture can be ignited by a suitable ignition source. Adding just a few percent of a liquid with a low flash point to a liquid with a high or no flash point can result in a mixture with a low flash point. The combustion process usually stops a short time after the ignition, since at this temperature not enough flammable vapors are produced to maintain the combustion . If the volume of the mixture is large enough, it can explode . Below the flash point, the flame front cannot spread away from the ignition source, as the heat from the oxidation is not sufficient to heat the mixture to the temperature required for combustion. A flammable liquid with a flash point that is in the range or below normal temperature of around 20 ° C is the most dangerous because it can be ignited with a spark at any time without additional heat supply. In the case of flammable liquids that are soluble in water (e.g. alcohol), the flash point depends on the concentration of the liquid. Dilution with water leads to an increase in the flash point. Literature values for flash points generally apply to an air pressure of 1013 mbar. At higher pressure, the vapor pressure increases slightly, but the flash point increases because the flammable vapor is "diluted" by more molecules in the air.
The ignition source (e.g. electrostatic spark or flame) must generate a minimum ignition energy (e.g. for methane 0.2 mJ ) and the atmosphere must have a minimum content of oxygen (e.g. for bisphenol A 2.0 vol. -%).
To maintain the combustion must also at least the heat of vaporization are applied (Many substances are at their flashpoint already in the liquid aggregate state , while others are firm and sublime, some are not stable at the flash point, so that the steam contains decomposition products). The higher vapor concentration required for this is created at a temperature a few degrees higher, the focal point . The ignition temperature , at which an ignition source is no longer necessary, must be distinguished from the flash point and fire point .
Diesel fuel or heating oil with a flash point of around 55 ° C can not be ignited with a burning match at room temperature . However, if the match is held to the liquid long enough, the temperature on the surface of the liquid rises locally, which means that the flash point is reached and the liquid is ignited locally. From here the flame spreads in a circle on the surface.
Flash point determination
There are various standardized devices for determining the flash point of a liquid:
- Pensky-Martens method (> 50 ° C; DIN 51758, EN 22719, currently standard equipment)
- Abel-Pensky method (<50 ° C; DIN 51755, closed crucible = cc closed cup )
- Cleveland method (DIN 51376, open cup )
- Marcusson method (DIN 51584, open crucible, outdated method from 1959)
In general, closed-cup methods provide lower flash points than the outdated open-cup methods. The latter were used in modifications to determine the focal point, which is no longer in use today .
Mixtures of flammable substances
In mixtures, the vapor pressure of the lowest-boiling substance determines the mixture's flash point.
- The petrol ( gasoline ) have recently been boiling ether (methyl tert -butyl ether, ethyl tert -butyl ether) added that its flash point and also its ignition temperature decrease.
- Wheat beer (= 5% by volume of ethanol in water) has a flash point of 81 ° C; d. H. At 81 ° C, 5 percent ethanol develops the concentration of flammable vapors of 3.5% required for ignition (= lower explosion limit).
This can also be calculated using Raoult's law on the partial vapor pressures of water and ethanol.
Note: 1.0 % by volume corresponds to 10,000 ppm
|substance||boiling point||Flash point||
|[° C]||[° C]||[% By volume]||[% By volume]||[° C]|
|Methane (natural gas)||-162||4.4||16.5||595|
|Methyl tert-butyl ether||55||-28||1.6||8.4||460|
|Ethyl tert -butyl ether||71||-19||1.2||7.7|
|Ethanol (denatured alcohol)||78||13||3.5||15th||425|
|Ethylene glycol dimethyl ether||84… 86||-6||1.6||10.4||200|
|n -heptane||98||-4||1.0||7th||215 = RON = 0|
|Isooctane , 2,2,4-trimethylpentane||99||-12||1.0||6th||410 = RON = 100|
|Propylene glycol monomethyl ether||119… 121||32||1.7||11.5||270|
|n - octane||126||12||0.8||6.5||210|
|Diglycol dimethyl ether||155 ... 165||51||1.4||17.4||190|
|Dipropylene glycol dimethyl ether||175||65||0.85||165|
|Dipropylene glycol monomethyl ether||185… 195||80||1.1||14th||205|
|Gasoline for vehicles (KW mixture)||70 ... 210||<−20||0.6||8th||200 ... 410|
|Diesel for vehicles (KW mixture)||150 ... 390||> 55||0.6||6.5||220 (approx.)|
|Biodiesel (FS methyl ester)||300 (approx.)||180||250 (approx.)|
|Jet-A1 aviation turbine fuel||150 (approx.)||38||0.6||6.5||220 (approx.)|
|Rapeseed oil (FS triglyceride)||350 (approx.)||230||300 (approx.)|
|Lighter fluid||113 ... 143||7th||0.7||6th||380|
The data for rapeseed oil are representative of all edible fats and oils. The flash point of rapeseed oil can be estimated quite reliably at approx. 230 ° C using the examples. Fires on the stove occur when the ignition temperature (approx. 300 ° C) of edible fats or oils is exceeded .
Pressure and concentration dependence of the flash point
The data in the table were determined under standardized conditions with pure substances. When diluted with inert gases and / or under pressure, it is likely that the values for the lower explosion limit decrease by 20% (per 100 ° C) and that the upper explosion limit increases by 10% (per 100 ° C). The lowering of the lower explosion limit by 20% corresponds approximately to a 5 ° C lower flash point (see saturation vapor pressure curve ).
- Roy Bergdoll, Sebastian Breitenbach: Burning and extinguishing (= Die Rote Hefte . Issue 1). 18th, revised edition. Kohlhammer, Stuttgart 2019, ISBN 978-3-17-026968-2 .
- E. Brandes, W. Möller: Safety parameters. Volume 1: Flammable Liquids and Gases. Wirtschaftsverlag NW, Verlag für neue Wissenschaft, Bremerhaven 2003, ISBN 3-89701-745-8 .
- BG RCI leaflet R003 Safety- related parameters. Jedermann-Verlag, Heidelberg, April 2016, (PDF download) .
- M. Kräft: Explosion protection with flame arresters. 2nd Edition. Mackensen, Berlin 2007, ISBN 978-3-926535-53-5 .
- Lothar Schott, Manfred Ritter: Fire Brigade Basic Course FwDV 2 . 20th edition. Wenzel-Verlag, Marburg 2018, ISBN 978-3-88293-220-1 .
- M. Kräft, Ex.CE.L occupational safety group: Selected methods of flash point determination.
- Representation of a device for determining the flash point of petroleum from 1883: The Abel'sche Petroleumprober
- Gisbert Rodewald: Fire theory . W. Kohlhammer Verlag, 2006, ISBN 3-17-019129-2 , pp. 172 ( limited preview in Google Book search).
- Henry Portz: Fire and Explosion Protection from AZ Explanation of terms and fire protection characteristics . Springer-Verlag, 2015, ISBN 978-3-322-80197-5 , pp. 68 ( limited preview in Google Book search).
- Practical manual for operational fire protection . WEKA Media, 2004, ISBN 3-8111-4471-5 ( limited preview in the Google book search).
- G. Wedler: Textbook of Physical Chemistry. 5th edition. Whiley-VCH, Weinheim 2004, ISBN 3-527-31066-5 .
- Olaf Eduard Wolff: Fire corpses - crime scene work and investigations, manifestations, causes, preservation of evidence . Richard Boorberg Verlag, 2017, ISBN 978-3-415-05888-0 ( limited preview in Google book search).
- Uwe J. Möller, Jamil Nassar: Lubricants in operation . Springer-Verlag, 2013, ISBN 978-3-642-56379-9 , pp. 124 ( limited preview in Google Book search).
- Henrikus Steen: Handbook of explosion protection . John Wiley & Sons, 2012, ISBN 978-3-527-66086-5 , pp. 60 ( limited preview in Google Book search).
- EU safety data sheet lighter benzine F001: In: pearl.de , Zippo GmbH, November 17, 2009, accessed on March 14, 2013 (PDF; 72 kB).
- E. Brandes, M. Thedens: Characteristics of explosion protection under non-atmospheric conditions. (PDF) (No longer available online.) In: PTB-Mitteilungen 113, Issue 2. Physikalisch-Technische Bundesanstalt , 2003, pp. 115–121 , archived from the original on September 30, 2007 ; Retrieved July 12, 2016 .
- E. Brandes, M. Thedens: Safety Characteristics at non Atmospheric Conditions. (PDF; 421 kB) (No longer available online.) Physikalisch-Technische Bundesanstalt , archived from the original on March 4, 2016 ; accessed on July 12, 2016 (English, presentation slides).