Flash point

from Wikipedia, the free encyclopedia

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.

more details

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 .

Fire test

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

Automatic flash point tester using the Pensky-Martens method with integrated fire extinguishing equipment

The flash point is decisive for the classification and classification as a hazardous substance or according to the BetrSichV .

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.

Examples

Note: 1.0 % by volume corresponds to 10,000 ppm

substance boiling point Flash point Lower
explosion
limit
Upper
explosion
limit
Ignition
temperature
[° C] [° C] [% By volume] [% By volume] [° C]
hydrogen 00-253 000004th 000077 000560
Methane (natural gas) 00-162 000004.4 000016.5 000595
Ethane 000-89 000-135 000003 000012.4 000515
acetylene 000-84 000002.3 000082 000305
propane 000-42 000001.7 000010.9 000470
butane 000000 000001.4 000009.3 000365
acetaldehyde 000020th 000-30 000004th 000057 000155
n -pentane 000036 000-35 000001.4 000008.0 000285
Diethyl ether 000036 000-40 000001.7 000036 000160
Carbon disulfide 000046 000-30 000001.0 000060 000102
Propionaldehyde 000047 000-40 000002.3 000021st 000175
Methyl tert-butyl ether 000055 000-28 000001.6 000008.4 000460
acetone 000056 000-18 000002.1 000013 000540
Methanol 000065 000011 000005.5 000037 000455
n -hexane 000069 000-22 000001.0 000008.1 000240
Ethyl tert -butyl ether 000071 000-19 000001.2 000007.7
Ethanol (denatured alcohol) 000078 000013 000003.5 000015th 000425
2-propanol 000082 000012 000002 000012 000425
Ethylene glycol dimethyl ether 000084… 86 0000-6 000001.6 000010.4 000200
n -heptane 000098 0000-4 000001.0 000007th 000215 = RON = 0
Isooctane , 2,2,4-trimethylpentane 000099 000-12 000001.0 000006th 000410 = RON = 100
1,4-dioxane 000101 000011 000001.7 000025th 000300
1-butanol 000117 000034 000001.4 000011.3 000340
Propylene glycol monomethyl ether 000119… 121 000032 000001.7 000011.5 000270
n - octane 000126 000012 000000.8 000006.5 000210
Diglycol dimethyl ether 000155 ... 165 000051 000001.4 000017.4 000190
Dipropylene glycol dimethyl ether 000175 000065 000000.85 000165
Dipropylene glycol monomethyl ether 000185… 195 000080 000001.1 000014th 000205
Glycerin 000290 dec. 000176 000400
Gasoline for vehicles (KW mixture) 000070 ... 210 0<−20 000000.6 000008th 000200 ... 410
Diesel for vehicles (KW mixture) 000150 ... 390 00> 55 000000.6 000006.5 000220 (approx.)
Biodiesel (FS methyl ester) 000300 (approx.) 000180 000250 (approx.)
Jet-A1 aviation turbine fuel 000150 (approx.) 000038 000000.6 000006.5 000220 (approx.)
Rapeseed oil (FS triglyceride) 000350 (approx.) 000230 000300 (approx.)
Lighter fluid 000113 ... 143 000007th 000000.7 000006th 000380

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 ).

See also

literature

  • 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 .

Web links

Wiktionary: Flashpoint  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. a b c d Gisbert Rodewald: Fire theory . W. Kohlhammer Verlag, 2006, ISBN 3-17-019129-2 , pp. 172 ( limited preview in Google Book search).
  2. 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).
  3. a b Practical manual for operational fire protection . WEKA Media, 2004, ISBN 3-8111-4471-5 ( limited preview in the Google book search).
  4. G. Wedler: Textbook of Physical Chemistry. 5th edition. Whiley-VCH, Weinheim 2004, ISBN 3-527-31066-5 .
  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).
  6. 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).
  7. Henrikus Steen: Handbook of explosion protection . John Wiley & Sons, 2012, ISBN 978-3-527-66086-5 , pp. 60 ( limited preview in Google Book search).
  8. EU safety data sheet lighter benzine F001: In: pearl.de , Zippo GmbH, November 17, 2009, accessed on March 14, 2013 (PDF; 72 kB).
  9. 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 .
  10. ^ 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).