Flash point

Flash point

Overview
The flash point of a volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...

 material is the lowest temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 at which it can vaporize to form an ignitable mixture in air. Measuring a 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
Autoignition temperature
The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion...

, which does not require an ignition source.

The fire point
Fire point
The fire point of a fuel is the temperature at which it will continue to burn for at least 5 seconds after ignition by an open flame. At the flash point, a lower temperature, a substance will ignite briefly, but vapor might not be produced at a rate to sustain the fire...

, a higher temperature, is defined as the temperature at which the vapor continues to burn after being ignited.
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Encyclopedia
The flash point of a volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...

 material is the lowest temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 at which it can vaporize to form an ignitable mixture in air. Measuring a 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
Autoignition temperature
The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion...

, which does not require an ignition source.

The fire point
Fire point
The fire point of a fuel is the temperature at which it will continue to burn for at least 5 seconds after ignition by an open flame. At the flash point, a lower temperature, a substance will ignite briefly, but vapor might not be produced at a rate to sustain the fire...

, a 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 dependent on the temperature of the ignition source, which is much higher.

The flash point is often used as a descriptive characteristic of liquid fuel, and it is also used to help characterize the fire hazards of liquids. “Flash point” refers to both flammable
Flammability
Flammability is defined as how easily something will burn or ignite, causing fire or combustion. The degree of difficulty required to cause the combustion of a substance is quantified through fire testing. Internationally, a variety of test protocols exist to quantify flammability...

 liquids and combustible
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

 liquids. There are various standards for defining each term. Liquids with a flash point less than 60.5°C (141°F) or 37.8°C (100°F) — depending upon the standard being applied — are considered flammable, while liquids with a flash point above those temperatures are considered combustible.

Mechanism


Every liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 has a vapour pressure, which is a function
Function (mathematics)
In mathematics, a function associates one quantity, the argument of the function, also known as the input, with another quantity, the value of the function, also known as the output. A function assigns exactly one output to each input. The argument and the value may be real numbers, but they can...

 of
that liquid's temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

. As the temperature increases, the vapour pressure increases. As the vapour pressure increases, the concentration of vapor of the flammable liquid in the air increases. Hence, temperature determines the concentration of vapor of the flammable liquid in the air.

A certain concentration of vapor in the air is necessary to sustain combustion, and that concentration is different for each flammable liquid. 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.

Measurement


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
Fire point
The fire point of a fuel is the temperature at which it will continue to burn for at least 5 seconds after ignition by an open flame. At the flash point, a lower temperature, a substance will ignite briefly, but vapor might not be produced at a rate to sustain the fire...

. The best known example is the Cleveland open cup
Cleveland open cup method
The Cleveland open-cup method is one of three main methods in chemistry for determining the flash point of a petroleum product using a Cleveland open-cup apparatus, also known as a Cleveland open-cup tester. First, the test cup of the apparatus is filled to a certain level with a portion of the...

 (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
Lower flammable limit
Lower flammability limit , usually expressed in volume per cent, is the lower end of the concentration range of a flammable solvent at a given temperature and pressure for which air/vapor mixtures can ignite. The flammability range is delineated by the upper and lower flammability limit. Outside...

 (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. The ATEX directive
ATEX directive
The ATEX directive consists of two EU directives describing what equipment and work environment is allowed in an environment with an explosive atmosphere...

 outlines three thresholds of danger at which vapor may ignite.

Methods for determining the flash point of a liquid are specified in many standards. For example, testing by the Pensky-Martens closed cup
Pensky-Martens closed cup
In the Pensky–Martens closed-cup flash-point test, a brass test cup is filled with a test specimen and fitted with a cover. The sample is heated and stirred at specified rates depending on what it is that is being tested...

 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.

Examples

Fuel Flash point Autoignition
temperature
Ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

 (70%)
16.6 °C (61.88 °F) 363 °C (685.40 °F)
Gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...

 (petrol)
-43 °C (-45 °F) 246 °C (495 °F)
Diesel >62 °C (143 °F) 210 °C (410 °F)
Jet fuel
Jet fuel
Jet fuel is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is clear to straw-colored in appearance. The most commonly used fuels for commercial aviation are Jet A and Jet A-1 which are produced to a standardized international specification...

>60 °C (140 °F) 210 °C (410 °F)
Kerosene
Kerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...

 (paraffin oil)
>38°–72 °C (100°–162 °F) 220 °C (428 °F)
Vegetable oil (canola) 327 °C (620 °F)
Biodiesel
Biodiesel
Biodiesel refers to a vegetable oil- or animal fat-based diesel fuel consisting of long-chain alkyl esters. Biodiesel is typically made by chemically reacting lipids with an alcohol....

>130 °C (266 °F)


Gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...

 (petrol) is a fuel for use in a spark-ignition engine. The fuel is mixed with air within its flammable limits and heated above its flash point, then ignited by the spark plug
Spark plug
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...

. In order to avoid preignition
Engine knocking
Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...

 by dint of the hot combustion chamber, the fuel must have a low flash point and a high autoignition temperature
Autoignition temperature
The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion...

.

Diesel fuel flash points vary between 52 °C and 96 °C (126 °F to 204 °F). Diesel is suitable for use in a compression-ignition engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

. Air is compressed
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...

 until it has been heated above the autoignition temperature
Autoignition temperature
The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion...

 of fuel, which is then injected as a high-pressure spray, keeping the fuel-air mix within flammable limits. There is no ignition source. The fuel is, therefore, required to have a high flash point and a low autoignition temperature.

Jet fuel
Jet fuel
Jet fuel is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is clear to straw-colored in appearance. The most commonly used fuels for commercial aviation are Jet A and Jet A-1 which are produced to a standardized international specification...

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 JP-4 have flash points between -23 °C and -1 °C (-10 °F to +30 °F ).

Standardization


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.