Flash point - Punto de inflamabilidad

The flash point or flash point is the set of conditions of pressure, temperature, gas mixture in which a combustible / flammable substance, normally a liquid, produces enough vapors that when mixed with air, they would ignite when applying a source of heat (called an ignition source) at a sufficiently high temperature.

Do not confuse the flashpoint with the ignition point or ignition , or the point of autoignition or spontaneous combustion . At the point of ignition the rate of vapor production is high enough that the flame sustains itself when the external heat source is removed. At the point of autoignition or autoignition, the substance begins to burn spontaneously, without the need for an external heat source. Neither the flash point nor the flash point depend on the temperature of the ignition source, which is usually much higher.

If normal pressure conditions are considered ( normal atmospheric pressure of 101.3 kPa ), these conditions are reduced to a minimum temperature and a certain proportion of fuel vapor in the ambient air, which can occur in a small part of it. Both the temperature and the mixing ratio are important. In fact, the temperature can be relatively low, most of the times lower than normal in the environment, but at that temperature liquid fuels begin to give off vapors that, when mixed with oxygen in the air or another oxidizer , can give the conditions , so that any spark that reaches the ignition temperaturenecessary, start the fire. Among these conditions, the ratio of gases to air is essential and, whether the ratio of gases is low or excessive, ignition will not occur.

The Pensky-Martens apparatus is used to measure the flash point.

Liquids

Flash point is a descriptive characteristic that is used to determine the fire hazard of liquids, and allows to distinguish flammable liquids from combustible liquids. Liquids that have a flash point lower than 37.8 or 60.5 ° C (100.0 or 140.9 ° F), depending on the standard used, are called flammable, while liquids that have a flash points above that temperature are called fuels.

Mechanism

Each liquid has a vapor pressure , which is a function of the temperature of that liquid. As the temperature increases, the vapor pressure increases. As the vapor pressure increases, the vapor concentration of the flammable liquid in the air increases. Therefore, the temperature determines the vapor concentration of the flammable liquid in the air.

A certain concentration of vapor in the air is necessary to maintain combustion, which is different for each flammable liquid.

Measurement

There are two basic types of flash point measurement: Open Cup and Closed Cup.

In the Open Cup configuration, the sample is contained in an open cup that is heated, and at different temperatures a flame approaches the surface. The flash point will vary with the height at which the flame is placed above the surface of the liquid. At a sufficient height the flash point temperature will coincide with the flash point.

There are two types of Closed Cup devices: unbalanced, where the vapors above the liquid are not in thermal equilibrium with the liquid, and balanced, where the vapors are considered to be in thermal equilibrium with the liquid. In both cases the cups are sealed with a lid through which the ignition source can be introduced. Closed Cup devices typically register lower values ​​for flash point than Open Cup devices (typically 5-10 ° C or 9-18 ° F lower) and are a better approximation to the temperature at which the Vapor pressure reaches the lower flammability limit.

The lightning point is an empirical measurement, not a fundamental physical parameter. The measured value may vary with the test equipment and protocol. Other variables are the time allowed for the sample to equilibrate, the volume of the sample, and whether the sample has been shaken.

Flash point of some substances

Combustible temperature
Ethyl alcohol 12 °C / 53.6 °F
Methyl alcohol 11 °C / 51.8 °F
Butyl alcohol 37 °C / 98,6 °F
Gasoline -40 °C
Petroleum naphtha -2 °C / 28.4 °F
Kerosene 38 °C a 72 °C / 100.4 °F a 161.6 °F
Diesel oil 52 °C a 96 °C / 125.6 °F a 204.8 °F
Benzene 20 °C / 68.0 °F
Hexane -28 °C / -18.4°F
Toluene 9 °C / 48.2 °F
Furfural 62 °C / 143.6 °F

References

Félix Esparza, Fire or Combustion , p. fifteen