Flash, Fire, And Autoignition Points Demystified
•Technical Data Sheet: Leakage in Thermal Oil Systems, Flash and Fire Points Demystified
Three major technical terms describe flammability conditions in hydrocarbon liquids and their vapors; flash point, fire point, and autoignition temperature.
Flash Point Defined
The lowest temperature at which a heated liquid's vapor/air mixture can be ignited ("flashed") by a flame or spark, or other ignition source placed above the liquid surface.
Fire Point Defined
The lowest temperature at which a heated liquid's vapor/air mixture will burn continuously when combustion is supported by ignition sources such as the above.
Autoignition Temperature Defined
The lowest temperature at which a heated liquid's vapors in air will selfignite and burn, without exposure to any ignition source.
Flash Point and Fire Point Testing
The liquid to be tested is heated in a cup and the rising liquid temperature is continuously measured. A small flame is mechanically passed back and forth just above the surface of the liquid. As the liquid gets hotter, more of it evaporates causing the fuel/air mixture above the liquid to gradually become richer. When the lower flammability limit is reached, the ignition source will ignite the vapor/air mixture, causing a pop. The observed temperature when the flame momentarily ignites the vapor/air mixture is the Flash Point. The ignitions repeat as the liquid temperature continues to rise. The observed temperature when the burning becomes continuous is the Fire Point.
Autoignition Point Testing
Liquid is heated, but without an ignition source. When the vapor/air mixture reaches a temperature sufficient to self ignite, the observed temperature is the Autoignition Point. For a flash-point-related fire to occur, all three conditions must be met:
- Vapor concentration - These combustion tests allow vapor to concentrate. In real life, the vapors turn to smoke as they encounter air and dissipate.
- Temperature - Thermal oils cool rapidly when exposed to air.
- Source of ignition - Thermal-fluid leaks are difficult to ignite unless a significant amount of very hot fluid leaks into a closed area where inadequate ventilation allows unreacted vapor to collect and mix with air. An exception occurs when fluid leaks onto an extremely hot surface such as the housing of a pump that is failing, or a rotary union that has seized. Technically, this is not a flashpoint- related problem but one of autoignition.
Heat Transfer Fluids in closed-loop systems, whether natural or synthetic, are routinely used well in excess of their flash and fire points, but never above their autoignition points.
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•Technical Data Sheet: Leakage in Thermal Oil Systems, Flash and Fire Points Demystified
SOURCE: Paratherm Corporation