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According to the TipSheet, in order to understand how a fire can occur, it is first necessary to understand the meaning of flashpoint and related technical terms.
- Flashpoint is the lowest temperature at which a heated liquid's vapors, when mixed with air, can be ignited ("flashed") by a flame or spark, or other ignition source.
- Fire point is the lowest temperature at which a heated liquid's vapors burn continuously when combustion is supported by ignition sources such as the above.
- Autoignition temperature is the temperature at which the vapor formed by a heated liquid will flash without a source of ignition.
During the autoignition temperature test, according to Paratherm, a sample is injected into a flask that is heated to the test temperature. If a "flash" is observed in the container, that temperature is the autoignition temperature. IF no flash is observed after a period of time, the flask temperature is increased and the test repeated. This method (ASTM E659-78) is valid only for fluids that are completely vaporized at the test temperature because the degradation products formed by any remaining liquid will affect the test result.
Given those definitions, Paratherm explained in its guide that for a flashpoint-related fire to occur, all three of the following 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.