For a flashpoint-related fire to occur, three conditions must be satisfied, according to "Thermal Fluid Leakage: Flash, Fire and Autoignition Temperature Demystified," part of the TipSheet series from heat transfer fluid maker Paratherm Corp., West Conshohocken, Pa.

Other technical guides in the series explain topics such as how to detect thermal fluid system leaks, why systems leak, and how to prevent and minimize leaks. Users of thermal fluids and systems can subscribe to the monthly publication at the company's web site.

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.
As explained in Paratherm's guide, during flashpoint and fire point testing, the liquid to be tested is heated in a cup, and the rising liquid temperature is continually 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 flashpoint. The ignitions repeat as the liquid temperature continues to rise. The observed temperature when the burning becomes continuous is the fire point.

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:
  1. Vapor Concentration. These combustion tests allow vapor to concentrate. In real life, the vapors turn to smoke as they encounter air and dissipate.
  2. Temperature. Thermal oils cool rapidly when exposed to air.
  3. 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.
Paratherm notes that heat transfer fluids in closed-loop systems, whether natural or synthetic, are routinely used well in excess of their flash- and fire points.