Thermal Fluid Leakage: Part 3 of 3
Parts I and II of this three-part TipSheet™ series covered detecting thermal fluid system leaks, why systems leak, and how to prevent and minimize leaks.
For further background, Part III will discuss the meaning of flash point and related technical terms. We hope this supporting detail will put recommendations from the first two TipSheets in clearer perspective to help you safely maintain your heat transfer fluid system.
PARATHERM TIPSHEET, LEAKAGE PART III: FLASH POINT, FIRE POINT, AND AUTOIGNITION TEMPERATURE DEMYSTIFIED.
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.
The lowest temperature at which a heated liquid’s vapors burn continuously when combustion is supported by ignition sources such as the above.
The temperature at which the vapor formed by a heated liquid will flash without a source of ignition.
FLASH POINT 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 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 TEMPERATURE TEST
A sample is injected into a flask which is heated to the test temperature. If a “flash” is observed in the container, that temperature is the Auto Ignition 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 since the degradation products formed by any remaining liquid will affect the test result.
SO, THREE CONDITIONS TO BE AWARE OF:
For a flash-point-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 flash-point-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.