Heat transfer oils are designed to move fast and transfer maximum heat at high temperatures. However, the properties such as high density and low viscosity, besides optimizing these performance characteristics, make it difficult to retain the fluids’ molecules at operating temperatures. These particular properties, which make the fluids better heat transfer media, also make them susceptible to leakage.
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Smoke emanating upon exposure of the hot fluid to air is a simple leak detector for thermal fluids. Criteria such as the size of the leak, the fluid temperature, and to a certain degree the airflow in the area determine the amount of smoke. In the case of minor oozing leaks, an enormous amount of smoke is generated as there is no adequate fluid for drop formation. Such a steady seepage causes smoking and then gets accumulated onto the metal close to the leak, thereby leaving dark stains, or a carbon crust over time.
In the case of larger leaks, the fluid normally gets cooled quickly as it is dripped or sprayed into the air. As smoke is caused by the reaction of the volatile low-boiling portions, i.e. smaller molecules, of the heat transfer fluid with the oxygen in air, the cooling minimizes the fluid vaporization, thereby reducing the amount of smoke. Nevertheless, if the leak is large enough for its oxidation to use up all the fresh air, or in cases where ventilation is inadequate, the vapor may get accumulated and lead to a probable fire hazard.
A major solution for avoiding safety issues due to leaks is to ensure that thermal fluid systems are not operated in enclosed areas that have insufficient ventilation. If there are chances for a considerable amount of leak, sufficient amount of fresh air flow must be ensured in such locations, e.g. valves, pumps, flanges, instrument ports, expansion tanks.
- System Maintenance: The length (and diameter) of a hot metal increases. Bolts get stretched. Piping runs get increased by up to 4" per 100' of length. Hot fluid is relatively much thinner than cold fluid, i.e. above 400°F, it is less viscous than water even at room temperature. Flanges are the major sources of leaks in old and new systems. Therefore, leaky flanges must be re-torqued. When a need arises to remove the insulation to reach the flange, instructions related to insulation fires provided in Paratherm’s Fire Prevention in Thermal Oil Heat Transfer Systems technical data sheet should be read and followed. Teflon tape or fluorocarbon-based thread sealant should be used on threaded fittings to tighten them.
- Prevent Operator Error: Care must be taken to close all the drain valves prior to adding the fluid and also to close all the block valves prior to opening a line. Isolating valves should be equipped in pressure gauges. The pressure gauges should be located such that they are not removed inadvertently by a fork truck. Replacement of leaking pump seals must be carried before the seals flush out the bearing grease. Prior to startup, the expansion tank level must be inspected.
Flash, Fire, and Autoignition Points Demystified
The flammability conditions of hydrocarbon liquids and their vapors are described by three major technical terms: autoignition temperature, fire point, and flash point.
Autoignition temperature is the temperature at which the vapor formed by a heated liquid flashes even in the absence of an ignition source.
Fire point is defined as the lowest temperature at which the vapor/air mixture of a heated liquid continuously burns when the ignition is supported by an ignition source that is placed above the liquid surface, or by a spark or flame.
Flash point is the lowest temperature at which the vapor/air mixture of a heated liquid can be ignited, i.e. “flashed,” by ignition sources such as those mentioned above.
Flash Point and Fire Point Testing
The to-be-tested liquid is taken in a cup and heated. The increase in liquid temperature is continuously measured. A small flame is made to mechanically move to and fro just above the liquid surface. Once the temperature of the liquid increases, more liquid gets evaporated and causes the concentration of vapor/air mixture above the liquid to slowly increase. Once the lower flammability limit is attained, the vapor/air mixture is ignited by the ignition source, thus causing a pop. The observed temperature at which the flame immediately ignites the vapor/air mixture is the flashpoint. The ignition is repeated along with an increase in the liquid temperature. The observed temperature at which the burning turns continuous is the fire point.
Autoignition Temperature Test
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A flask containing the sample is heated to the test temperature. The temperature at which a “flash” is observed in the container is the autoignition temperature. Upon observing no flash even after a specific period of time, the test is repeated by increasing the flask temperature. However, this method, i.e. ASTM E659-78, is valid only in the case of fluids that get entirely vaporized at the test temperature because the test result may be affected by the degradation products formed from any leftover liquid.
The following three conditions should be satisfied for the occurrence of a flash-point-related fire:
- Temperature: Thermal oils are quickly cooled upon exposure to air.
- Vapor concentration: The combustion tests normally allow vapor concentration. However, in real-time, the vapors get converted to smoke upon exposure to air and are dissipated.
- Source of ignition: Thermal fluid leaks are highly difficult to ignite unless a substantial amount of the very hot fluid leaks into an enclosed area in which insufficient ventilation enables unreacted vapor to accumulate and react with air. A deviation from this phenomenon happens if the fluid leaks onto an extremely hot surface such as a rotary union that has seized or the housing of a pump that has failed. In technical terms, this is an autoignition issue and not a flashpoint-related one.
Natural or synthetic heat transfer fluids in closed-loop systems are regularly used well in excess of their fire and flashpoints.
This information has been sourced, reviewed and adapted from materials provided by Paratherm.
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