Thermal Fluid Leakage

The smoke that is generated when hot fluid is exposed to air is considered one of the simplest leak detectors for thermal fluid. The volume of smoke depends on temperature of the fluid, the size of the leak, and to a certain extent, the airflow in the area.

"Oozers", also known as small leaks, can generate an exaggerated quantity of smoke because there isn't enough fluid to develop a drop. This stable weeping smokes and then cooks onto the metal close to the leak.

Generally, the fluid cools rapidly as it sprays into the air or drips with larger leaks. As smoke is the reaction of the fluid’s volatile low boiling portions (smaller molecules) with oxygen in the air, cooling decreases the fluid’s vaporization, which helps to reduce the amount of smoke.

However, vapor can accumulate and lead to a fire hazard if ventilation is inadequate, or if the leak is large enough that it uses up all the fresh air.

One significant way to prevent safety issues from leaks is to ensure that thermal fluid systems are not operated in enclosed areas without sufficient ventilation. It is also necessary to ensure sufficient flow of air in varied locations, such as expansion tanks, pumps, instrument ports, flanges and valves, where there are risks for a major leak.

Minimizing Thermal Fluid Leaks

Thermal fluid leaks should be minimized for several reasons:

  • Screwups – The following tips are derived from real case histories. It is important to close all of the drain valves before adding fluid and all of the block valves should be closed before opening a line. Pressure gauges should comprise of isolating valves and be positioned to prevent them from being accidentally removed with a fork truck. It is necessary to replace the leaking pump seals before they flush out the bearing grease. Before startup, it is important to check the expansion tank level.
  • Bolts stretch – Hot metal increases in length and diameter. Piping runs increase up to 4" per 100' of length. Also, hot fluid is extremely thinner than cold (thinner than water above 400°F). In old and new systems, flanges are considered to be the largest source of leaks. Sometimes the carbon crust that ultimately develops will seal the leak but why take the chance? The solids may also get trapped in the gasket. In this case, it is important to use the wrench and re-torque the flanges. A thorough knowledge about insulation fires is essential for removing the insulation to access the flange. Teflon tape or fluorocarbon-based thread sealant can be used on threaded fittings in order to tighten them down.

Leakage and Combustion Issues

This section looks at the meaning of flash point and related technical terms. The aim is to provide a clearer perspective to help customers safely maintain their heat transfer fluid system.

Paratherm Tipsheet

Flash Point

Flash point is the lowest temperature at which the vapors of a heated liquid mixed with air can be ignited (“flashed”) by ignition sources such as a spark or flame.

Fire Point

Fire point can be defined as the lowest temperature at which the vapors of a heated liquid burn constantly when combustion is aided by ignition sources.

Auto-Ignition Temperature

The temperature at which the vapor produced by a heated liquid will ignite/flash without an ignition source.

Flash Point and Fire Point Testing

The liquid to be analyzed is heated in a cup and the increasing liquid temperature is constantly measured. A small flame is mechanically passed back and forth just above the liquid surface. As the liquid becomes hotter, more of it evaporates resulting in a mixture of fuel/air above the liquid to steadily become richer.

On reaching the lower flammability boundary, the ignition source will ignite the mixture of air/vapor, causing a pop. Flash point refers to the observed temperature when the flame briefly ignites the air/vapor mixture. The ignitions repeat as the liquid temperature continues to increase. Fire point refers to the observed temperature when there is continuous burning.

Autoignition Temperature Test

A sample is injected into a flask that is heated to the test temperature. When a “flash” is seen in the container, that temperature is the auto ignition temperature. If no flash is noticed after a period of time, the temperature of the flask is increased and the test is performed again.

This technique (ASTM E659-78) is suitable only for fluids that are fully vaporized at the test temperature, given that the degradation products produced by any remaining liquid will influence the test result.

Three Conditions to Be Aware of

The following conditions should be met to trigger a flash-point-related fire:

  • Temperature – Thermal oils cool fast when it is exposed to air.
  • Vapor concentration – These combustion tests facilitate vapor to concentrate. However, in real life, the vapors change to smoke as they come in contact with air and dissipate.
  • Source of ignition – Thermal-fluid leaks are hard to ignite unless a large amount of extremely hot fluid leaks into a closed area where insufficient ventilation allows unreacted vapor to gather and combine with air. An exception happens when fluid leaks onto a very hot surface such as a rotary union that has seized, or the housing of a pump that is failing. Technically, this is an auto- ignition problem and not a flash-point-related one.


Heat transfer fluids in closed-loop systems, whether synthetic or natural, are regularly used well beyond their fire and flash points.

This information has been sourced, reviewed and adapted from materials provided by Paratherm.

For more information on this source, please visit Paratherm.


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