Flash and Fire Points in Heat Transfer Fluids

The temperature at which a fluid produces adequate vapor to be ignited is known as a “flashpoint”, or “flash.” There are two common techniques for testing the generation of “flash”: Open Cup, or ASTM D92, and Closed Cup, or ASTM D93. In both methods, the sample in a small cup is heated, an ignitor is inserted into the vapor, and the temperature at which the vapor “flashes” is noted down.

The basic difference between the Closed Cup and the Open Cup techniques is that the cup is concealed in the Closed Cup test, thus preventing the vapor from dissipating into the air, and making the concentrated vapor to directly vent into the ignition source. As there is no cover in the case of the Open Cup, it records higher results as the sample should be hotter to produce adequate vapor to compensate for the amount of vapor that gets dissipated into the air. Usually, the temperature of the Open Cup flashpoint is 20- 30°F greater than the Closed Cup flashpoint.

Flash Point Versus Fire Point in Running Systems

Vapor flashes must not be mistaken as fire because only the accumulated vapor gets ignited; once the vapor is used up, the flash is over. Following are the criteria for burning of a thermal fluid:

1. The thermal fluid must stay adequately hot to continuously produce vapor. This is called the Fire Point and is typically 30-40°F greater than the temperature of Open Cup flashpoint for a novel fluid.
2. The ignition source should be positioned within the vapor cloud.
3. The system should be exposed to adequate air to assist the combustion but not so adequate to convert the vapor into smoke.

What Prevents the Majority of Leaks from Becoming Fires?

1. Hot vapors generally react with air and generate smoke which is not ignitable, thus limiting the area in which ignition can take place.
2. Organic heat transfer fluids have specific heat and comparatively low density, so they cool down rapidly when exposed to air, thus restricting the amount of vapor generated around a leak.
3. Generally, leaks invariably take place in the open where there is adequate air both for reacting with any vapor and for cooling the thermal fluid. Therefore, air, which is one of the major requirements for a fire to occur, can actually minimize the potential for a fire in a significant way.

Very few fires have been caused in thermal fluid systems due to a low flashpoint. This point is supported by the fact that Factory Mutual refers to flash point only once in its FM 7-99 Property Loss Prevention Sheets on Heat Transfer by Organic and Synthetic Fluid.

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

For more information on this source, please visit Paratherm.