Significance of Flash Fire Points

Flash and Fire Points in Heat Transfer Fluids

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What are Flash & Fire points?

A heat transfer fluid’s flash and fire points are the temperatures at which the fluid’s vapors, mixed with the right proportions of air, will ignite in direct and intimate contact with a spark or open flame.

Difference Between Flash & Fire Points

Fluid to be tested is placed in a cup along with a temperature probe. The cup is placed on a hot plate and an ignition source (gas flame or electric arc) is located just above the cup. The hot plate is turned on. As it heats up, the fluid produces vapors. When enough vapors are produced so that the cloud is ignited by the open flame (the cloud “pops”), the technician reads the thermometer. This is the flash point. Continuing to heat, the fluid produces more and more vapor.

When the “popping” becomes a continuously burning flame, the technician again reads the thermometer. This is the fire point.

Until the fire point is reached, should the ignition source be removed, the fluid merely sits there and oxidizes (it smokes).

What prevents the majority of leaks from becoming fires?

Unlike high-pressure hydraulic systems, thermal oil systems are not generally pressurized. The typical atmospheric closed-loop heat transfer system employs a vent line that runs from the headspace in the system’s expansion tank to a catch container beneath. Even though these systems are normally unpressurized, there are occasional leaks. The leaks that do occur are found mostly in threaded fittings, joints, valves, and pumps — the fluid will slowly weep rather than gush or spray. This “weeping” is similar to the way bolt loosener works its way through tiny openings.

Upon contact with outside air, the hot fluid oxidizes (it will smoke). This is nearly identical to the smoke vegetable oil produces when it is overheated on the kitchen stove. Leaking heat transfer fluids will typically smoke rather than burn, even at temperatures in excess of their flash and fire points. The smoking will continue until all that remains on the piping is a dark stain.

  1. Hot vapors will react with air to produce smoke which is not ignitable. This limits the area where ignition can occur.
  2. Organic heat transfer fluids have relatively low density and specific heat and so cool rapidly when exposed to air.  This limits the amount of vapor produced around a leak
  3. Leaks almost always occur in the open where there is sufficient air both to cool the fluid and to react with any vapor. So one of the requirements for a fire – air- actually can significantly reduce the potential.


It is very important to quickly find and eliminate leaks in heat transfer systems. We suggest you locate all potential leak points in the system, and, at these points, specify the use of high-temperature closed-cell insulation (Pittsburgh Corning Foamglas or equal) — or no insulation at all. Most important, you should never fail to perform regular system checks.

Like many other thermal liquids, the Paratherm NF®HE® and OR® fluids have proven exceptionally safe. For years they have been used in a broad range of demanding systems where bulk fluid temperatures well exceed the fluid’s flash and fire points.

Final note : There have been very few fires in thermal fluid systems caused by a low flashpoint. This is reinforced by the fact that Factory Mutual only mentions flash point once in its FM 7-99 Property Loss Prevention Sheets on Heat Transfer by Organic and Synthetic Fluid (and that section deals with heater location.) 

Questions? We’d like to hear from you. Call toll-free, 610-628-2672 — or fax or e-mail.

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