Silicone oil flashpoint reduction
In the US an incident with three fatalities occurred while a Syltherm heat transfer fluid system was being drained for maintenance/repair. Specific details as to the cause of the incident are not available. However it is known that heat transfer fluids degrade on extended thermal exposure, as the following reports show.
Experience with heat transfer fluids
- In USA
Plant engineers of another chemical company in the US found that the actual flash point of the Syltherm heat transfer fluid in use was approximately 102°F (39°C). Dow Chemical lists an "as supplied" flash point for Syltherm of 320°F (160°C). This type of reduction is typical with Syltherm, and is discussed in detail in their product technical data. Heat transfer fluids, such as Syltherm (a dimethyl polysiloxane), can present hazards when their vapours can form flammable atmospheres and come in contact with an ignition source. This risk is increased especially during shutdown, maintenance/ repair and restart cycles. Even if Syltherm fluid is used in the temperature range specified by the supplier, it undergoes a slow rearrangement of the silicone-oxygen bonds in each molecule of material. This rearrangement generates low molecular weight silicone compounds that can be flammable. Over time the generation of these flammable silicone compounds can lower the flash point of the heat transfer fluid significantly. Other heat transfer fluids, such as Dowtherm A, Dowtherm G, Therminol 66, etc. also have relatively low flash points compared to their normal operating temperatures and should be handled accordingly.
- In UK
Dowtherm was disposed after at least 3 years in service, and the flashpoint had dropped from 145°F (~63°C) to ~120°F (~49°C). For Syltherm a decrease in the flashpoint from 320°F (160°C) to about 90°F (32°C) was observed.
- In Switerland
During the regular checks carried out in a plant, it was found that the flashpoints of Marlotherm L and Marlotherm S had decreased within one year upon exposure to temperatures at the upper end of their application range.
- Marlotherm L decreased from 120°C (248°F) to 60°C (140°F)
- Marlotherm S decreased from 190°C (375°F) to 85°C (185°F)
- A different site in Switzerland
In a laboratory a reaction mass containing potassium hydroxide was melted in a silicon oil bath at 240°C. During lunch break a fire developed in the fume hood, which fortunately could be extinguished in short time by the fire brigade. The investigation in the safety laboratory revealed the following:
- Silicon oil decomposes slowly at 280°C forming volatile products.
- Silicon oil reacts violently with potassium hydroxide resulting in foaming by volatile products.
- The silicon oil in the oil bath had a flashpoint of 110°C, while the flashpoint of new oil is 250°C.
- Furthermore it was found out that the sort of oil used is not recommended for such high temperatures.
- Heat transfer fluids may decompose under thermal stress, especially if they are used at temperatures higher than the recommended application range. The flashpoint decreases and the viscosity might increase
- The decomposition of heat transfer fluids may be accelerated by contamination and oxygen.
- Regular evaluation of the heat transfer fluid is necessary (either by the supplier or by internal tests according to the suppliers recommendations). E.g. Dow Chemical Company recommends in their product technical data that this evaluation should be conducted annually.
- Anyone working on heat transfer systems, including contractors, must be made aware of the potential hazards associated with these materials, especially during maintenance and repair operations.