Over time, the molecules in a thermal fluid will break down by a process called thermal cracking and oxidation, creating volatile light ends, and heavy ends and organic acids. As well as causing the thermal fluid to degrade, thermal cracking and oxidation also produces carbon molecules that stick to the pipes and system, which reduces efficiency.
Furthermore, organic acids which results from the oxidation process are both corrosive and help to accelerate the production of carbon and fouling. Light ends lower flash points and increase risk of fire.
If a fluid and system are left unattended the fluid will potentially thermally degrade at a much faster rate. You may only notice this once it begins to impact production.
Once this occurs you must test and analyse a sample of the fluid and respond to the data with the appropriate intervention. If necessary, you must shut down production, drain, clean and flush the system and then purchase new fluid, which is a costly and time-consuming process.