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During after-sales service for customers, I have encountered such situations. The new heat transfer oil installed by Customer A worked fine at first, and the test results for the oil samples taken within the specified time period showed no significant increases. So why did the oil consumption suddenly rise and become difficult to circulate? Upon checking the indicators, it turns out that they all meet the criteria for scrapping. What’s going on here? Let’s first take a look at what causes the degradation and deterioration of heat transfer oil. 1. After being heated, heat transfer oil first undergoes thermal cracking and thermal polymerization reactions, which cause its color to gradually darken until it turns black. Moreover, the physical and chemical properties of the products resulting from these cracking or polymerization reactions, such as thermal stability, thermal conductivity, flash point, and dynamic viscosity, all **decrease**, thereby affecting the performance of the heat transfer oil. ； 2. When heat transfer oil comes into contact with oxygen in the air, an oxidation reaction occurs, producing organic acids. These organic acids can further accelerate the polymerization reaction. As the temperature rises, the oxidation rate increases, and the color of the heat transfer oil gradually turns darker, along with an increase in its parameters ； 3. Impurities have entered the heat transfer oil circulation system (including the mixing of the material to be heated into the heat transfer oil due to pipe damage, the addition of contaminated new oil, or simply the use of oil that is not suitable as heat transfer oil) ； Now that we know the reason, let’s analyze it. First, we can rule out the first scenario, because for heat transfer oil used within the maximum allowable operating temperature, its thermal pyrolysis is a very slow process (except in the case of low-quality heat transfer oil) ； Next is the third scenario, which requires specific determination through component analysis of the oil sample and inspection of the pipelines ； Let’s focus on analyzing the second scenario. Tests have shown that when the temperature of the heat transfer oil is above 60°C, for every 10°C increase in temperature, the rate of its oxidative degradation doubles, and as a result, its service life is also reduced by half. The sump is the only pathway through which the heat transfer oil comes into contact with oxygen; especially when the auxiliary exhaust valve is forgotten to be closed, the high-temperature hot oil flows into the sump where it is rapidly oxidized and degraded, causing the specifications of the heat transfer oil in use to become unsuitable in a short period of time (this is the reason for the deterioration of heat transfer oil at Customer A’s site). Please regularly check whether the auxiliary exhaust valve is closed, and monitor the temperature of the heat transfer oil in the sump. It is advisable to apply a nitrogen seal or liquid seal to the sump in order to prevent oxidation of the heat transfer oil.