TAN increase in hydraulic unit

Hi everyone.
about 30 tests have been done on a hydraulic unit for a walking beam furnace.
TAN increase:from 1 to 6
viscosity increase:from 46 to 55(oil grade is 46)
Fe and Cr increase
the oil has been changed when reaching these critical values and again it seems the problem is going to repeat as I see new oil trend.this problem has been observed in some other units.
is there a common problem.
Original Post
The oils always degrade but if it a stable one takes longer and if it a metastable one will taker less.
For example hydrocarbons are relatively stable, especially as higher the structure as it happens for oils. So its degradation is so slow that even temperature cannot raise the kinetics of the reaction enough to be extremely significant.
According to the Arrhenius index, each 10ºC above 60ºC will double the speed of the degradation reaction. Oxygen degrades by forming peroxides which then breack forming smaller molecules (ketones, aldehides, etc.) which give smell to the oil. The presence of light interferes by activating the oxygen, so it is more reactive.

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