Hydraulic Oil Oxidation Explained

suctionstrainerclogged-resizeOne of our members wrote me about the following problem:

“We are facing the following problem with our European made moulding machines. If we fill the hydraulic tank with brand new oil, within less than 24 hours oil colour turns dark brown and gives burnt smell. But if we test the oil it shows fit for further usage. Initially we were using Shell Tellus 46 and recently we changed to Mobil DTE 10 Excel-46, which is a near to synthetic grade. But the result is the same with both oils. When we emptied the hydraulic tank we found thick, dark brown sludge in the bottom of the tank. Similar dark brown sludge was found on the heat exchanger coils too.”

If the hydraulic oil darkens, smells burnt and varnish and/or sludge forms, this indicates oxidative degradation of the oil. And there are two ways this can occur. The first involves intense local heating of the oil. The cause of the intense heat can be collapse of entrained air bubbles, micro-dieseling, high watt-density tank heaters, a continuous pressure drop in the system, for example, oil leaking across a relief valve, and less commonly, electrostatic discharge. This process is sometimes referred to as thermal oxidative degradation.

The other form of oxidative degradation of the oil, or ‘normal’ oxidative degradation if you like, is accelerated by high bulk oil temperature in the presence of air. But the intense local heating of the oil described above is absent. Importantly, the chemical process of the two forms of oxidative degradation are different.

Degradation that involves intense heat forms carbonaceous nitro-nitrates in the oil. And this normally shows up as nitration on the oil analysis report (FTIR). On the other hand, oil that degrades through the ‘normal’ oxidation process produces metal carboxylates and carboxylic acids. The amount of these acids, and therefore the remaining oil life, is what the Acid Number (AN) test seeks to quantify.

So when this member says that oil analysis indicates his oil is fit for further service, this is presumably based on the result of an AN test. And so from this we can deduce that ‘normal’ oxidative degradation is not the problem. Which means thermal oxidative degradation is the likely culprit. And this should be confirmed by checking for nitration using FITR.

In terms of identifying the root cause, a heat gun or thermal camera can be helpful in identifying any ‘hot spots’ in the system. Micro-dieseling and pressure-induced bubble collapse are both consequences of entrained air. So the system needs to be examined for all possible causes of aeration. If the system has a tank oil heater, its heat density needs to be checked. As do the setting and proper operation of all relief valves in the system. And it’s worthwhile inspecting all filter elements for signs of electrostatic discharge as well.

But the moral of this story is: not having an effective oil analysis program in place can be a costly mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.

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