"Do you have any suggestions for problems with the solubility of oxidation products in turbine oil at low temperatures? Recently, my clients have had a problem with the solubility of oxidized products in turbine and hydraulic oils. At operating temperatures (60-80 degrees С), they are dissolved, but in stoppage (i.e., temperatures below 25 degrees C), they become insoluble and begin to deposit on working surfaces. This is a problem with the hydraulic piston pumps, and it does not matter the type of turbine (gas/steam/etc. or manufacturer) or the working hours."

Based on your comments, you may be dealing with varnish formation, which is a frequent problem in high-temperature and high-pressure systems such as steam turbines or high-performance hydraulic systems.

Varnish is the accumulation of oil oxidation and degradation compounds on machine surfaces or components. It can be the result of several possible root causes, including high temperatures, electrostatic discharges, lubricant degradation and microdieseling. Varnish can produce a number of problems related to machine operation, such as valve stiction, lubricant flow restriction, clogged filters, etc.

Varnish begins as dissolved impurities. When these impurities accumulate and reach the saturation point, they migrate to the surfaces of the lubrication system. If these deposits remain on the surfaces, they cure (harden) with time, causing failure of the lube system and lubricated components.

Oxidation resistance and solubility are two important lubricant properties to consider. Oxidation resistance refers to how molecules resist the chemical reaction with the oxygen in the air. Oxidation degrades oil and is one of the main reasons to change it. The greater the oxidation resistance, the longer the oil life.

Solubility is the property that allows a lubricant to hold polar substances like varnish in suspension without damage to the machine. Oil solubility increases at higher temperatures. Group III oils also have lower solubility than Group II and Group I oils. There have been many instances of machines experiencing varnish deposits due to lower solubility of the oil after switching from a Group I oil to a Group II or III oil.

If you are facing varnish deposits, two actions are recommended to control it. First, identify the root causes. This will require a systematic study of the possible factors supported by oil analysis. Next, remove the existing varnish in the machine. This can be achieved by adding solvent or detergent additives to the oil, using a synthetic product with high natural solvency or installing varnish removal systems. In cases of hardened varnish, the solution will be mechanical and may simply involve changing the components.