Minimising the Oxidation of Hydraulic Oil

Minimising the oxidation of hydraulic oil is crucial for maintaining the efficiency and longevity of hydraulic systems.

What is oxidation in oil?

Oxidation is the chemical reaction with oxygen, which permanently changes the molecular structure and properties of oil. Depending on the extent of oxidation, other chemicals such as alcohols, aldehydes, ketones and acids replace the oil molecules. This can lead to an increase in oil viscosity, deposit formation, varnishing and sludge.

What are some of the causes of accelerated oxidation?

Exposure to Oxygen

Hydraulic oil is susceptible to oxidation when exposed to the air. As the oil circulates through the system, it is exposed to oxygen, which initiates the process.

High Temperatures

High temperatures are common in hydraulic systems due to the friction and pressure generated during operation, and accelerate the oxidation of hydraulic oil.

Contamination

Contaminants or impurities in the oil such as water, acids, or other fluids and particles such as wear metals (e.g. copper) can speed up the oxidation process.

What impact does oxidation have on hydraulic systems?

Oxidation reduces the amount of functional oil in the system and replaces it with chemicals such as acids, which increase corrosion and create a loss in foam control. This also means that the oil becomes more viscous, and promotes deposit, varnish and sludge formation. These issues restrict the oil flow by clogging filters, strainers and valves.

Varnish build up can also act as an insulating film, increasing operating temperatures and accelerating the rate of oxidation, while deposits and sediment cause wear in the system. The most severe result of oxidation is a system breakdown, most often caused by pump failure.

What is the best way to stop oil oxidising?

Hydraulic oil oxidation is unavoidable. However, by selecting the right oil and implementing strict oil maintenance schemes, such as using inherently stable base oils and anti-oxidants, the rate of oxidation can be slowed and managed. By taking regular oil samples, the presence of contaminants can be identified and remedial action including filtration can be taken.

Millers Oils Millmax hydraulic oils are specially formulated to resist oxidation. Using a blend of base oils with naturally high oxidation resistance, paired with the latest additive technology, they provide longer service life and extended drain intervals.

How can you tell if an oil has oxidised?

Two indications of hydraulic oil oxidation include foul odours and darkening of colour, as well as the more obvious presence of sludge and varnish. Millers Oils Fluid Management Division will help you to identify evidence of oxidation through oil sample analysis.

Over 80% of damaged hydraulic components can be attributed to contamination of hydraulic oil, which is why oil sampling is a critical tool to help protect capital equipment. Oil sampling also keeps your maintenance team informed on the condition of the oil and machine components, so maintenance can be scheduled around operations, minimising downtime, and saving both time and money.

What do the lab test for?

There are a number of ways to determine the level of oxidation by oil sample analysis in the lab.

Total Acid Number (TAN)

Fresh hydraulic oil should have a TAN of around 0.2 mg KOH/g, however, this will increase as acids are produced in oxidation reactions. Once the oil has a value of 1 mg KOH/g greater than the initial value, an oil change is recommended.

IR Spectroscopy

Oxidation products such as alcohols, aldehydes, ketones and acids can be identified in the oil by using infrared spectroscopy. A greater presence of these molecules indicates an accelerated level of oxidation.

Viscosity

During oxidation, condensation products are formed which causes an increase in density and oil viscosity.