Air contains oxygen, and when it is mixed with oil at high temperatures, the oxygen converts oil molecules into acids — a process known as oxidation. These acids can thicken the oil, which will decrease pump efficiency. The thickened oil may also result in cavitation, which can cause catastrophic failure. In some cases, acid buildup forms deposits that can block filters and strainers. In general, an oil’s oxidationrate doubles with every 10° C increase in temperature.

To combat oxidation, good quality hydraulic oils are formulated with antioxidants, which interfere with the oxidation process (air reacting with the oil molecules). As a result, the fluid lasts longer and is less likely to increase in viscosity.

Air in hydraulic oil also can lead to foam-related problems. Foam can cause pump cavitation and decrease lubricity, which shortens component life. Foam carried by the fluid will deteriorate system performance and usually can be prevented by eliminating air leaks. Surface foam can be eliminated with proper reservoir design or by using a defoaming additive.

Ship LoadingAnother general problem related to air contamination is entrained air. Entrained air is made up of bubbles suspended in the fluid. It can be introduced during release of dissolved air within the fluid when pressure is decreased, from leaks on the suction side of the pump, from splashing in the reservoir, or from contamination Entrained air can result in spongy controls, cavitation, noise, and loss of horsepower. The best prevention for entrained air is a fluid formulation designed with proper additive and base oil selection.

Varnish
Hydraulic systems often operate at elevated temperatures. For mineral oil-based hydraulic systems, temperatures from 140° to 180° F are considered normal. If the oil temperature goes higher, oxidation rate increases and oil life decreases. The problem is exacerbated by the use of more powerful systems with smaller reservoirs, which have become a trend in the design of many of today’s hydraulic systems. Over time, the additives and oil can degrade to form sludge and varnish.

Sludge and varnish are by-products of oxidation and thermal degradation of the fluids that are meant to protect expensive hydraulic systems. Varnish is a dark, sticky deposit that adheres to metal parts. It can increase friction of moving parts to cause erratic operation, especially in valves, or prevent valves from shifting altogether. Sludge is a thick deposit that can accumulate through a hydraulic system. It can also cause erratic operation and often clogs filters, necessitating premature element replacement. In extreme cases, sludge and varnish can collect in orifices and narrow passageways and block them.

These problems prove costly not only in the expenses associated with labor and component replacement, but with productivity lost due to down time. Furthermore, the cost of replacing the degraded fluid with fresh fluid, disposal costs, and possibly having to flush the system can also be substantial.

So it is best to keep varnish from forming in the first place. To do that, a combination of carefully selected additives can provide all the traditional benefits expected in a properly formulated hydraulic oil. They help prevent varnish from forming on surfaces, thus keeping the system clean.

Other contaminants
Other contaminants, such as gear oils, motor oils, and even cleaners, can contaminate hydraulic oils. This often happens if poor transfer methods are used to fill systems or if fluids or equipment are not labeled correctly. The result can vary depending on the contaminant.

Often, viscosity is an issue if a different viscosity grade of another fluid is added. Viscosity increase can result in sluggish pump performance. The viscosity can be too low if a low viscosity contaminant is added by mistake. If contaminated with motor oil, water separation properties can be compromised, leading to potential wear and/or rust issues.

Hydraulic oil contamination is a hot topic with hydraulic systems. The impact on the system varies depending on the type of contaminant. Effective filtration systems help remove particulates in order to minimize performance- related issues. However, hydraulic oil additive chemistry can also be effective in reducing the effect of potential contaminants. Overall, this helps to extend the life of the fluid and boost reliability of the equipment.

For more information, contact the authors at robert.profilet@lubrizol.com or dave.oesterle@lubrizol.com or visit www.lubrizol.com.