Why Hydraulic Oil Changes color

When hydraulic oil turns from that golden honey color of new oil to a dark brown, does that mean it must be changed immedeately?. Is the system suffering from lost lubricating properties or gross contamination when this occures, or is this a normal aging characteristic to be dismissed so long as the oil analysis results are within acceptable parameters?

These types of questions are often asked whenever hydraulic fluid maintenance is discussed. Many people compare the oil in their industrial hydraulic systems to that of their automobile, assuming if the oil has turned to a dark brown that it must be changed as soon as possible regardless of how long it has been in service.

Its easy to forget that the oil in an industrial hydrulic system is kept in a much different environment than the oil is an internal combustion engine. A color change in hydraulic oil is a good reason to be alert but not a good reason to go running for the oilskid to replace it right away. you first need to determine why the oil has changed the color.

The two most common causes of oil darking are thermal stress and oxidation, neither of which will necessarily require that the oil be replaced. The first step is to take a representative sample of the oil and have it analyzed. I have seen hydraulic oil that has darkened considerably but was still perfectly good to remain in service. I have also seen hydraulic oil that has retained its original color but could not meet the parameters neccessary to provide adequate system protection. In short a change in oil color alone tells you nothing about the serviceability of the oil.

Howerever, darkening of the oil can direct you to potential problems that many need to be addressed. Perhaps a system has one or more HOT SPORTS where the oil is heated up significantly in a localized area, but the temperature is brought down again once it reaches the relatively color reservoir. I once found a value that had failed, forcing oil through a small orifice with a significant pressure drop. This generated a relatively large amount of heat, but it was localized in only a very small amount of the system oil. The only symptom was a darkening of the oil.

When a sample of the oil was analysed it was determined that the acid number and viscosity had not changed, ranging from brillant yellow to inky black. There are a number of factors that include formulation, operating conditions and contaminants, any of which can cause a considerable color change without significants oil degration. Although the color change can be alarming, the can still retain good antioxidant potential, since a series of these reaction can occur before becoming truly exhausted. Again, the only way to be cetain of the eliminating the likelihood of oil oxidation and suggesting that the change in color was the result of thermal degradation. Inspection with an infrared camera located the overheating valve in very short order. The valve was replaced and significant varnish was noted at the point where the heat was generated. oil analysis showed that the oil was perfectly suitable for continued service, but since there was no discernible change in system operation, the valve failure may very well have gone unnoticed until it became a system outage had there been no color change in the oil.

While oxidation, the chemical union of oil and oxygen, is a common reason that hydraulic oil stability is reduced, the amount of color change is not a good indication of the level of oxidation. The presence of metal catalyst particles, heat, oxigen and water all contributer to oil oxidation. As the acid level increses, corrosion of components will become more likely. Viscosity will increse as soluble contamination mix with oil. This will leave sludge, varnish and tar deposits as thin, insoluble film throughout the systems internal surface. The degradation process accelerates with continued exposure to thease elements.

System pressure can make a difference too. As Pressure increase, so does fluid viscosity, which causes an increase in friction and heat generation.Also incresed pressure results in an increse of entrained air (and therfore oxygen). Contamination are another factor that can affect oxidation.

Conclusion :

When you find that your hydraulic oil has darkened in color, donont assume that it needs to be replaced. It is quite likely that there are years of service remaining in your fluid. Get a good representative sample and have it analysed. The most representative sample will be taken immediately downstream of the pump. Only then will you know the condition and serviceability of your hydraulic fluid.