When to replace lubricating oil in hydraulic systems

When the color of fluids used in hydraulic systems is changed from the original colorless or golden (honey) color to dark brown, what immediately changed? Will the moving components of the hydraulic system suffer from lack of lubrication or has there been gross contamination of the hydraulic fluid? Or is there a natural degradation process (“aging”) occurring in the hydraulic fluid that can be accepted as long as the analyzes of the physical-chemical properties are within acceptable parameters?

These questions are common when discussing maintenance of hydraulic systems. Many professionals in the field of mechanical maintenance or machinery operators may compare the service life of fluids used in hydraulic systems of mobile or industrial equipment with the lubricating oils that are used in Otto Cycle/4T Diesel Cycle internal combustion engines.

Using this inadequate comparison, many users or maintainers of hydraulic systems have the dogmatic conviction that as soon as the lubricating oil charge used in the hydraulic system acquires a dark brown color it must be immediately replaced, regardless of how long it has been in service. We must not forget that the fluid used in hydraulic systems works in a very different environment from the lubricating oil used in Otto Cycle/4T Diesel Cycle internal combustion engines. Rapid changes in the color of fluids used in hydraulic systems are a good reason to be alert, but still, it is not a sufficient reason to replace, on impulse, the lubricating oil charge with a new one. It is first necessary to determine through careful investigation the root cause of the color change.

CAUSES OF DARKENING HYDRAULIC LUBRICANTS

The two most common causes of darkening of fluids used in hydraulic systems are oxidation and thermal stress. However, neither of these causes is a reason to immediately replace the lubricating oil charge. To elucidate what is actually happening in the hydraulic system, firstly, a representative sample of the hydraulic fluid in use must be collected for analysis. Many fluids used in hydraulic systems that have a considerably darkened appearance can still remain in service safely. Conversely, fluids used in hydraulic systems without changing their original color may not meet the necessary parameters to provide adequate protection. The color change, quite simply, does not indicate whether or not a lubricating oil can continue in service.

The darkening of the fluid used in hydraulic systems can give us indications of potential problems that need to be addressed. There may be some so-called “hot spots” in the hydraulic system, localized areas in which the hydraulic fluid undergoes significant heating. However, after passing through the heated area, the temperature decreases again when reaching the lubricating oil reservoir where temperatures are considerably lower.

An interesting fact can serve as an example, where a directional valve with faulty operation forced the hydraulic fluid to pass through a very small hole, which caused a significant drop in Differential Pressure. This failure led to the generation of a large amount of localized heat in an extremely small area of ​​the hydraulic system and, as a consequence, caused the hydraulic fluid to darken. However, when analyzing the sample, it was found that the Acid Number (AN) and Kinematic Viscosity had not undergone any change, which eliminated the possibility of oxidation of the hydraulic fluid and suggested that the change in color was the result of thermal degradation.

Inspection carried out on the hydraulic system with a thermographic camera quickly located overheating in the directional valve. When replacing the defective directional valve, it was found that a significant amount of varnish had formed in the area where excess heat was being generated. Physicochemical analyzes showed that the hydraulic fluid was in perfect conditions of use and as the system did not present any noticeable change in operating condition, the failure in the directional valve could very well go unnoticed until there was an interruption in the functioning of the hydraulic system, not was the change of color in the hydraulic fluid (darkening).

Since the phenomenon of oxidation - chemical union between the molecules of a lubricating oil and oxygen - is a common cause of reducing the chemical stability of fluids used in hydraulic systems, the same cannot be said about the change in their color, due to the change in color is not a reliable indication of its degree of oxidation. Antioxidants will react chemically to fulfill their function of delaying the phenomenon of oxidation and will often induce color variations from bright yellow to black.

There are a significant number of factors that cause considerable changes in the color of fluids used in hydraulic systems and that are not due to the oxidative process. For example, formulation, operating conditions, contaminants, etc. Although changes in color in fluids used in hydraulic systems may cause concern, it may occur that the product, despite darkening, still retains good antioxidant potential and a series of chemical reactions will still have to occur before the oxidative process becomes critical, the oxidation resistance has been completely exhausted and changing the lubricating oil charge becomes urgent. Again, it is worth highlighting: the safest way to check whether oxidation is at critical levels is through analysis of the hydraulic fluid in use, when we will have an increase in Kinematic Viscosity and Acid Number (AN).

The presence of metal catalyst particles, heat, oxygen and water will contribute to the oxidation of the hydraulic fluid. As the Acid Number (AN) increases, there will be a greater propensity for corrosion in metallic components. The increase in the Kinematic Viscosity of the hydraulic fluid begins to occur as soluble oxides begin to form that dissolve in the fluid load and lead to the formation of deposits in the form of varnish, sludge and resins that are deposited as a thin film of insoluble material in the entire extent of the internal metallic surfaces of the hydraulic system, with the oxidation process being accelerated due to continued exposure to the aforementioned causative elements.

Oxidation can be kept under control through simple maintenance practices. The speed of chemical reactions, including oxidation, approximately doubles for every 10ºC increase in service temperature. For most hydraulic systems that use mineral-based hydraulic fluids, the maximum recommended service temperature is 60ºC. An increase in the operating temperature of the hydraulic fluid to 70ºC, for example, would be enough to cause a significant reduction in its useful life as well as accelerate the formation of deposits in the form of sludge and varnish.

The operating pressure of the hydraulic system makes a lot of difference, too. As the operating pressure increases, there will be a momentary and sudden increase in the Kinematic Viscosity of the hydraulic fluid in the area under pressure and, consequently, an increase in internal friction and heat generation. The increase in operating pressure also leads to an increase in air (oxygen) trapped in the hydraulic fluid and the additional oxygen present will accelerate the chemical reactions that cause oxidation. It is recommended, if possible, that service pressures in hydraulic systems are maintained as close as possible to those recommended by OEMs (as low as possible), with a view to maintaining good efficiency, reliability and availability of the machinery, as well as service life. extended hydraulic fluid service.

Contaminants (internal and external) also contribute to increasing the oxidation rate. The presence of 1% of sludge in hydraulic fluid can double the oxidation rate if we compare it to the rate in hydraulic fluid without any sludge present.

Certain metallic elements are powerful catalysts for oxidation reactions. The presence of water and copper particles, very common occurrences when there are leaks in heat exchangers, greatly enhances the oxidation phenomenon.

When the fluid used in the hydraulic system becomes dark, you should not immediately think that it needs to be replaced. It may be that the Remaining Useful Life (RUL) of the product is still sufficient for several years of service. In these cases, a representative sample must be collected and sent for analysis. The so-called Primary Sampling Point (P) in order to collect a representative sample of hydraulic fluid would be after the hydraulic pump and the mobile components of the hydraulic system (hydraulic cylinder, hydraulic motor, blocking and directional valves, etc.). Representative collection point, for example, would be in the lubricating oil return pipe to the hydraulic reservoir and before the lubricating oil filter. A sample collected in a hydraulic fluid reservoir can be used to check the service condition of the hydraulic fluid, and is not representative for checking the actual wear condition of the moving components. Collection by drainage log is not the best option for collecting samples in a fluid reservoir for use in hydraulic systems, the ideal being collection at an intermediate point of the hydraulic fluid level, so as not to collect sediments (sludge, emulsified or free water and solid particulate matter).

An interesting periodicity to start the Sampling Plan for hydraulic fluids is every three months, and this frequency can be adjusted during the program according to the results obtained by the analyses. With the results of the analysis reports, trend curves can be drawn relating to the physical-chemical properties of the product, contamination status and wear rate of mobile components. This way, it will be possible to know the real condition and operability of the hydraulic fluid.

What does the color change represent?

1) The color change of hydraulic fluids is a very useful parameter to be considered when monitoring the operating condition of hydraulic systems through rapid and frequent sensory inspections.

2) Sudden change in color in a period of time much shorter than that normally perceived in situations of normal operation of the hydraulic system is a reason for immediate and detailed investigation of the root cause of this occurrence.

3) The condition in service must be monitored through physical-chemical analyses, with its darkening being part of the process of gradual loss of properties and natural degradation (“aging”).

Marcos Lobo, Petrobras