In 2008, workers at Wieland-Werke’s plant in
“Because of the volume of fluid in the machine, the replacement cost would have been significant,” Wolfgang Ott, lubricants project engineer for Wieland, said in February at the OilDoc Conference in
Instead Wieland collaborated with its lubricant supplier, Hermann Bantleon GmbH, to find another solution. Recognizing that the hydraulic fluid was still adequate in most respects, the companies decided to address the one area in which it was lacking. They added antioxidant additives to the existing fluid.
It was a novel approach, the companies say, but appears to have been successful. The old fluid continued performing satisfactorily during more than a year and a half of follow-up monitoring. Wieland and Bantleon are so pleased with the results that they plan to expand the program to other machines and sites.
Oxidation: How Lubes Age
In his presentation to the OilDoc conference, Ott was joined by Bantleon’s Manfred Jaumann, who explained that oxidation is one of the main factors limiting the service life of lubricants. The oxidation process ( See page 36 ) begins with the formation of free radicals (chemically respresented as RO°) – reactive molecules that may have positive, negative or neutral charge and which, in the presence of heat, mechanical stress and metal surfaces, react with oxygen to form peroxy radicals (ROO°).
Peroxy radicals then react with the hydrocarbons (the core molecules of the lubricant, designated as RH) to form hydroperoxides (ROOH) and free radicals. Hydroperoxides can break into alkoxy radicals and hydroxyl radicals (RO° and HO°), which can combine with hydrocarbons to produce more free radicals. So the process can be summarized as the coinciding breakdown of lubricant hydrocarbons and formation of free radicals.
Base stocks have different levels of oxidation stability, so the choice of base stock can help stave off oxidation. Otherwise the formulator’s main tools are antioxidant additives. These fall into one of two general categories, depending on the method by which they inhibit the oxidation process, Jaumann said. Primary antioxidants react aggressively with peroxy radicals and less aggressively with alkoxy radicals. Secondary antioxidants work on hydroperoxides, breaking them into by-products that are less reactive than radicals.
Primary antioxidants are typically amines or sterically hindered phenols, Jaumann said, while phosphites are an example of secondary antioxidants.
A Fluid Reaches the End
Wieland is one of the world’s largest suppliers of copper and copper alloy specialty products and semi-finished materials. Headquartered in
The Vohringen plant, according to Wieland, is
P19 holds 25,000 liters of hydraulic fluid, and Wieland fills it with Avia RSL 46 fluid supplied by Bantleon. Avia is an international association of independent European oil jobbers, including Bantleon, that cooperate to supply Avia branded fuels and lubricants. In addition to supplying hydraulic fluids and other lubricants, Bantleon provides fluid monitoring services in order to determine when fluids need to be changed, as well as to watch the condition of machines. In his presentation to the OilDoc conference, Ott was joined by Bantleon’s Manfred Jaumann, who explained that oxidation is one of the main factors limiting the service life of lubricants.
There are a number of ways of gauging a lubricant’s oxidative stability and level of oxidation. As part of its fluid monitoring program, Bantleon, which is also based in
Until the second half of 2008, these tests detected nothing amiss in P19. The color of the fluid was a bit darker than preferred, but this was not a cause for concern, Ott said. Tests also showed significant levels of copper particles in the fluid, but these were from the copper work pieces, not the result of wear to machine parts. In short, all seemed well.
But as Ott explained, the companies recognized that this type of standard chemical analysis is less than ideal in watching for oxidation.
“A highly increased oxidation number or [acid] neutralization number can only serve as an emergency signal to start an already overdue oil change,” he said. “These values only show significant changes when it may already be too late.”
In light of this, in September 2008 Bantleon expanded its analysis by adding a Ruler (remaining useful life evaluation routine) test, which actually measures the level of antioxidants in the fluid. This method is based on the idea that lubricants in service consume antioxidants, and that once they are gone an increase in oxidation levels cannot be far behind.
The timing turned out to be fortuitous because it gave Wieland an early warning: the Ruler test detected no antioxidant additives. Without antioxidants, Ott said, the hydraulic fluid could be expected to deteriorate significantly in 2009. Wieland would need to replace the fluid, and, at a price of €2 per liter, the bill would be high.
Instead, Wieland and Bantleon decided to try an alternative solution. Since the fluid remained in acceptable condition except for the depletion of antioxidants, why not simply add antioxidants? The cost would be much less and, if it the fluid’s one shortcoming could be addressed, replacement could be postponed.
It took a few months to work out the details.
“Wieland and Bantleon chose a suitable antioxidant package with amine and phenolic contents,” Ott said. “The cost was approximately €1,000 – far less than the cost of completely replacing the fluid.”
Replenishing the fluid required 40 liters of additives, but Bantleon pre-mixed it in a 220-liter drum in order to ensure proper distribution inside P19. At the end of February 2009, the additives were put into the machine.
Bantleon immediately conducted a Ruler test to verify the distribution of the additives. Two weeks later, another ruler test found that the concentration of the antioxidants had decreased by 21 percent. After that, the rate of antioxidant depletion slowed to a rate of approximately 20 percent per year. Another Ruler test in October 2010 recorded their level at 48 percent of initial concentration.
As a cross-check, Bantleon extracted oil samples and performed chemiluminescense tests to check for oxidative stability. As expected, the results improved immediately after the addition of the antioxidants. Results for these tests did deteriorate unexpectedly in September 2009 but stabilized thereafter. Ott said the stabilization may have been due to the fact that 840 liters of fresh oil was added in December 2009 to top off the machine.
In April and September of 2009, Bantleon performed an RPVOT (Rotating Pressure Vessel Oxidation Test) and compared results to fresh oil scores of products on the market. The results showed that the oxidation stability of the fluid was about 45 percent of that of fresh fluid. Based on this data, Jaumann said the fluid in P19 still had quite a bit of life in it. He also posed the possibility that Wieland and Bantleon might be able to continue putting off replacement.
“We may eventually have to change it because of depletion of some of the other additives, such as extreme pressure additives or detergents,” he said. “Or we may be able to add those additives, too, and in doing so allow the fluid to last even longer.”
Ott indicated that Wieland will continue adding additives – and not just to the hydraulic fluid in P19.
“The project can be considered very successful,” Ott said. “The oil values… in the autumn of 2010 were much better than at the beginning of the project in the autumn of 2008. This was achieved by the addition of additives and motivated us to continue the project. In the future, the oil will continue to be monitored every six months with the objective that the oil remains in use for several years, if necessary, by using a second addition of additives.
“We now plan to extend the program to other machines at Vohringen and to other plants.”
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