1 Introduction Oil condition monitoring programs have significantly improved in the last decade, to allow the users of hydraulic oils to measure oil degradation products. Tests such as; the Membrane Patch Colorimetry and Particle Counting have allowed them to identify soft contaminants in their fluids. These are usually responsible for many mechanical issues, such as pump failures, sticking valves, decreasing cooler capacity, etc. The frequency of hydraulic oil failures as a result of deposit formation is due to a confluence of events. There has been a constant evolution in the formulation of hydraulic oils. API Group I base oils formulated with ZDDP additives are being replaced by next generation base oils and ashless antiwear technologies. Although these formulations may perform better in the field, it is essential that their degradation products are measured using a condition monitoring program. Additionally, thermal stress continues to increase in hydraulic oils as oil reservoirs shrink in size, while operating temperatures and pressures increase. Hydraulic system deposit formation needs to be addressed, in an economical and sustainable way. This paper will present the principle and use of a novel solubility enhancing technology. Case studies will illustrate how this approach can be used both reactionary; to resolve immediate mechanical issues, and proactively; to provide long-term deposit protection. Applications covered in this paper are from the automotive industry, steel manufacturing, injection moulding, mining and marine industry. 2 Lubrication requirements and developments for hydraulic applications Both lubricants and their applications continue to evolve. There has been a drive within the manufacturing and hydraulics industry towards more globalization and efficiency improvements. These have set off a chain of events, impacting all aspects of an operation. The industry is demanding faster, smaller, more efficient machines. The Original Equipment Manufacturer (OEM) responds, delivering a more compact, efficient machine however, in most cases, this results in placing higher thermal and mechanical stress on the lubricant. In the case of a hydraulic oil, the fluid is now expected to perform at higher operating pressures and temperatures, in smaller capacity systems, for longer periods of time and in the presence of many potential contaminants. Oil manufacturers have responded by reformulating their products to meet these new demanding specifications. The most consequential change to lubricant formulations over the last two decades is the use of more highly refined base stocks, such as API Group II and III. Fully formulated Group II and III lubricants have superior oxidative resistance because virtually all hydrocarbon molecules are saturated. A drawback of Group II formulated lubricants is reduced solubility. This often requires the use of a solubility enhancer, or cosolubilizer, to keep the additive package in solution. As the oil degrades, the reduced solubility properties mean a limited ability to keep oil degradation products in solution. Over the last couple of decades, the combination of the new generation of lubricants and the increased operational conditions of industrial hydraulic systems, has resulted in new oil degradation mechanisms. During maintenance overhauls, operators often report that valves or oil reservoir walls are coated with a dark brown/ amber deposit, or oil reservoirs have more problems with foaming control. These oil-derived deposits are commonly known as varnish. (Figure 1) Varnish is typically defined as a thin deposit in a lubrication system that is difficult to remove by wiping and comprised primarily of organic residue. The chemical composition of varnish can be extremely varied and classified by chemistry and degradation mechanisms. Typi- Research 25 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0030 Managing hydraulic oil deposits by using novel solubility enhancing technology Jo Ameye, Greg Livingstone, Cristian Soto* * Jo Ameye Fluitec NV, Brussels, Belgium Greg Livingstone Fluitec International, Bayonne, NJ, USA Cristian Soto Fluitec International , Bayonne, NJ, USA Condition monitoring, soft contaminants, sticking valves, decreasing cooler capacity, automotive industry, steel manufacturing, injection moulding, mining, marine industry. Keywords friction . In some control systems, the energy input of a valve is measured. Valves that are sticky due to oxidation products can be identified by the increased amount of energy required to move the valve. Besides loss of hydraulic control, valves that are coated with varnish are often considered to be malfunctioning and are prematurely replaced. Although this may immediately solve the valve sticking issue, the newly replaced valve will be quickly coated with varnish, causing reoccurring issues. (Figure 2) Research 26 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0030 cally, sludge is considered an easy to wipe, gooey substance that also contains moisture. Varnish has a more cured, shiny appearance and is not easy to wipe. Varnish is primarily caused by the continual process of oxidation which is accelerated by temperature and various metallic components as well as gaseous catalysts. Everything is susceptible to degradation due to the presence of oxygen, lubricants not excluded. Oxidation on hydraulic oils is a combination of 3 types of contaminants: dirt contamination (silt, wear metals), water, and air. Over the last 15 years significant efforts have been made in the hydraulic industry to control contamination (ISO particle counting), as well as to reduce water content (via the use of breathers, vacuum dehydradation). In parallel, significant efforts have been taken to protect the hydraulic base oils, by applying a wide range of antioxidants. Antioxidants are formulated into hydraulic oils because they are more reactive species than the base oil. The oxygen and free radicals more readily react with the antioxidants, who sacrifice themselves to protect the base oil and significantly extend the life of the oil. In addition to oxidative stress, high temperature thermal events may also be a contributing mechanism for fluid degradation. Thermal degradation is created by high temperature events without the influence of oxygen, such as Micro-dieseling (due to increased air release values) or Electrostatic Spark Discharge (ESDoften with very fine filtration). 2.1 The Impact of Deposits on Control Valves Varnish deposits are sticky in nature. This impacts the performance of proportional control valves or servo-valves, which rely on clean, deposit-free lubricants to function as designed. Often, oil flow to parts or all of the control valve is intermittent. This results in the oil cooling in temperature, allowing degradation products to come out of solution, adhering to the internals of the valve. The formation of deposits in valves has been shown to cause valve lock due to resulting high levels of static Figure1: Deposits formation on oil reservoir Figure 2: A Failed valve coated with varnish deposits 3 Condition Monitoring Programs and standards for hydraulic oils The operators of hydraulic systems are acutely aware of the need for realizing optimum reliability and availability of their assets. Monitoring the condition of the lubricant by selecting the correct tests and analysing the fluid at the right intervals helps the operator understand if the fluid is suitable for service. It is also the primary tool to identify incipient lubricant failure, allowing the operator to take proactive actions. Guidance on oil condition monitoring strategies is often provided by OEMs, international standard bodies and lube oil experts. As the formulation of hydraulic lubricants has evolved, so too have the tests required to monitor them. Traditional oil analysis methods for hydraulic oils, such as viscosity, elemental spectroscopy, acid number, particle count and water content will provide value, however they cannot be reliably used for detecting fluid failure. This is mainly for two reasons. First, the polar products formed from hydraulic oil degradation are smaller than one micron in size - which is undetectable with routine analysis. Secondly, many of today’s new lubricant formulations no longer degrade in a linear fashion thus making it more challenging to predict when the lubricant will begin to rapidly develop deposits. To achieve an optimum value for the performed oil analysis programs, the following oil analysis matrix (in complement to viscosity, particle counting (ISO 4406) and acid number), will help operators to be more proactive: 3.1 The health of a lubricant’s antioxidant system will largely determine the life of the oil. Directly monitoring individual antioxidants has demonstrated to be a very good predictive method to monitor antioxidant depletion and provides a more thorough understanding of how fluids degrade. The RULER ® is specifically engineered to measure and trend individual antioxidants. Unlike other testing methodologies that can detect antioxidant molecules, such as FTIR, the RULER is not influenced by the presence of other additive components. An example of a RULER test can be seen below (Figure 3): Once the antioxidants in a lubricant start to degrade, the first physical impact to the lubricant is the generation of extremely small, sub-micron degradation products. These contaminants may consist of degraded base oil molecules, but at the early stages of development, most often consist of the degraded antioxidants. 3.2. MPC test - The most common test method to detect oil degradation products is Membrane Patch Colorimetry (MPC), conforming to ASTM D7843. Other tests such as measuring the gravimetric weight of insolubles or ultracentrifuges have shown promise, but may also be influenced by larger contaminants. The MPC test is a relatively straight forward procedure. Fifty milliliters of sample are mixed with an equal amount of solvent (usually petroleum ether) and filtered through a 0.45-micron patch. The color of the patch is then analyzed with a spectrophotometer and the total amount of color is reported. The results are reported on the CIE LAB ∆E color scale, examples of which can be seen below. (Figure 4) Other test methods that may also be of value in a hydraulic oil analysis program are: 1. FTIR analysis - The Fourier Transform Infrared (FTIR) practice is a refined infrared spectroscopy method, which can be used to monitor molecular changes in the fluid, potentially identifying its mode of degradation or oil mixtures. 2. Air Release - Measuring the fluid’s ability to dissipate air bubbles can be of value if the lubricating oil is also expected to provide hydraulic work, such as moving a valve or lifting a bearing shaft. 3. Elemental Spectroscopy by ICP -can identify wear in various machinery components, or oil mixtures (for example to identify if zinc-containing oils are mixed into ashless hydraulic oils). In all of the above test methods, it is important to have a correct oil sampling interval. The most common sample periodicity for hydraulic oil systems is every 3 months. Oil sampling frequency shall also be adapted to the function of the condition of the oil, in order to establish a trend for the individual oil parameters. Variations from fluid condition trends may be further investigated for root cause determination. 4 A solution to remedy deposit formation on hydraulic oil systems Fluitec has developed a solution to manage deposits already formed in hydraulic and lubricating systems to prevent their further development by use of a solubility Research 27 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0030 Figure 4: Example Patches and values from the Membrane Patch Colorimetry (MPC) test. A value of 50 would be considered critical, warranting immediate action. A result of 9 would be considered acceptable Figure 3: RULER directly measures the health of individual antioxidants, revealing the health of the lubricant. The red line is the used sample and the blue line is the new oil reference sample Solvancer ® technology was developed after an exhaustive categorization of the various deposit chemistries found in hydraulic systems. Three essential properties were studied in this categorization exercise: polarity, hydrogen bonding and dispersive forces. Solvancer ® has been optimized to take these three characteristics into account to ensure that lube oil deposits are quickly dissolved back into the lubricant. Solvancer ® is typically added a treat rate between 3 and 5 % to an in-service lubricant. It has no adverse effects on the performance of the fluid and goes to work immediately dissolving deposits, and preventing further varnish from forming. The case study below shows both the immediate and long-term impact of adding Solvancer ® technology to an in-service hydraulic system. This graph shows the results from a 6-week oxidation test (120 °C), without and with adding Solvancer ® . (Table 1, Figure 5) Research 28 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0030 enhancer. This patent pending technology is referred to as Solvancer ® . (Figure 6) A solubility enhancer should only be considered under the following conditions: 1. The solution must be added to the machine while operating. 2. It must be compatible with in-service oil and other system materials. 3. It can’t affect the oil’s interaction with contaminants, inhibit corrosion, anti-wear performance and must work under intense conditions. 4. Doesn’t give into oxidative stress. 5. Usage of this solubility enhancer eliminates the need to install ANY other mitigation system. 6. Works even under extreme oxidative conditions and doesn’t produce deposits when burnt. Table 1: TOPP test results hydraulic oil with and without Solvancer treatment Figure 5: Effect of Solubility enhancer on solubility 5 Case studies hydraulic oil systems treatment 5.1 Case 1 - Injection moulding company This case study covers an injection moulding company equipped with more than 50 injection moulding machines, which have oil reservoirs from 250 to 2500 liters of hydraulic oil. In a period of 2 years, the maintenance department reported a sudden increase of sticking servovalves, shorted filter life, and failing hydraulic pumps. Through a visual inspection of one of the failed pumps, the maintenance engineers noticed light brown deposits on the pump components (Figure 6) 1. Drop of MPC values averaging an MPC of 51 d-E before treatment and MPC of 12 d-E after treatment 2. Filter life time increased to the normal lifetime of 4-6 months, without changing the retention rate of the filter (Beta 200 6 micron) 3. Over a period of 1.5 years, no problems associated with the sticking servo-valves were reported. Additionally, no hydraulic pump failures occurred. 5.2 Case 2 - Aerospace forging company For this second case study, we cover a leading aerospace component manufacturing company, which noticed a decrease in ram speed with their hydraulic radial forging press. This resulted in an inconsistent system performance which risked product quality. The Aerospace Component Manufacturer’s hydraulic radial forging press comprises of 4 x 42,000 liters reservoirs with hydraulic fluid from each reservoir powering one of four cylinders. Upon having performance-related issues, an inspection later revealed a manifold riddled with varnish deposits. Deposits were also found in pumps, valves and system internals. Further oil analysis revealed a high varnish potential (MPC >50 d-E). (Figure 8) A concentration of 3 % DECON™ was added to the in-service hydraulic oil and circulated throughout all four reservoirs. Within one week, the varnish potential was significantly lowered and DECON™ had restored system performance while simultaneously cleaning system internals. Research 29 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0030 Figure 7: MPC trend before and after DECON™ treatment Figure 6: Deposit on hydraulic pump component Figure 8: Pictures of varnish on press components Once the maintenance team added MPC analysis to their existing oil analysis program, high values of MPC were recorded for 75 % of their machines. It was also noticed from the oil analysis reports that on a few injection moulding machines, the acid number as well as the zinc-content was increasing above the expected values. After a further investigation, it was found out that the oil storage tanks for the zinc-free and zinc-containing oils had been mixed. This probably resulted in the mixing of small quantities of zinc-containing oil into the ashless oils. After consultation with the OEM, as well the oil company, the company decided to add 3 to 5 % of the solubility enhancer (DECON™ formulated with Solvancer ® ), which resulted in the following performance (Figure 7): nish deposits on valves, coolers and other lube system components. The impact of these deposits in a hydraulically controlled valve include valve sticking and locking. It is possible to monitor the health of these fluids, by optimizing the oil analysis program with methods that monitor the development of varnish such as RULER and MPC tests. One solution to the formation of deposits in hydraulic oils is the use of a solubility enhancer. Fluitec has developed its Solvancer ® technology, specifically for a wide range of industrial lubricants. It has been shown to provide immediate relief to systems suffering from varnish and long-term deposit control protection for components susceptible to failure from varnish. Research 30 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0030 DECON™ is also currently preventing the formation of additional deposits. The manufacturer is continuing to monitor the fluid on a monthly basis and has included the MPC (Membrane Patch Colorimetry) test. The manufacturer will add approximately 3 % DECON™ along with new oil whenever the system needs to be topped-up to maintain superior performance. 6 Conclusions Reliable hydraulic system operation is essential for the manufacturing industry. These fluids are under increasing levels of thermal stress and may be exposed to metallic, air and water contamination causing further acceleration to its failure. The results of hydraulic oil degradation are polar degradation products that form as var-