1 Introduction What turbine oil should I buy? This is a frequent question of anyone operating turbomachinery. Although there may be up to 20 physical characteristics of a turbine oil found on the product specification sheet, there is no value on this document that predicts the longevity or performance of the fluid. Virtually all the formulations available on the market meet one or more OEM specifications. A new oil’s Rotating Pressure Vessel Oxidative Test (RPVOT) value is often suggested as a proxy for oil longevity, however, as will be discussed later, there is often no real correlation between high initial RPVOT and the oil’s longevity. Far too often, a turbine oil is selected by price as purchasing agents view all products meeting the required specification as equivalent. Turbine oils are not commodity products. Nor are they consumable products. They are just as integral in the operation of a turbine as a bearing. As when purchasing other assets, considering the total cost of ownership will yield the best investment. So how do you select a turbine oil? Or rather, how do you select a good turbine oil and how can you tell when often the field performance does not live up to the marketing hype? What we proposed and undertook was simulating accelerated operating conditions to conduct stress tests for various commercially available turbine oil formulations. This approach eliminates the inevitable variabilities in machinery and operating conditions which hinder a fair comparison between different oils’ performance and longevity in the field, creating instead a controlled testing environment where the only remaining variable is the oil formulation. 2 Turbine Oil Performance Prediction (TOPP) Test There are multiple accelerated aging tests specified by ASTM, such as: • D943 - Standard Test Method for Oxidation Characteristics of Inhibited Mineral Oils • D2272 - Standard Test Method for Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel • D4310 - Standard Test Method for Determination of Sludging and Corrosion Tendencies of Inhibited Mineral Oils • D6186 - Oxidation Induction Time of Lubricating Oils by Pressure Differential Scanning Calorimetry (PDSC) Although there are multiple degradation pathways a turbine oil faces when subjected to operational stress, the dominant failure mode in an operating system is oxidation. While each of the above stress tests exert oxidative stress on a turbine oil, they will not necessarily predict the performance of the fluid. To evaluate the performance of turbine oils, one needs to measure both the oxidative performance and the varnishing tendencies. One ASTM test method attempts to do this: D7873 - Standard Test Method for Determination of Oxidation Stability and Insolubles Formation of Inhibited Turbine Oils at 120 °C Without the Inclusion of Water (Dry TOST Method). However, the reporting system of the test procedure uses metrics not typically used in condition monitoring, so the results are not necessarily comparative to actual turbine oil perfor- Research 13 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0028 Applying knowledge from accelerated turbine oil aging tests to oil management programs Elona Rista, Greg Livingstone* Although there are multiple degradation pathways a turbine oil faces when placed in operation, the dominant failure mode is oxidation. By analyzing a turbine oil placed under accelerated oxidation conditions in a laboratory, much can be learned about how that oil will perform when placed in service. This paper reviews the test results of over a dozen commercially available turbine oils and identifies several findings that can be applied to better managing in-service oils. The paper also proposes a benchmarking tool to predict which turbine oil will perform better in the field. Keywords in-service oils, benchmarking tool, test methods, TOPP test, RPVOT correlation, ASTM D7873 Abstract * Elona Rista Solar Turbines, San Diego, USA Greg Livingstone Fluitec, Bayonne, NJ It is often assumed that varnish formation follows the following process: Oxidation -> Antioxidant Depletion & Diminished Oxidative Stability -> Varnish formation. However, this is not necessarily the case. As can be seen in Table 1, there is not necessarily a correlation between oxidative stability and varnish formation. Turbine Oil 1 was able to retain the vast majority of its RPVOT value throughout the experiment however it generated significant MPC values. Turbine Oil 2 showed the opposite results where the RPVOT value decreased significantly, yet the MPC values were still very low. In this case, Turbine Oil 1 should be expected to outlast Turbine Oil 2 in the field, however it will generate a lot more deposits. Turbine Oil 2 will have a shorter life; however, it should provide strong resistance to varnish while in-service. Below is a radar chart illustrating the comparative performance of 14 different commercially available turbine oil formulations. In this case, Oil Life corresponds to an average of the RPVOT and RULER values at the end of the 12-week test, deposit control represents the MPC value, and varnish production is a visual assessment of the deposits on glassware from the test cell after 12-weeks. It is interesting to note how many oils show strong oxidative performance yet produce higher levels of varnish. Research 14 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0028 mance. The authors have found that by using the same accelerated oxidative process as specified in D7873 yet using standardized test methods such as the MPC, RULER and RPVOT tests to report the results, the findings can be correlated to actual field performance. The test is referred to as the Turbine Oil Performance Prediction (TOPP) test and a general overview of the test conditions are listed below. • Four (4) 350ml samples of new turbine oil are placed in glass test cells containing a steel/ copper coiled wire catalyst conforming to ASTM D5846 specification. • The test cells containing the turbine oil and catalyst coil are placed into a 120 °C (248 °F) solid-block temperature bath. • The samples are allowed to equilibrate to 120 °C (248 °F) for 20 min. After equilibration, dry atmospheric air is bubbled through the test oil at a rate of 3L/ hr to accelerate aging throughout the duration of the test. • One sample cell is removed at three (3), six (6), nine (9) and twelve (12) week intervals for testing. • After each stress period, testing includes: • Viscosity, 40C: ASTM D445 • Acid Number, mg KOH/ g: ASTM D664 • Rotating Pressure Vessel Oxidation Test, mins: ASTM D2272 • RULER, antioxidant health, ASTM D6971 • Membrane Patch Colorimetry, dE, ASTM D7843 • These results are compared to and trended against test data obtained as a baseline for the new oil (Week 0) 3 Not all turbine oils are created equally The first observation made when performing this test is that there is little change in viscosity and acid number in most formulations throughout the test period. This is consistent with what we see in the field. The difference between formulations is found in the oxidative stability and deposit control capabilities of the turbine oils, where we see significant differences in performance. Below are examples of two different turbine oils that have been stressed for 12-weeks. This example highlights the difference in values between oxidation stability and varnish control. Turbine Oil 1 Turbine Oil 2 12 - Weeks 12 - Weeks RPVOT, % 94% 40% Membrane Patch Colorimetry 56 8 Table 1: Comparison of two different turbine oil formulations after 12-weeks of stress in the TOPP accelerated aging test Figure 1: Radar Chart comparing the performance of 14 commercially available turbine oils after 12weeks in TOPP experiment. Each oil is ranked 1-14 according to its performance in each measured category (1-Best, 14-Worst) This is a common theme throughout TOPP testing. Most turbine oils tested either performed well from an oxidative stability perspective or from a deposit control perspective, but not both. The search for a turbine oil that provides long life without producing deposits has proven elusive to date, however with the rate of advancements in turbine oil formulations such products are expected to become available soon. 4 Interesting findings from TOPP testing The TOPP testing encompassed many different formulations which facilitated objective benchmarking of various turbine oils’ performances. In addition to allowing various formulations to be benchmarked, below are some other interesting findings which provide guidance on monitoring turbine oils. 4.1 Oil color is not related to oil condition It is often believed that the color of in-service oil is directly correlated to its degree of degradation or even performance. Evidence from TOPP testing however suggests that there is no such correlation. Below is an example of two different oils after 12-weeks of accelerated aging and what their corresponding MPC value is. Oil A has a color of 8, according to ASTM D1500, however the MPC rating is only an 8 ΔE. Oil B also has a color rating of 8, however its MPC value is 62 ΔE. Notating the color of in-service oil still provides value. If one observes a sudden change in color, an event has contributed to this change warranting further investigation. However, the color by itself should not be used as an indication of the health of in-service turbine oil, and should be evaluated in combination with other oil health parameters. Some oil formulations use additive components that are photosensitive and will react in the presence of light, especially UV light. If such an oil sample is exposed to sunlight, it’s not unusual to see the color of the oil change to blue, purple or black. Sufficient exposure to UV light has been known to degrade the oil causing increases in acidity and MPC values and a consumption of antioxidants. 4.2 RPVOT Correlation to Performance The Rotating Pressure Vessel Oxidation Test has been a standard method to measure the oxidative stability of inservice oils since the 1960s. Back then, the oil formulations consisted of API Group I base oils and the RPVOT value of new oil was typically below 500 mins. The precision of the test according to ASTM D2272 shows a reproducibility value of 22 % for oils with values below 1000 mins. This test has been an adequate measurement of the health of in-service turbine oils for decades. The introduction of more advanced base oils in turbine oil formulations has resulted in significantly higher starting RPVOT values. This has contributed to much lower test accuracy, especially with reproducibility. There are multiple articles purporting that high RPVOT values correspond directly to oil performance. In fact, General Electric referenced this in one of their older specification revisions (GEK 32568F) by stating, “some studies show RPVOT to be a good indicator of performance”. There were even some that stated that their new oil formulation with an RPVOT of 1500 mins had 3X the life compared to their previous formulation with an RPVOT of 500 mins. There’s no reliable and direct correlation between an oil’s initial RPVOT value and the life of the product or its varnish formation and deposition tendencies. These facts were also confirmed by the results of the TOPP test experiments. The image below shows the RPVOT trend of two different oils from TOPP testing. Turbine Oil 1 starts with an RPVOT value twice that of Turbine Oil 2. However, at the end of the 12-week test, Turbine Oil 1’s Research 15 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0028 Figure 2: Comparison of two different oils, both of which turned very dark in color, but didn’t have corresponding increases in MPC values Figure 3: Initial RPVOT results are not as indicative of oil life as RPVOT Retention during the life of the oil without any significant change to the system. TOPP testing provides one possible explanation for these puzzling field observations and illustrates why higher MPC values do not necessarily equate to higher levels of oil degradation. The image below shows the MPC results of the same oil throughout the 12-week test. The MPC value starts at 5 ΔE, peaks at a value of 66 ΔE after Week 9 and then drops precipitously to 36 ΔE at Week 12. One can also observe the glassware, where the level of visible deposits sharply increases between Week 9 and 12. From this example, one can deduce that as the oil degradation rate accelerates, so does the rate of varnish deposition which can lead to “false low” MPC results. This condition highlights a potential downfall of overreliance on MPC results to determine oil health. Additionally, it underscores the importance of building a robust trend for oil degradation parameters through frequent and consistent oil sampling and analysis, through which a “false low” can be detected. Research 16 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0028 RPVOT value is at 45 % compared to 94 % of Turbine Oil 2. ASTM D4378 suggests that a turbine oil condemning point is 25 % RPVOT value compared to new oil. Following this guidance, Turbine Oil 2 will have a significantly longer life than Turbine Oil 1, even though its initial RPVOT value is quite low by comparison. When interpreting RPVOT values, it is more important to weigh RPVOT retention instead of initial RPVOT values. There is also a misconception about oxidative stability versus varnish production. One may believe that the higher the oxidative stability, the less the oil will oxidize, and the fewer deposits will be generated. TOPP testing is consistent with in-service oil samples from the field in disproving this hypothesis. The image below shows the performance of two different oils in the TOPP test. Turbine Oil 1 had a high initial RPVOT value of 2796 mins and maintained strong oxidative stability throughout the 12-week test. At the conclusion of the test however, its MPC was 56 ΔE. Turbine Oil 2 on the other hand, had an initial RPVOT value of 1125 mins and experienced consistent depletion of oxidative stability throughout the test. However, after 12-weeks of accelerated aging under the same conditions, it only had an MPC of 8 ΔE. This data breaks the connection between RPVOT and MPC values. Figure 4: Illustration showing lack of correlation between an oil’s oxidative stability and varnish performance Figure 5: The higher the MPC doesn’t necessarily mean more oil degradation. Severe deposits are often associated with lower MPC results. Any MPC over 30 should be considered critical and justify remedial action. Figure 6: MPC values of new oils are not always low. When interpreting in-service MPC values, it is valuable to consider the MPC of the new oil baseline. 4.3 Higher MPC values don’t necessarily mean more oil degradation When monitoring in-service turbine oils, the more extreme a parameter gets, the more severe the situation. For example, the higher the particle count, the more contamination. This logical principle applies similarly to acid number, viscosity, water content and the other condition monitoring tests. It is intuitive that the same principle would apply to MPC testing. The expectation is that the higher the MPC value, the more degraded the oil and the higher its propensity to form deposits. However, many oil samples from the field do not follow this principle. It is possible to see a sharp drop in MPC values 4.4 Remember to account for Baseline MPC A worthwhile consideration when assessing MPC test results is the importance of a baseline. While it is expected that most new oils will have a negligible MPC measurement, this is not true across the board. Figure 6 below depicts the varying baseline MPC levels for three different turbine oil formulations. For example, when assessing oil analysis results for two samples both showing in-operation oil MPC levels of 30 ΔE, without consideration of the baseline MPC we would be unaware that Oil C is only experiencing an increase of 12 ΔE while for Oil A the increase of 29 ΔE is much more significant. 4.4.1 MPC is not an accurate measurement for deposited varnish It is undeniable that MPC testing has become an indispensable tool in varnish detection and treatment, however we have to acquiesce that it also only provides a partial view of the entire varnish picture within the oil system and equipment. The test evaluates the amount of varnish that is present in the liquid oil at the time of sampling. While this measurement is very valuable, it does not address two other aspects of varnish that are commonly encountered, especially in severely contaminated systems. When varnish levels surpass the oil’s carrying capacity (solubility), excess varnish drops out of the oil solution and often settles in areas of low circulation and mixability areas such as the bottom of lube oil tanks, oil coolers, low velocity piping etc. The typical oil sample is understandably extracted from regions where the oil is well mixed and thus not representative of any insoluble soft varnish that might have settled within low oil circulation areas. The conditions for varnish to drop out of the oil solution are exacerbated even more when a high temperature delta is encountered such as in the case of oil coolers, which become some of the prime varnish deposition points within the oil system. As warm oil encounters a surface of low temperature and is therefore cooled (this being the primary function of the oil cooler), its solubility point drops which allows more varnish to come out of solution. This situation creates an opportunity for varnish to plate on the metal surface, creating over time a hard and lustrous surface which is typically what we first envision as “varnish”. Among other detrimental effects on the equipment, one aspect of plated varnish is its insulating property which results in impaired heat rejection across metal surfaces. This effect results in diminishing cooler efficiency as well as excessive heat retention within the metal, the latter a detrimental condition when varnish plates on bearing surfaces. In both situations discussed above, MPC testing is “blind” to the amount of sludge or varnish that is part of the system and potentially affecting operation and equipment health, albeit also not present in the bulk liquid oil and thus not measurable. During our study however, we had a luxury seldom afforded in the field: We were able to observe the amount of varnish deposited on the glassware used for aging the oil. While an objective scale to determine deposition levels is not available, it is nonetheless possible to draw some conclusions based on the color darkness and haziness level progression of the glassware throughout the test. Figure 7 below illustrates the lack of interrelationship between MPC and deposit formation. While the oil on the right presents with a significantly higher MPC patch result, its corresponding test glassware indicates that minimal varnish deposition has occurred. By contrast, the oil on the left has deposited significantly more varnish on the glassware despite its lower MPC test results. By MPC results alone, we might conclude that the system using the oil on the right is experiencing higher levels of varnish presence and thus calls for more urgent and drastic intervention, when in reality the system using Oil A is likely at higher priority for mitigation. Research 17 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0028 Figure 7: Illustration showing that the MPC result is not always indicative of actual varnish formation Differences in an oil’s solubility point and ability to “carry” varnish could in the future become important considerations alongside the MPC test results when determining contamination severity and planning mitigation steps. 4.4.2 A Model for Benchmarking Turbine Oil Formulation After gathering oil performance data using the TOPP testing procedure, it is useful to benchmark various formulations. A useful method of benchmarking oils is by generating a Decision Matrix (often called a Pugh matrix, selection matrix, or criteria-based matrix). This is done by prioritizing which data point is of highest importance and establishing weighted criteria. This method for benchmarking is valuable because there is not an oil that is “the best” as the application plays a significant role. A Decision Matrix can be customized depending on the customer’s application. For example, in one case, Varnish Production may be the most important criteria to weigh as this is the test which most closely correlates to system reliability. In another case, Oil Life may be the most important criteria for a customer, because they already use a varnish mitigation technology and are less concerned about the oil generating deposits. way of making a purchasing decision. Nor is there a correlation between price of an oil and its field performance. The best method for selecting turbine oils is to generate comparative test data in a controlled experiment. Testing turbine oils can be done by stressing the oil following ASTM D7873’s procedure. Samples pulled after 3, 6, 9 and 12 weeks are then analyzed by standard condition monitoring tests. Throughout a 12-week aging period, the only significant changes in oil properties are its oxidation stability and varnish potential. Therefore, the tests that provide the most revealing data are the RULER (ASTM D6971), MPC (ASTM D7843) and RPVOT (ASTM D2272). Many things have been learned by performing this test, such as: • There are significant performance differences between formulations. • Formulating an oil with both long-life and low deposits has been a challenge, however the rate of improvement in both properties suggests that this elusive goal will be met soon. • The color of the oil is not related to its performance or varnish tendencies. • Initial RPVOT value doesn’t necessarily determine the life of the oil or deposit control characteristics. An oil’s RPVOT retention properties are more important to track. • A higher MPC doesn’t necessarily mean more degradation or more deposits. An MPC greater than 30 suggests that the oil is susceptible to form deposits and warrants attention. Finally, a Decision Matrix can be made from the test data and the various performance criterion weighted based on a customer’s application and needs. This allows various oil formulations to be benchmarked and provides a better selection guide for purchasing new turbine oils. Research 18 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0028 It is also important to note that an actual oil’s performance in the field is also dependent upon how it is maintained. If a customer purchases a best-in-class oil, but then doesn’t manage contamination or perform condition monitoring, the oil will likely not perform as well as an oil that benchmarked far lower but is monitored and treated as needed. Below is an example of twelve oils that have been benchmarked using the Decision Matrix tool. In this case, four different criteria are weighted, as can be seen in Table 2. Then the results of the TOPP test are evaluated and compared to each other, developing a ranking of each oil. Finally, these values are multiplied by the established criteria weight to come up with an overall rank. An example is listed below in Table 3. 4.5 Summary Selecting a turbine oil based on physical values in its product data sheet or marketing claims is not an effective Criteria Weight Meaning Deposit Control 1.5 The highest MPC rating recorded over the 12-week test Varnish Production 2.0 Visual Assessment of the 12week test cell and comparison to the other test cells Oil Life 1.0 Average of the RPVOT and Antioxidant Levels compared to New Oil at Week 12 Oil Price 0.5 Cost of the oil for the customer Table 2: Example of Measurement Criteria that could be used to help benchmark and select turbine oils Deposit Control Varnish Production Oil Life Price (Scale 1-4) Weighted Rankings Overall Rank Turbine Oil A 1 3 11 2 23.0 1 Turbine Oil B 2 2 12 4 24.0 3 Turbine Oil C 3 4 10 1 28.5 4 Turbine Oil D 4 6 1 4 29.0 5 Turbine Oil E 5 1 9 3 23.5 2 Turbine Oil F 6 7 13 2 47.0 8 Turbine Oil G 7 8 7 2 46.0 7 Turbine Oil H 8 5 6 4 39.0 6 Turbine Oil I 9 11 3 3 55.5 9 Turbine Oil J 10 13 4 2 64.0 10 Turbine Oil K 11 12 14 2 73.0 12 Turbine Oil L 12 14 2 1 68.5 11 Attribution Weight 1.5 2.0 1.0 0.5 Table 3: Decision Making Matrix to benchmark turbine oil formulations and be a criterion for oil selection