Today, gas turbine owner/operators are under great pressure to advance operational productivity by optimizing equipment performance and longevity while minimizing downtime.
One major challenge to effective turbine operation and reliability is the common phenomenon that can significantly reduce engine reliability and performance known as “varnish.”
A 2010 survey of 192 U.S. gas turbine power plants- operating a combined total of 626 gas turbines – illustrated just how common varnish issues are in the industry, revealing that approximately 40 percent of plant operators reported current or historical varnish issues within the first six years of oil service life.
However, by developing and implementing a proactive maintenance strategy that includes the use of gas turbine oils that have been specifically formulated to prevent the formation of varnish, gas turbine owner/operators can minimize the impact of varnish on their equipment, helping boost the productivity of their operations and reduce unscheduled downtime.
Varnish: An Introduction
To understand how to mitigate the impact of varnish, it’s important to first recognize what varnish is and how it can impact turbine reliability, performance and longevity.
Varnish has historically been used as a catch-all term for deposits, which can appear in the form of sludge or varnish, but this is not an accurate description. Where sludge is a soft, pliable, organic residue that can be easily removed by wiping, varnish is a harder, oil-insoluble organic residue that cannot be removed by wiping alone. However, for the purposes of this article, I will be using the term varnish to refer to both types of deposits.
Three main factors contribute to the formation of varnish: thermal degradation, oxidation, and oil contamination.
Thermal degradation describes molecular cracking that takes place at temperatures above 570°F (300°C). In turbine operations, Electro Static Discharge (ESD), which provides arcing across filters, can generate localized temperatures of up to 1830°F (1000°C) and, similarly, adiabatic compression (a.k.a. microdieseling), compresses entrained oil in hydraulics and can generate localized temperatures of 1100°F (600°C). In both situations, these high temperatures, well beyond the threshold for molecular cracking, can lead to oil degradation.
Oxidation describes a chemical reaction between the lubricant and the oxygen in the air, which results in the decomposition of the oil. Though this process can be accelerated by temperature exposure of 480°F (250°C), it may also take place at lower temperatures; however, at lower temperatures, the rate of decomposition is slower. Oxidation is possible at bearing surfaces.
Oil contamination can occur as a result of both external and internal sources. External contamination is often the result of poor turbine system flushing during equipment commissioning or by re-use of the flush oil. Internal contamination occurs when sub-micron oxidized oil agglomerates to leave deposits.
While some instances of varnish and deposits can be expected in all turbine operations to varying degrees, certain types of turbines are more susceptible to varnish and its effects.
For example, turbines with common hydraulic and bearing reservoirs are more susceptible to unit trips or no-starts as a result of varnish, as compared to turbines with segregated reservoirs. This is because in common reservoir systems – which rely on a single reservoir and oil to lubricate bearings and hydraulics – the same turbine oil that supports the bearing load at elevated temperatures must also flow through three-micron tolerance servo valves.
The turbine oil, which supplies servo valves through external tubing, does not circulate while the unit is shut-down, causing the oil to cool. As the oil cools, insoluble products of oxidation can settle out of solution and, subsequently, foul the hydraulic elements. As a result, these cooler, low-flow, tight-tolerance areas are the most prone to varnish problems.
By comparison, steam- and gas-turbine journal and thrust bearing clearances are around 200 microns in size. Because of the larger size, mild varnish can build on journal and thrust bearings with little-to-no impact on bearing temperatures or shaft rotation. Unit trips or no-starts in these systems as a result of varnish formation are rarely, if ever, reported, as they have little effect on operations. For these reasons, varnish prevention and detection focuses primarily on turbines with a common control- and turbine-oil reservoir.
Varnish formation is influenced by a myriad of factors, including unit operation, ambient and bearing temperatures, system flush, turbine oil chemistry, filter selection and hydraulic system pressures. Considering this, treating the symptoms of varnish needs to be approached through a double-barreled approach that includes selection of the right turbine oil and the adoption of a proactive maintenance strategy that includes appropriate technical services.
Selecting the Right Turbine Oil
To combat varnish formation, operators should take a close look at the chemistry of the turbine oil that they are using to ensure they select an oil specifically formulated to address varnish.
When selecting a well-balanced gas turbine lubricant, maintenance personnel should consider a lubricant’s deposit control, oxidative stability, air release and foam control, filterability, rust and corrosion and wear protection, as these factors will mitigate and manage varnish formation.
Deposit Control: Turbine oils formulated specifically to combat varnish will help keep deposits in suspension. Because it is easy to measure deposit formation, you can see the deposit control attributes of various oils in the photograph below.
- Oxidative Stability: As previously described, the combination of high operating temperatures and contamination from wear metals and entrained air can lead to oxidation, and thus to varnish formation. Selecting oils with high oxidative stability is important.
- Air Release and Foam Control: Oils not specifically formulated to combat varnish are more susceptible to micro dieseling, as entrained air in the oil may be compressed in turbine bearings or high pressure hydraulics
- Further, excessive surface level foaming can accelerate oxidation and lead to operational issues, such as the inability to correctly measure lubricant levels or reservoir overflow from vents.
- Filterability:This is an important characteristic, as it defines an oil’s ability to pass through a filter with minimal pressure drop. An oil with poor filterability will foul filters faster than an oil with good filterability, often leading to more frequent filter changes.
- Anti-Rust and Corrosion Protection: Rust and corrosion – in addition to generally having a negative impact on system performance – can contribute to oxidation and the formation of contaminant-based varnish. Oils formulated to minimize rust and corrosion will reduce the likelihood of varnish formation.
- Wear Protection: Wear on high pressure hydraulics and on accessory gearbox gears, generator reduction gear or turning gear, can directly impact operation of gas turbines. Wear materials from these components can indirectly lead to varnish formation since the wear metals will act as an oxidation catalyst.
In addition to understanding the formulation of a specific oil, it’s also important to understand how different oils interact with one another, as this aspect of lubrication will play a major role in preventing lubricant contamination.
Mixed reservoirs of two different oils may contribute the formation of varnish due to additive incompatibility, but using ASTM D 7155 Standard Practice for Evaluating Compatibility of Mixtures of Turbine Oils will help to screen products to ensure that they are compatible. And, it is important to remember that while base oils may be compatible, the additives may not be. Working closely with your lubricant supplier may be helpful to determine instances in which such incompatibilities exist.
Delivering a Balanced Formulation
Formulating oil that achieves key performance goals – including protecting equipment against the formation of varnish – without sacrificing other attributes has its own challenges.
The leading attribute of a low varnish oil is deposit control, and achieving step-out performance in that area can often result in lubricants which in which some other performance characteristics, such as demulsibility, are less than optimal.
As part of a balanced formulation approach, particularly in the case of gas turbine oil formulation which will be used in turbines with bearing and reservoir operating temperatures that will volatize minor water ingression, less than optimal demulsibility is considered an acceptable trade-off for a significant improvement in deposit control.
Understanding which trade-offs are acceptable requires knowledge of the oil’s application and a balanced formulation.
For example, ExxonMobil Research and Engineering has designed and constructed turbine oil developmental test rigs, called Valve Varnish Rig Tests (VVRT), which simulate real world service and form varnish. These test rigs are used to develop next generation turbine oils and to evaluate market available oils in use today.
These test rigs utilize hydraulic components typical of a gas turbine and technicians operate the rigs in cyclical on/off service with servo valve movements at start-up and shutdown. Technicians then take voltage measurements during valve extension and retraction, and compare voltage signals with response time, overshoot and undershoot. In these tests, high varnish forming oils generate issues with stick slip friction and valve hunting at fewer operating hours than low varnish forming oil. Additionally, EMRE technicians examine components of the test rig for signs of varnish and turbine oil degradation.
Testing oils in a rig, as described above, offers evaluations closer to real world conditions than typical glassware testing of RPVOT (ASTM D 2272) or TOST (ASTM D 943), which are tests traditionally used in the industry. Candidate turbine oils should, at a minimum, exceed OEM and new oil acceptance standards.
The graph above compares seven commercially available oils against an ExxonMobil Research & Engineering developmental oil (Oil A), showing hours of operation until the onset of valve sticking.
Implementing a Proactive Maintenance Approach
Once you’ve selected the most appropriate lubricant for your gas turbine, it is important to ensure optimal performance of your equipment through a proactive oil analysis program.
Generally, sampling and testing should be done at least quarterly, and it is usually beneficial to perform analyses more frequently as the oil condition degrades. By comparing results of these tests over time, maintenance personnel can gain valuable insights into the condition of the oil, the condition of the equipment and the remaining service life of both.
However, it’s important to note that today’s available historical oil analysis testing methods, including relative RPVOT (ASTM D-2272), total acid number (TAN) increase (ASTM D-664) and ISO Cleanliness Code 4406, cannot accurately predict varnish. The industry has been working to identify or develop methods to better predict varnish in gas turbine hydraulics systems with a combination of Ultra Centrifuge (UC), Membrane Patch Colorimetry (MPC) and Linear Sweep Voltammetry (RULER) tests that are gaining credibility and acceptance in the industry.
Ultra Centrifuge: UC is a varnish prediction test used to quantify the level of contamination present in an in-service lubricant. In this analysis, a test tube of used oil is spun at a high velocity (17,500 RPM) at an ambient temperature for 30 minutes. The tubes are visually examined for residue and given a rating of between one and eight, with one being the cleanest. A rating of four indicates caution for further use and that maintenance personnel may continue using the oil, but proceed utilizing additional equipment inspections, if available. Oil ratings should be trended and confirmed with field inspections, but as a guide, a rating of six or more indicates that the oil may be a candidate for change or conditioning. UC scores continue to correlate well with field varnish observations.
Membrane Patch Colorimetry (MPC): The MPC test is designed to identify the contamination level in used oil related to oil degradation and potential varnish development. This test entails pulling an oil sample with a vacuum pump through a membrane nitro-cellulose patch. The resulting stain on the patch is numerically rated, based on the increased absorbency of specific visible light waves. Simply stated, the darker the stain, the more light that is absorbed in the stain and the higher the MPC value. Specific actions based on the patch rating are very much application and oil dependent. Some oils generate darker color bodies than others as they naturally age, even though the total amount of deposits may be less. These color bodies do not necessarily indicate varnish formation, but should be used for trending.
RULER: The RULER test identifies levels of antioxidants, typically phenol and/or amine. In-service oil antioxidant levels are compared to those of new oil. As antioxidants age, they can convert to new species that have antioxidant properties, called intermediates. These intermediates can be measured by this test, but may require additional analysis processing. In gas turbine operation, the phenols will deplete first, followed by the amines.
In steam turbines and other lower temperature applications, it is not uncommon to see amines deplete at the same rate as phenols, or in some cases even faster. ASTM International Subcommittee D02.C00 on Turbine Oils is considering a revision to ASTM D 4378 “Standard Practice for In-Service Monitoring of Mineral Turbine Oils for Steam and Gas Turbines” to adjust the turbine oil warning guideline to read “25 percent of the remaining antioxidant.” This is important to note because as seen in gas turbine operation the phenol may convert early to an intermediate antioxidant that is not directly identified in R.U.L.E.R. In gas turbine operation the end user should closely monitor the depletion amines.
These tests are helpful in predicting the likelihood of varnish formation, but operators should take care when developing plans of action based on test results. Action plans from these tests should be application, oil, and equipment specific.
When developing action plans, one should take into account the application, or the type of gas turbine used. As described earlier, combined hydraulic and bearing reservoirs can be more sensitive to varnish formation than a steam turbine with separate hydraulic reservoir. Action plans should also consider the formulation chemistry of the turbine oil tested to help determine if an alternative oil should be used. Finally, operators should develop equipment specific action plans based on equipment inspection, maintenance history and valve stroking considerations.
The test results may require the use of different interpretative methods, depending on the presence of various additive components, so it is therefore advisable for operators to consult with their oil suppliers for specific recommendations.
Lubrication Key to Optimizing Performance
Knowing the impact of varnish, where it occurs, how it forms, how to predict whether it will form in specific applications and how to minimize its formation through oil selection should provide plant personnel with tools to enhance equipment reliability and allow them to focus on operation.
As this article has described, there is no doubt that a major component of trouble-free operation, relative to varnished-caused servo valve sticking, is the use of a high-quality turbine oil formulated specifically to reduce the impact of varnish. Further, proper turbine oil development requires that you validate formulations in field-like conditions.
In addition to selecting the right lubricant, it’s equally important to develop and implement a proactive maintenance approach that includes regular oil analysis and monitoring. By evaluating the lubricant’s key features through validation rig performance and using innovative tests to monitor its in-service performance, maintenance professionals will gain the insight needed to optimize gas turbine operation and minimize unscheduled downtime.
Working with expert lubricant manufacturers and oil analysis providers who have the application expertise needed to help vanquish varnish will help gas turbine owner/operators boost productivity and maximize the life of their equipment.