Mitigating the formation of varnish-hard, oil insoluble organic residue that cannot be easily removed from mechanical components-is one of the biggest challenges facing gas turbine operators in the power generation industry today.
Abundance of natural gas has led to an increase in gas-powered turbines. Some gas turbines use the same lubricant for the bearings and the hydraulic circuit and are more prone to varnish-related operating issues due to tight servo valve clearances. Further, precise air-to-fuel ratios, necessary for proper operation in dry, low nitrogen oxide burners, has increased the sensitivity to servo valve varnish.
Approximately 40 percent of the gas turbines in an ExxonMobil survey were reported to have current or historical issues with varnish within six years of oil service life. These turbines were filled with oils from all major oil suppliers. And, as turbine operators know first-hand, varnish formation can significantly impact an operation by leading to decreased equipment performance, unplanned downtime and reduced productivity.
There are three main causes of varnish formation: oxidation, a reaction that acts to decompose the oil; thermal degradation, which can take place at temperatures above 300 degrees Celsius; and contamination through either internal or external sources.
While implementing varnish detection and prevention practices is important to help mitigate these causes, the best defense against all three is using a turbine oil engineered to prevent varnish from forming in the first place. The right base oils and additives can help deliver greater deposit control, minimize oxidation, prevent electrostatic discharge and maintain filterability, all of which can help mitigate varnish formation.
Base Stock Basics
The fundamental ingredient for any oil is the base stock. API Group I, Group II and Group III oils are the most commonly used base stocks today. Group IV oils (polyalphaolefins) are synthetic base oils that offer extreme temperature performance and superior oxidation stability at a higher cost premium. Group V oils are specialty base stocks that have been successfully used in conjunction with other base oils.
Historically, Group I oils have been the leading type used in turbine oils, with a viscosity index range of 90 to 120. They are refined using a simple solvent-refining process.
Group II oils have a similar viscosity index range to Group I oils, but because they are refined using a hydrocracking process, they have a higher hydrocarbon saturation percentage by weight. As a result, these oils offer better oxidation resistance.
Group III oils are more advanced, refined using hydrocracking or gas-to-liquid processing. They have a viscosity index above 120, meaning they can perform across a wider temperature range.
Turbine bearing temperatures can approach 120 C, and in combination with equipment metals, contaminants and entrained air, these high temperatures contribute to the oxidation process. Higher grade base oils can deliver enhanced performance at high temperatures, thus helping protect against oxidation.
In the Lab
With the exception of field testing, rig tests can be the most accurate indicators of varnish-related performance. These tests replicate real-world conditions, so varnish deposits that form in the test are chemically similar to those that form in the field.
Valve sticking is the key failure criteria for a valve varnish rig test. Valve sticking is when the sliding pool element, which controls the valves position, responds irregularly. This is caused by an increase in current, which makes the valve overshoot its specific position.
Primarily, the test measures how long it takes to reach onset of valve sticking. However, the test also determines system cleanliness (in both the filter and reservoir) and in-service oil condition by measuring deposits.
In ExxonMobils proprietary rig testing, higher quality base oils outperformed lower quality base oils. Turbine oils formulated with API Group III base stocks and carefully selected additive technology outperformed those formulated with Group I and Group II base stocks by delivering at least 500 hours of extended performance before the onset of valve sticking.
While rig tests are more indicative of real-world performance, glassware testing can still provide valuable insights. Glassware aging studies test oil performance at elevated acid number and temperatures to provide indicators of long-term oil performance.
In an aging study conducted by ExxonMobil, an oil formulated with a Group III base oil outlasted an oil formulated with a Group I/II base oil blend.
The Group I/II base oil blend started showing signs of degradation after 1,344 hours of testing. At that time, the oil contained unacceptable levels of insoluble contaminants, which can contribute to varnish formation.
Conversely, the Group III base oil did not show any sign of meaningful degradation until 2,352 hours after the testing began.
The results of the glassware aging study echo the results of the valve varnish rig test, with Group III base oils consistently outperforming Group I and Group II oils.
In the Field
While laboratory testing is an important indicator of real-world performance, nothing beats field results. And, unsurprisingly, Group III base oils continue to outperform lower grade base oils when in service.
In one scenario, five GE Frame 7EA gas turbines in cyclical service were initial-filled with industry leading Group I turbine oils in 1992. After only 6,000 hours, these turbines experienced servo valve stickiness and last-chance filter fouls. Last-chance filters, which are installed just ahead of the jets that spray oil onto the bearings, catch any contaminants that get past the main system filters. As service continued, the turbines required frequent servo valve and last-chance filter changes. At about $3,200 per filter and downtime for the changes, the cost added up quickly.
In the late 1990s, these turbines were transitioned to Group II turbine oils, and performance improved. The first signs of lubricant aging (servo valve sticks and last-chance filter fouls) occurred at 18,000 hours, or after three years. However, the operators still needed to conduct frequent servo valve and last-chance filter changes to mitigate varnish.
In 2009, operators started using a Group III turbine oil, and the results were even better. After 36,000 hours, or nearly seven years, there are still no signs of servo valve sticks or last-chance filter fouls. These turbines continue to successfully operate and anticipate a minimum of 48,000 hours of operation.
Group IV base oils delivered similarly impressive performance in GE Frame 6B turbines being run in base load. Three of the turbines were initial-filled with Group I oils in 2002 and experienced servo valve sticks and last-chance filter fouls after only two years of operation (13,000 hours). After that, the turbines required monthly servo valve and last-chance filter changes to mitigate varnish formation.
In 2006, the operator began using a Group IV oil (PAO) in these turbines, and the results were staggering. There were no servo valve sticks or last-chance filter fouls even after nine years, or 65,000 hours.
As these two examples show, there is clear improvement in lubricant and equipment performance when using oils formulated with Group IV base oils, which is why it should be a key consideration when selecting a turbine oil.
Conductivity
Many experts point to electrostatic discharge (ESD) as a prime cause of varnish formation. The primary influencer of ESD is conductivity.
Oils with conductivity lower than 50 picosiemens per meter are vulnerable to ESD, which can lead to sparking and cause localized hot spots that can accelerate oxidation. To minimize ESD, oils should have high conductivity, above 50 pS/m.
Some experts believe that formulating with certain base oils will lead to a turbine oil with low conductivity, but a base oils influence on conductivity is actually minimal compared to that of the additive.
Oils need to use the right additive technologies to ensure high conductivity, meaning operators can still get the performance benefits of Group III base oils without increasing the risk of ESD.
In the end, field performance tells the story. We have examples of low conductive base oils (less than 50 pS/m) providing over 60,000 hours of reliable service in gas turbines that had been susceptible to varnish. We also have examples of high conductivity base oils (greater than 50 pS/m) that generate more varnish deposits than lower conductive oils.
Much is being said around the industry about the relative influence of base oils and additives on turbine varnish formation, but it is often too general and misinformed. Based on the results presented here, we can see that higher quality base oils with the proper additives offer significant performance benefits by minimizing varnish formation.
Jim Hannon is a product technical advisor for ExxonMobil Research & Engineering. He is a Professional Engineer and an STLE Certified Lubrication Specialist with a degree in marine engineering from Massachusetts Maritime Academy. He can be reached at james.b.hannon@exxonmobil.com or (609) 259-6580.
Tara Nickels is an advanced researcher for ExxonMobil Research and Engineering, specializing in industrial lubricant formulation. She earned her Ph.D. in analytical chemistry from the University of Oklahoma. She can be reached by email at tara.m.nickels@exxonmobil.com.