By: Vyacheslav Romaschenko, Industrial Marketing, Europe, Africa & Middle East,
ExxonMobil Lubricants and Specialties
Varnish can have a significant detrimental impact on gas turbine operation, but by selecting the right oils companies can help minimise its build-up and extend gas turbine life.
Varnish is a catch all term for deposits found in gas turbine operation either in the form of sludge or varnish. According to a recent ExxonMobil Lubricants & Specialties survey of 192 gas turbine power plants with a combined total of 626 gas turbines, approximately 40 per cent reported current or historical varnish issues within six years of oil service life.
Estimates put the current cost of a utility trip or no-start to be $100,000 per event, but these costs are expected to increase dramatically as the same turbines are dispatched for back up to renewable energy generation. In refineries, the economic impact of a turbine trip can be measured in millions of dollars.
These problems are often perceived as unavoidable. However, clients should be made aware that the selection of an oil specifically formulated to minimise the impact of varnish, together with improved maintenance strategies, may extend gas turbine/oil service life.
Compared to 20 years ago, there is now much greater awareness of varnish issues. Based on limited experience, some clients may hold the incorrect view that newer technology turbine oils have caused increasing amounts of gas turbine hydraulic varnish. However, direct field experience, combined with test rig comparisons, confirms that advancements in some turbine oil formulations have improved turbine oil life with less varnish generation. For example, ExxonMobil Lubricants & Petroleum Specialties has demonstrated that newer turbine oils have performed with improved reliability in the same service as older, now retired turbine oil formulations.
Understanding where varnish comes from
One of the problems facing power companies is understanding why some turbines are more susceptible to varnish than others. The answer lies in varnish formation theory which looks at the different ways varnish can occur. There are three main mechanisms of varnish formation: thermal degradation of oil which can take place at temperatures above 300°C; oxidation, a reaction that acts to decompose the oil; and contamination of the oil, through either internal or external sources.
While treating the symptoms of varnish through mitigation technologies may extend service life, the important factors for reliable operation are starting with a clean system and using a turbine oil designed to prevent varnish from forming. A well-balanced formulation that utilises high-performance base stocks and advanced technology additives is the first line of defence against the formation of sludge and varnish.
What to look for in turbine oil?
By selecting an oil with highly refined base oils and a proper balance of advanced technology additives, operators are less likely to see their oil compromised during long-term service. In general, higher group base stocks blended with advanced technology additives offer the strongest defence against varnish.
In finding a well-balanced gas turbine lubricant, maintenance personnel should consider analysing a lubricant’s deposit control, oxidation stability, air release and foam control, filterability, rust and corrosion, and wear protection in their selection of an appropriate lubricant that will mitigate and manage varnish formation.
As mentioned earlier, varnish can be generated by thermal degradation, oxidation and contamination. Some oils generate more deposits than others, but advanced turbine oils are formulated to limit the generation of sludge and varnish, while keeping deposits in suspension.
Turbine bearing temperatures approaching 250°C, in combination with equipment metals, contaminants and entrained air all contribute to oxidation, which precedes varnish formation. Operators should look for higher level turbine oil base stocks and advanced antioxidants which can provide protection against oxidation when properly formulated.
Air Release and Foam Control
Entrained air in an oil with inferior air release performance may be compressed in turbine bearings or high pressure hydraulics and cause adiabatic compression (a.k.a. micro dieseling). Adiabatic compression could cause localised elevated oil temperatures that may promote the formation of varnish. In addition, inferior air release can result in less than precise control in system hydraulics.
Similarly, excessive surface-level foaming can accelerate oxidation and can lead to operational issues, such as the inability to measure lubricant levels correctly, or lead to reservoir overflow from vents. Oils formulated to have rapid air release and minimal foam formation will provide superior protection against the formation of varnish.
The filterability characteristic of a fluid can be defined as its ability to pass through a filter with minimal pressure drop. An oil with poor filterability will foul filters faster that an oil with good filterability. This often translates into more frequent filter changes.
Anti-Rust and Corrosion protection
Rust and corrosion can also contribute to oxidation and the formation of contaminant-based varnish. Oils formulated to minimise rust and corrosion will reduce the likelihood of varnish formation.
Wear on high pressure hydraulics, the gears of the accessory gearbox, generator reduction gear or turning gear, can directly impact operation of gas turbine performance.
Wear material from these components can indirectly be a source of varnish formation since the wear metals will act as an oxidation catalyst.
A Balanced Formulation
Varnish formation and management are greatly impacted by the oil’s formulation. It is anticipated that a gas turbine utilising a lubricant formulated with highly refined base oils and a proper balance of advanced technology additives will be less likely to be compromised during long-term service. In general, higher group base stocks blended with advanced technology additives offer the best first line of defence against varnish.
One of the main challenges is formulating an oil that achieves key performance goals without sacrificing other oil attributes. The leading attribute of a low varnish/sludge oil is deposit control - however, achieving this may lead to some performance characteristics being less than optimal, like demulsibility.
Less than optimal demulsibility is considered an acceptable trade-off for a substantial improvement in improved deposit control since gas turbines operate with bearing and reservoir operating temperatures that will volatise minor water ingression.
Rig-testing turbine oils
To help further its understanding of varnish and oil performance, ExxonMobil Research and Engineering has designed and constructed turbine oil developmental test rigs, called Valve Varnish Rig Test (VVRT), that simulate real world service. These test rigs are used to develop next generation turbine oils and to evaluate oils in use today.
Testing oils in a rig offers evaluations closer to real world conditions than the typical glassware testing traditionally used in the industry.
Monitoring and predicting varnish
Once the appropriate lubricant is selected, it is imperative that its performance is monitored through a proactive oil analysis programme. Sampling and testing should be done at least quarterly, and it is usually beneficial to perform it more frequently as the oil condition degrades. By trending the results of these tests, maintenance personnel can gain valuable insights into the condition of the oil, the equipment and the remaining service life of both.
Today’s available historical oil analysis testing methods cannot accurately predict varnish, although the industry has been working to develop methods to better predict varnish in gas turbine hydraulic systems with a combination of Ultra Centrifuge (UC), Membrane Patch Colorimetry (MPC) and Linear Sweep Voltammetry (RULER) tests.
While these tests can be helpful, care should be taken regarding action plans based on test performance. Action plans from these tests should be application and oil specific. “Application specific” action plans consider that gas turbines with combined hydraulic and bearing reservoirs are more varnish sensitive than a steam turbine with separate hydraulics. “Oil specific” action plans refer to the formulation chemistry of the turbine oil.
Making the right choice
Understanding how varnish is made and its impact should provide plant personnel with tools to enhance equipment reliability and allow them to focus on operation.
The key to trouble-free operation is selecting a high quality turbine oil that has been validated in field-like conditions. Setting the stage with a well-formulated oil in a properly flushed turbine and utilising the proper prediction tools will offer years of reliable operation. But selecting the right oil is not a simple choice, and clients should work with expert lubricant manufacturers and oil analysis providers who have the application expertise needed to help vanquish varnish.
For more information about ExxonMobil’s range of gas turbine oils or other Mobil-branded lubricants and services, please contact the ExxonMobil Lubricants Technical Help Desk on TechDeskEurope@exxonmobil.com or +420 221 456 426, or visit www.mobilindustrial.com