Power plant owners have discovered that lubrication health plays a big role in keeping turbines running smoothly. Identifying varnish in turbine oils and getting rid of it are key to maintaining machinery reliability.
Mineral-based turbine oils lubricate most of the power generation industry’s stationary gas and steam turbines. As gas turbine technology continues to improve, the stress on the turbine oil increases and requires improved base oils and additives to handle the higher temperatures and loads. Turbine oil manufacturers have responded by using Group II-finished products with improved additive characteristics. The new Group II turbine oils show much improved oxidation stability as measured by rotating pressure vessel oxidation test (RPVOT) and turbine oil stability test (TOST) over the older Group I formulated products that have been in place since World War II. Turbine manufacturers believe that oxidation stability is extremely important and have listed these two oxidation tests in their specifications along with tests that determine other properties including viscosity, water separability (especially for steam turbine oils), load carrying ability, corrosion protection, resistance to foam, air release and cleanliness levels.
However, now that Group II-based turbine oils have been in many turbine systems for more than 10 years, new challenges are arising regarding sludge, varnish and deposit formation for many gas and steam turbine operators. These contaminants are causing problems with turbine operations and, when left alone, can create operational issues with critical bearing and servo applications. These problems lead to reduced efficiency and production capability. Due to this new phenomenon, a better understanding of varnish is needed.
What is varnish?
Varnish is a thin, insoluble film deposit that is usually found on bearings and servo-valves. It is a high molecular weight substance that is insoluble in oil. Varnish insolubles are more than 75 percent soft contaminants that are less than 1 micron in size and are not measured by traditional particle counts. Insoluble compounds have polar affinities and, over time, begin to migrate from the body of the base oil to machine surfaces, based on system and oil conditions. Initially, the surfaces start to show a gold/tan color building to darker gum layers that develop into lacquer. The chemical compositions of these insoluble materials vary from case to case. For example, the composition of a varnish on a gas turbine servo valve may not be the same as a deposit found in a steam turbine oil system. Since insoluble compounds are less stable in Group II, III, and IV base oils due to their high purity, it is important to optimize the additive package with the base stocks and to maintain system cleanliness to retard varnish and sludge formation.
How is varnish formed?
Varnish formation is an operational and reliability issue. All turbine oils will create insoluble materials given severe and/or unusual operating conditions. These insolubles create lubricant imbalance due to factors such as oxidation, cross- and chemical-contamination, microdieseling and adiabatic compression. The tendency and speed at which turbine oils produce these products is greatly influenced by the formulation of the product, the stress on the oil and system contamination levels. Figure 1 shows the deposit tendency of commercial turbine oils. As illustrated, the synthetic turbine oil produced the second highest amount of insoluble materials mainly due to “mismatching” of additives with the synthetic base stocks. Synthetic base stocks are an excellent platform; however, if they are not properly formulated, optimum performance will not be achieved, resulting in increased varnish formation.
Harmful Effects of Varnish
The varnish deposits that form on machine surfaces cause numerous operational issues by interfering with the reliable performance of the fluid and the machine’s mechanical movements. They can also contribute to wear and corrosion or simply just cling to surfaces. In severe cases, varnish build-up could prevent hydrodynamic lubrication of a bearing surface, resulting in bearing failure. Other potential problems include:
Varnish buildup on a shuttle valve. Photo courtesy of Cort Johnsen, Chevron Corp. Refinery
• Restriction and sticking in moving mechanical parts such as servo or directional valves
• Increased component wear due to varnish’s propensity to attract dirt and solid particle contaminants
• Loss of heat transfer in heat exchangers due to varnish’s insulation effect
• Catalytic deterioration of the lubricant
• Plugging of small oil flow orifices and oil strainers
• Increase of friction, heat and energy because varnish acts as a heat insulator
• Reduction in filter efficiency and potential filter plugging
• Damage to mechanical seals
• Journal-bearing failure
• Increased maintenance costs due to cleanup and discard of oil
Varnish can occur in oils that appear healthy and clean, with no visible signs for concern through normal oil analysis. It cannot be identified by the typical in-service turbine oil tests. Low total acid number (TAN), a low ISO particle count and a high RPVOT does not guarantee that the lubricant is immune from varnish. However, varnish potential can be monitored by Fourier transform infrared spectroscopy (FTIR - nitration procedure), gravimetric analysis, color patch with spectrophotometer analysis (quantitative spectrophotometric analysis, QSA) and ultracentrifuge tests. Varnish potential may be monitored in the infrared spectrum of in-service oil. If the 1630 cm-1 peak in the infrared spectrum rises over time, this is an indication of higher varnish potential. Gravimetric analysis (similar to the procedure of ASTM D893) measures the weight of residual components. QSA utilizes gravimetric analysis and spectrophotometric analysis of the patch. A proprietary calculation gives the ultimate varnish potential rating. The ultracentrifuge test uses gravity to force oil sediment to the bottom of a test tube. This sediment is rated and given a numerical rating that corresponds to varnish tendency (Figure 2).
There are two general methods for removing insoluble materials in rotating equipment systems. The most common method is through electrostatic purification and the other (less common) is through chemical cleaning. The electrostatic filtration apparatus can be set up tank- or system-side and hooked up quite easily with limited impact on system operations. The unit itself places a small charge on the diverted oil stream. The small varnish precursors are charged and are sent through an ionic filter. The charged particles (positive and negative) are removed from the oil (Figure 3). As more varnish precursors are removed, the oil is able to absorb additional varnish molecules from those in the system that have already plated out and they begin the process of cleaning the system from varnish. The removal of varnish from system components is a long process. For electrostatic purification to work, it is critical for the lubricating oil to have a moisture content of less than 500 ppm water along with a low particle count. An important benefit of electrostatic filtration is that it helps remove particulates up to 2.5 microns, which helps improve system cleanliness and equipment reliability.
Chemical cleaning is another method for removing varnish. This method requires a significant amount of monitoring and often requires a system to be shut down. Cleaning chemicals are typically flushed through the system to dislodge varnish from components. These chemicals will soften and remove insoluble materials and the flushing action will remove them to fine filters. This process is usually performed for several hours to several days, depending on the size of the system and the extent of the varnish build-up on components. Once the flush and chemical treatment is complete, the system must be flushed again with an appropriate flush fluid to remove all residual chemicals and ensure no contamination finds its way into the new lubricating oil. Although this process is more intensive, it does allow for quicker removal of varnish versus the electrostatic method, especially with a large system. p
Varnish buildup on main gas turbine bearing. Photo courtesy of Analysts Inc.
Leonard J. Badal, Jr. is a Business Solutions Specialist for ChevronTexaco and Mark Okazaki is a Product Specialist for Industrial Oils Technology at ChevronTexaco.
UAS Kleentek's electrostatic fluid cleaner. Photo courtesy of UAS Kleentek