Turbine technology | Vanquishing varnish
Varnish attacks turbines from the inside, destroys performance and threatens operations. But to defeat varnish, you must first understand it. Texaco offers some guidance
Varnish is a common issue in circulating systems where oil is used to provide a continuous flow of lubricant to bearings, gears and other components of industrial equipment. Even the smallest amount of varnish can result in reduced system performance and equipment failures. Sludge or varnish adhering to servo or thermostatic valves can cause the valves to stick, bearings to overheat, and poor heat exchange performance, often producing elevated oil temperatures. Solid particle contaminants that imbed into varnish can also lead to increased component wear in valves, gears and bearings. Varnish build-up in turbine systems can have a serious adverse effect on system performance, equipment longevity, operational flexibility, and lubricant longevity. Over the life of a piece of equipment, this can equate to huge financial losses.
What is varnish?
A lubricating oil degrades when it undergoes thermal and mechanical stress. The rate of degradation is accelerated by many factors, such as: oxidation; thermal stress (equipment hot spots, bubble adiabatic compression, micro dieseling); contaminants such as water, solvents, gas, air, and dirt; additive depletion or drop out due to operating conditions or over filtering, electrostatic discharge from filters and cross contamination or fluid incompatibility. Any of these factors could play a significant role in oil degradation, creating varnish precursors that, over time, form layers of varnish. Once formed, varnish is a thin, oil-insoluble deposit composed primarily of organic residue, mixed with metals, inorganic salts, and other contaminants that cannot easily be removed by wiping with a dry, soft rag. It forms a coating that adheres to internal surfaces and can take on different characteristics, from a sticky coating to a hard lacquer, and ranges in colour from grey to brown to amber.
How is varnish identified? The most definitive means to assess varnish formation in equipment is through visual inspection of the parts inside the equipment casing. However, this usually requires a partial or full-system shutdown, therefore, is not always feasible outside planned system maintenance. While not a definitive list, the following symptoms may be indicative of the effects of varnish: erratic component operation, including valve and hydraulic cylinder sticking; sub- synchronous vibration in high-speed bearings and seals; decrease in oil inlet flow; increased frequency of filter replacement. System temperature can often also be a warning, with system alarms triggered due to
higher temperatures, main bearing or header temperature increase, and poor heat exchange performance in an oil cooler, leading to high overall oil temperatures or the derating of the equipment to stay below alarm limits. And finally, the oil condition itself – oil darkening, foul odours, acid number increase, viscosity increase, evidence of sediment and/or sludge, increased particulate count, high MPC (membrane patch colorimetry) values and rapid depletion of lubricant oxidation life – all indicate issues within the system.
How to prove varnish is the problem?
Oil analysis test methods fall broadly into two types: those that predict the life of the oil; and those that measure the current performance of the oil. The first test type will only indicate the varnish potential of the oil. At best, the second test type will only indicate the amount of varnish and varnish precursors in the oil. Neither test type provides an indication of varnish deposits on equipment surfaces nor whether such deposits are problematic. Oil analysis is therefore merely suggestive as to whether a system’s performance issues are varnish related.
In addition, all oil analysis tests used for identifying varnish problems present challenges
because: ● the oil sample tested may not be representative of the oil in the whole system;
● the potential for varnish may change through the year depending on system utilisation rates as degradation products often come out of solution to form deposits in an idle system;
● varnish precursors are difficult to measure as particle sizes are very small (sometimes < varnish precursors in suspension, while the precursors are oil-soluble and can easily move in and out of solution depending on sample handling;
● they only measure the overall ‘health’ of the oil or the additive package or the quantity of varnish precursors in the oil, as a proxy for varnish-potential which itself is a proxy for varnish deposition.
There is no direct, definitive way of identifying varnish-related system issues that do not involve measuring critical parameters like valve hysteresis or oil cooler effectiveness or even tearing down the system to visually ascertain the root cause.
What can be done? The VARTECH holistic solution
Many engineers believe that varnish can only be managed by continuous varnish filtration and a combination of mechanical and chemical cleaning
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www.modernpowersystems.com Above: Cycle of failure
during planned downtime. However, using varnish filtration and standard cleaning practices to remove varnish is often not effective, especially on severe varnish that has been depositing for years in both hot spots and cooler areas of equipment. In fact, by continuing to follow an approach based on downtime cleaning with standard varnish filtration, it is likely that more frequent, unexpected shutdowns and operation halts will be needed, as turbines get older. When the subject experts at Texaco looked at ways to mitigate varnish, they believed a more holistic solution to the problem was required. Texaco VARTECH technology is a two-step process. The first step is designed to help eradicate the varnish that has infiltrated the turbine system using VARTECH Industrial System Cleaner, and the second step then helps to prevent the formulation of new varnish through the use of lubricants formulated with VARTECH technology.
Nathan Knotts, portfolio architect for industrial products at Texaco, says: “VARTECH technology was created in response to our research around products currently in the market and their efficiency. We looked at key areas such as the use of solvents in cleaners. While they help to remove varnish, they also have a low flashpoint, and that low flashpoint could have implications for increased fumes or temperature limit as well as a greater fire or explosion risk. “We investigated whether harsh cleaners were causing damage to more vulnerable components such as seals and potentially causing leaks. We also reviewed whether cleaners were compatible with differing types of oil or flexible in the length of time they are kept in a system. Generally, the longer a cleaner is in a system, the more varnish is removed. High flow areas are often cleaned quickly, potentially in just an hour, while heavy varnish deposits, cool temperatures or low flow
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