Mitigating Turbine Oil Varnish: Finding Proactive Solutions to a Sticky Problem

By Paul Sly, Global OEM Industrial Sector Specialist – Chevron

Equipment reliability and performance are critical for maximizing productivity in power plant operations. Unfortunately, varnish buildup in turbine engine oil systems can be a drag on reliability. At a minimum, persistent and unchecked varnish resulting from oil degradation can cause costly downtime. In the worst case, it can damage equipment severely enough to require expensive repairs or even replacement.

Mitigating the impact of varnish and reducing the risk of formation are maintenance priorities. In this article, we’ll look at steps you can take to do both. First, though, it’s helpful to understand what varnish is, what causes it, and the effects it can have on equipment.

Just What is Varnish?

The term “varnish” encompasses various types of deposits in oil systems. Some varnish types are hard, others sticky, and still others soft and thick, commonly referred to as sludge. Regardless of the varnish type, the cause is usually the same: the breakdown of the turbine oil, typically triggered by excessive heat. As the oil degrades, impurities form. As more and more of these impurities accumulate, they begin to cluster, forming insoluble, suspended submicron particles. Ultimately, these particles become polar, or attracted to metal surfaces. When they start sticking to those surfaces, they become varnish.

Cross-contamination of oils with incompatible additives can also trigger varnish. If an operator introduces a new oil with a different additive formulation from the existing oil, the additives can react with each other, destabilizing the lubricant and causing degradation that ultimately leads to varnish.

In turbines, varnish may deposit everywhere but tends to have significant impact on servo valves, causing them to stick sometimes called “hysteresis” and “trip” the turbine out of operation, triggering a shutdown. In a peaking turbine, one that is held in reserve for peak demand, stuck servo valves can result in failure to start.

Monitoring and Managing Varnish Formation

One of the challenges in trying to mitigate varnish is that it is difficult to detect before you start to experience its effects. Consistent oil sampling and analysis is the most efficient way to stop varnish problems before they cause a major failure. Particulate matter identified through oil analysis is an indicator the oil may be breaking down, a precursor to varnish. The Membrane Patch Colorimetry or MPC test under the ASTM D7843 standard is a common method for quantifying varnish potential. In the MPC test, a sample of in-service oil is run through a fine membrane patch designed to capture insoluble contaminants. Any substances that are polar, sticky, or larger than around 0.4 microns will adhere to the patch and give it color. A spectrophotometer is then used to analyze the color, which yields a measurement of varnish potential in the oil.

If the precursors of varnish are present, various filtration and separation technologies can be employed to remove harmful degradation byproducts, reducing the concentration of precursors and resulting in cleaner working oil.

Of course, the most efficient solution is to take action even earlier, even before the oil starts to break down. By proactively keeping lubricants clean, cool and dry, it is possible to cut down on the root causes of varnish formation. An oil can experience contamination at a number of points during shipping, handling and storage before it actually enters the turbine system. It pays to work with a supplier who provides clean oil consistently and certifies cleanliness upon receipt.

Preventive and Proactive System Cleaning

Despite best efforts to keep oil clean, detect oil degradation, and filter out varnish precursors, varnish may still form. In that case, a solution is needed for cleaning and clearing varnish out of the system and, ideally, minimizing the risk of its recurrence. A variety of chemical solvents, dispersants and detergents are commonly used to remove varnish. Though they may be effective to varying degrees, they also raise some issues. Specifically, they may impede the natural separation of water from oil, interfere with antioxidant properties in the oil, or have an adverse impact on rubber seals and rings. Moreover, they most likely do not address the issue of varnish recurrence.

As turbine technology has evolved, Chevron has worked closely with turbine OEMs to develop varnish treatment and lubrication products that stand up to the stresses of modern turbine engines. Chevron’s VARTECH Industrial System Cleaner provides turbine operators with a non-solvent-based alternative for the removal of varnish and sludge. Chevron has also introduced the GST Advantage family of turbine oils, formulated with an advanced chemistry designed to reduce oil degradation and inhibit the precursors of vanish formation. Together, these two products are intended to complement each other in a two-step, “clean and control” solution to varnish mitigation. The first step is to clean away existing varnish using VARTECH Industrial System Cleaner in advance of a planned oil change. The second step is to fill the newly cleaned system with the appropriate GST Advantage oil for the application, thus minimizing the risk of varnish recurrence.

A deep, thorough and effective system cleaning process, followed by an oil formulated to mitigate varnish formation, should enable operators to maximize turbine reliability, performance and uptime. Regardless of the detection, testing or mitigation methods you employ in your fight against varnish, a proactive approach to oil cleanliness and varnish removal is the key to improving equipment reliability and avoiding costly unplanned maintenance. Work closely with your lubricant suppliers, equipment vendors and service providers to ensure you have the right tools to help mitigate varnish problems in critical turbine systems.