Operators no longer have to wait for a servo trip before identifying and dealing with varnish in gas turbines.
By Doug Muennich, UAS/Kleentek
For power plant operators, varnish-the sticky sludge that plagues bearings and servo valves-is a lot like the common cold virus. The cold virus can live in a person’s system for days or even weeks before any physical symptoms emerge. Similarly, varnish is often present and wreaking havoc on gas turbines long before damage is detected. Even though operators are becoming increasingly aware that varnish is a serious threat to reliability, most still don’t diagnose the problem until they’ve experienced a servo trip.
But unlike the common cold, a vaccine and a cure exist for varnish formation in turbine oil. Recently introduced large-capacity electrostatic oil cleaning systems have a filtering volume of 10,000 gallons and a flow rate of 5.5 gallons per minute to circulate oil and remove the insoluble contaminants that cause varnish. This ounce of prevention often is literally a fraction of the cost of a complete system flush or unscheduled outage due to a servo trip. Besides prevention, catching the common symptoms of varnish formation and implementing a solution early can also save the turbine owner thousands of dollars in equipment repair costs and system downtime.
Varnish build-up on bearing surfaces reduces clearances and can prevent hydrodynamic lubrication of a bearing surface, leading to bearing failure.
The earlier varnish is detected in turbine oils, the better the chances of saving the oil and avoiding expensive equipment wear. Because varnish acts as a catalyst that shortens the lubricant’s life, letting the problem go undetected can lead to a total system flush, which can cost upwards of $100,000. Also, the increased friction caused by varnish build-up results in reduced turbine performance, slower start-ups and loss of control stability. Eventually a servo trip can occur, leading to expensive unscheduled downtime, turbine repair and servo valve replacement. The industry standard cost for a servo trip averages $80,000 to $90,000 in wear to the unit. Depending on the application, a servo trip can occur just once a year for a unit that operates in baseload configuration, or as frequently as every quarter for a peaking unit.
No Problem Yet? Just Wait
The bad news is that varnish build-up is inevitable in turbine oil lubrication systems. It will occur, even in well-maintained systems using thermally robust oils that are still relatively new. Thermal degradation-the process by which friction and heat combine to degrade lubricant oil and produce varnish-takes place in every system each day and will lead to turbine failure if left undiagnosed and untreated.
Varnish is the result of the base oil and additive system deteriorating in the lubricant. Heat, an unavoidable factor, is among turbine oil’s biggest enemies and is a primary factor in creating degradation by-products. When oil temperatures reach 572 F, the hydrocarbon molecules begin to crack and break apart, thereby polluting the lubricant.
Beyond general heat-related degradation, several additional root causes of varnish in gas turbines exist, including static discharge from mechanical filters, a shared reservoir for hydraulic and lube oil circuits, hot spots in the system, additive depletion, implosion of air bubbles, recent formulation and base oil changes in turbine oils and low flow hydraulic circuits with temperature differentials.
Beyond Routine Oil Analysis
The most common response to a varnish build-up diagnosis is: “It can’t be varnish. Our test results say our oil is fine.”
The task of catching varnish problems before they cause equipment damage is made more difficult by the fact that varnish is often present in oil that appears clean and tests healthy. The onset of varnish cannot be predicted with routine oil analysis. Unfortunately, the standard oil analysis tests used to determine lubricant health, like the rotating pressure vessel oxidation test (RPVOT) and total acid number (TAN) indicate that oxidation potential exists in the oil, but only after varnish has affected the system. The precursors to varnish formation are free radicals, or broken hydrocarbon molecules, which are too small for these tests to identify. Because the signs of varnish cannot be seen on these routine tests, most people assume it simply isn’t there.
Over time, varnish particles attach themselves to surfaces throughout the turbine, like this load gear, producing a sticky coating.
The good news is that tests are available to determine if varnish is present in turbine oil and, if so, how severe the problem is. The following tests are offered by most reputable labs and should be performed annually or semi-annually.
Fourier transform infrared spectroscopy
In Fourier transform infrared spectroscopy (FTIR), various degradation by-products found in the oil cause absorptions in specific regions of the FTIR infrared spectrum. The higher the level of contamination in the sample, the higher the degree of absorption in the characteristic region. In this case, rising absorption peaks in the 1630 cm-1 region are an indication that oxidation is taking place and that a high potential exists for varnish formation.
A colorimetric test will indicate the varnish problem’s severity by analyzing the insoluble contaminants in turbine oils. The procedure involves drawing oil and soluble additives through a 0.08 micron filter patch, which will leave only the insoluble portion, or contaminants, behind. The severity of the varnish problem is indicated by the color and shade of stain on the patch. A spectrophotometer analyzes the light reflectance of this color and shade of stain and then compares it to a clean, unused patch. The resulting color difference is charted by the colorimetric value and cleanliness level. The higher the colorimetric index, the more prone the oil is to varnish formation.
Remaining useful life test
A remaining useful life (RUL) test is used to determine how much life is left in the antioxidant additives. If varnish formation goes undetected for too long, the free radicals that form varnish will consume all the antioxidant additives in the turbine oil. Regular RUL tests will catch the varnish problem before the oil degrades to the point of needing to be replaced.
A standard ISO 4406:99 particle count can also help identify varnish if the person performing the test knows what to look for. An increase in the volume of particles in the two-micron to five-micron range can be a sign of varnish. This is because varnish is polar, so it has a propensity to stick together and grow. As it grows, it turns into particles with enough size to be measured in this range. When varnish is identified at this stage, it is time to examine filters for varnish build-up and start the FTIR and colorimetric tests immediately to determine the problem’s severity.
The ultracentrifuge test uses gravity to force varnish-forming insoluble contaminants to the bottom of a test tube. The amount of contaminants is assigned a rating, which corresponds to its varnish-formation tendency. An ultracentrifuge rating of six or above signals moderate to severe varnish potential.
By monitoring several critical areas and implementing a solution at the first sign of trouble, operators can catch their varnish problems before real damage starts. Here are some of the early warning signs of varnish formation in turbine systems.
Last-chance filters on the servo valves
Varnish in turbine oils can plug oil strainers, so increased consumption of both last-chance and hydraulic oil filters is an early sign of varnish. If a system that typically operates eight to 12 months between filter change-outs begins receiving warning signs every few months, this signals a likely varnish problem. It is essential to pull these last-chance filters on a regular basis and inspect them for the brown, gooey varnish accumulation. Varnish on these filters is the earliest known warning sign. Once varnish is detected on these filters, a colorimetric test should be performed to determine the problem’s severity and then an oil cleaning system should be installed.
An electrostatic oil cleaner helps prevents varnish from forming.
Monitoring the life and appearance of these filters is the easiest way to catch varnish early, but it requires extra attention among those doing the change-outs. Many facilities simply discard the old filter and insert a new one without noting its condition. Keeping accurate records detailing when the filter was changed, who changed it and whether any brown or yellowish accumulation could be seen on the used filter is important. By analyzing these records a varnish problem can be spotted in the early stages and the system can be protected from damage.
Increasing bearing temperature
Varnish build-up on bearing surfaces reduces clearances and can prevent hydrodynamic lubrication of a bearing surface, leading to bearing failure. High bearing temperature is a signal of decreased lubrication and a sign of a severe varnish problem. Although the definition of a “high” bearing temperature varies by application, an increase of 5 C or more should be considered a warning sign of varnish build-up. At this point, the tests mentioned above should be conducted to determine the problem’s severity.
The inlet guide vane (IGV) servo valve is the component most likely to be affected first by varnish build-up. In a peaking station, intermittent poor performance from the IGV valve could be caused by varnish build-up. The varnish causes the valve to stick, slowing or stopping its response to positioning signals. If not addressed, this will lead to a servo trip and system shutdown.
Implementing a plan to monitor and respond to these early warning signs should be a key element of every operation’s reliability program. The cost of conducting the necessary tests is minimal when compared to the cost in damage and downtime of a turbine trip.
In Fourier transform infrared spectroscopy (FTIR), various degradation by-products found in the oil cause absorptions in specific regions of the FTIR infrared spectrum.
The cost of prevention-installing an electrostatic oil cleaning system-is also minimal when compared to letting varnish damage a system before taking action. An electrostatic oil cleaner prevents varnish from forming because, unlike traditional filtration, electrostatic cleaning removes both large, hard contaminants and the insoluble soft contaminants that cause varnish. For a standard high output gas turbine, an electrostatic oil cleaning system cost about $25,000. The system draws all particle size contaminants out of the oil and traps them on the surface of a pleated collector. Electrostatic oil cleaning units can be installed with little impact on system operations. When installed on a system with an existing varnish problem, they can also eliminate existing build-up in as little as 14 days after installation.
Doug Muennich is technical sales manager for UAS/Kleentek, the exclusive licensed manufacturer of Kleentek varnish removal systems in North America. He has been a lubrication engineer for 20 years, a designation achieved through specialized training programs within the oil industry. Muennich can be reached at 800-252-4647 or [email protected]