Understanding Varnish
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.
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 through 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 color from gray 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 of the equipment casing. However, this usually requires a partial or full-system shutdown, therefore, is not always feasible outside of 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 cylinders sticking; sub-synchronous vibration in high-speed bearings and seals; a decrease in oil inlet flow or the 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 do I 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 with respect to confirming that a system’s performance issues are varnish related.
In addition, all oil analysis tests used for identifying varnish problems present challenges such as:
- 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 (a) they are very small (sometimes < 0.1μm) and (b) test methodologies measure only soft varnish precursors in suspension, but the precursors are oil-soluble and can easily move in and out of solution depending on sample handling
- As a proxy for varnish-potential which itself is a proxy for varnish deposition, tests measure the overall ‘health’ of the oil or the additive package or the amount of varnish precursors in the oil
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?
Many engineers believe that varnish can only be managed by continuous varnish filtration and a combination of mechanical and chemical cleaning 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 more frequent, unexpected shutdowns and operation halts will be needed, as turbines get older.
Texaco VARTECH technology – a holistic solution
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 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 areas can need extended cleaning durations up to one month.”
Mr Knotts continues, “Our analysis of data from laboratory testing and field trials of the Texaco VARTECH Industrial System Cleaner has shown a number of benefits in contrast to other conventional cleaners, highlighting our intention to introduce a system that could clean efficiently without creating operational constraints. Firstly, VARTECH is compatible with many in-service oils at the recommended concentrations of 5 - 20% by volume for the recommended cleaning durations (allowing longer residence times and generally removing the need for flushing). It has also been shown to help reduce filter plugging and contribute to deposit removal effectiveness as well as being designed to help dissolve varnish deposits from internal surfaces which then helps stabilise them for removal with the used oil. Finally, it has a viscosity of 53 cSt which helps make it effective in ISO 32-68 viscosity oil as it will not significantly affect oil viscosity at the recommended concentrations and has a minimal impact on both lubricant water separability and on long term new oil performance in residual amounts up to 2%.”
VARTECH technology field trials
One of the greatest difficulties with varnish is that much of the time it can go unnoticed within a system. Components, reservoirs, and filters continue to function, but may not be functioning as efficiently as they once were. As varnish worsens, a vicious cycle is often set in motion. The coating insulates metal surfaces which prevents efficient oil cooling and raises the temperature. This causes more degradation and less effective lubrication. More precursors are created, more varnish layers are formed and the problem spirals towards equipment failure.
This was the case in one of VARTECH’s first field trials in the US. Operating gas turbine equipment in challenging climactic environments such as the high temperatures experienced in California’s Central Valley (typically between 30º and 40ºC), is bound to produce additional obstacles. Chevron San Joaquin Valley Business Unit (SJV) operates more than 16,000 wells, producing 159,000 barrels of crude and 53 million cubic feet of natural gas a day. SJV has world-class heavy crude oil steam flooding expertise, simultaneously producing steam and electricity. The steam facilitates oil recovery using the steam flooding process and the electricity is sold to a power utility. During summer the gas turbine bearings were running too hot to operate at its full capacity, in fact the turbine could only produce 3.3MW of electricity, significantly below its 5.5MW design capacity. High ambient temperatures were part of the problem, but the team suspected varnish was also interfering with efficiency to the oil coolers, an issue subsequently confirmed following a visual inspection.
To address the ongoing problem, the Texaco VARTECH team, led by Paul Sly, Global Industrial OEM Specialist changed the oil filters, replaced 15% of the used in-service oil with Texaco VARTECH Industrial System Cleaner and then restarted the system. As the gas turbine approached full capacity, it was apparent that something had radically changed. All system temperatures remained within the acceptable range and no alarms were triggered. The gas turbine continued at full capacity for two weeks with no high-temperature alarms, then the mixture of used in-service turbine oil and VARTECH Industrial System Cleaner was drained. The system was flushed, the filters replaced, and the system refilled with Texaco GST turbine oil. The result is that SJV can operate its co-generation equipment at full capacity year-round no matter the ambient temperature, thereby gaining over $350,000 of previously lost revenue per year.
A further opportunity to trial VARTECH technology came about in one of the largest oil fields in the Gulf of Mexico. Loss of power generation in any circumstance is costly and to be avoided where possible, but when it is powering a spar floating platform sitting in 4,100 feet of water 190 miles off the US coast, loss of electricity could lead to the shutdown of the whole operation. When hydrocarbon production was threatened by oil headers in the generator sets consistently running hot and triggering high-temperature alarms, Texaco’s new VARTECH technology was instrumental in providing a long-term solution. Using VARTECH technology has not only helped the customer save up to $81,000 in annual maintenance costs but has also helped to reduce their safety exposure and environmental risk by not having to remove equipment. 12 months after first employing VARTECH technology at the Tahiti spar platform there have been no high temperature alarms and therefore no premature system shutdowns.
Paul Sly, Global Industrial OEM Specialist concludes, “We are all fully aware that the lost revenue caused by unplanned or longer than expected shut down can be extensive. Our challenge was to create a solution that moved away from relying on varnish filtration and dated cleaning methods. The utilisation of the VARTECH two-step programme, in conjunction with our LubeWatch oil monitoring has seen companies successfully change their maintenance process to remove deposited varnish more effectively before they fill their systems with new oil.
“Our VARTECH holistic varnish solution has shown strong deposit removal effectiveness and deposit resistance with GST Advantage Turbine oil products in extensive comparative laboratory and field testing. This improves system deposit control and cleanliness, helping to deliver longer oil life and helping to improve turbine operation.”
