Varnish, lacquer, deposits—no matter which word you use, it is never one that maintenance managers want to hear.

Varnish is the thin, sticky brown or black film deposit occurring on the interior of lubrication systems that can eventually lead to maintenance problems. These hard-to-remove deposits are formed when your lubricant breaks down or decomposes, either due to age of the oil, equipment condition, maintenance practices or a combination of these factors.

Varnish deposits are nothing new, but varnish has become more of a concern as we push our equipment to higher operating extremes. Once varnish attacks a circulating system, maintenance problems can occur and may worsen if left untreated.

Numerous factors can contribute to creating varnish and depositing it on system surfaces. Hot spots, cool spots, air entrainment (which, upon compression, may lead to microdieseling), metal catalysts, water and oil contaminants can lead machinery further down the slippery slope of varnish deposits.

Deposit formation can lead to a domino effect within machinery causing:

• Sticking of moving parts, especially those with fine tolerances
• Increased damage to bearings, seals, valves and gears
• Reduced oil life
• Loss of heat transfer in heat exchangers
• Decreased filter life.

Sticking of critical controls such as servo valves can lead to unplanned downtime and machinery maintenance. In the end, the problem may lead to less production and greater maintenance costs. In power plants, turbine trips due to servo sticking may lead to unexpected repair costs. Extended downtimes can result in losses of revenue, so plants cannot afford to let varnish go untreated.

Many power suppliers are installing gas or combined cycle turbines rather than steam turbines in an effort to increase efficiency; however, these systems may produce more severe conditions for the oil. Higher oil operating temperatures, increased oil flow rates and more complex lubrication systems have the potential to create an environment for varnish formation.

When faced with these harsh conditions, oil quality is a crucial part of the success of the lubricant’s ability to fight against oxidation and varnish. Irrespective of the application, the careful selection of base stocks and additives is critical to developing a high performance product. That’s where the product development teams play a critical role in matching oil performance to application needs. As an example, emerging industry specifications mean that turbine and hydraulic oils, among others, require increasingly higher levels of performance in the area of control over deposit formation. In addition to using a powerful lubricant, Shell recommends a consistent oil testing program.

Formation of varnish can be very difficult to predict. Signs of varnish can include servo valve sticking, filter plugging, oil darkening and odor and high particle count in the oil. However, these signs may not appear until varnish is already prevalent in a system.

Standard bulk oil tests often do not identify problems. For instance, rotating pressure vessel oxidation tests (RPVOT) used by the power industry for more than 40 years to determine the condition of in-service turbine oils provide no detail on the deposit-forming characteristics of the oil. Other tests such as total acid number (TAN) likewise often do not highlight the formation of the precursors to deposit formation. Varnish can remain under the radar despite a rigorous program of oil condition monitoring with conventional testing:

• RPVOT
• Total acid number
• Viscosity at 40 C
• ISO cleanliness
• Elemental analysis (wear, additive, contaminant levels)
• Water content
• Fluid color
• Oxidation/nitration by Fourier-transform infrared (FTIR) spectroscopy

Such tests and observations measure bulk properties of the oil and may not effectively measure the levels of varnish pre-cursors in the oil. Moreover, some systems with varnish issues may not show any changes in oil condition for quite some time.

The shortcomings of these tests in detecting varnish have led companies to adopt test procedures that have yet to be approved by the American Society for Testing and Materials (ASTM). For instance, many power utilities are adding newer tests, such as the ultra-centrifuge, colorimetric patch, gravimetric patch and others. Many plant engineers are also adding the quantitative spectrophotometric analysis (QSA) test to their oil analysis programs. The test isolates and measures the specific degradation byproducts contributing to sludge and varnish formation. During the QSA test, the lubricant is treated with a chemical mixture that isolates the insoluble material in the oil. Once isolated, a spectral analysis of the byproducts of oil degradation is conducted. Using a rating scale of 1 to 100, a propensity for the lubricant to form varnish is determined. Though tests such as QSA should not be used in place of standard oil testing procedures, many plants are using them to add another perspective to make more informed decisions about their lubrication.

Beyond oil testing, many plants, especially those using critical hydraulic or turbine systems, are utilizing electrostatic filters. These fluid-conditioning systems are typically installed in a kidney-loop and have shown to be active in removing varnish precursors from degraded oils. The filters employ ultra-fine filtration and electrostatic cleaning to remove varnish particles.

When faced with varnish, the decisions that plant managers confront are often based on the plant’s bottom line. A balance is needed in an effort to curb maintenance and monitoring costs while ensuring effective equipment operation. Unfortunately, many do not identify the hard deposits on the interior of their lubricating systems until the system has shut down and requires major repairs. Such shutdowns are costly not only because of machinery repair but also because of the resulting downtime. Varnish can best be controlled by selecting the proper lubricant after identifying the equipment’s operating demands, maintaining the lube system properly and controlling temperature, using new oil analysis tests for varnish and varnish filtration when necessary.— Author: Felix Guerzoni, product application specialist with Shell Global Solutions.