One of the most difficult aspects of dealing with lubrication systems is that you can’t see what is happening inside of them. Taking an oil sample allows you to gather information about the trace amounts of wear metals, oxidation deposits, additive chemicals and other contaminants that tell you about the system. Just like blood tests don’t always tell a doctor the whole story, traditional oil analysis testing doesn’t give the entire story when it comes to varnish.
The ASTM International standard, ASTM D7843, defines varnish as “a thin, hard, oil-insoluble deposit, composed primarily of organic residue and most readily definable by color intensity. It is not easily removed by wiping with a clean, dry, soft, lint-free wiping material and is resistant to saturated solvents. Its color may vary, but it usually appears in gray, brown, or amber hues.”
More importantly, varnish is a degradation by-product and a potential problem for any moving machinery components. Varnish buildup reduces heat transfer and increases stiction of valves and seals. Varnish buildup on bearings results in reduced clearance and traps abrasive particles, increasing wear and reducing component life. As the buildup gets brittle, it can break apart and cause catastrophic damage to pumps, bearings, valves and other moving components. The potential problems that varnish causes makes minimizing oxidation and varnish formation critical.
The dangers of varnish make it crucial to have fluid analysis tools that can identify potential issues. The best fluid analysis tools for varnish testing today are a combination of the membrane patch colorimetry (MPC) test, as indicated by ASTM D7843 and the remaining useful life evaluation routine (RULER™) test, as directed by ASTM D6971.
Table 1 – Effects of Varnish on System Components
| Problem | Result |
|---|---|
| Servo Salve Sticking | System trip and system outage, resulting in production loss |
| Heat Exchange Plate Out | Reduced heat exchange and higher operating temperature and increased oxidation |
| Varnish Buildup on Bearings | Varnish captures abrasive particles, creating a sandpaper effect that increases bearing wear and reduces component life |
| Additive Depletion | The base oil is left unprotected and oxidation occurs, accelerating the formation of degradation by-products |
| Seal Failure | Oil leaks and cylinder problems |
| Filter and Strainer Clogging | Oil starvation, resulting in damage to components, such as pumps and bearings |
MPC Test
The MPC test provides a visual indication of insoluble oxidation products or varnish precursors that are currently suspended in the lubricant. The higher the MPC value, the more severe the potential is for varnish buildup in the system.
RULER™ Test
With the RULER test, antioxidants are measured relative to the new oil. The test is an alternative to the rotating pressure vessel oxidation test (RPVOT), which has been the accepted method for estimating lubricant service life. However, ASTM D7348 has been updated to recommend RULER over RPVOT for current oil formulations. The amount of additive present is reported relative to the amount of additive in the new oil.
Testing Challenges
Among the challenges of varnish testing is that the MPC and RULER tests are not part of a standard lube oil analysis package. Consequently, the first varnish test performed on a system is typically after someone already suspects varnish. This poses a problem because, like most sample analysis, a single test can only tell you so much. While it can indicate poor or good health of your system, it doesn’t tell the story of how your system got to that point. With varnish, the story is critical for root cause diagnosis and pinpointing the corrective action needed to prevent equipment failure.
Varnish Mitigation Solutions
Preventive Measures
Understanding lubricant degradation can help you address some of the underlying causes of varnish formation. Lubricants degrade by two pathways: thermal and oxidative degradation. Thermal degradation occurs when the fluid encounters hot metal surfaces, electrostatic discharge (ESD), or air entrainment leading to micro-dieseling. ESD occurs when the oil passes through filters. Many manufacturers sell antistatic filters to address this problem. Entrained air may be the result of low oil levels, return lines above the oil level, or insufficient dwell time. Performing an air release test, as indicated in ASTM D3427, may be useful. The air release time will increase if the oil is contaminated or degraded.
Specialty Filtration
A variety of specialty filtration options are available that claim to address varnish with a slipstream filtration skid. Balance charged agglomeration, electrostatic precipitator, cellulose depth media and ion exchange resins are some of them. Each has applications that have shown success, while each has also encountered varnish that is resistant to its specific approach. When selecting a filtration media, it is recommended to consult with a vendor that is experienced with the various varnish mitigation and filtration technologies. This approach, regardless of type, is an active way to manage the amount of oxidation products or varnish precursors in the lubricant. If varnish precursors are kept out of the lubricant, they cannot plate out on pumps, bearings, valves, etc. Depending on the severity of the varnish buildup on the internals of a system and the amount of oxidation by-products in the oil, this approach can be very effective. The specialty filtration method has the advantage of no machine downtime while it is performed.
Oil Change Out
Performing an oil change is a tried-and-true method for many maintenance personnel. By removing oil that is contaminated and replacing it with new lubricant, you are typically adding good oil to the system and buying time to find a root cause or a more permanent solution. However, if this is the only action taken, the root cause will never be addressed and, over time, a lot of healthy lubricant will be wasted. In fact, if varnish has built up on the components in the system, oil will degrade more quickly than if it was placed into a system that had been cleaned. Oil change outs are a necessary practice, but not the total solution.
Chemical Cleaning
The most aggressive option for removing varnish deposits inside oil systems is the use of a chemical cleaning process. This approach uses a variety of chemicals, solvents, detergents and acids to aggressively break down and dissolve any oil oxidation products throughout the system.
This process can be performed two ways:
- Brief Downtime: Adding the chemical solvent or detergent while the system is still running at a prescribed percent by volume and allowing the solvent or detergent to interact with all parts of the system. The system continues to operate for two to three days, followed by a drain out, rinse, or flush to get the chemical out of the system prior to adding fresh lubricants.
- Extensive Downtime: Shutting the system down completely and using either an aqueous solution with acid or an oil solution with solvents and detergents while performing an oil flush on the system.
Both techniques are the most aggressive, expensive and time-consuming compared to oil change outs, filtration and filtration with chemicals. However, when the criticality of the equipment is paramount, these approaches are used to help ensure that varnish is not the eventual cause of major failures and additional downtime.
As long as lubricants are the lifeblood of systems, varnish will not vanish!
Solutions Summary and Long-Term Management
Varnish is an extreme danger to your system. Understanding your system and the available testing options can ensure a long, healthy equipment life. The operating conditions of your machinery are key to choosing a solution for your application. Once a solution has been selected, it is critical to establish a routine oil analysis program that includes the MPC and RULER™ tests on a semiannual or annual basis. Testing needs to be performed more frequently if test results indicate a potential problem.
Along with oil analysis, contamination control methods should be evaluated to prevent contaminants from getting into the oil. Generally, the cleaner the oil, the fewer opportunities for oxidation to occur that lead to varnish. Lastly, established baselines for oil analysis reporting and actions to take upon reaching those baselines should be implemented.
As long as lubricants are the lifeblood of systems, varnish will not vanish! However, you can do your best to keep your systems from failing as a result of varnish.