Lubricating oil in gas turbines and hydraulic systems is unfortunately subject to the ravages of varnish. It is well-documented that varnish is a thin insoluble contaminant comprised of oil degradation by-products and sometimes depleted additive molecules. It is generally caused by some type of thermal (heat-related) stress placed on the oil. The debilitating effects of varnish include the loss of operating clearances within machinery and a loss of heat transfer due to thermal insulating. As more operators face the prospect of varnish in their lube oil systems, they are turning to oil analysis labs for answers.

The Destruction of the Lube Oil System
Whether tending to a gas turbine or a large hydraulic system, the mere mention of varnish can cause one's spine to tingle. A number of explanations for the increasing occurrence of lube oil varnish have been postulated. Tighter filtration requirements, higher lube oil flow rates, higher operating temperatures, and the switch to Group II base stocks in oil formulations have been offered as potential culprits in the decimation of lube oil systems. Varnish can often lead to unplanned outages and costly downtime; therefore, understanding and responding to varnish with remedial filtration is critical. Unfortunately, the ability to measure varnish potential has remained elusive in routine testing.


Figure 1. Varnish on Spool Valves

Measuring Varnish Potential
Insight Services, an industrial oil analysis lab with headquarters in Cleveland, Ohio, has developed a varnishing potential analysis solution to alert customers of developing lube oil varnish potential. This new analysis combines multiple testing technologies to measure a lubricating oil's propensity to create varnish deposits.

This new system combines the results of several individual tests to provide a complete analysis of a lube oil's varnishing potential. The ultra centrifuge (UC) test subjects a lube sample to G-forces that yield oil-degraded insoluble contaminants typically associated with varnish. The density of the agglomerated material is compared to a rating scale to obtain a UC value. Microscopic particle counting is used as a tool to trend particles in the microscopic size ranges and alert users of potential varnish anomalies. Because these soft contaminants are less than one micron, this technique allows a technician to visually count particles with a high-intensity microscope and monitor the submicron ranges for increases in levels.

Fighting Back Against Varnish
Mike Barrett, Insight Services' director of marketing, explains the benefits: "Initially, we thought the colorimetric analysis was an adequate solution. Upon digging deeper, it became apparent that the ultra centrifuge test and microscopic particle counting are just as valuable in determining varnishing potential."

This varnishing potential analysis offers a solution to detecting lube oil's propensity for varnish. Once aware of varnish potential, active measures can be taken before destructive varnish ravages a system. Visit Insight Services at to find additional information on varnishing potential analysis.

1. Ultra Centrifuge Test: A small amount of oil in a test tube is run for 30 minutes at 18,000 RPM in an ultra centrifuge. By subjecting the sample to G-forces, the oil degraded insoluble contaminants that are too small to be detected by normal particle counting or be removed with mechanical filters can be extracted. The density of the agglomerated material is compared to a rating scale to obtain the UC value (1 to 8). When the UC value exceeds 4, a marginal condition is noted.

2. Microscopic Particle Count: Varnish is a thin, insoluble contaminant comprised of oil degradation by-products and an occasional depleted additive molecule. Because these soft contaminants are less than one micron, microscopic particle counting is used as tool to trend particles in the microscopic size ranges and alert potential varnish anomalies. With this technique, particles are counted with a high-intensity microscope and the submicron ranges are monitored for increases in levels.

3. Colorimetric Analysis: Another tool in determining the varnish potential of an oil. The process of making a patch isolates and agglomerates insoluble by-products associated with varnish. The color of the membrane patch provides a guideline concerning the extent of varnish potential. The more yellows and blacks on the patch, the higher potential for varnish. In determining severity, focus is on the values B (black) and Y (yellow).

4. Physical Properties: While these tests may not indicate a direct varnish threat, an increasing trend may place the in-service oil at risk for future varnish potential. As oil begins to oxidize, an increase can be seen in oxidation by-products. The FTIR detects these by-products in the form of increasing carbonyl peaks. The FTIR reports an oxidation number which is compared to the value of a reference oil to determine the varnish potential risk. Additionally, a significant increase in the acid number is a possible indication of rising carboxylic acids associated with an oxidation condition. Monitoring the acid number provides an alert to an increasing risk of oxidation and varnish. Thermal degradation and its resulting by-products have different effects on the oil. By-products of thermal degradation exhibit increases in the nitration peaks of the FTIR. The nitration number of the used oil is compared to the value of the reference oil to determine the varnish potential risk. Water acts as a catalyst to speed up the oxidation process. In addition, electrostatic filters, often used to clean up varnish in oil, fail to operate at optimum levels in the presence of water.

A. Customer Information, Machine Information, Sample Information.

B. Ultra Centrifuge: Trend graph of the UC value with points moving left to right.

C. Colorimetric Analysis: Trend graph of the color value with points moving left to right.

D. Varnish Status: Normal, At Risk, Marginal, Critical.

E. The overall condition of the fluid.

F. Images of the ultra centrifuge tube used to determine UC value.

G. Images of the color patch used to determine color value.

(Click Image to Enlarge)

Figure 2. Varnishing Potential Analysis Report Key