eSource Varnish Potential

eSource Varnish Potential

Posted 01 April 2016
by Ed Eckert
Varnish mitigation is an important tool to avoid premature bearing and gear wear and filter clogging. Understanding the conditions that contribute to the creation of varnish and monitoring for its presence can help avoid costly breakdown.

What is varnish? Varnish starts as a high molecular weight soft contaminant that remains soluble in the oil when at system temperature. If an excessive amount of these soft contaminants are present, they will agglomerate and adhere to cooler internal surfaces, low flow areas where more agglomeration can take place, and tight clearance areas where these varnish precursors are trapped, becoming an insoluble, amber colored, sticky substance called varnish.  Varnish is created due to oxidative and thermal degradation of the oil. 

Oil oxidative degradation by-products are typically the primary source of varnish pre-cursors, and are created by the presence of oxygen, heat, catalysts like metals (Iron and Copper), and water. Even contact with other chemicals and fine metal particles will create oxidation by-products.

Thermal degradation is caused by adiabatic compression, also known as micro-dieseling, and ESD (electro-static discharge). Adiabatic compression is when trapped air-bubbles in the oil travel from a low pressure to a high pressure area (typically a bearing surface where the oil is “squeezed”), and due to the extreme pressure the air bubble will collapse on itself imploding and creating extremely high temperatures and localized hot-spots. ESD is a common problem when R&O type fluids with low electrical conductivity pass through a systems oil filters, and generate electrostatic charges. If these charges are not dissipated, they can lead to electrical arcing from the filter media to the support tube.

The effect of varnish in a system are numerous:

  • Adheres to cool areas, low flow, and low clearance areas
  • Impede proper cooling of the oil by coating heat exchangers and other cool surfaces
  • Excessive varnish will cause filter clogging
  • Clogging of oil flow lines
  • Can trap hard contaminants on surfaces creating abrasive wear
  • Cause premature bearing and gear wear
  • Cause sticking servo control valves, causing a system to trip off-line, or a no-start situation
  • Cost of potential downtime
  • Cost of manpower
  • Cost of system flushing and cleaning

Mitigation, as defined by is “minimization of the potential impact of a threat or warning”.  Varnish is definitely a “threat” to maintain proper function of a turbine, hydraulic system, or compressor.  Routine (annual or semi-annual) monitoring using ALS Tribology’s Varnish Potential test package, will provide a cost effective measure of varnish mitigation tactics, by providing the necessary data to take any corrective measure(s) required to reduce the “potential impact” of varnish in the system.

The ALS Tribology Varnish Potential test package includes seven tests for a multiple parameter approach in determining a rating for the level of varnish in a lubricant. The seven tests are:

  • Membrane Patch Colorimetry (MPC), by ASTM D7843
    • MPC - Dark discoloration of a 0.45 micron membrane nitro-cellulose filter patch from soft contaminants (varnish precursors). Reported numeric color rating of the filter patch is based on the CIE_ΔE (International Commission on Illumination delta energy values), using a spectrophotometer. Please note that the ΔE rating is different than other labs membrane patch color rating not using ASTM D7843.
  • Millipore Filter Particulate Weight (ASTM D4055 Pentane Insolubles by Membrane Filtration - modified) and Particle Count ISO 11500/4406
    • The same membrane filter patch from the MPC is used for particulate weight. This is useful in determining the varnish potential rating when comparing the high molecular weight of soft contaminants vs. hard contaminants seen optically in the particle counter. 
  • Color by ASTM D1500
    • Color is a good screen for excessive oxidative degradation by-products in the fluid

  • Blotter Test
    • The test is performed to support the presence of excessive varnish precursors by looking for a dark inner ring circle from a single oil drop on No.2 Whatman paper.
    • A rating of 1 - 5 is given using the ASTM D4740 rating chart.
  • Ultra Centrifuge
    • 42 mls of sample are placed in a clear plastic centrifuge tube, spun @ 15,000 RPM’s for 40 minutes at a regulated temperature of 20 – 25 C. The extreme centrifugal force will extract and imbed the soft contaminants to the bottom of the test tube. After spinning in the centrifuge, the test tube is inverted to view the amount of soft contaminants present at the bottom. Dependent on the amount present, a UC rating is given based on a scale of 1 – 8.

varnish image

  • RULER (Remaining Useful Life Evaluation Routine) – Linear Sweep Voltammetry – ASTM D7590
    • The RULER instrument quantitatively analyzes the relative concentrations of Hindered phenolic and Aromatic amine antioxidants using linear sweep voltammetry. It compares the level of primary antioxidants in the new oil vs. the in-service sample. Please note that not all oil contains both phenolic and amine antioxidants. Some just contain one or the other.
      • In the Varnish Potential test kit, a separate 4 oz. bottle is provided to supply the new reference fluid for the RULER test.

Based on these seven tests performed, a varnish potential rating is determined and provided on the report. The rating consists of four levels, NORMAL – MODERATE – ELEVATED – HIGH. A report with a MODEARATE level may require only more frequent monitoring, depending upon the time on the fluid plus data from other physical/chemical tests, like Acid Number and RPVOT. A rating of ELEVATED or HIGH should ensue some action taken in accordance with a varnish mitigation plan.  ASTM D6439 – “Standard Guide for Cleaning, Flushing, and Purification of Steam, Gas, and Hydroelectric Turbine Lubrication Systems” offers guidance in this area.

The ALS Tribology Varnish Potential test kit comes with a 32 oz. bottle for the in-service fluid and a 4 oz. bottle for the new reference fluid required for the RULER test. Turnaround Time for the Varnish Potential tests is typically 4 – 5 days. Please note that once the sample arrives at the lab, it is required to experience a 3 day waiting period at room temperature in the lab after receipt. This in order to allow the fluid to auto-oxidize (auto-degradation). The auto-degradation will allow the soft contaminants to transition from a soluble to more of an insoluble condition, for accurate varnish potential determination. 

Written By:

Edward F Eckert, CLS, OMA I
Diagnostics Manager, Tribology
North America


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