Lubricant Varnish

Lubricant varnish is a thin, insoluble film that deposits throughout the internal surfaces of a lubrication system.  Varnish is a widespread problem in many sensitive lubrication applications, and comes in a wide range of colors and consistencies.  The presence of varnish in hydraulic and lubrication systems cause serious and expensive problems.

Varnish formed on the Inlet Guide Vane
Valve of a Gas Turbine

Why does varnish seem to be a more noticeable contamination control problem then ever before?

1. Machines are running at hotter temperatures placing higher demands on the lubricants
   
   
2. Users attempt to maximize oil drain intervals
   
   
3. Use of different base stock technologies may contribute to varnish formation, especially if two different base stocks are blended together
   
   
4. Reliability Engineers are performing more root cause analysis on failed components
   
   
   
   

   What Causes Varnish?

   Lubricant varnish is a contaminant comprised of agglomerated degraded and/or polymerized oil and additive molecules.  Polymerized oil molecules are a result of lubricant degradation, a process that alters the hydrocarbon molecule.  While there are a variety of mechanisms that contribute to oil degradation, the three most common methods are oxidation, thermal degradation and chemical degradation.

   Oil degradation by-products, also referred to as "soft contaminants", are the evidence of lubricant degradation and form as polar components.  Soft contaminants may be either soluble or insoluble in the oil.  The polar nature of insoluble soft contaminants means they are unstable in a non-polar oil environment.  Soft contaminants will combine and adsorb onto dipolar metallic surfaces forming varnish.  These soft contaminants agglomerate on valve spools and sleeves, bearing surfaces, gears and other internal surfaces of the lubricant system. 

	Varnish at 500X	Varnish at 1000x

Varnish Problems

• Reduced clearance zones affecting lubrication regimes.  Often, this means a transition from hydrodynamic lubrication to boundary lubrication, which increases wear rates of pumps, bearings and gears.
  
  
• Increased friction in components.  This friction will result in higher energy requirements and can cause valves to stick or seize.
  
  
• Higher operating temperatures. Varnish acts as an insulator, lowering the effect of heat exchangers and lessening the ability of the lubricant to cool.  The impact of friction and reduced clearance zones also contribute to higher temperatures.
  
  
• Restriction or impedance of oil flow.  Varnish can cause valves, strainers and filters to clog.
  
  
• Increased wear rates. Varnish captures hard contaminants creating an abrasive surface that will accelerate wear.  Varnished surfaces often appear like sandpaper when examined under a microscope.

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Detection

Due to the potentially high costs associated with varnish, it is important for maintenance and reliability personnel to have a predictive tool to measure the lubricant's varnish potential.  Determining the varnish potential of a fluid enables the user to investigate the root cause and implement corrective action before a catastrophic failure occurs. However, routine oil analysis fails to identify varnish potential.  The following table lists some of the oil analysis tests that are currently performed and why they may not assist in determining varnish potential.

Analytical Test	Method	Typically Detects	Limitations In Detecting Varnish
Spectro Metals	AA, RDE, ICP, XRF	Metallic wear particles. May also be used to measure metallic additive levels.	Although metal particulates can act as a catalyst in lubricant degradation, the by-products responsible for varnish are non-metallic and therefore cannot be directly identified or measured by this method.
Water ( % volume)	KF, Crackle	Water level.	This test is useful for detecting conditions that may accelerate oil degradation, but cannot be used to directly measure varnish propensity.
ISO Particle Count	Automatic or Manual – ISO cleanliness rating	Particles >5 microns in size.	Degradation by-products are typically less than 1 micron size range. Conventional particle counting methods measure particles greater than 2-microns in size. While an excellent tool for monitoring larger “clearance sized” contaminants and filter efficiencies, this test is of limited use for measuring varnish contributing contaminants.
Acid Number (AN)	ASTM D644, D974	Acidic constituents.	AN measures the lubricant’s acidic properties.   This test is ineffective for measuring varnish potential, as varnish may occur well before an increase in acid number is observed.
Rotating Pressure Vessel Oxidation Test (RPVOT)	ASTM D2272	Oil’s resistance to oxidize under prescribed conditions.  Can sometimes be correlated to levels of anti-oxidants.	This test method is useful in measuring a lubricants remaining useful life.  RPVOT cannot be used alone to measure the fluid’s varnish potential, as the oil still may have considerable useful life while simultaneously having a high varnish potential.
Infrared	FT-IR	Molecular fingerprint.	Although this test may be useful in determining the method of fluid degradation, FT-IR does not provide a quantifiable level of insolubles.
Viscosity	ASTM D445	Oil’s resistance to flow.	A lubricant’s viscosity can increase from hydrocarbon chain polymerization.  While a useful indicator that degradation is occurring, varnish propensity cannot be determined from changes in viscosity alone.

Quantitative Spectrophotometric Analysis (QSA^SM)

Quantitative Spectrophotometric Analysis is a technique of purposely isolating and measuring the specific lubricant degradation by-products responsible for varnish formation.  The process begins by treating the lubricant sample with a specific chemical mixture designed to isolate insoluble by-product material.  Next, a separation process collects the varnish forming insoluble degradation by-products including the sub-micron species.  The process concludes with Quantitative Spectrophotometric Analysis on the isolated degradation by-product.  The results are reported on a scale of 1-100 and indicated the Varnish Potential Rating of the lubricant.

QSA^SM has been demonstrated to have a high correlation with the varnish potential of the lubricant.  It is sensitive, repeatable and reliable.

QSA^SM is a Service Mark of Analysts Inc.

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Solution

Preventing Varnish

What about switching to a better oil to reduce varnish?  Oil manufacturers have developed new oils to resist varnish formation.  These products are composed of an API Group II (hydro-cracked) or higher basestock. Although API Group II and III formulated lubricants outperform API Group I (solvent refined) oils in virtually all categories, the research to date is inconclusive concerning varnish prevention.

Reasons that switching to API Group II or III lubricant may not help:

1. Lower solvency means that the lubricant has a lower tolerance for degradation by-products.

2. Several antioxidants are combined in oils to maximize RPVOT and TOST values.  Some inexperienced formulators use some antioxidant combinations In Group II and III products that have a high propensity to form sludge when depleted.

3. Topping off a lubrication system that has an API Group I product with an API Group II or III product often produces severe varnish problems, even if the oil is from the same manufacturer.

4. All of the laboratory research in this area has been performed by exerting oxidative stresses on the oil.  Varnish is often caused by a different degradation mechanism (i.e. thermal degradation), that is not measured by the scope of current research.

Most times, varnish is an inherent problem with lubrication systems.  However, it is possible to prevent varnish from forming with certain technologies that are capable of removing sub-micron particles.  Clarus Technologies has examined many of the products commercially available and come up with a technology that solves varnish problems.  This has been demonstrated with dozens of customer ranging from power plants, chemical factories and marine applications.

Please contact Clarus Technologies for more information.

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