Varnish is the thin, insoluble film deposit that forms on fluid-wetted surfaces inside a turbine lube system, including bearings and servo valves. The material is comprised of a wide range of oil additives and high molecular weight thermo-oxidative fluid breakdown compounds that have limited solvency in the base fluid. These compounds are polar in nature and begin to migrate from the base fluid to the wetted surfaces over time, based on the system and fluid conditions and their polar affinities.
Initially, the surfaces show a gold/tan color, building to darker gum-like layers that eventually develop into a hard, lacquer-like material. The chemical compositions of these insoluble materials vary depending on the turbine operating conditions, the fluid base stock and additive type.
Adsorption Compared to Electrostatic Separation
Lubricants and oils that have experienced severe oxidation and heavy varnishing may not be adequately reclaimed or filtered to continue useful service. However, in most cases varnish and other soft contaminants can be removed from the oil and the oil returned to service.
Two common methods of varnish removal are electrostatic separation and adsorption. How does adsorption compare to electrostatic separation?
Electrostatic Separation
Electrostatic separators operate in a low-flow, off-line or kidney-loop installation. They operate on the premise that charged particles precipitate to a collection media or plates of the opposite charge.
Disadvantages reported when using electrostatic separators include:
- High purchase cost and operational costs
- Low varnish holding capacity of collection media
- Efficiency drops with the presence of water in the oil (500 ppm or greater)
- Low flow rates
- Complex control systems
Adsorption Systems
Adsorption systems (Depth Media Filtration) are also installed in an off-line or kidney-loop manner. Unlike electrostatic units, adsorption units can provide a higher range of flow rates, from low flow (<5 gpm) to high flow (>50 gpm). The adsorption media commonly used in off-line filters is a high-density, depth-type cellulose filter. With these filters, it is possible to combine particle, moisture and varnish removal in one system. The downside comes from the oil needing to be “cold” filtered to effectively remove the soft varnish particulate. In most cases up to 72 hours of cooling needs to take place before running through depth media filters.
However, removing varnish particles requires other forces besides ordinary physical forces employed in particle and moisture removal (impaction, absorption, etc.).
Absorption Systems
We recommend the absorption method form of Ion-exchange for varnish mitigation or removal. Selective Ion-exchange resins are mixed and formulated to absorb varnish within its porous structure. This absorptive nature is due to polar attraction between ion exchange resign and varnish contamination. There are large amounts of surface area by volume, making this process highly effective. Selective Resins are mass produced which make them a cost-effective solution. Our systems use large amounts of ion-exchange resin media combined with oversized high efficiency post filters.
- Combination Kidney Loop Filtration for Low Particle Counts with Varnish Absorption
- High performance particulate removal with Beta(x) > 1000 Post Filtration – for Low Particle Counts
- High varnish holding capacity
- Cartridge replacement does not require a winch or lifting system.
- Presence of water in the oil (500 ppm or greater) is not detrimental to performance. Recommendations are to keep applications lower that 100 PPM to maximize lubricity of oil and minimize varnish propagation.
- Variety of flow rate options from 1 to 50 GPM
- Simple control systems
- Cost effective operational and purchase cost
Sources:
Duke Cooper Hydrocarbon Filtration, LLC. (1/2018)
Khalid Farooq Machinery Lubrication (8/2012)
Justin Stover Practicing Oil Analysis (3/2008)
Larry B. Jordan – PetrolinkUSA (3/2018)