Varnish formation in hydraulic systems: causes and mitigation

The formation of varnish in hydraulic systems happens in the process of thermally degrading and oxidizing the molecules of hydraulic fluid, polymerizing to form sticky insoluble deposits that cling to the surfaces of internal components. In contrast to sludge, varnish forms an extremely thin, hard, lacquer-like layer that is formed in fluids that are clean and clear, which makes it difficult to discern by routine inspection of fluids. Varnish is deposited on spools of valves and orifices of servo valves along with bearings, which can cause the valve to stick, which can cause erratic actuator performance and diminished heat transfer, as well as premature failure of close tolerance components. Prevention focuses on controlling the temperature of the oil by limiting air entrainment and electrostatic discharge and keeping the fluid within the limits of its oxidative lifespan by employing proactive methods for monitoring varnish, like using the Membrane Patch Colorimetry (MPC) test, which is used in conjunction with traditional oil analysis.

What is varnish, and why is it created?

Varnish is a byproduct of degrading hydraulic fluids at the molecular levels. When base oil and additives are broken down by the stress of oxidation and heat, they develop sub-micron degradation products known as soft contaminants. They are sticky, polar molecules that don't dissolve completely in base oil. Instead of dissolving into apparent sludge, they remain suspended until they come across an area that favors deposition, which is typically high temperatures on metal surfaces with low flow rates, like bearing housings or valve spool clearances and orifices for servo valves.

When the polar molecules touch the surface, they begin to oxidize further and cross-link to form an unhardened film. The film forms layers over time as tolerances gradually decrease that are designed for micron-level accuracy. The most dangerous aspect of the varnish could develop even when oil analysis yields adequate viscosity, content of water, and particle counts. This fluid could appear "fine" on standard reports even as varnish precursors are building up.

The primary causes for the formation of varnish

The degradation of thermal properties and the high operating temperatures

The temperature of the fluid is the primary driver for the formation of varnish. Any 10°C increase over the optimal operating temperature of the fluid approximately doubles the rate of oxygenation, in accordance with the well-known Arrhenius relation that is applied to the chemistry of lubricants. Hot spots that are localized that are caused by inefficiencies in pumps, poorly sized coolers, or limited circulation through valves for relief are able to reach temperatures above the overall reservoir temperature, which can accelerate the development of localized varnish precursors even if the overall temperatures are considered to be acceptable.

Oxidation resulting from air infiltration

Air entrainment introduces oxygen to the fluid precisely at temperatures and pressure that encourage the process of oxidation. Cavitation and return lines that are aerated and inadequately sized reservoirs all increase the risk of exposure to air dissolved and entrained. Micro-dieseling, where the air bubbles that have been entrained break down under compression and briefly increase to temperatures that are extremely high, is an extremely dangerous contributor since it thermally breaks oil molecules nearly instantly.

Electrostatic discharge

High-pressure hydraulic systems with fine filtration may create static charge when the fluid flows through the filter media at a high speed. If the charge discharges abruptly and causes localized temperatures that are similar to micro-dieseling, it reduces the fluid as well as triggers the chain of precursors to varnish. This phenomenon, also known as Electrostatic Spark Discharge (ESD), has been more frequent because systems have moved towards cleaner filtration and higher pressures.

Additive depletion and incompatible mixing of fluids

Antioxidants and detergent-dispersant ingredients are used up over the time that the oil has. After depletion the base oil ceases to be able to neutralize free radicals and maintain degradation byproducts in suspension, speeding up the process of forming varnish. The addition of an incompatible fluid formulation or mixing different fluid additives could weaken the additive package and cause premature formation of varnish.

Catalytic and contaminant metals

Fine particulate matter and water contamination are catalysts for the oxidation reaction. Wear metals like iron and copper are particularly catalytic. Even trace quantities can dramatically increase the rate at which oxidation occurs in the fluid surrounding it, resulting in feedback loops in which early wear encourages varnish; the sticking of valves caused by varnish can lead to further wear.

What effects does varnish have on the system's performance?

Varnish deposition on valve spools can reduce the effective clearance between the bore and spool, causing friction and causing valves to stick in an intermittent manner, which is a problem that operators typically describe as irregular or slow actuator performance, particularly at the beginning of the process or following intervals of idle. Proportional and servo valves that depend on micron-level clearances to ensure accurate metering. They are especially susceptible to this issue and frequently show performance fluctuations even before mechanical failure becomes obvious.

Varnish films also insulate heat transfer surfaces within coolers and also reduce clearances of bearings, thereby aggravating the problems with heat that led to this varnish being present in the beginning. This results in a self-reinforcing loop that is heat triggers varnish, it reduces cooling efficiency, and it can increase friction. Both of these factors increase the temperature of the system further.

The detection of varnishes before they cause failure

Colorimetry of the membrane patch (MPC)

MPC is a standardization of ASTM D7843 and is the most widely used method to measure the potential of varnish. The sample of fluid is then filtered through a membrane, and its color on the patch is analyzed to give the numerical MPC value that indicates the severity of the varnish. Fluids that have a small particle count and normal viscosity may nevertheless yield high MPC numbers, which is the reason this test is now an integral part of hydraulic fluid condition monitor software instead of a substitute for the traditional analysis.

Ultracentrifuges and testing of RULER

The ultracentrifuge method (ASTM D7843 predecessor methods) is a method for separating soft contaminants by density, and the Remaining Utilizable Life Evaluation Routine (RULER) tests are used to determine the remaining antioxidant additive levels using linear sweep voltage measurement. When combined, the tests offer the complete picture. MPC displays the current varnish potential as well as RULER, showing how long the current fluid will withstand further oxidation until intervention becomes necessary.

Operational and visual indicators

Beyond lab testing, users must be aware of early signs of operation such as amber or gold staining on valve spools during teardown, sticky residue on housings for filters, a gradual shift in the proportional response of valves, and an increase in the differential pressure of filters, despite a reasonable particle count.

Mitigation strategies

Temperature control

Sizing coolers to meet the worst-case conditions, such as duty cycle and ambient temperatures, and keeping clean heat exchanger surfaces in addition to setting up alarms when large temperatures of oil can all decrease the burden of oxidation for the liquid. Limiting the temperature of the reservoir to the recommended threshold of the manufacturer, which is typically at 60°C for a long fluid life, although this can vary according to the formulation. This significantly prolongs the time between varnish-related actions.

Air management

Eliminating air ingress from the suction side as well as ensuring that the reservoir has a sufficient dwell time and using well-developed return line diffusers can all decrease the amount of air that is entrained and the consequent micro-dieseling risks. Reservoir sizing that permits enough time for residence time so that air can escape prior to the time that fluid is drawn back into the pump's inlet is an essential design element.

Technology for additives and fluid selection

Fluids that are formulated with high-performance antioxidant packages and varnish-inhibiting additives have become increasingly available for systems with a documented history of varnish. Group II as well as Group III base oils typically offer higher oxidative stability compared to older Group I formulations, and choosing a product that is rated for the actual temperature and pressure duty cycles—instead of the historical default setting—can pay dividends through extended fluid and component lifespan.

Filtration strategy

Beyond the particulate filtration process, specially designed varnish removal filters made of ion-exchange resins or cellulose-based materials can effectively pull soft particles and varnish precursors from circulation prior to depositing on the surfaces. They are generally used as kidney loop filtering systems that continually clean a slipstream of fluid without affecting the central circuit.

Proactive fluid management

The creation of a testing schedule that incorporates RULER and MPC along with the standard particle count and viscosity analyses permits maintenance teams to plan the removal of varnish or fluids prior to performance degradation and not react once the appearance of actuator drift or valve sticking is already evident.

What's the difference between sludge and varnish in the hydraulic fluid?

Varnish is a slender, hard, lacquer-like, and hard deposit made up of the oxidation of polar molecules that bind to metal surfaces. The sludge forms a soft and bulkier collection of larger degrading particles that usually settle in areas with low flow, such as reservoirs, instead of adhering as films.

Do varnishes form in hydraulic fluids that meet the standards of examination of oils?

Yes, the varnish precursors are sub-micron and polar. Therefore, they aren't likely to impact the viscosity, water content, or the standard particle count. That's why MPC tests are recommended to identify the possibility of varnish.

What operating temperatures increase the chance of forming varnish?

Temperatures of bulk oil that exceed 60°C for long durations significantly increase the rate of oxidation; however, hot spots that are localized due to cavitation or a restricted flow could cause damage even if the bulk temperature appears normal.

How can varnish be removed from a hydraulic system?

Varnish removal is usually a kidney-loop filtration using ion exchange resin or cellulose, which continuously polishes the fluid. Sometimes, it is coupled along with system flush or fluid change if the MPC and RULER numbers indicate a higher degree of degradation.

Can switching hydraulic fluid brands prevent the formation of varnish?

Moving to a fluid that has an enhanced antioxidant package as well as improved oxidative stability may aid; however, abruptly switching without confirming compatibility with the current fluid may also weaken additives. This should be done according to the fluid's compatibility guidelines.