Varnish formation in hydraulic systems: causes and mitigation

Varnish develops within hydraulic systems when they are oxidized and the thermally degraded fluid molecules form into sticky, insoluble deposits that encase metal surfaces within the system. It's caused by electrostatic discharge, thermal breakdown, and additive depletion, which is often enhanced by extreme operational temperatures, air infiltration, and water pollution. The mitigation process requires a proactive analysis of the fluid (MPC and RULER tests) as well as appropriate filtering (including special filters specifically designed for the varnish), temperature control, and prompt fluid changes prior to when degradation becomes too severe.

What is varnish, and why is it important?

Varnish is a fine, amber-to-brown lacquer-like layer that develops when hydraulic fluid is broken down chemically and its byproducts of degradation cannot remain dissolved within the oil. In contrast to sludge, an elongated, loose layer, it is hard and adsorbent that bakes on surfaces in the same way that cooking oil bakes on the frying pan after several times.

It's because varnish doesn't declare itself in the same way that an unavoidable failure might. It is created silently, covering servo valve orifices, spools of proportional valves, and bores for cylinders as well as bearing surfaces. As it builds and builds up, valves react sluggishly or stick and restrict flow through small clearances, encapsulate heat transfer surfaces, and increase wear by causing interference with the oil film, which should be protecting contact points between metal and metal. Once the varnish is evident on a dipstick or on a sight glass, the system may have had a problem for several months.

The main causes behind the formation of varnish

1. Oxidation

Hydrolic oil is a reaction product of oxygen dissolved in time, particularly in the presence of heat or catalytic metals such as iron and copper. Oxidation results in acidic byproducts, ketones and aldehydes, that transform into larger, insoluble molecules, the basic components of varnish. All hydraulic liquids have a limited time to oxidize, and when the antioxidant additive package has been exhausted, the degradation process accelerates dramatically instead of sluggishly.

2. Thermal degradation

Systems that operate at a high temperature—especially above 65 to 70 degrees Celsius (150-160°F) for extended durations—will degrade the fluid much faster than the manufacturers' rated life expectancy would suggest. In general oil life is cut by about half when temperatures rise 10 degrees above the operating temperature that is recommended. Hot spots in the local area, like those around pumps operating at high pressure or in reservoirs that are not sized, are prone to causing thermal damage even if temperatures for the bulk of oil are good.

3. Electrostatic discharge (ESD)

Modern high-pressure systems featuring fine filtration, the flow of oil through the tight passages produces a static charge. When this charge discharges abruptly and is absorbed by the micro-scale, it causes an electrical arc, which can raise local temperatures up to a few thousand degrees in just a few seconds, enough to break the oil molecules as well as seed varnishes. This phenomenon, also known as micro-dieseling in the context of the collapse of air bubbles, is a more well-known problem as systems have shifted to higher pressures and better filtering.

4. Micro-dieseling and air entrainment

The air bubbles that are trapped in the hydraulic fluid break apart violently under pressure. The implosion produces extreme, localized heat similar to the principle of compression ignition in diesel engines that burns the surrounding oil and generates varnish-forming chemicals. A poor reservoir design, incorrect return line installation, and low levels of fluid all increase the risk of air entrainment.

5. Particulate and water contamination

Water increases the rate of oxidation reactions. It can hydrolyze specific additive chemistries and release byproducts that create varnish. Particulate pollution exacerbates the issue through the provision of catalytic surfaces and wear that abrasives expose to new metal that further catalyzes the process of oxidation.

6. Incompatibility and depletion of the additive

Antioxidants, detergents, as well as dispersants are consumed while they perform their functions. When the liquid is depleted, it is unable to hold degradation byproducts in suspension and is soluble. Consequently, varnish precipitation speeds up. Mixing different types of fluids and topping them off using a distinct composition from the base charge may cause the additive to destabilize in a short time.

How can you tell if varnish is present before it causes an issue?

Using only visual inspection detects varnish only when significant damage is caused. Two lab tests have become industry standards to identify early signs of damage:

• Membrane patch colorimetry (MPC) filters an oil sample through a membrane and then determines the color and darkness of the leftover residue, resulting in a numerical score that is associated with the potential for varnish. An MPC score that is higher than 30-40 is usually regarded as an indication of danger, but thresholds differ by OEM guidance.
• RULER (Remaining Effective Life Evaluation Routine) is a method of measuring remaining antioxidant additive levels by voltammetry, providing an early warning prior to the depletion that causes accelerated depletion and accelerated oxidation.

Routine oil analysis software that tracks acid numbers (AN) and viscosity trends and particle counts in conjunction with MPC and RULER provides the most precise image of the risk of varnish over time and not one single image.

Mitigation strategies

Control operating temperature

Maintain the temperature of bulk oil within the suggested range, usually between 60 and 65 degrees Celsius, for a longer life of the fluid. This could mean resizing coolers, modifying reservoir design to improve heat dissipation, or fixing components that aren't sized correctly, which can make the system more efficient than originally planned.

Make use of varnish-specific filtration

In addition to standard particulate filters, specially designed varnish removal systems employ ion exchange resins or other specialized media to draw sub-micron and soluble varnish precursors from circulation prior to allowing them to plate onto the surface. They can be particularly useful as a remedy in systems that have high MPC readings. They are often used as kidney loop filtration systems without shutting down operations.

Minimize air entrainment

Sizing the reservoir correctly, not having to separate suction and return flow, having a sufficient enough space for the air escapes, as well as properly sized return lines all help lessen the turbulence and cavitation that cause micro-dieseling.

Control water pollution

Make sure you meet ISO purity and moisture requirements suitable for the system, make use of desiccant breathers for reservoirs, and take care to address any water intrusion sources such as leaky seals and condensation that occurs in moist conditions.

Make sure you choose the right fluid formula

Premium hydraulic fluids have strong, well-balanced additive packages and better oxidative stability (often determined by RPVOT/rotating pressure vessel tests for oxidation) and can withstand the formation of varnish longer than common fluids, even with a greater upfront cost. For systems that have an established history of varnish, changing the formulation of the fluid in conjunction with mechanical repairs addresses the root chemical issue, not just the symptoms.

Create an oil analysis routine that is proactive

Instead of adhering to fixed calendar-based intervals for oil changes or a schedule of maintenance that is driven by condition, routine MPC, RULER, and particle count tests detect degradation trends in time to take action in any way—whether it's kidney-loop filtering, partial fluid replacing, or even a complete cleaning of the system prior to when varnish leads to friction in the valve or leads to premature wear on components.

The final line

Varnish formation is a chemical issue with mechanical implications. It is caused by an interaction of oxidation, electrostatic discharge, heat, air entrainment, and additive depletion. It does not always show up when sticky valves or an unidentified performance issue causes the issue to be resolved. Systems that incorporate temperature control, appropriate filtering, management of contamination, and a condition-based analysis program detect the problem at the chemistry level and far before it turns into an issue with mechanical origins.

1. What is the difference between sludge and varnish within a hydraulic system?

Varnish is a firm, thin, adsorbent film that bakes on surfaces. It is difficult to remove. In contrast, the sludge forms a softer, looser layer that usually is found in areas with low flow, such as reservoir bottoms. Both are caused by degradation of fluids However, varnish can cause more serious operational problems due to the fact that it coats surfaces that are close to tolerance, like valve spools.

2. What operating temperature causes the formation of accelerated varnish?

The majority of hydraulic fluids start degrading more rapidly above 65-70 degrees Celsius (150-160°F), and oil life is reduced by a factor of halving each 10°C rise above that recommended temperature range. The presence of hot spots around pumps or in poorly constructed reservoirs can lead to degradation even if the overall temperature is normal.

3. Is it possible to remove varnish after it has been formed?

Yes, most of the time. Varnish removal systems that utilize ion-exchange resins or other specially designed adsorptive materials can be used in a kidney loop configuration to draw dissolved and soluble varnish precursors from the fluid over time, slowly decreasing existing deposits, without the complete shutdown of the system.

4. How often do you test hydraulic fluid to determine the possibility of varnish?

For high-pressure or critical systems, MPC and RULER testing every quarter. Tests are a standard for establishing a basis, and regular testing is when trends are showing increasing levels. Systems with lower criticality may be tested every two years, but the best time frame is determined by the operation temperature, duty cycle, and the type of fluid.

5. Can the switch to premium hydraulic fluids prevent varnish completely?

It is not possible for any fluid to be completely invulnerable to varnish formation; however, superior fluids with more powerful antioxidants and greater antioxidant stability are able to resist it longer than common fluids. Quality of fluid reduces the risk of forming varnish and prolongs the life of your service; however, it must be combined with appropriate filtering, temperature control, and management of contamination for optimal outcomes.