Resin-like or solid coating deposits that form in a fluid system are often referred to as varnish. The cause of these varnish deposits is related to oil aging or oil degradation, which can be determined by lab testing the oil. Reducing varnish in a hydraulic system is accomplished with oil care measures, including dewatering and degassing the fluid and avoiding electrostatic discharge and elevated oil temperatures.

To care for hydraulic systems appropriately, it’s important to know:

• Why varnish builds up.
• How to determine if varnish is the primary issue in a system.
• Current strategies to remove and reduce varnish deposits.
• Field applications for varnish removal systems.

The condition of oil in lubrication and hydraulic systems indicates the health of the entire system. For ensuring productivity, avoiding malfunctions, and reducing operating costs, it is essential to:

• Fluid condition monitoring.
• Timely maintenance for lubrication and contamination control.

What Is Varnish?

In the context of lubricating and hydraulic systems, varnish refers to resin-like or solid varnish-like deposits that form in fluid systems.

Varnish causes:

• Elevated temperature in turbine bearings due to increased friction in the bearing.
• Operational control problems with the control valve due to increasing deposits in the gap between the piston and the housing.
• Cooling problems because the varnish impedes the heat transfer.
• Shorter filter change intervals.

The cause of these lacquer-like varnish deposits is oil aging. However, in most cases, the above malfunctions are not correctly attributed to the real cause. This results in ineffective and often very expensive repair work.

Causes of Increasing Varnish Formation

Today, system operators are confronted with the fact that the production of the base oils used is changing or has already been changed. Previously, oil was produced exclusively in crude oil distillation (Group I oils). Now, processes reduce the content of substances hazardous to health (e.g., aromatics). The change in the oil production process has resulted in base oils with a lower content of unsaturated/polar hydrocarbons (Groups II, II+, and III). With Group I base oils being polar, polar substances were more easily dissolved.

The changes in oil production have led to oil with less polarity; therefore, there is a reduction in the amount of polar substances being dissolved in a hydraulic system’s oil. Varnish is a polar substance that no longer easily dissolves in newer base stocks.

Polar substances tend to dissolve more readily in other polar substances. If the proportion of polar hydrocarbons in the oil is reduced, oil degradation products – often includes varnish – cannot dissolve as easily. The effect is oil turbidity or deposits in the system.

The Effects of Increasing Varnish

These changes usually start once the oil has been operating for a few years. As soon as the solubility limit for varnish is exceeded, it precipitates and forms conglomerates, which lead to deposits in the system. Varnish is not heavier than oil; as a result, it deposits on metal surfaces and at the system's colder locations (tank, cooler, valve body) – not on the tank bottom.

Due to the low proportion of polar substances, these oils also have low electrical conductivity. If this oil flows through the filters in the hydraulic system, an electrostatic charge can be generated. Electrostatic discharge (ESD) occurs in turbine lubrication systems from friction between the fluid and the system components. An indication of ESD is an audible clicking sound as the accumulated charge discharges, causing sparking within the system.

Less apparent effects involve movement of the electrical charge downstream of the filter, which damages system components and the filter. The amount of charge generated by oil flowing through a filter is related to several fluid and filter properties. Charge generation/accumulation generally increases with increasing flow rates (velocity through the filter element). Reduced fluid conductivity, certain additive packages, and lower temperatures (higher oil viscosity) can also lead to increased electrostatic charge in the oil.

The subject of oil degradation is not new by any means; in fact, it has always been an issue. The characteristics of oils have changed due to the introduction of more highly refined base oils. In the past, oils had a life of up to 20 years. Today, the service life of modern oils has been reduced while facing increased demands. Most oils have a service life of less than 10 years. The hydraulic oils are expected to handle higher temperatures, be more efficient, and have reduced levels of hazardous components.

This means that fluid monitoring and conditioning are becoming increasingly important.

Today's technical data sheets for oils do not always provide information on the base oil used. Since oil names are often not changed when the oil type is changed, this may mean old oils are inadvertently mixed with new oils during refilling. Such mixing can result in chemical reactions, which, under certain circumstances, may lead to the precipitation of reaction products and deposits within the system.

How to Recognize Varnish

Varnish in oil cannot be detected through routine lab analysis. Instead, labs utilize Membrane Patch Colorimetry (MPC) testing. Varnish can be seen as a dark precipitate on a filter membrane, and MPC tests record these changes in color on filter membranes with a 0.45µm filtration rating. Critical system conditions occur if the MPC value is over 40.

Laboratory particle counting will differ significantly from online measurements that use a portable particle counter; for example, a laboratory measurement may give 24/23/17 while an online measurement will be 16/14/10. This is because the solubility of the varnish in the oil depends highly on temperature.

When the oil sample is cooled down, the solubility limit in the oil is exceeded. This is mainly counted in the ISO code's 4- and 6-micron ranges, resulting in a significant difference compared to the 14-micron code. Usually, there is a difference of up to 4 codes between 6 and 14 microns. As soon as varnish is present, this difference can be more than five codes in the laboratory particle count at room temperature. This exceedance of the solubility limit is reversible; when the temperature increases, the varnish goes back into the solution.

How to Reduce Varnish Formation

An oil’s degradation rate can be reduced with oil care measures. These include:

• Offline Filtration: Limits the growth in particle size.
• Dewatering: Minimizes additive reduction due to leaching.
• Degassing: Reduces contact with air and, therefore, with oxygen.
• Avoid electrostatic discharge in the oil and avoid premature degradation due to local temperature spikes and hot spots.
• Monitor the oil temperature balance to detect elevated friction in the bearing or cooler overgrowth early on.
• Regular oil analyses and trend the remaining useful life (RUL) help to avoid critical system states such as jammed control valves for the steam control of a turbine and unsafe machine operation.

Systems Used to Reduce the Effects of Varnish

The removal of varnish from system components is a relatively slow process. Oil degradation products are initially individual particles less than 0.1 microns in size. As a result, they can pass through the filter initially without impairing the valve function. During further oil aging or when the oil is cooled down (e.g., during a system shutdown), these particles agglomerate, become larger, and block the valve function and the filter medium.

It is recommended that varnish removal systems (usually affixed as offline filtration skids) are operated over a long period of time or installed permanently. The system varnish removal process can be sensitive to elevated moisture levels in the fluid and the presence of high levels of metal wear particles.

Several technologies and methods can be employed in a hydraulic system to reduce or remove varnish, including:

• Fine filtration
• Electrostatic purification
• Adsorption by disposable media
• Chemical rinsing or flushing of equipment

The functional principle of offline filtration is that polymerized varnish in the range of 2 microns is removed using very fine filtration. An offline filtration system is installed on the hydraulic reservoir for continuous filtration. Cooling the oil with a cooler or chiller prior to a filtration unit allows the varnish precursors to form and be filtered out. The fine filtration will remove the free varnish and is often combined with a purifier or water removal unit.

ESD filter media was introduced to eliminate potential electrostatic charging problems with hydrocarbon fluid filtration. Extensive testing in controlled laboratory conditions and on operating equipment in industrial applications has shown this filtration media significantly reduces damage and the electrostatic charge generation compared to a typical glass-fiber filtration medium. Filter elements are made using a special mesh pack structure to reduce fluid electrostatic charge and are deployed in filter housings.

The use of offline filtration and ESD filter media can result in higher operational safety since sparking, oil degradation, and sludge formation are eliminated. Longer oil service intervals resulting from fine oil filtration can lead to significant cost savings through fewer oil changes, filter element changes, and system breakdowns.

Varnish mitigation units are designed to remove the soft contamination both in suspension and solution. They have a simple design that is easy to use in a wide fluid temperature range. The functional principle of the varnish mitigation unit is to accumulate particles in the range of < 1 micron on the active surface of an ion exchanger.

Accumulating very fine particles improves the solubility of varnish in the oil; the oil becomes "varnish-hungry," and soft oil degradation products already deposited on the surfaces in the system are dissolved back into the oil. The varnish mitigation units don't impact the removal of any hardened oil aging contaminates that have formed on the surface. The advantage of a varnish mitigation unit is that the structure of the ion exchanger provides a large separating surface while reducing the system's operating costs.

The chemical cleaning/flushing method removes varnish by utilizing cleaning chemicals circulated through the system to dislodge varnish from components. The chemicals are added to the hydraulic oil to soften and dissolve the insoluble materials. Then the flushing action suspends the hard deposits in the fluid, which are removed when the fluid is drained. This process is usually performed for several hours or days, depending on the system size and the extent of the varnish build-up. Finally, the system must be flushed with clean oil to remove any traces of the chemicals and then refilled with new oil before use.

Conclusion

Varnish is soft particles of < 0.1 microns that form conglomerates and cause resin-like or solid coating deposits to hard lacquer-like deposits. The change in the production of base oils has contributed to increased varnish formation in the oils due to changes in solubility. However, oil care can reduce varnish formation. Filtration, dewatering, air removal, and degassing extend the oil's lifetime. If there is a varnish problem, a varnish mitigation unit can help remove "free" and "dissolved" varnish and reduce deposits in the system by improving the oil's dissolving behavior.

This paper was provided as supporting materials for Kristine Mikulan’s speaking session at the Reliable Plant Conference. To learn more about attending Reliable Plant Conference, click here.