Varnish Elimination

Varnish Animation

Oil degradation products / varnish

Oil degradation products or "varnish" in hydraulic and lubricating oil units can lead to malfunctions and increased warehouse temperatures. The result is unplanned downtime and high costs. The oil degradation products can be like gels or resins or be present in the unit as solid residue - the color scale here ranges from gold-yellow to dark black.

The main cause for the formation of oil degradation products is oxidation, that is, the reaction of the oil with oxygen in the air. Furthermore, contaminants in the oil, regardless of whether they are solid, liquid or gas, accelerate oil aging. Here, the oil reacts either directly with the contaminants or the contaminants function as catalysts. Electrostatic discharges can crack the oil or additives in it. These cracked oil components react chemically to the larger molecules, which then deposit as oil sludge in the unit.

Oil sampling at room temperature with slight clouding

Consequences of varnish

There are numerous problems with oil degradation products in hydraulic and lubricating oils. They settle at different areas in the unit. For example, if deposits should collect at valves, the valves may fail completely. Since the oil degradation products deposit to a greater degree on cold surfaces, crusts can often build up in the cooler, which then prevent heat transfer. If the oil can no longer be cooled with the full power, the oil temperature in the unit will rise. In addition, oil degradation products will collect at areas which are under compressive stress. This phenomenon often means that warehouses are affected. In them, deposits roughen up the surfaces, thereby leading to increased wear.

In the past, the life expectancy of one oil filling, depending on use, was between 15 and 30 years. With modern hydraulic oils, it is considerably shorter; usually under ten years. The life expectancy of the filling nowadays depends more than ever on oil maintenance. Alongside the classic maintenance measures like the removal of particulate contamination using oil filters or dehydration by vacuum dehydration units, the removal of oil degradation products is receiving more and more focus.

Whether or not oil degradation products exist, and if they do, to what extent, can be determined in oil analyses. In general, they can be detected by measuring the contamination class using particle counters at different temperatures. If the oil is cold, it has low solubility for oil degradation products - they are then present in free form. If the oil is heated, the solubility will increase and they will dissolve.

An optical particle sensor also only detects free oil degradation products as small particles. Dissolved oil degradation products cannot be detected using an optical particle sensor. If the particle counter shows considerably more particulate contamination when it is cold compared to when it is warm, it can be assumed the oil contains oil degradation products.

MPC Test

In the case of turbine lubrication, the content of oil degradation products can be determined from the color of the oil residue. With the MPC test, oil will be drawn across a diaphragm, the color of the residue on the diaphragm analyzed and a non-dimensional derived: The MPC value.


Antioxidants like phenols and amines are additives which slow down the time deterioration of the oil. One cause of oil aging is radical substances which chemically attack the oil. Antioxidants react with the radical substances in the oil and render them harmless. In this reaction, the antioxidants consume themselves and their concentration in the oil sinks over time. Once all of the antioxidants have been used up, the oil aging will proceed very quickly and oil degradation products will swiftly start appearing in the oil. In this case, maintenance measures for removing oil degradation products are no longer efficient as too many oil degradation products come about for the time unit. For this reason, the VGB, in its VGB-S-416-00-2014-08-DE standard, recommends replacing the oil as soon as the residual content of antioxidants drops below 25% phenols and 40% amines. As long as the concentration of antioxidants is above this limit value, oil maintenance for removing oil degradation products is the most economical and safest method to keep the oil in good condition.

Removal of oil degradation products

With conventional hydraulic or lubricating oil filters, it is difficult to remove the oil degradation products from the oil to any great degree. Hydraulic and lubricating oil filters are designed in such a way as to remove particles from the oil. With the subsurface structure, the particles, depending on their size, are retained within the structure of the filter medium. Gel-like oil degradation products already stick to the surface of the filter element, thereby blocking the subsurface structure of the filter medium. In this case, the differential pressure increases quickly. In some cases, the life expectancy of the filter element will drop to just a few hours. For this reason, there are more suitable options applicable for the removal of oil degradation products from hydraulic or lubricating oil:

1.    Removing oil degradation products with ion exchangers

Ion exchangers have a similar structure to bypass filters and work 24 hours/day, 365 days/year. The separation of oil degradation products is done via the accretion of a specific type of resin. Once the adsorption capacity of the resin is exhausted, the number of oil degradation products in the oil rises again. If an increase in the quantity of oil degradation products is detectable in the oil analysis, the resin-filled elements will be replaced. With this, a zig-zag profile come about over time.

Due to the increasing fluctuations in the MPC value or the high fluctuations of the particle counts when cold as well as the high oil analysis requirements connected with this method, a different procedure for clearing the oil of degradation products is finding greater and greater usage.

2.    Removing oil degradation products by reducing solubility and subsequent filtration (VEU-F)

With this technology, the solubility of oil degradation products decreases with the cooling down of the oil. Subsequently, these can be separated through filtration. This method can be applied in all hydraulic and lubricating oil applications which are run on mineral oils. This type of purification functions in a similar way to offline filtration and works 24-hours/day, 365 days/year. The oil is cooled down to below 30 °C in a single run. There are various options available when it comes to cooling. The most efficient option is cooling with cooling water in a plate heat exchanger, but cooling with a cooling unit is also common practice.

This separating procedure has distinct advantages compared to the ion exchangers. Like with normal hydraulic filters, a required element change is detected by the rise in the differential pressure at the element. In contrast to this, complex oil analyses are required with ion exchangers in order to find out the optimum moment to change the element. Thanks to its greater efficiency, the "cooling & filtering" technology is asserting itself more and more on the market.


VarnishElimination Unit VEU-F

Sizes:60/1, 15/5, 30/10, 45/15, 60/20
Versions:stationary, mobile
Cooler / heat exchanger:plate heat exchanger, compressor cooler

gear pump, without pump

Preferred models with shorter delivery times available.

Further technical data can be found in the technical data sheet VEU-F under DOWNLOADS.