Removing varnish in hydraulic oils efficiently

The term "varnish" refers to thin, hard, glossy and oil-insoluble deposits from organic residues. It is an oil degradation product which appears in different forms. Varnish can sometimes resemble a coat of paint on tank walls and piping, while other times it can also appear in the form of sludge adhering to tank floors, valves and pumps. The colour of varnish ranges from black to brown to golden yellow.

• Oxidation, in other words, the reaction with oxygen, is the main cause of oil degradation. This process leads to the formation of oil degradation products.
• Contaminants in oil also lead to the formation of oil degradation products. Whether it's a solid, liquid or a gas – contaminants react with oil or work act as a catalyst.
• High temperatures and pressure in the system accelerate the degradation processes. In the case of low-viscosity oils, electrostatic discharges (or the high temperatures caused by discharge flashes) can crack the oil or additives in the oil.

• Varnish accumulates in lubrication gaps, among other places. The lubrication gap becomes narrower;friction and temperature increase leading to increased wear on bearings.
• If varnish accumulates in valves, this can lead to malfunctions.
• Varnish that accumulates on cooler walls inhibits heat transfer there,reducing the cooler's efficiency.
• Varnish can also accumulate in filter elements, leading to shorter service lives and more frequent element changes.

Oils have a degree of solubility for varnish, similar to that for water. If the solubility limit is exceeded, the varnish precipitates out of solution and becomes free. Dissolved varnish can't be filtered. Free varnish, on the other hand, can under certain circumstances form deposits on the filter element. This relatively quickly leads to a standard filter element becoming blocked, a filter that's designed to separate particulate contamination.

In general, the new oils in API Group II or III are more resistant to degradation than the old oils in API Group I. These new degradation-resistant oils are very non-polar, while the varnish that accumulates is very polar. As soon as these oils age, the varnish immediately precipitates out of the solution and causes disruptions in the system as polar and non-polar substances do not mix.

While it is true that the old oils in API Group I degrade more rapidly, they do exhibit a degree of polarity due to the chemical contaminants in the oil (e.g. caused by sulphur or aromatic compounds). These oils therefore produce more oil degradation products, although larger quantities also remain in solution and cause no further disturbance.

1. Determining the MPC value
   The varnish content is usually determined on the basis of the colour of the oil. Oil is drawn through a membrane for this purpose and the residue is characterised on the basis of the colour. A dimensionless number is calculated from the yellow and red content and the intensity: the MPC value (Membrane Patch Colorimetry). If the MPC value increases to more than 30, there is an acute risk of a varnish deposit.
    
2. Comparison of cleanliness classes
   A further method for determining the presence of varnish in oil is to compare the cleanliness classes of an oil sample at different temperatures. For this purpose, the cleanliness class is determined at an oil temperature of 20°C and 80°C with the aid of a visual sensor.
   If the class determined at 20°C is considerably worse than the one for 80°C, varnish contamination can be assumed. This is because a majority of the varnish is in solution at 80°C and will not be detected by visual particle sensors. Varnish becomes free in the same oil sample at 20°C and is recorded using the particle sensors.

The HYDAC VEU-F is installed in the tank bypass flow. First, the oil temperature is lowered. This results in a reduction in the solubility for the polar oil degradation products in the oil and they precipitate from the solution. After this,the oil degradation products are removed from the oil with a polar filter element. The filter elements must be replaced when the differential pressure rises above 2 bar.

If varnish is cleaned away until its content is considerably lower than the solubility limit in the oil, the oil will have a tendency to absorb additional varnish. A portion of the varnish on tank walls and valves could dissolve again in such cases under certain circumstances.