Removing varnish in hydraulic oils efficiently

In the last few years, there have been changes to base oils. These changes have led to base oils which are more thermally stable and chemically pure. While in the past the degradation of lubrication oils and hydraulic oils led straight to oil carbon, today there are precursors. These are mostly gel-like substances which lead to brown deposits in "cold" parts of the system. Valves, cylinders and bearings are affected by this. The consequences are higher valve positioning forces, leaking check valves, seal wear on cylinders and increased bearing temperatures in turbine bearings.

That's why there's an increasing focus on the removal of oil degradation products, know as "varnish". We'll explain how you can use oil analysis to detect and effectively combat varnish and avoid unscheduled downtimes caused by oil sludge.

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Varnish in hydraulic systems ­& lubrication systems

System protection: why are deposits a problem?

Oil degradation products in hydraulic systems and lubrication systems can lead to malfunctions and increased bearing temperatures. The result is unscheduled downtime and high costs. Gel-like, resinous or solid oil degradation products are present in the system as residues. They accumulate at different points in the system where deposits on valves lead to increased wear and complete failure. Electrostatic discharges accelerate the oil degradation by cracking the oil. The resulting products react chemically to form larger molecules. This oil sludge leads to noticeable system contamination.

Oil analysis: how do I detect varnish in hydraulic oil?

To detect oil degradation products, oil analyses need to be performed. Visual particle sensors, MPC tests and the concentration of antioxidants give an indication of the extent of oil degradation. The main cause for the formation of varnish is oxidation – the reaction between the oil and the oxygen in the air. Furthermore, contaminants in lubrication oil and hydraulic oil accelerate oil degradation– regardless of whether they are solid, liquid or gas. As an expert in fluid conditioning and oil conditioning, HYDAC helps you to control varnish effectively and ensure sustainable system protection.

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Our solution: HYDAC Varnish EliminationEfficient system protection with the right separation method

Long service lives, low service costs and maintenance costs and high system availability are goals for every system operator. The aim is to minimise varnish and remove it without leaving any residue. Conventional lubrication oil filters and hydraulic oil filters are not particularly well suited to this task. This is because gel-like oil degradation products stick to the surface of the filter medium, blocking its depth structure. Due to the rapidly increasing differential pressure, the filter element service life drops to just a few hours in some cases.

The successful removal of varnish and the associated oil sludge in hydraulic oil can be achieved via a separation process with ion exchangers as well as by reducing the solubility and following this with filtration.

HYDAC Ion eXchange Unit IXU Removing varnish with ion exchangers

Efficiently remove varnish from phosphate-ester (HFD-R) and polyolester (HFD-U) based fire resistant hydraulic fluids and lubrication fluids with the HYDAC ion exchanger unit.

Our ion exchanger is constructed in a similar way to a bypass filter and works 24/7. The varnish is separated via accumulation onto a specific resin. This enables the varnish to be eliminated reliably. Increase your system's productivity – we're happy to help you.

More information on the HYDAC IXU

HYDAC VarnishElimination Units VEU-F Removing varnish with cooling & filtration

For the removal of solid or gel-like oil degradation products: the service-friendly HYDAC VarnishElimination Units VEU are used to condition mineral oils and effectively remove varnish. With this technology, the solubility of the contamination decreases as the oil cools down.

This method is suitable for all lubrication applications and hydraulic oil applications which are operated with mineral oils. Compared to ion exchangers, element changes are possible without complex oil analyses; the higher cost-effectiveness is making the "cooling & filtering" technology increasingly lucrative. HYDAC makes your system protection simple and cost effective.

More information on the HYDAC VEU-F

HYDAC VarnishElimination Unit VEU-F-5/3 Removing varnish from small tanks

The easy to service VarnishElimination Unit VEU-F-5/3 effectively removes oil degradation products from mineral oils. With this new unit, HYDAC has just the right solution for system tanks <1000 litres.
Thanks to the special two-stage filter element, oil degradation products can be removed from your oil even without cooling.

The VEU is installed in a matter of minutes with the tank connection kit and is ready for operation immediately. Extend the service life of your fluids – we are happy to help.

More information on the HYDAC VEU-F

HYDAC is your partner for sophisticated fluid conditioning solutions

System availability

Increased system availability

Reduce your system failures and increase your system availability with clever varnish removal.

Conservation of resources

Conservation of resources

Extend the service life of your hydraulic oils and fluids and save valuable resources.

Reduced costs

Cost optimisation

Thanks to reduced downtime, extended oil change intervals and increased component service lives, you can optimise your costs in a sustainable way.

Increased productivity

Productivity increase

With efficient fluid conditioning, you can enhance your machine performance and actively protect your system.


What is varnish?

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.

How do oil degradation products such as varnish come about?

  • 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.


Why is varnish undesirable?

  • 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.

Can oil degradation products be removed from oil using a filter?

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.

Are there oils that don't age?

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.

What is the limit value for oil degradation products in oil?

  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.


How does the HYDAC VarnishElimination Unit VEU-F work?

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.

Is varnish also removed from tank walls and valves using the HYDAC VEU-F?

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.

Downloads Find more information on our solutions in brochures and data sheets – with our solutions, there's nothing standing in the way of efficient removal of oil degradation products in hydraulic oil.

Let's protect your systems from varnish

Do you want to know how you can best protect your specific system? Discuss your individual requirements with us in a consultation – we'll work with you to find your tailor-made, cost-efficient solution. We look forward to working with you!

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