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Varnish in turbine oils

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The condition of the oil in lubrication and hydraulic systems is indicative of the health of the entire system. To ensure productivity, avoiding malfunctions and reducing operating costs, we need to monitor the fluid condition and operate a continuous maintenance of operating equipment.

Varnish refers to oil ageing products that form deposits in the fluid system which have a gel/resin-like consistency or resemble solid varnish. These oil ageing products are readily deposited on cool surfaces such as the tank, valve housings or coolers. This causes an increase in bearing temperatures, malfunctions in hydraulic valves and cooling problems. In most cases, however, the above malfunctions are not correctly attributed to the real cause. This results in ineffective, and often very expensive, repair work.

The base oils generally used today for the production of turbine oils have changed in recent years. They reflect the increased demands with regard to higher turbine efficiency, higher bearing temperatures and reduced levels of hazardous substances. Whereas oils used to be produced exclusively by the distillation of crude oil (ASTM Group I oils), for some years now further refinement processes have been employed.

But after 3 or 4 years of operation, and 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. This can lead to sparking in the system. In addition to damaging the filter elements, it can cause failures in measurement and control systems, and deflagrations may occur in return lines or in the tank.

The Varnish Elimination Unit (VEU) produced by HYDAC is an efficient solution for cleaning the varnish in the oil.

This system is similar in construction to an offline filter and works 24h per day, 365 days per year. The service-friendly Varnish Elimination Units VEU prepare and refurbish damaged mineral oils. The removal of varnish is based on reducing the oil solubility for varnish with subsequent filtration.


  • Removal of solid or gel-type oil ageing products
  • Operating reliability of the system is increased because there are fewer deposits in hydraulic valves
  • Increase in the oil service life
  • Available as a complete unit for retrofitting to existing systems, as well as a modular system for new systems

For more information, click here

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