Klaus Defren of Pall GmbH reported on a new varnish removal method at the 18th International Colloquium Tribology at Esslingen in January. The technique uses a fibrous medium that removes and retains the varnish-forming material. The filter technology has been shown effective in both removing varnish precursors from the oil and also cleaning up the loosely adhered varnish deposits in turbine lubrication systems.

A Sticky Problem

Varnish deposits in turbine lube systems cause a number of problems, especially the restriction and sticking of moving mechanical parts such as servo- or directional-control valves. Defren related that typical varnish removal methods include chemical cleaning and flushing, electrostatic charge-induced agglomeration, and adsorption by an adsorbent medium.

Palls new technique is an absorption method that uses a fibrous filter with an affinity for varnish-forming materials in the oil. The filtration medium is a composite cellulose with specially formulated, temperature-cured binder resins Defren said. In testing on operating turbines, it reduced varnish levels from the critical range to below normal range in a relatively short time.

Defren, who is based at Palls office in Dreieich, Germany, explained that the solvency of varnish in turbine oil is temperature dependent, with the transition point being in the range of 54 to 57 degrees C. Temperatures frequently fall below this level in the hydraulic section, allowing varnish to deposit on valve components. Gas turbines in peaking service are known to have developed varnish deposits within two and a half years after commissioning, Defren said.

The biggest problem with varnish contamination is that the material plates out on servovalve surfaces, leading to valve sticking. It also plugs the last-chance filters that are part of the servovalve assembly. Precision servovalves have operating clearances in the 2- to 5-micron range, Defren said. So it takes only a thin layer a few microns thick to clog the clearances and cause jamming.

In addition, the last-chance filters are typically made of sintered metal or fine screens. They provide a convenient surface for varnish formation because they are located in the low-flow, colder hydraulic control section, Defren explained. Lower temperature promotes varnish formation because the material is less soluble, causing it to come out of solution and deposit on the filter.

Filters made with glass fiber, typically used for full-flow filtration of the lubricant, normally are not plugged by varnish. Full-flow filters as fine as 6 microns are known to have no varnish-related premature plugging, even when the fluid had elevated levels of varnish forming material, said Defren.

According to Defren, the plugging of metallic filters but not glass-fiber filters is likely due to a number of factors, including:

the difference between the interaction of the varnish material with metal versus glass fiber;

lower temperatures in the hydraulic section;

reduced flow in that section resulting in longer contact time and lower drag forces.

In addition to servovalve and filter deposits, Defren said that varnish forms on mechanical seals, Babbitt sleeve bearings, thrust-bearing pads, orifices and the walls of heat exchangers and reservoirs. The latter deposits reduce heat transfer and result in higher operating temperatures.

Removal Strategies

Defren explained that currently available solutions for varnish removal from turbine lubricants can be divided into three categories: electro-static purification; chemical cleaning and flushing; and adsorption by disposable media.

The electrostatic method operates in a kidney loop off the main tank. It subjects the oil to an electrical field, causing varnish precursors to agglomerate into larger particles. These particles are captured by a filter mat or attracted to a charged, disposable surface, he said.

Electrostatic devices first remove varnish precursors from the fluid phase. Then, as the fluid cleans up, soft varnish is removed from system components. Because the removal rate is relatively slow, Defren said, electrostatic devices must operate over an extended time period, or be installed permanently. Whats more, electrostatic devices are sensitive to elevated moisture and to high levels of metallic wear particles.

Chemical cleaning and flushing begins with shutting down the system and draining the lubricant. Next the system is flushed to dislodge varnish from components. These chemicals soften and remove insoluble materials, Defren explained, and the flushing action suspends the deposits in the fluid, which are then removed by fine filtration. This process usually runs for several hours to several days, depending on the size of the system and the extent of the varnish build-up. Downtime is the disadvantage of this method.

Once the chemical and flush treatment is complete, the system must be flushed again with an appropriate fluid to remove all residual chemicals and to ensure no contamination finds its way into the new lubricating oil. Although this process is more intensive, it provides for quicker removal of varnish versus the other methods, said Defren, especially in a large system.

The adsorption method uses adsorbents with large surface areas and high void volume, relatively low fluid flux and, in some cases, adsorbents with electrochemical affinity for varnish precursors. Many materials can be used as adsorbents, Defren said, including compressed cellulose, cotton liners, and macroporous media such as resin beads, fullers earth and activated carbon.

Finding a Filter Fix

Pall says that its new method can avoid the creation of varnish by removing its precursors, although it does not remove varnish that already exists. The method is based on physical adsorption, a process in which an adsorbent material attracts varnish molecules through weak molecular forces such as van der Waals forces and hydrogen bonding. He explained that Pall employs a filter made of a cellulose fiber matrix and other materials. The composite has an open structure and a high void volume.

The open fiber matrix is highly permeable, allowing the fluid to come in contact with a large surface area of fiber that absorbs the varnish precursors. Resins bonded to the fiber give the filter medium high affinity for the polar varnish precursors, resulting in high removal efficiency and retention of the material suspended in the fluid phase, said Defren. The system includes a cooler designed to keep the oil below the temperature at which the precursors to varnish dissolve.

Pall tested the filter using samples of degraded fluid from operating turbines that had reported high levels of varnish and related problems. The test entailed circulating the fluid sample through the filter at ambient temperature, except for one test conducted at 70 degrees C. The accompanying table shows varnish ratings of the samples before and after filtration. Ratings reflect a variety of conditions that can affect varnish formation. The higher varnish rating of the sample at 70 degrees C indicates lower varnish removal performance. This is likely due to the higher solvency of the varnish precursors in the fluid at elevated temperatures, Defren said.

Tests were also conducted to assess the filters effect on additives in the fluid. The results of one such test on fresh fluid indicate essentially no change in the levels of two antioxidant additives after the sample was filtered 20 times. The absence of depletion suggests that the filter has no noticeable adverse effect on the fluid.

Following laboratory validation, the varnish removal filter was tested on two operating turbines. The filtration system was temporarily installed in a kidney loop that took fluid from one end of the reservoir and returned it to the opposite end continuously. A forced air cooler maintained fluid temperature at the level necessary to optimize filter efficiency.

Both turbines showed a high tendency to produce varnish, but they differed significantly in the level of actual varnish deposits in their lubrications systems. One, an Alstom turbine, showed a heavy brownish coating of varnish inside the housing of the main flow filter. No such deposits were observed in the other machine, a GE Frame 7FA turbine.

After filtration, the operators of the Alstom turbine reinstalled an electrostatic cleaner used before treatment. An oil sample taken from this turbine about six months after the varnish removal treatment had elevated varnish levels. In contrast, a sample from the GE Frame 7FA had low varnish levels similar to those recorded at the time treatment was terminated.

The two field trials indicate that the amount and type of deposits in the system has a bearing on how quickly the varnish can be cleaned up and how long it will remain free of varnish, said Defren. Other variables such as the type of varnish material, temperature, duty cycle, fluid type and deposit state (soft or hardened) can also influence the rate of varnish removal and the dissolving of deposits back into the fluid.