Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/120548
COVER STORY and additive degradation. As such, operating with varnish within the oil allows new varnish to form more quickly and shortens the service life of the oil. Complications/Variations Figure 2. Varnish can occur in any system. (Courtesy Paul Sly, Chevron) design property that allows it to hold and accumulate varnish. In addition, existing system sludge and varnish that have adhered to surfaces within a turbine oil system are not readily dislodged with a system flush. As a result, when new, clean oil encounters this existing varnish within a system, it begins to chemically react with the varnish and remove this material from the walls, causing the new oil to approach a point of saturation. Within a relatively short period of time, this "chemical cleaning" phenomenon can render the benefits of a new oil change moot in terms of oil performance and design. In other words, the removal of system varnish may reduce the new oil to a "failed oil" status not long after it is placed in service. The same condition occurs after a varnish-saturated oil is cleaned of varnish residue, but the cleaning is stopped prior to the system itself becoming clean. The newly cleaned oil will again accumulate varnish materials from the system to once again approach saturation levels. Remember, varnish acts as a catalyst to speed oil The consequences of accumulated varnish within a turbine system can include power de-rating and damage to the operating equipment. When observed problems begin to occur, the concentration of varnish within the oil or system can vary greatly. Factors such as system design, variations in the system operating temperature and fluctuations in the system operating conditions all affect varnish formation. Systems that use turbine oil as control oil are highly susceptible to issues. The control systems include tight orifices located in lower temperature regions. These conditions allow hot, saturated oil to accumulate deposits at these important system locations. The choice of oil also impacts the rate of varnish generation and accumulation, as some oils are more prone to varnish dropout than others. Figure 4 shows two samples with similar Figure 4. Different oil formulations can have very different varnish conditions but appear highly similar. (Courtesy Dave Wooton, Wooton Consulting) Figure 3. Varnish deposits were found within this system. 16 | March - April 2013 | www.machinerylubrication.com laboratory settling times. While these oils have a highly similar visual appearance, they have dramatically differing levels of varnish load, which is related to the influence of oil additives, base oil and the chosen formulation. Oils from different manufacturers are known to have different additives and base oils. Depending on the in-service operations, their differences can influence the severity of a developing varnish issue. Because these oil design variations are considered proprietary information by the lubricant manufacturers, the consumer of the oil is unlikely to be able to determine which oil formulation is least likely to cause varnish problems. When system contaminants mix with the oil in service and further degrade it, the design properties such as water separation,