Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/120548
COVER STORY A temperature limit of oil saturation could become an important criterion. This could be measured by the patch test but at lower limits than those presently used in order to avoid machinery issues. This approach could then retain the present criteria as a higher severity warning for varnish and separate criteria for its potential challenge to machine operation. Mitigation Since oil that is in operation at a fully saturated level can be expected to leave varnish deposits, mitigation strategies to manage this condition should be directed at keeping varnish levels at a concentration where deposits would not be expected to form. The following are viable preventive maintenance strategies that have been demonstrated to benefit the system oil condition or varnish saturation level: While this approach could be beneficial if implemented in a manner that keeps the oil in a less than saturated condition, managing this would require frequent laboratory tests to ensure the oil condition. It is also questionable whether oil consumers would be sufficiently sensitive to the rate of varnish accumulation or change in the new oil to properly implement this method. However, it would produce a net benefit if performed periodically, as the cleanliness of the system in terms of existing varnish deposits can be improved with time. Filtration with Cellulose Media The best time to filter varnish material retained within oil is when the oil is at an ambient temperature for a few days. This allows the varnish to agglomerate and be collected by the filter material. Filter replacement is a must if this strategy is employed, as varnish material removed from the oil at ambient temperature returns to solution if the filter is not replaced prior to returning the oil to the higher operating temperature. This is due to the greater solvency of the oil at the higher operating temperature. The primary limitation of this approach is that the filtration is limited to the reservoir oil and cannot be expected to reduce the volume of varnish within the main turbine system. When the oil returns to service, its inherent design is as a solvent, thus the results are removal of system varnish. Once again, due to existing system deposits, a saturation of the operating oil can reoccur. In addition, varnish can migrate from a less harmful area to an undeFigure 5. The MPC test measurement of the sample on the left was 24, sirable location. The overall cost of while the sample on the right measured 156. this strategy can also be expensive, as (Courtesy Dave Wooton, Wooton Consulting) frequent filter replacements could be required to remove system varnish from the varnish-laden filters. Partial or Full Oil Changes As new oil does not have any retained varnish products, it would However, this method does produce a net benefit when performed not be expected to cause new deposits when added to the system. periodically, and the cleanliness of the system in terms of varnish However, the benefit of this method is severely limited by the quan- deposits can be improved. tity of varnish deposits within the system. When new (or newly cleaned) oil is placed in service, its inherent design results in System Chemical Cleaning This method is the quickest way to improve system cleanliness cleaning the existing system varnish, which then goes into the oil. After this has occurred, the oil can again become saturated, and and allow the oil to function as it was designed. While a clean new varnish deposits can commence once saturation levels have system will extend the useful service of the new replacement oil, it been reached. cannot be expected to prevent reoccurrence of varnish. Its cost to A secondary problem can develop from this mitigation strategy implement can also be high. if varnish is removed from a less harmful area and then re-deposited in more undesirable locations. Another drawback in making Ion Filtration With ion filtration, oil is processed through resin beads, which frequent oil additions including full oil replacement involves the chemically attract the varnish component and remove it from the expense, as large volumes of oil can be costly. 18 | March - April 2013 | www.machinerylubrication.com