Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1412702
32 | September - October 2021 | www . machinerylubrication.com LUBRICANT SELECTION The varnish removal systems are recommended to be operated over a long period of time or to be installed perma- nently. e removal of varnish in a system can be sensitive to elevated moisture levels in the fluid and to the presence of high levels of metal wear particles. Several different technologies can be employed in a hydraulic system for removing varnish. Methods commonly used for the reduction or removal of varnish in a system are: • Fine filtration • Electrostatic purification • Adsorption by a disposable media • Chemical rinsing or f lushing of equipment e functional principle of offline filtration is that polymerized varnish in the range of 2 microns is removed by means of very fine filtration. An offline filtration system is installed on the hydraulic reservoir for continuous filtra- tion. Using a cooler or chiller prior to a filtration unit to cool the oil allows the varnish precursors to form and be filtered out. e fine filtration will remove the free varnish and is often combined with a purifier or water removal unit. Electrostatic dissipative (ESD) filter media was introduced to eliminate potential electrostatic charging problems in the filtration of hydrocarbon fluids. Extensive testing in controlled laboratory conditions and on operating equipment in industrial applications has shown this filtration media to eliminate filter damage and significantly lower charge generation compared with the typical glass-fiber filtration medium. Filter elements are made using a special mesh pack structure to reduce the charging of the fluid and are used in filter housings installed inline. The use of off line filtration and ESD filter media can result in higher operational safety since sparking, oil degradation and the formation of sludge are eliminated. Longer oil service intervals resulting from fine filtration of the oil can lead to significant cost savings through fewer oil changes, filter element changes and system breakdowns. Varnish mitigation units are designed to remove the soft contamination that are both in suspension and in solution. Typically, they are a simple design that is easy to use on a wide fluid temperature range. e functional principle of the varnish mitigation unit is to accumulate particles in the range of < 1 micron on an active surface of an ion exchanger. e advantage of accumulating even very fine particles is that the solubility of varnish in the oil is improved. As a result, the oil becomes "varnish-hungry," and soft oil aging products that have already been deposited on surfaces in the system are dissolved and absorbed by the oil. e varnish mitigation units don't have any impact on removing any hardened oil aging contaminates that have formed on the surface. e advantage of a varnish mitigation unit is that the structure of the ion exchanger provides a huge separating surface, and the operating costs of the system are reduced. The chemica l cleaning/f lushing method is used for removing varnish utilizing cleaning chemicals that are typically circulated through the system to dislodge varnish from components. e chemicals are added to the hydraulic oil to soften and dissolve the insoluble materials. e flushing action suspends the hard deposits in the fluid, which are then removed with the fluid when it is drained from the system. is process is usually performed for several hours or several days, depending on the system size and the extent of the varnish build-up on components. e chemical cleaning method will require the system to be flushed with clean oil to remove any traces of the chemicals and then refilled with new oil before using. Conclusion and Summary Varnish is soft particles in the range of < 0.1 microns that form conglomerates and cause gel-like to hard lacquer-like deposits. e change in base oil has contributed to the increase of varnish formation in the oils. e formation of deposits is increased if the temperature drops and deposits are formed at "cold" points in the system or if the pressure increases. In general, it is harder for more highly refined oils to keep varnish in solution. Identifying these oils is difficult; it is not immediately apparent from the oil data sheet. e consequences of varnish deposits are not usually traced to the altered base oil properties. Expensive and usually ineffective measures are taken, such as performing mechanical cleaning or replacing the oil. The formation of varnish can be reduced through oil care. Filtration, dewatering, air removal, and degassing extend the oil lifetime. If there is a varnish problem, a varnish mitigation unit can help to remove "free" and "dissolved" varnish and reduce deposits in the system by improving the dissolving behavior in the oil. ML References HYDAC International, Sulzbach, Germany HYDAC Technology Corporation, Bethlehem, PA, USA Schroeder Indus- tries, Leetsdale, PA, USA Varnish potential rating