Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1412702
ML www.machinerylubrication.com | September - October 2021 | 31 ML "varnish") cannot dissolve as easily. e effect is oil turbidity or deposits in the system. ese changes usually start once the oil has been in operation for 3 to 4 years. As soon as the solubility limit for varnish is exceeded, it precipitates and forms conglomerates, which lead to deposits in the system. Varnish is not heavier than oil and deposits on metal surfaces and at colder locations (tank, cooler, valve body) in the system — not on the tank bottom! 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. Electrostatic discharge (ESD) occurs in turbine lubrication systems as a result of friction between the fluid and the system components. An indication of ESD is a clearly audible clicking sound as the accumulated charge discharges, causing sparking internally within the system. Less apparent effects involve movement of the electrical charge down- stream of the filter, which produces damage to system components and the filter. e amount of charge generated by the flow of oil through a filter is related to several fluid and filter properties. e charge generation/accu- mulation generally increases with increasing flow rates (velocity through the filter element). Reduced fluid conductivity, certain additive packages and lower temperature (higher oil viscosity) can also lead to increased elec- trostatic charge in the oil. e subject of oil aging is not new by any means; in fact, it has always been an issue. e characteristics of oils have changed due to the intro- duction of more highly refined base oils. e hydraulic oils are expected to handle higher temperatures, be more efficient and have reduced levels of hazardous components. is means that fluid monitoring and fluid conditioning are becoming more and more important. Today's technical data sheets for oils do not give information on the base oil used. Since oil names are often not changed when the oil type is changed, it may mean the old oils are inadvertently mixed with new, more modern oils during refilling. Such mixing can result in chemical reactions, which, under certain circumstances, may lead to the precipitation of reaction products and to deposits within the system. How Do I Recognize Varnish? e presence of varnish in oil cannot be detected through routine lab analysis. In laboratory reports, varnish is found as a dark precipi- tate on a filter membrane. is testing is called an MPC (Membrane Patch Colorimetry) which records the changes in the color of a filter membrane with 0.45µm filtration rating. Critical system conditions occur if the MPC value is over 40. e effect is more deposits occurring in the system. Laboratory particle counting will differ greatly from online measurement using a portable particle counter; for example, a labo- ratory measurement may give 24/23/17, while an online measurement will be 16/14/10. e reason for this is that the solubility of varnish in the oil depends highly on temperature. When the oil sample is cooled down, the solubility limit in the oil is exceeded. is is mostly counted in the 4- and 6-micron ranges of the ISO code and results in a large difference when compared to the 14-micron code. Normally, there is a difference of up to four codes between 6 and 14 microns. As soon as varnish is present, this difference can be more than five codes in the laboratory particle count at room temperature. is exceedance of the solubility limit is reversible; when the temperature is increased, the varnish goes back into solution. How Do I Reduce the Formation of Varnish? e aging rate of the oil can be reduced through oil care measures. ese include: • Offline filtration: limits the growth in particle size. • Dewatering: minimizes additive reduction due to leaching. • Degassing: reduces contact with air and therefore with oxygen. • Avoid electrostatic discharge in the oil and avoid aging due to local temperature spikes and hot spots. • Monitor the oil temperature balance in order to detect elevated friction in the bearing or overgrowth of the cooler early on. • Regular oil analysis and tracking of the oil aging help to avoid critical system states such as jammed control valves for the steam control of a turbine and unsafe machine operation. Systems Used to Reduce the Effects of Varnish Removal of varnish from system components is a relatively slow process. Oil aging products are initially individual particles less than 0.1 microns in size. As a result, they can pass through the filter at first and initially do not impair the valve function. In the course of further oil aging, or when the oil is cooled down (e.g., during a system shutdown), these particles agglomerate, become larger, and block the valve function and the filter medium. Varnish deposits on hydraulic pump (1), filter element (2) and tank walls (3 & 4) 1. 3. 4. 2.