Lubricating oil viscosity trending
Oil analysis is a valuable tool for determining the condition of lubricating oil and the equipment it lubricates. Arguably the most important property of a lubricant, kinematic viscosity measures the oil´s resistance to flow under gravity. Kinematic viscosity is measured in centistokes (cSt) and is typically reported at 40¦C and 100¦C. Basic functions of the lubricants are negatively impacted if the viscosity of the oil falls outside of the intended viscosity range. Hence, trending of viscosity data is important as deviations may indicate base oil degradation, additive depletion or the use of an incorrect lubricant. If a lubricant´s viscosity is too high or too low, problems such as excessive friction, wear and heat typically occur, potentially leading to equipment damage or failure. Usually, the viscosity of a lubricant in use increases due to oxidation, high operating temperatures, the presence of water, wrong oil top ups, etc. Decrease in oil viscosity is attributed to degradation of the viscosity index improvers in multi-grade oils, shearing, thermal cracking, contamination/top up with a lubricant of lower viscosity, etc.
Following is a case history from a cement plant focusing on change of lubricant viscosity during use. Periodic samples were being collected from roller lubrication system of a vertical mill as part of their oil condition monitoring programme.
After ruling out the possibility of sampling/testing errors and wrong oil top ups plant maintenance personnel tried analysing other aspects which can result in decrease of lube oil viscosity. During investigations it was found that the lube oil circuit was fitted with high wattage heater elements, exposing the oil to high localised temperatures resulting in thinning/cracking of the oil viscosity. Subsequently, heaters were replaced and new oil was charged.
Trending of viscosity data is important as deviations could be a cause for excessive wear out and catastrophic failure. Root cause analysis of abnormal change in viscosity readings resulted in finding and correcting the underlying cause to avoid reoccurrence, instead of simply changing the oil.
Viscosity @ 40¦C was very low at 120cSt. for the sample against a normal range of 612 - 748cSt. Immediate resample advised. Bearing wear particles of size up to 40 microns were observed in Analytical Ferrography.
Second Sample received after 20 days Viscosity @ 40¦C dropped further to 88.8cSt. indicating abnormal change. The Oil Analysis Report was rated Critical with a recommendation to change the oil immediately and investigate the probable causes for decrease in Viscosity.
Analytical Ferrography revealed increase in Size and concentration of Bearing wear particles up to 150 microns along with the presence of copper alloy particles probably due to metal to metal contact because of low viscosity.
The case study is authored by Masood, CEO, Fluid Analysis Consulting & Trading Co (FACT), Hyderabad. He is a free lance consultant on lubrication.