Testing in-service lubricants for determining lubricant and equipment integrity
Diagnosing equipment conditions and evaluating in-service lubricant test results includes looking at both the health of the equipment and the integrity of the oil.
By David Doyle, CLS, OMA I, OMA II
Companies with mechanical maintenance needs might be familiar with used oil testing for their equipment, but many have not fully grasped the relevance of and the value incurred from implementing an oil testing program. Companies currently using testing services still have questions regarding test reporting and its applicability. This article provides an overview of the practice of testing in-service lubricants and clarifies how testing and analysis benefits companies with no operating equipment investments.
Diagnosing equipment condition and evaluating in-service lubricant test results includes looking at both the health of the equipment and the integrity of the oil. Trending of the test data over a period of time is a valuable tool to monitor ongoing equipment condition and predict corrective action before interruption to operations or increased maintenance costs occur. Using proper trending analysis will minimize equipment downtime, permit more efficient maintenance scheduling, protect warranty claims and increase equipment resale value.
Predictive maintenance is a key advantage of analyzing in-service lubricants. Scheduled maintenance alone, without using in-service lubricant testing, may occur too frequently or not allow enough coverage between services. Optimizing maintenance schedules leads to reduced costs and a greater return on investment. Companies that take in-service lubricant analysis to another level concentrate on root cause analysis. This allows monitoring contaminants, such as dirt or water, so that root causes are controlled and kept at a level in which resulting detrimental effects are minimized.
Evaluating a used lubricant sample is not always based on a single test parameter. Equipment or lubricant conditions may be flagged based on the range of acceptable limits for multiple parameters. In addition, it may not be necessary to wait for test parameters to reach a condemning or warning limit across the board, as changes in a combination of parameters may also lead to condemning the oil or flagging an equipment condition. Trained diagnosticians must understand equipment application and design, oil formulation and costs involved in draining and replacing in-service oil. The diagnostician must also be trained to recognize the integrity or applicability of the data in order to make judgments regarding the oil’s condition.
When looking at establishing a testing program, consult with a laboratory and your lubricant supplier for the most cost-effective test package relevant to your needs. More is not always better. Acquire information that can be used effectively. Decide how the testing relates to and complements your maintenance needs. Look at tests that meet your particular needs. An important consideration is working with your laboratory to optimize the service in order to obtain the most cost-effective results and create successes that lead to increased equipment reliability. Typical testing programs are comprised of standard routine tests that can meet most service requirements. Laboratories providing your testing service should also have the capability to provide specialized testing and technical services in order to address non-routine needs.
Viscosity is the most important lubricant property measured. Proper viscosity is required to maintain an adequate oil film that will separate and protect moving parts. The selection of a proper viscosity grade is dependent upon temperature, speed and load. Too high of a viscosity will cause wear during start-up and excessive heat during operation. Too low of a viscosity will not create a sufficient oil film to protect moving parts. Viscosity increases due to oxidation and oil degradation, or soot in the case of diesel engines. Viscosity decreases due to contamination with a less viscous product (e.g., different oil, solvent, or fuel) or shearing of the viscosity index improver. Viscosity changes are monitored in conjunction with other test parameters (e.g., fuel dilution, soot and oxidation). Viscosity can also be judged as application-specific. For example, if an excessive mixture of natural gas in the oil is common, higher viscosity grade oil may be used to compensate for anticipated viscosity thinning.
Spectrochemical analysis determines the level of dissolved and suspended metal concentrations in an oil sample with a size range of 10 micron or less, which represents metalorganic additives, contaminants and wear. Test results are expressed in parts per million (ppm). This determination is used to trend wear rates in equipment as well as to monitor changes in certain additive and contaminant levels.
Combinations of metals are analyzed to determine which components are wearing. Different equipment designs and makes will have characteristic wear metals as well as limits for wear metal concentrations. Wear metal alarms do not always have to reach an upper limit. A sudden increase in the wear metal concentration over previous sample trends should also generate a warning flag.
Samples requiring spectrochemical analysis with large metal particulates would normally require special preparation in order to account for the large particles that do not stay in suspension or are not detected by the spectrometer.
One way of monitoring oxidation in oil is by infrared analysis (FTIR), which is reported in absorbance per centimeter. Test results are based on differences in the used oil chemistry detected by infrared analysis when compared to new, unused oil.
Values for oxidation represent a relative level of oil degradation constituents. Evaluation of a lubricant’s oxidation level is best compared with and complements other measurement parameters of oil degradation. It is not necessarily considered a condemning parameter by itself but is a good trending tool when compared to other test results, such as viscosity, acid number and other oxidation tests.