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Energética India | September / October 2015

POWER SECTOR largest amount of residual sediment. A result of 4 to 6 is cause for concern that the oil in service has the potential to lay down performance-robbing deposits in the system. Particle Count (ISO 4406) Particle Counting and ISO Cleanliness ratings define the concentration of particles in the oil and relate this back to the ISO Cleanliness scale. The results are reported as the number of particles greater than 4 microns/6 microns/14 microns per ml of fluid. The ISO Cleanliness Code relates the number of particles per ml to a logarithmic scale with code number for each range. A typical result would look like 18/16/13 where 18 means there is 1,300 to 2,500 particles per ml greater than 4 microns in size, 320 to 640 greater than or equal to 6 microns, and 40 to 80 greater than 14 microns. Refer to Table 1. Particle counts are subject to a wide range of variability due to sample preparation, oil formulations, contamination of the sample container, and location and method of sampling. There are also differences in the equipment used to measure particle counts between light dispersion techniques and filter pore blockage methods. Care should be taken to ensure that the samples used for Particle Counts are representative and consistent. The particle count results are only good as a relative measure of contamination and no ASTM standard exists for this test. Ultimately, particle count does give a good indication of overall system cleanliness. OEMs do offer some guidelines for new and used oils, but in general an ISO Cleanliness code of 18/15/13 or lower is an acceptable result. Colorimetric Analysis Colorimetric analysis is designed to measure the insoluble materials in the turbine oil which often lead to varnish deposits. The process includes treating the lubricant sample with a specific chemical mixture designed to isolate and agglomerate insoluble by-product material, and collect this material on a filter patch. The color spectra of the collected material are then evaluated and depending on the intensity of specific colors or color ranges, a varnish potential rating may be derived. The filter patch may also be weighed as a means to determine insoluble concentration in the lubricant. Several commercial labs utilize this technique, each with their own specific method. Currently, this is not covered by an ASTM standard, but an ASTM method is currently being developed based on this concept. Foam Tendency and Stability (D892, Sequence I) The presence of some foam in the reservoir is normal and not a cause for concern. Excessive foaming is generally not related to the oil, but rather to mechanical issues that cause excessive amounts of air to be introduced to the oil. Contamination and oil oxidation can also have an effect on the foaming tendency and stability. Excessive amounts of foam are a concern to the turbine operator for two reasons. First is a safety and housekeeping issue if the foam overflows the reservoir. Second, excessive amounts of air in the oil can lead to more rapid oxidation and phenomenons know as micro-dieseling. Micro dieseling is caused when an air bubble in the oil is rapidly and adiabatically compressed causing extreme local temperature increases. These large temperature increases are known to cause thermal and oxidative degradation of the oil leading to deposit formation. Performance Properties Corrosion Inhibition (ASTM D665 A and B) ASTM D665A uses distilled water and a steel test spindle at 60°C. ASTM D665B uses synthetic seawater and is a more severe test not commonly used for turbine oils. If rust is detected on the steel test spindle in the test, the test is considered a failure. However, a failing ASTM D665 test does always correlate to a rust issue in the system. Demulsibility (ASTM D1401) Demulsibility is a measure of the oil’s ability to separate from water. The 40 ml of the subject oil and 40 ml of distilled water are mixed and then allowed to settle. The amount of time for full separation of the oil and water is recorded or after 30 minutes, the amounts of oil water and emulsion are recorded. ASTM does not offer a warning limit for demulsibility, but a result Suggested Schedule for Oil Analysis of Turbine Systems Test Steam Gas New Oil Baseline Frequency - Used Oil Suggested Limit Viscosity - ASTM D445 x x x Monthly +/- 5% of new oil value Total Acid Number - ASTM D664 x x Monthly Caution = 0.1 to 0.2 mg KOH/g over new oil value; Warning = 0.3 to 0.4 over new oil value and check against RPVOT value RPVOT - ASTM D2272 x x x quarterly 25% of new oil value; If close to 25%, increase frequency of test Water Content (visual) x x x Daily Check for haziness Water Content - ASTM D1744 x x x Monthly Greater than 0.1% in steam turbines; Greater than 0.05% in gas turbines ISO Cleanliness x x x Monthly Target 18/16/13 or better Rust Test - ASTM D665 A x x x Only if corrosion issues Pass Foam - ASTM D892, Esq. I x x x Only if foam is an issue Esq. I exceeds 450 tendency, 10 ml stability Demulsibility - ASTM D1401 x x x Only if water separation is a concern 15 ml of emulsion after 30 minutes Ultra Centrifuge x x x Monthly to Quarterly UC rating of 4 to 6 Varnish Potential Rating x x (Gas Turbine Only) Monthly to Quarterly Varnish potential rating of 50 or more 6 energetica INDIA · SEP | OCT15


Energética India | September / October 2015
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