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Energética India | September / October 2015

sults should be +/- 5 percent of the new oil viscosity. Total Acid Number Total Acid Number (TAN) is the measure of the oil’s acidity and is measured by titrating the oil with a base material (KOH) and determining the amount of base required to neutralize the acids in the oil. The results are reported as mg KOH/g of the oil being tested. TAN measures the acidic by products formed during the oxidation process. ASTM D4378 (In Service Monitoring of Mineral Turbine Oils for Steam and Gas Turbines) recommends that a 0.3 to 0.4 mg KOH/g rise above the new oil value as the warning limit. Any significant change in TAN should be investigated as the acids in the oil can cause corrosion of bearing surfaces that result in irreparable damage. However, care should be taken in reacting to a single high TAN result. The TAN test is not a precise method (+/- 40 percent by ASTM Standard) and is subject to variability of operators. Poor maintenance of the buffer solution or electrodes used in the titration can also yield false results. Oxidation Stability by Rotary Pressure Vessel Rotary Pressure Vessel Oxidation Test (RPVOT and formerly known as RBOT) is a measure of remaining oxidation life when compared to new oil. The test is not intended to draw comparisons between two different new oils or oils of different chemistries. In fact, oils with very high new oil RPVOT values have been seen to have the shortest life in laboratory rig testing. ASTM D4378 defines 25 percent of the new oil RPVOT value as the lower limit. When the oil is approaching the 25 percent of new oil value in conjunction with an increasing TAN, ASTM D4378 recommends that plans should be made to replace the charge of oil. Contamination Measurements Water Content Turbine oils are subject to water contamination from several sources. Steam turbines can have leaking gland seals or steam joints. All turbines can become contaminated with water from atmospheric condensation in the reservoir or leaking heat exchangers. The turbine oil should be inspected daily for water. Looking at the sample, it should be clear and bright. A cloudy or hazy appearance indicates that water may be present. An on-site water test can be performed such as the hot plate crackle test where the subject oil is dropped on a heated metal surface. Bubbling and crackling indicate that water is present. In the laboratory, water is typically measured by Karl Fischer Titration (ASTM D1744) and reported as a percent or in parts per million. ASTM D4378 identifies 1,000 ppm or 0.1 percent water as a warning limit. However, some OEM’s have defined 500 ppm as the warning limit. Keep in mind that the Karl Fisher method does not measure free water, so daily visual inspections of the turbine oil are recommended. Metals by Inductively Coupled Plasma (ICP) Metals concentration in turbine oil can give early warning of wear conditions, changes in equipment operation or potential contamination issues. Keep in mind however, that the size of the metals detected by this POWER SECTOR ISO Cleanliness Particle Count Ratings Range Code Number of Particles per ml More Than Up to & Including 24 80,000 160,000 23 40,000 80,000 22 20,000 40,000 21 10,000 20,000 20 5,000 10,000 19 2,500 5,000 18 1,300 2,500 17 640 1,300 16 320 640 15 160 320 14 80 160 13 40 80 12 20 40 11 10 20 10 5 10 9 2.5 5 8 1.3 2.5 7 0.64 1.3 6 0.32 0.64 5 0.16 0.32 4 0.08 0.16 3 0.04 0.08 2 0.02 0.04 1 0.01 0.02 0 0.005 0.01 0.0 0.003 0.005 method is limited to very small metal particles, typically less than 8 microns in size. That means catastrophic failures can occur where large pieces of wear metal are generated and not detected by this test. There is no specific limit on the amount of metals for turbine oils. The trend of metals concentration is often the most important aspect of this test. Ultra Centrifuge Rating The Ultra Centrifuge test detects finely dispersed or suspended particles in the oil. The subject oil sample is centrifuged at 17,500 rpm for 30 minutes. At the end of this period, the test tube is drained and the remaining sediment is rated against a standard as shown in Figure 1. The primary use of this test is to give an early indication of deposit precursors in the oil. The results of the test are reported on a scale of 1 to 8, where 8 indicate the Figure 1. 5 energetica INDIA · SEP | OCT15


Energética India | September / October 2015
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