Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1289708
18 | September - October 2020 | www . machinerylubrication.com OIL ANALYSIS sample and their total volume is then measured. However, doing so disregards the fact that the hazardous effects of oil-soluble varnish contamina- tion are far greater than those of oil-insoluble varnish. e soluble varnish will be able to circulate f ur t her, reaching t he sma llest openings and clearances of control valves due to its solubility in oil. en, as the oil temperature drops for any reason, the varnish parti- cles join together and are dissolved into the oil. e resulting viscous sludge mass can easily interfere with the normal operation of the turbine and even cause sudden stops. In addition, the accumula- tion of these contaminants in the turbine can cause serious damage to mecha n ic a l pa r t s such a s bearings (4). If a comparison can be made between the volume of soluble and insoluble varnish present in the lubricant, an estimate of the actual hazards in the gas turbine can be more accurately determined. In the following method, an index will be introduced that can measure oil-soluble varnish and insoluble varnish separately and its benefits will be examined more closely. The procedure for measuring soluble and insoluble varnish 1. In accordance with the instruc- tions given in ASTM D7843, the oil sample is kept at 65 ° C for 24 hours. 2. e first patch test will then be performed. Since only the insol- uble varnish present in the oil can be identified at this stage, the results indicate the insoluble varnish present in the oil. 3. en the oil sample is kept at about 20 ° C for 72 hours. 4. e second patch test is then p e r f o r m e d , a n d b o t h membranes are evaluated via spectrophotometer. 5. e result obtained from the first membrane is the index of the insoluble varnish present in the oil sample (X_MPC) 6. e result obtained from the second membrane is the sum of the soluble and insoluble varnish present in the oil sample (MPC). Evaluating results: With the aid of the oil insoluble varnish index (X _MPC) and the soluble and insoluble varnish sum index (MPC) the R index value can be determined according to the formula below: R = MPC + (5 - X_MPC) It should be mentioned that 5 units have been added to the formula to help eliminate the effects of new oil color on a standard membrane. Now that R has been deter- mined, it is possible to eliminate insoluble particles from the test results providing a more relevant measurement of real world risk. If R is less than 15, the turbine condi- tions are acceptable. From 15 to 30, turbines should be monitored continuously, and if more than 30, turbines are in an abnormal or crit- ical condition and require immediate action. Solutions for cleaning up the varnish in affected gas turbines is a topic for another article. Fields practical observations: What prompted me to introduce a new index was my experience dealing with the different effects of the contaminants discussed in this article despite relatively similar MPC results. As shown in Figure 2, the varnish potential of the turbine oil employed in a petrochemical plant is MPC = 65. Another example shown in Figure 3 is a turbine working in a thermal power plant, showing a varnish potential of MPC = 63. A lthough both oil samples appear to be at high risk of varnish contamination, the calculation of the R index indicates that the oil used in the thermal power plant turbine has a higher risk. Here is what this method of testing looks like in prac- tice: Calculation of Varnish Index R1 for Petrochemical Plant gas turbine. R1 = MPC + (5 - X_MPC) = 65 + (5 - 45) >>> R1 = 25 Calculation of R2 Varnish Index of ermal Power Plant gas turbine. R2 = MPC + (5 - X_MPC) = 63 + (5 - 17) >>> R2 = 51 e MPC value for both samples are nearly identical and both appear to indicate serious risk. In fact, FIG 2. Sample of oil patch for a gas turbine in petrochemical plant. FIG 3. Sample of oil patch for a gas turbine in a power plant.