Page 79

Energética India | September / October 2015

of 15 ml or greater of emulsion after 30 minutes is a fair warning limit. Contamination and oil age are factors that negatively affect demulsibility. Care should be taken when evaluating demulsibility as the preparation of the glassware and the quality of the water used can yield false or failing results. 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 a phenomenon known 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. How, Where and When to Sample There is no correct answer as to how and where to sample turbine oil. The selection of sample location is dependent on what data is required from the oil analysis. For example, if information about wear metals is the primary concern, sampling after the filter is not a good location since the desired data would have been lost through the filtration process. A better sample point in this case would be prior to the filters or the bearing return line. If information about contamination is the primary concern, pre and post filter samples can be useful. Comparing the ISO Cleanliness code of a sample before and after the sample gives specific insight into the effectiveness of the employed filtration and the degree of contamination ingress. For most purposes, a sample obtained prior to the filters is most desirable for general testing. While there are many appropriate locations to obtain a sample, there is almost universal agreement on how to sample. To get representative oil samples, the unit should be up to normal temperature and operating condition or just following shutdown. The sampling point should be clean and purged of all stagnant or dirty oil that may be in the line and valve. Make sure the sample container is clean and dry. Correctly and completely fill in the sample labels and mail as soon as possible to the appropriate lab. Delays in sending samples can add variability to test results, especially those concerned with insoluble content in the oil. A suggested schedule for oil analysis is shown in Table 2. POWER SECTOR Conclusion The reliable operation of a power generating turbine and its associated equipment is dependent on the health and well being of the lubricant in use. Regular oil analysis and condition monitoring is one tool that should be used to maintain the turbine in peak operating condition. The oil analysis program should include the basic tests outlined in this document to assess the physical and chemical properties, contamination issues and performance characteristics of the fluid. Operators should consult with the equipment manufacturer for further guidance on the interpretation of the used oil analysis data 7 energetica INDIA · SEP | OCT15


Energética India | September / October 2015
To see the actual publication please follow the link above