Mechanical vibrations in hydraulic machines - Suggested vibration limits based on analysis of the IEC TK4 vibration database
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This report is an updated version of chapter 6 in the previously published Elforsk report 12:70. For the revision and integration of the current mechanical vibration standards for hydraulic power generating and pumping plants ISO/IEC 7919-5 and 10816-5 IEC and ISO are supporting an international workgroup (ISO/TC 108/SC 2 & IEC/TC 4 - JWG1 Vibration of Hydraulic Machines). To support the working group, analysis of the IEC TK4 vibration database has been performed by the Swedish national workgroup. The purpose of the analysis has been to form a statistical foundation for a recommendation on vibration limits in a new integrated vibration standard. The fourth step in the total scope of work represent the additional analysis that has been performed and added to the report in this version. As a statistical foundation for the revised standard an international vibration database have been developed. The analysis performed in this report are based on the database versions Vib_DB_ Revision_E , Vib_DB_Revision_F and Vib_DB_Revision_J . Revision E contains 2392 rows, revision F contains 2472, whereas revision J contains 7355 rows. Every row corresponds to one measurement. The database contains measurements on all types of hydraulic power generating and pumping machines and commonly more than one measurement on each machine. During step 4 (Analyze the most recent database with methods established in earlier steps in order to propose action limits for the different machine types) of the work scope in the total project, database revision J of the database was considered. Database revision J has been improved with a vast number of measurements. The filtered database revision J was analyzed with the same method as for step 3. The median values for vibration level and shaft oscillations are presented in a table where also the suggested action limit values are presented. The main conclusions from the previous work remain: • No clear correlation between vibration values and the unit specific parameters such as head, rotational speed, runner diameter and radial bearing clearance can be observed which implies that both shaft oscillations and vibration velocities are relevant parameters. • Several shortcomings can be identified for the parameter utilized dynamic bearing clearance (UDBC). High magnetic unbalance in the generator and high hydraulic unbalance in the turbine gives small shaft oscillations and small UDBC, bearing load can however be very high. A poorly aligned shaft arrangement can also give low values on shaft oscillations and UDBC, although high bearing load. • The future standard has to distinguish at least between turbine type (Francis, Kaplan, Bulb, Pelton and Pump) and between bearing location (turbine bearing and generator bearings). Preferable is also a separation into shaft orientation (horizontal and vertical). • Reference values are suggested to be based on the median values, at least for turbine and generator bearings. • Actions are suggested to be undertaken if the actual vibration value exceeds 1.6 and 2.5 times the reference value: o 1.6 times the reference value corresponds roughly to the 75 percentile. o 2.5 correspond approximately to the 90 percentiles. • “Problem units” are evenly distributed over the complete range of measured vibration values. Surprisingly, machines labeled as ”problem” in the database do not have exceptionally high vibration levels if compared to other measured values which are considered to be ”normal”. However, the proportion of machines marked as “problem” increases with higher vibration values. At approximately 2.5 times the median value the proportion of problem units increases more radically. This could prove that the proposed boundary levels 2.5 and 1.6 times the median value makes sense. The suggested boundaries are significantly lower than the current boundary zones in the existing standard. The suggestion was supported by analysis that shows that the median value for the Burr-distributions is very close to the mean values for the datasets. And since the median value method objectively excludes extreme values, this method is promoted for finding the adequate reference values. The method is also verified through comparisons of the median value for a dataset with only measurements in best operating range and an unfiltered dataset. The resulting median values is nearly equal for the two compared datasets. During 2013-2014, analysis of database revision J was also conducted. Since the number of measurements was increased with this version the aim of the analysis was to find reference values which could be used for producing action limits. These limits were produced by using the boundary levels recommended from earlier analysis and the median values calculated from the database revision J. The recommended action limits from this analysis is generally at the same level as calculated from previous revisions of the database. Exceptions are shaft oscillations for Bulb units which results in increased values compared to previous database revisions. Suggested future work: • Verify the suggested action limits with more or improved data if the database is revised. • Refine the “problem” definition in the database. • Identify relevant bearing groups from the database for parameter correlation. • Identify the relationship between the vibrations of the generator guide bearings and the turbine guide bearing. • Explain the measured shaft movements that is larger than the specified available bearing clearance. • It is also suggested to identify the impact of the current requirements of IEC/ISO of 30 μm p-p for stationary parts. Current ISO 10816-5 action limits of 30 μm p-p is more severe for the majority of large-scale turbines than the future recommended limits that will be expressed in mm/s.
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