In-situ running bucket vibration test of an intermediate-pressure steam turbine

Vibration test and analysis results of running buckets are presented. A design modification is made on the 9th stage wheel dovetail of a high-intermediate pressure (HIP) turbine rotor for a 500 MW fossil power plant that necessitates the use of new long-shank buckets for the row. A bucket vibration test is necessary to verify whether the new 9-th stage buckets have adequate frequency margin from a nozzle passing frequency when running at speed. A finite element analysis (FEA) has been performed using commercial S/W to approximately estimate the difference between the modified and the normal bucket natural frequencies at various rotational speeds. After a row of the new buckets has been assembled on the HIP rotor for the vibration tests using dynamic balancing facilities, deceleration tests have been conducted. The test results are compared with the FEA and our empirical formulas, and show that the modified design meets the frequency-margin requirements.