Identification of Flow Induced Vibration in Steam Exhaust Hood-Duct System

Vibration and noise of concern were observed at the hood-duct of the exhaust steam turbine at a power generating facility. The field personnel noticed this problem after a startup of the unit which followed the maintenance replacement of the expansion joints. The author was contacted by the field personnel and asked to formally evaluate this problem and advise on the steps needed to eliminate it. Several similar units were scheduled for the same maintenance work. The analysis carried out was based on the acoustic and structural dynamic modeling of the critical parts of the hood-duct system in question. Both the flow induced excitations and mechanical structural responses were considered. The results of the analysis combined with field measurements indicate a vibration generation mechanism which is flow induced in nature and is coupled with the acoustic field and enhanced by the high stream-wise flow velocities. Consequently, there are two categories of solutions identified to the observed problem. The first considers addressing the problem at the source and the second suggests path treatment. Addressing the problem at the source by means of aeroacoustic optimization of the flow path is recommended. Detuning the mechanical response from the flow excitation(s), i.e. the vibration path treatment, involves an aspect of greater or lesser trial and error since the responses of the studied structure depend on many variables; therefore, even the most accurate calculations might not be sufficient to give the expected results. Both recommendations are discussed.