Steam turbine retrofits often result in an increase of turbine size, aiming for more power and higher efficiency. As the existing LP steam turbine exhaust hoods are generally not modified, the last stage rotor blades frequently move closer to installations within the exhaust hood, such as baffles or ribs.
To assess the influence of supporting ribs on the vibration behavior of the last stage rotor blades, tests with two rib configurations were performed in a single stage LP model steam turbine at the Institute of Thermal Turbomachinery and Machinery Laboratory (ITSM) at the University of Stuttgart, Germany.
At design load and overload operating conditions no significant change in blade vibration amplitudes is observed in case of a supporting rib in close vicinity to the rotor blades compared to the reference case without installations. However, at part load operating conditions a remarkable reduction in blade amplitudes is found rather unexpectedly.
The present work shows that changes in the pattern and the frequency content of the flow within the diffuser, caused by the interaction between supporting rib and steam flow is evidently responsible for this.