Turbine Instability Solution-Honeycomb Seals.

LecturePg. 47-56A 40,000 hp steam turbine-generator in a utility service experienced a history of high vibration alarms and trips from an online vibration monitoring system. The excursions of high vibration restricted the operational load performance of the turbine. The cause of the vibration was determined to be a 'rotor instability,' identified as a subsynchronous vibration in the form of steam whirl of the rotor. This whirling excites a rotor critical speed resulting in a very unstable rotor system. The instability could lead to destructive levels of vibration. A rotordynamics model of the rotor-bearing-support system was developed. Then, using the computer model, a stability analysis was performed and the sensitivity of the instability was evaluated for various system parameters (such as seals, rotors, and bearings). Numerous possible solutions were investigated, of which two exhibited significant increases in the stability of the system. First, a· newly designed rotor and second, replacing the rotor interstage labyrinth seals with honeycomb seals. The latter had a high probability of success and a greatly reduced capital cost. The turbine underwent a major overhaul in August 1992 when the honeycomb seals were installed. The machine is steadily running at loads well above the expected project justification levels. Aspects of the project, including before and after vibration analysis, rotordynamics analysis, and seal technology will be discussed