Analysis of Steam Turbine Instabilities of a 100 MW Combined Cycle Power Plant

Abstract

An analysis of the vibration field of the steam turbine in a combined cycle power plant has been used to identify the causes of the plant rejection at commissioning. The steam turbine group consists of a high pressure turbine connected through a gear box to a medium and low pressure turbine. The high vibration level in the main steam turbine journal bearings caused the plant can’t reach nominal power 100 MW, and at approximately 43 MW the plant was rejected. Vibration is measured through proximity non-contact sensors (two per bearing at 90°), which give relative displacements between shafts and the bearing housing. Accelerometers are also located in the bearing housing. The analysis carried out included: • Field measurements; • Critical speeds dynamic model (API 684 guidelines); • Comparison with the API 684 stability test. As a result of this study, the authors found that excessive vibration was caused by a rotor instability phenomenon, “steam whirl”, and that it led to plant rejection. The main conclusions from this work are: • The vibration rejection of the combined cycle plant was due to an excess in the maximum permissible vibration value at synchronism speed, which always occurred at 43 MW. • The excessive vibration level was caused by rotor instabilities at the rotor shaft in the high-pressure steam turbine. The main vibratory energy was concentrated in a frequency range 0.38–0.41 × rpm. • The main instability phenomenon was identified as “steam whirl”. • The protocol used as a tool in the stability analysis of the turbine, defined in API 684, shows that the logarithmic decrement at the first modal frequency in the most dangerous situation (minimum gap in bearings) has a value of 0.149, which is higher than the stability threshold defined in the API 684 specification (0.1). In this way, and according to the stability analysis, the steam turbine design would be safe despite the fact that the instability problem appeared.