This fossil fuel-fired electric generating station utilizes a 78-megawatt, General Electric (GE) 7EA gas turbine. It has a 17-stage axial compressor and three turbine stages and is rated at 3,600 rpm. It is an on-demand peaking unit and was commissioned almost 30 years ago.
Past inspection intervals were within manufacturer parameters and in the year before this incident, a borescopic hot gas path inspection showed good results.
The following sequence of events occurred:
- The unit was started and loaded to about 40 MW over a 10-minute timeframe.
- 30 minutes later, the first of several significant power output spikes was noted.
- About 80 minutes later, a power drop to -19 MW (motoring) occurred.
- This momentary motoring (reverse power) did not trip the master protective trip function as would have been expected.
- Vibration levels began to steadily increase over the next 60 minutes.
- Operators reported hearing a loud bang followed by a final power spike resulting in power output dropping to 0 MW.
- The alarm history indicates that only at this point were trip signals received for loss of flame, overtemperature, generator breaker opening and high vibration.
Based on the alarm history, the operators then apparently attempted to reset the diagnostics/alarms and restart the turbine twice within several minutes.
The unit was shipped to a GE repair facility for examination and damage assessment. The 17-stage compressor rotor, stationary vanes and casing all sustained impact damage due to liberated blades passing through the unit. Upon dismantle, damaged and missing blades were noted throughout the compressor section.
The turbine and the generator did not appear to be damaged but replacement of the compressor rotor and casing was required. The property damage is estimated at US$ 4.5 million.
It is believed that uncontrolled pressure surges within the compressor caused high cycle fatigue and eventual blade failure.
This event underscores the importance of maintaining and testing safety devices, alarms, and interlocks. It also shows the importance of trained operators that can properly respond to emergency situations. In this case, both the safety devices and the operators failed to respond properly. Operators should not have tried to restart the unit without first determining the reasons for the abnormal operation and unit trip.