This cogeneration power plant consists of two 25-megawatt (MW) aeroderivative gas turbine generator systems with heat recovery steam generators. Steam from heat recovery is injected in the combustion chambers of the gas turbines for power augmentation and NOx (emissions) control.
This facility is operated on a peaking basis and is contracted for an “hour-ahead” dispatch, meaning it must ramp up to full load within one hour of notification.
After receiving the dispatch to come online, the unit was started as normal and ramped up to full load. The control room received an exhaust temperature spread alarm after operating for just over one hour. This alarm was quickly followed by steam injection high differential temperature and steam injection overflow alarms. The unit tripped offline six minutes after the first alarm.
Operators proceeded to restart the unit approximately 15 minutes after the unit tripped. Shortly after startup, a steam injection high differential temperature alarm was received. This alarm was quickly followed by steam injection overflow and exhaust temperature spread alarms. The unit tripped offline again and a loud “boom” was heard from the turbine compartment nine minutes after the first alarm.
A borescope inspection revealed severe compressor airfoil damage beyond row six. Subsequent gas generator top case removal revealed further damage to the high pressure (HP) rotor, and case and turbine midframe. The unit was shipped to the manufacturer’s shop for dismantle, full inspection and repairs.
Alarm management is of great importance. Exhaust temperature spread alarms may indicate a host of issues. Mechanical concerns include plugged fuel nozzle orifices or fuel assembly problems. Additional concerns include low, high, or uneven atomizing air flows, reignition/flashback, and/or structural deficiencies such as insulation breakdown. However, in this loss event the other alarms received in the control room were indicative of potential anomalies with the steam injection system.
This loss event emphasizes the vital need for operators to understand what alarms are indicating and the importance of good operator decisions. In this case, the operators continued to operate the unit with steam injection despite indications of a damaging malfunction of the system. Moreover, the unit was restarted without first determining the reasons (root cause) for the initial alarms and trips.