Gas Turbine Generator (GTG) availability is one of the most critical requirements for any plant production in oil & gas industries. The subject case study provides the information and root cause analysis done on the number of trips happened on a dual fuel Gas Turbine due to high exhaust temperature spread while operating on liquid fuel, that affected plant power production requirements.
An exhaust temperature spread refers to a high differential in temperature readings between the thermocouples placed radially around the exhaust of a gas turbine. Spread indicates the difference between maximum and minimum temperatures recorded by exhaust temperature thermocouples. If spread is allowed without correction, it will result in reduction of residual life of downstream components of the Gas Turbine. The worst exhaust temperature spreads occur when the hottest and coldest spot are nearer i.e. exhaust thermocouple readings are grouped very closely adjacent. Most exhaust temperature spreads are the result of combustion section problems that can lead to premature failure of turbine blade /bucket.
Site is configured gas turbine running on dual fuel that is fuel gas and liquid fuel driving generator for power generation. During the commissioning and unit start-up the exhaust spread was normal on both fuel gas and liquid fuel. After running the unit for almost a year there was a trip event. Evaluation of the trip data indicated that the exhaust temperature spread is very high when the unit was running on Liquid fuel.
It was agreed to start the unit again and perform the fuel transfer from gas fuel to liquid fuel at lower load of 5 MW, the unit tripped again when it was switched to liquid fuel due to same issue of high exhaust spread.
Further checks were performed, and it was established that the high exhaust spread is due to hardware issue, related to the fuel nozzles and liquid fuel or purge air system check valves
There are number of factors that caused the high spread exhaust issue, all of them are verified by engineering in details and recommendations are provided for the future operation of the machine without the subject issue of high exhaust spread. Most probable causes identified was the Clogging of Fuel Nozzles and issues with the Liquid Fuel check valves and Atomizing Air Purge System Check Valves.
High exhaust temperature spread is one of the major concerns in liquid fired Gas Turbines leading to low reliability of Gas Turbine & more down time. In this case study some of the rare reasons for the high spread issues & troubleshooting are analyzed and presented in detail. The information contained in this case study is broadly applied in Oil and Gas plants where Gas Turbines are installed for power generation.