Advances in Direct Measurement of Gas Turbine Exhaust Temperature With Multi-Element Thermocouple Rakes for Diagnosing Performance Issues and Characterizing Turbine Upgrades

Heavy-duty gas turbines are designed to deliver maximum performance within their respective technology class and emissions limits. In order to achieve performance goals consistent with hot section durability constraints, it has become more critical than ever for engineers to have an economical, dependable, and accurate measurement of the average exhaust gas temperature and the associated profiles. Simple thermocouples “rakes” have been used for many years to meet the basic need of measuring planar average temperature. In addition, recent testing experience has shown that the measured radial temperature profile data from these same “rakes” can play a key role in the diagnosis of performance issues and also in the characterization of hardware upgrades. For example, high technology hot section spin-offs from F, G, and H class turbines have been applied as upgrades to older B and E class turbines with dramatic impact on the exhaust temperature pattern. Another example has been the use of pressure/temperature exhaust “rakes” in F class turbines to diagnose changes in the radial temperature profile that result from combustion system upgrades. In both cases, the careful measurement and interpretation of these temperature patterns is crucial to the proper setting of control algorithms that govern performance levels and exhaust emissions. Advances in the design and arrangement of exhaust thermocouple rakes, and in the analysis methods used to interpret the resultant test data, are presented. Several recent cases of using rakes to diagnose performance issues and to characterize the temperature pattern for the purpose of optimizing control settings are discussed.