Prediction and Mitigation of Thermally Induced Creep Distortion in Gas Turbine Combustors

In an effort to maximize efficiency and decrease emissions, modern gas turbine combustors are exposed to extreme operating conditions which if not accounted for during the design process, can lead to premature failure of the combustion components. Of interest to this article are some operating conditions that, in many instances can expose the gas turbine combustion chambers (liners) to asymmetric thermal loads. Highly asymmetric thermal loads at high temperatures can inflict severe distress on combustion liners attributing to thermal creep distortion and Thermo-Mechanical Fatigue (TMF). Modern low emission pre-mix combustion systems such as the Dry Low NOx (DLN) 2.6 in the GE F Class machines and PSM’s FlameSheet combustor employ firing curves that involve “staging” when the gas turbine is ramping up or down in load or is simply operating in part-load condition. During such staging process, the flame resides in only certain sectors of each combustor while the other sectors are cold, these part load conditions can cause high thermal gradients leading to high thermally induced stresses in the liners. High thermal stresses at high metal temperatures can induce severe visco-plastic (creep) geometric distortion in liners upon prolonged exposure to such conditions. Extreme thermally induced creep distortion can eventually lead to liners’ catastrophic failures due to buckling and/or rupture. Under mild circumstances permanent creep distortion of liners can lead to non-optimal combustion and hence attributing to non optimal operation of the gas turbine. Several means can be employed during the design process to avoid and/or account for creep distortion, some of which are discussed in this article. Although linear elastic analysis is usually used by design engineers to predict liner thermal deflection under part load conditions, it is important to note that even though the resulting stresses may be within the material’s elastic range, creep relaxation leading to permanent liner deformation may still occur over time causing non-optimal base load operation and degradation to the gas turbine efficiency. In most cases predicting thermally induced creep distortion over time can only be done using iterative numerical techniques such as FEA coupled with the material specimen creep testing. A case study involving a F class FlameSheet liner will be discussed and used for illustrative purposes. ANSYS non-linear creep FEA modeling was used to predict the creep deformation results over time using Haynes 230 specimen test data. The predicted numerical analytical results matched well with actual hardware characterized data, thus validating the analytical technique.