Steam Turbine Thermal Modeling for Improved Transient Operation

Sammanfattning: The growing shares of renewable energy sources in the market and solar thermal power applications have set higher requirements on steam turbine operation.These requirements are related to flexibility during transients. A key aspect sought of such flexibility is the capability for fast starts. Due to the varying temperature gradients during start-up, the speed at which the turbine can start is constrained by thermal stresses and differential expansion. These phenomena either consume component lifetime or may result in machine failure if not carefully controlled. In order to accomplish faster starts while ensuring that lifing requirements are preserved, it is important to analyze the thermal behavior of the machine. For this, a transient thermal model was developed with a focus on adaptability to different turbine sizes and geometries. The model allows for simple and fast prediction of thermo-mechanical properties within the turbine metal, more importantly, of the temperature distribution and the associated thermal expansion. The next step of this work was to validate the assumptions and simplifications of the model. This was done through the study and comparison of two turbines against measured operational data from their respective power plants. Furthermore,validation studies also included comparisons concerning the geometric detail level of the model. Overall, comparison results showed a large degree of agreement with respect to the measured data and between the geometric detail levels. The validated model was then implemented in studies related to reducing start-up times and peak differential expansion. For this, the potential effects of turbine temperature maintaining modifications were investigated and quantified.The modifications studied included: increasing gland steam pressure, increasing back pressure and increasing barring speed. Results yielded significant improvements starting from 9.5% in the start-up times and 7% in the differential expansion.