Investigation Into the Thermal Limitations of Steam Turbines During Start-Up Operation

Liberalized electricity market conditions and concentrating solar power technologies call for increased power plant operational flexibility. Concerning the steam turbine (ST) component, one key aspect of its flexibility is the capability for fast starts. In current practice, turbine start-up limitations are set by consideration of thermal stress and low cycle fatigue. However, the pursuit of faster starts raises the question whether other thermal phenomena can become a limiting factor to the start-up process. Differential expansion (DE) is one of such thermal properties, especially since the design of axial clearances is not included as part of start-up schedule design and because its measurement during operation is often limited or not a possibility at all. The aim of this work is to understand DE behavior with respect to transient operation and to quantify the effect that such operation would have in the design and operation of axial clearances. This was accomplished through the use of a validated thermomechanical model that was used to compare DE behavior for different operating conditions of the machine. These comparisons showed that faster starts do not necessarily imply that wider axial clearances are needed, which means that the thermal flexibility of the studied turbine is not limited by DE. However, for particular locations, it was also obtained that axial rubbing can indeed become a limiting factor in direct relation to start-up operation. The resulting approach presented in this work serves to avoid over-conservative limitations in both design and operation concerning axial clearances.