On-line monitoring and control of thermal stresses in steam turbine rotors

The requirement of high operational flexibility of utility power plants creates a need of using on-line systems for monitoring and control of damage of critical components, e.g. steam turbine rotors. Such systems make use of different measurements and mathematical models enabling calculation of thermal stresses and their continuous control. The paper presents key elements of the proposed system and discusses their use from the point of view of thermodynamics, heat transfer and solid mechanics. Thermodynamic relationships, well proven in design calculations, are applied to calculate on-line the steam temperature at critical locations using standard turbine measurements as input signals. The model predictions are compared with operational data from a real power plant during a warm start-up and show reasonably good accuracy. The effect of variable heat transfer coefficient and material properties on thermal stresses is investigated numerically by finite element method (FEM) on a cylinder model, and a concept of equivalent Green's function is introduced to account for this variability in thermal stress model based on Duhamel's integral. This approach was shown to produce accurate results for more complicated geometries by comparing thermal stresses at rotor blade groove computed using FEM and Duhamel's integral. Finally, the applicability of Neuber's and the Glinka-Molski rule with ideal elastic and bilinear material model to estimating elasto-plastic strains at the rotor groove was investigated by FEM. For the considered groove geometry, Neuber's rule with bilinear material model resulted in most accurate strain predictions.