To ensure a reliable power supply with a minimum environmental impact in the future, further increases in efficiency and flexibility of fossil-fired power plants are major challenges to address in recent R&D activities. In order to counterbalance substantial fluctuations in the electricity grid due to the rising share of renewable resources, frequent start-ups and shutdowns of turbomachinery accompanied by quick and heavy load changes will be claimed by the market. Hence, along with the development of advanced materials for elevated steam parameters, contemplations with respect to altered boundary conditions of currently used materials, e.g. for bolted pipe flange connections, are required.
In this paper, a selection of results from a recently finished research project on stress relaxation will be presented. Together with European power plant component manufacturers, a newly developed test rig comprising a scaled IP steam turbine pipe flange now allows investigations under near-service conditions. Before, throughout and after performance of the experiments, an extensive measurement setup enables the examination of effects that cannot be studied in standardized relaxation tests. For example, deformation of the flange plates caused by the interaction of creep and stress relaxation can be considered as such an effect. Within this project, a loss in bolt pretension of up to almost 50% over a comparatively short period of experimental time was observed. By means of numerical calculations, creep deformation in the transition areas between pipe and flange plate sections was identified to be a major originator of these pretension drops. Particularly, good correlations between experiments and FEM-simulations could be achieved through the implementation of enhancements concerning the fitting process of the Graham-Walles-type creep model which was used.