Abstract
The changing energy landscape leads to a rising demand of more flexible power generation. A system for steam turbine (ST) warm-keeping provides the ability to shutdown conventional power plants during periods with a high share of renewable power. Simultaneously, these power plants are ready for grid stabilization on demand without an excessive consumption of lifetime during the start-up. One technical solution to keep a ST warm is the use of hot air, which is passed through the turbine. In addition, the air supply prevents corrosion during standstill and also enables the prewarming after maintenance or long outages. This paper investigates the warm-keeping process of an intermediate pressure (IP) ST (double-shell configuration) through the use of dynamic numerical finite element (FE) simulations. As a representative test case, warm-keeping calculations during a weekend shutdown (60 h) are conducted to investigate the temperatures, their distribution, and gradients within the rotor and the casing. For this purpose, an improved numerical calculation model is developed. This detailed three-dimensional FE model (including blades and vanes) uses heat transfer correlations conceived for warm-keeping with low air mass flows in gear mode operation. These analytical correlations take heat radiation, convection, and contact heat transfer at the blade roots into account. The thermal boundary conditions (BCs) at the outer walls of the rotor and casing are determined by use of experimental natural cool-down data. The calculation model is finally compared and verified with this dataset. The results offer valuable information about the thermal condition of the ST for a subsequent start-up procedure. The warm-keeping operation with air is able to preserve hot start conditions for any time period. Most of the heat is transferred close to the steam inlet of the turbine, which is caused by similar flow directions of air and steam. Thus, temperatures in the last stages and in the casing remain well below material limits. This allows higher temperatures at the first blade groove of the turbine, which is highly loaded during a turbine startup and thus crucial to the lifetime.
