The growing share of volatile renewable power generation poses a new operating scheme of conventional power plants. This operating scheme, which presuppose the frequent start-ups, shutdowns and steeper load gradients, inevitably leads to excessively high thermally induced stresses in the main components of watersteam cycle, i.a. in rotors and casings of high/intermediate pressure steam turbines. With consideration of maximum allowable temperature gradients, the start-up time of turbine therefore strongly depends on its initial thermal state. Hence, the turbines, which may be equipped in either concepts aimed to minimize heat losses into the environment during idle times (warm-keeping solutions), or to gradually raise the temperature from cold/warm state (prewarming solutions), benefit from lower thermal stresses and a shorter start-up time. This in turn results in reduced lifetime consumption and increased electricity generation. Hernce, motivated by the urgent need for flexibility and start-up capability improvements of conventional power plants in addition to extending their life cycle, General Electric provides its customers with a product to pre-warm steam turbines using hot air.

In this paper, the transient thermal and structural analyses of a 19-stage IP steam turbine in various start-up operating modes are discussed in detail. The presented research is based on previous investigations and utilises a hybrid (HFEM – numerical FEM and analytical) approach to efficiently determine the time-dependent temperature distribution in the components of the steam turbine. The simulation strategy of the HFEM model applies various analytical correlations to describe heat transfer in the turbine channel. These are developed by means of extensive unsteady multistage conjugate heat transfer simulations for both startup turbine operation with steam and pre-warming operation with hot air. Moreover, the complex numerical setup of the HFEM model also considers the thermal contact resistance (TCR) on the surfaces between vane and casing as well as blades and rotor. Prior to the analysis of turbine operating modes, the typical start-up turbine process is calculated and validated against an experimental data as a benchmark for subsequent analysis. In addition to heat transfer correlations, the simulation of turbine start-up from cold state uses an innovative analytic pressure model to allow for consideration of condensation effects during first phase of start-up procedure.

Finally, the presented thermal investigation focuses on the comparison of transient temperature fields in the turbine for different startup scenarios after pre-warming with hot air and provides the subsequent structural investigation with required boundary conditions. As a result, the values of the highest stress are numerically determined and compared to the values of stress obtained by means of cold start-up simulation. The final evaluation clearly displays the technical potential and the benefits of the pre-warming concept.


Thermo-Structural Analysis of Steam Turbine Start-Up With and Without Integrated Pre-Warming System Using Hot Air

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

Turbo Expo - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.