Adaptability of coal-based power generating units to accommodate renewable energy sources is becoming increasingly important. One of the main challenges given by frequent start-ups, shutdowns and steeper load gradients is the lowering of lifetime of main components like thick-walled components in steam turbines. New operation scenarios leading to local high temperature gradients and hence high thermal induced stress. In order to improve flexibility, reduce start-up time and extend the life cycle, General Electric has developed a product to pre-warm/warm-keep steam turbines using hot air.

Pre-warming uses a hot air through flow in the turbine flow path to pre-heat to warm or hot start conditions starting on cold or warm state. Warm-keeping sets in during natural cooling at a high temperature level and is utilized to hold this level till a power plant start-up is conducted.

In this paper two main contributions to optimizing the warming arrangement are presented. Firstly, the calibrated model of a 19-stage IP steam turbine is analyzed regarding time-dependent mass flow rates at pre-warming mode. The influence to a duration time of a process and to the thermally induced stress is investigated. This investigation utilizes a detailed 3D hybrid (HFEM-numerical FEM and analytical) model of the turbine including the rotor, inner casing and blading for computationally-efficient determination of transient temperature fields in individual components. The thermal boundary conditions are calculated by means of heat transfer correlations developed for this purpose. Moreover, a separate FEM model allows for the implementation of a structural mechanical analysis. Using an increasing mass flow rate over time the maximum thermal induced stress can be reduced drastically by a flattening of the transient stress curve. This allows for an even higher increase in mass flow rate which therefore leads to a reduced pre-warming duration.

Secondly, selected pre-warming strategies are compared against warm-keeping scenarios for different power plant standstills. With use of a modeled natural cooling transient temperature distribution both scenarios can be modeled to reach a specific characteristic temperature after the standstill time. The analysis is aimed at a minimum thermal energy use required for reheating of air in the corresponding warming arrangement. For a vast data basis some typical start-up temperature levels are defined to reach for subsequent start-up.  Hence, the pre-warming and warm-keeping operating strategies are evaluated with regard to their energy demand before start-up. Thus, based on the duration of standstill, the most energy-efficient turbine warming strategy can be chosen to ensure the specified start-up conditions.


Comparison of Steam Turbine Pre-Warming and Warm-Keeping Strategies Using Hot Air for Fast Turbine Start-Up

Division / Sponsor:
International Gas Turbine Institute (IGTI)

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

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