Using 1D Models to Predict the Thermal Growth and Stresses During The Start up and Shutdown Phase of a Steam Turbine

Steam turbines are not just restricted to conventional or nuclear power plants, they are widely used in combined cycle power plants, concentrated solar thermal plants and also geothermal power plants. The operational requirements of a steam turbine in the combined cycle and CSP’s means that they operate under transient conditions. Even in conventional steam turbines, the market requirements are changing with requirements for faster and more frequent start-up which can result into faster deterioration of the equipment and reduced lifespan. During the startup phase, significant heat exchange takes place between the steam and the structural components that include the valves, rotor and casing. The accuracy of the life prediction is strongly affected and dependent on the accuracy of the transient thermal state prediction [1].

Though the expansion of steam takes place in the nozzles and blades, the influence of the leakage steam during the startup phase is significant with steam expanding through the labyrinths resulting in expansions, condensation, and increased velocities which may even reach supersonic levels. During cold start, the flow is minimal, the temperature of the metal is at room temperature and heat exchange happens between the steam and metal parts resulting in thermal stress.

Every designer is interested in making a prediction that is as accurate as possible. This requires modelling the entire flow path with all the intricate details which means generating a complex 3D model,use of extensive computational resources and so on, which ultimately results in more time and cost. Even if one has the luxury of using a complex 3D model with all the intricate details, the question is how to get the appropriate boundary conditions to be applied for the 3D simulation and how to reduce the number of iterations required between the flow analysis and structural analysis. The flow parameters and the area between sealing fins in the labyrinth (refer Fig.1) is varied, not only in each stage, but also within each component which means the heat transfer coefficient being applied to each of these locations is also different.

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   Figure 1: Sealing fins in a turbine stage

 

As seen in Fig. 2, the entire flow through the seals and cavities can be modelled as a 1D model considering both the flow and heat transfer between the fluid and metal. During design phase of the flow path, a streamline analysis tool such as AxSTREAM® can give the leakage flow parameters which can be further detailed using AxSTREAM® NET, the 1D flow and heat transfer module. The results from the 1D module, which gives the thermodynamic parameters and heat transfer coefficients at different zones in the seal, can be used to apply the boundary conditions for performing thermos-structural analysis of the steam turbine.

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                    Figure 2: 1D model of the flow and heat transfer in sealing fins of a turbine stage

 

To learn more about how the AxSTREAM® platform can be used for designing steam turbines, from concept to optimizing operating curves, please contact info@SoftInWay.com

 

 

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