Session: 21-07 Mechanical Integrity and Materials for Steam Turbines

Paper Number: 101142

101142 - Constitutive Model Based Efficient Creep-Fatigue Damage Computation Technique for Steam Turbine Rotors to Enhance Flexible Operation Capabilities 

Reduction of greenhouse gas emissions from the conventional sources in the energy sector and increasing the share of renewables in the energy mix, at present, is considered as a key lever to avert the effects of climate change. Instabilities of the electrical grid caused by incorporation of renewables necessitate conventional steam power plants to operate in a flexible mode. Steam turbines have thick-walled components which operate at high temperature by design and the advent of enhanced flex-cycling significantly increases the damage caused due to creep-fatigue interaction. Accurate damage prediction of such components is necessary to ensure high availability, reliability and long service life in the new era of flex-operating regime.

Conventional life assessment techniques based on uncoupled linear damage accumulation, which are widely used at present, are conservative and have to be implemented with high factors of safety for design. The non-linear coupling of creep and fatigue damage variables, in more advanced cyclic viscoplastic constitutive material models, accurately depict the mutual accelerating effect of the interacting phenomena, this has enabled a unified and accurate prediction of damage. However, high computation time is currently a major disadvantage of these complex natured constitutive models.

In this paper various start-up and shut down sequences of a typical high temperature steam turbine rotor using transient heat transfer is studied. This is followed by damage evolution study using finite element implementation of constitutive model based on modified Chaboche kinematic hardening model to include the damage parameter, modified Chaboche and Rousselier isotropic hardening model including damage parameter, Norton type visco-plastic flow model, Lemaitre’s damage potential function and Kachanov-Rabotnov creep damage law.

To substantially reduce computation time a novel approach of damage accumulation using the constitutive models coupled with damage is presented here using a finite element method (FEM) based representative inelastic strain input cycle (RIC) concept. The multi-axial inelastic strain history obtained from the FEM for various start types i.e. cold, warm, hot starts and load changes is used for local computations of damage using the RIC and the same constitutive models, using a non-iterative Asymptotic Numerical Method (ANM). To advantage, only initial few cycles of the sequence have to be computed using the time-consuming finite element based techniques. Subsequent cycles are computed efficiently using the proposed method. It is shown that the reduction in computational efforts is substantial at very minimal loss of accuracy. The practicality and efficiency of the proposed method makes it possible to be used in daily industrial business. The current state of damage prediction accuracy and consequently capabilities of flexible operation of steam turbine rotors is greatly enhanced.

Presenting Author: Suvadeep Sen Siemens Ltd.

Presenting Author Biography: Suvadeep Sen is an Advisory Expert in structural integrity of steam turbines at Siemens Energy. His notable contributions at Siemens Energy includes ideation and implementation of various new design concepts with a credit of two patents. In recent years he has been involved in active research in the field of creep-fatigue interaction with constitutive modeling for steam turbine components to address the need of flexible operation with energy transition.

Authors:

Suvadeep Sen Siemens Ltd.
Henning Almstedt Siemens Energy Global GmbH & Co. KG