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Research Papers: Gas Turbines: Structures and Dynamics

Application of an Advanced Creep–Fatigue Procedure for Flexible Design of Steam Turbine Rotors Based on Fracture Mechanics Methods

[+] Author and Article Information
Shilun Sheng

Siemens AG, Energy Sector,
Rheinstrasse 100,
Mülheim an der Ruhr D-45478, Germany
e-mail: shilun.sheng@siemens.com

Henning Almstedt

Siemens AG, Energy Sector,
Rheinstrasse 100,
Mülheim an der Ruhr D-45478, Germany
e-mail: henning.almstedt@siemens.com

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 11, 2014; final manuscript received August 5, 2014; published online October 7, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 137(3), 032506 (Oct 07, 2014) (8 pages) Paper No: GTP-14-1377; doi: 10.1115/1.4028461 History: Received July 11, 2014; Revised August 05, 2014

The demand for steam turbine components is driven not only by high efficiency but also by high plant operational flexibility. Steam turbine rotors are therefore exposed to increased temperatures and increased number of stress cycles. These aspects should be considered for life-time prediction. Fracture mechanics methods are usually applied when crack like defects are detected not only for new rotors but also for rotor components in service. Based on the findings, a decision has to be made with respect to acceptability considering high temperature effects as well as the expected future operating regime. For defect analysis in the high temperature range, crack initiation and crack propagation under combined creep and fatigue loading need to be taken into account. Based on fracture mechanics methods and long-term testing data, an advanced creep–fatigue procedure for the evaluation of crack initiation and crack growth has been developed within the German Creep Group W14 for creep crack growth (CCG) behavior. Furthermore, recent studies show that the crack size for creep crack initiation (CCI) depends on material ductility and creep strain in the ligament. This paper demonstrates the industrial application of the abovementioned method for steam turbine rotor assessment, which has a focus on crack initiation and crack growth under creep–fatigue conditions. For crack initiation, a simplified approach based on defect size and material ductility is compared to a standard approach—two-criteria-diagram (2CD). For the advanced evaluation concept, the CCI criterion is combined for analysis with a creep–fatigue crack growth (CFCG) procedure. The benefit of the method especially for ductile material will be highlighted.

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Figures

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Fig. 1

Creep crack initiation time of 10Cr rotor material

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Fig. 2

Creep crack growth of 10Cr rotor material

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Fig. 3

Evaluation of CCI based on 2CD diagram

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Fig. 4

Influence of creep ductility on 2CD diagram

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Fig. 5

Definition of ath for DENT specimen

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Fig. 6

Definition of crack initiation criterion

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Fig. 7

Determination of uniform creep strain of uniaxial tests

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Fig. 8

Comparison of calculated creep strain with the reference strain at failure location

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Fig. 9

Iterations for determination of ath

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Fig. 10

Crack initiation size ath in dependence on rupture strain of 1Cr material

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Fig. 11

Crack initiation size ath in dependence on creep rupture strain

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Fig. 12

Crack initiation size ath depending on creep strain in ligament

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Fig. 13

Typical IP turbine module

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Fig. 14

Typical start-up/shut-down behavior of a steam turbine component: n: speed, P: power, T1: rotor surface temperature, Tc: rotor center temperature, tdwell: dwell time, and σth: thermal stresses in the rotor center (related to Tc see also Fig. 13)

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Fig. 15

Loading situation of a crack in an unbored rotor due to hoop stresses at steady state and during transient operation (start-up)/Fig. 13 section A—A

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Fig. 17

Comparison of creep crack-initiation times creep strain (a) 0.7% and (b) 0.35%

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Fig. 18

Principle sketch of creep–fatigue crack growth with and without CCI

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Fig. 19

Relative improvement by including creep crack-initiation in the analysis

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