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Probabilistic assessment of the potentialcreep damage at the surface of steam turbinerotor shafts
Ralf Voß, Henning Almstedt
2014-09-26
Unrestricted © Siemens AG 2014 All rights reserved.
Page 2 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shaftsTable of content
• Motivation
• Deterministic design rules
• Implementation strategies ofProbabilistic Design
• Exemplary application: Potentialcreep damage in steam turbinerotor shafts
• Resume
2014-09-26
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Page 3 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shafts
Requirements from steam turbine customers:• Due to changes in the energy-mix increased flexibility
necessary:Ø Increasing number of start-upsØ Faster start-upsØ Part-load-operation
• Increasing cost-pressure on steam turbine manufacturers(e.g. US: low gas priceà efficiency less important; EU: powerplants are less time in load-operation à worse ROI)
Motivation
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication
► Resume
à Increasing mechanical utilization of steam turbinecomponents required
2014-09-26
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Page 4 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shaftsDeterministic design rules
State-of-the-art in the design of steam turbines: Deterministicmechanical design methods using „Worst-Case“-assumptions:• Nominal steam temperature and pressure• Minimum material parameters (e.g. creep rupture strength)Judgement:
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication
► Resume
• Is this really the limit?• What risk of failure belongs to this utilization?• How robust is the result when inputs variate?
SRSR
Em
CalcEmCalc
1
52
52 £Û£s
s
Inner of rotor: 1/S1/S
2014-09-26
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Page 5 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shafts
Curves of equalrisk level
Implementation strategies of Probabilistic Design
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication
► Resume
TurbineModel
Material
Geometry
Load regime
Turbine life timeconsumption(e.g. LCF-damage)
Inherent uncertainties(stochastic model)
Uncertainties in parameterestimation (depends on
available data)
PoF(Probabilityof Failure)
Numberof starts
Accepted PoF(e.g. 10%)
Approach 1: Development of Probabilistic Design methods thatare released in internal design rules or norms
2014-09-26
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Page 6 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shaftsImplementation strategies of Probabilistic Design
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication
► Resume
Approach 1: Development of Probabilistic Design methods thatare released in internal design rules or norms
Necessary aspects for realization:• Update of design rule:Ø Calculation procedureØ Acceptable risk level
• Development of adequate tools• Knowledge building in affected areas (e.g. mechanical design
department)
à Challenging and time-consuming implementationprocess!
2014-09-26
Unrestricted © Siemens AG 2014 All rights reserved.
Page 7 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shafts
Curves of equalrisk level
Implementation strategies of Probabilistic Design
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication
► Resume
Approach 2: Usage of Probabilistic Design methods to determinerisk-based safety factors for deterministic design. The designrules are updated with those new safety factors.
PoF(Probabilityof Failure)
Numberof starts
Accepted PoF(e.g. 10%)
Value calculated according todeterministic design rule
Sdet Sprob
2014-09-26
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Page 8 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shaftsImplementation strategies of Probabilistic Design
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication
► Resume
Approach 2: Usage of Probabilistic Design methods to determinerisk-based safety factors for deterministic design. The designrules are updated with those new safety factors.
Aspects for realization:• Update of design rule:Ø Safety factor
• Some conservativities still need to be kept (due to simplifications)• Changes in boundary conditions of probabilistic calculations need
to be checked regularly
à Quick implementation possible (but potential for furtherimprovement)!
2014-09-26
Unrestricted © Siemens AG 2014 All rights reserved.
Page 9 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shaftsExemplary application: Introduction
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication► Introduction
► Mechanics
► Sensitivtiyanalysis
► Probability offailure
► Risk assessment
► Challenges
► Resume
det
1
SDDDD
c
cf
£
<+=
w P
q&q&
Background:High-temperature rotors are exposedto life-time consumption in the inletstress-relieve groove due to:• Dc: creeping (steady state effect)
and• Df: Temperature gradients during
start up and shut down limiting thenumber of start-ups (low-cycle-fatigue damage)
2014-09-26
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Page 10 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shaftsExemplary application: Mechanics
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication► Introduction
► Mechanics
► Sensitivtiyanalysis
► Probability offailure
► Risk assessment
► Challenges
► Resume
w PApproach: Simplificationby a cylinder
( ) ÷÷ø
öççè
æ××
×+×××= 322
2222 3max
tfV Rf
PRfp
prs
Centrifugal force(50/60Hz)
Torsion caused bytransmittable power
),,(max
TimeOperationTfD VC s=Þ
2014-09-26
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Page 11 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shafts
Sensitivity analysis using Monte-Carlo-Simulation:
Exemplary application: Sensitivity analysis
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication► Introduction
► Mechanics
► Sensitivtiyanalysis
► Probability offailure
► Risk assessment
► Challenges
► Resume
Calcu-lationmodel
Rf (Geometry)
P (Power)
T (Temperature)
(MaterialProperty)
),(max
Tf Vs
CD
Obs
erve
dfre
quen
cy
CD
Normed share of rangcorrelation coefficientaccording to Spearman:
Material Property(Variation of creeprupture strength)
Temperature
2014-09-26
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Page 12 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shafts
Application to a statistical load regime:
Values used fordeterministic
designPD
F
Design target1)
Creep Damage Fraction Dc
Exemplary application: Probability of Failure
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication► Introduction
► Mechanics
► Sensitivtiyanalysis
► Probability offailure
► Risk assessment
► Challenges
► Resume
Share of valuesabove Sdet (here:
1.3%)
Sig_min = 0.8*Sig_median (Mpa)T_nom = 550ºC1)
1): Siemens-internalvalues differ from thesevalues!
detS
2014-09-26
Unrestricted © Siemens AG 2014 All rights reserved.
Page 13 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shafts
Application to a statistical load regime:
Values used fordeterministic
designPD
F
Design target1)
Creep Damage Fraction Dc
Exemplary application: Probability of Failure
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication► Introduction
► Mechanics
► Sensitivtiyanalysis
► Probability offailure
► Risk assessment
► Challenges
► Resume
Share of valuesabove Sdet (here:
3.8%)
Sig_min = 0.8*Sig_median (Mpa)T_nom = 550ºC1)
1): Siemens-internalvalues differ from thesevalues!
detS probSSx =× det
2014-09-26
Unrestricted © Siemens AG 2014 All rights reserved.
Page 14 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shafts
Curves of equal risk level 1)
Exemplary application: Probability of Failure
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication► Introduction
► Mechanics
► Sensitivtiyanalysis
► Probability offailure
► Risk assessment
► Challenges
► Resume
1
2
3
Scenario
Scenario
Scenario
4Scenario
1
2
3 4
1): Siemens-internal value differs from this value!
2014-09-26
Unrestricted © Siemens AG 2014 All rights reserved.
Page 15 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shaftsExemplary application: Probability of Failure
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication► Introduction
► Mechanics
► Sensitivtiyanalysis
► Probability offailure
► Risk assessment
► Challenges
► Resume
Challenges:• Adequate determination of the input variations and frequent
verification of changes in the variations• Development of efficient and problem-specific simulation
methods• Determination of an acceptable risk level (Health&Safety,
financial)
2014-09-26
Unrestricted © Siemens AG 2014 All rights reserved.
Page 16 Voß / E P SU R&D TEC COT 1
Probabilistic assessment of the potential creepdamage at the surface of steam turbine rotor shaftsResume
► Motivation
► Deterministicdesign rules
► Implementationstrategies ofProbabilisticDesign
► Exemplaryapplication
► Resume
Resume:
• Presentation of limitations ofdeterministic mechanicaldesign rules
• Introduction of two implementationstrategies of Probabilistic Design
• Exemplary application: Analysis of thepotential creep damage at steamturbine rotor shaftsØ Sensitivity analysisØ Probability of FailureØ Risk assessmentØ Challenges of the approach
w P
Turbine Model
Material
Geometry
Load regime
Turbine life timeconsumption(e.g. creep-damage)
PoF (Probabilityof Failure)
e.g. Operationtime
Accepted PoF(e.g. 10%)
Design target
Creep Damage Fraction Dc
Inner of rotor: 1/S1/S
Sdet
Sprob
detS probSSx =× det