Failure Investigation Principles for Combustion Failure Investigation Principles for Combustion Turbines- Combining Science and ArtTurbines- Combining Science and Art

Presented by

Ron Munson, P.E. & Dr. Swami SwaminathanPrincipal Engineers

Mechanical & Material Engineering, LLC

Westinghouse 501F/G Users MeetingSan Diego, CA January 30, 2007

IntroductionIntroduction

What is a failure?-What is a failure?-– Inability of a turbine to perform its function Inability of a turbine to perform its function

with reasonable safetywith reasonable safety ExamplesExamples

– Unscheduled disassembly of a turbine?Unscheduled disassembly of a turbine?– Fractured Turbine Blade?Fractured Turbine Blade?– Cracked Turbine Blade?Cracked Turbine Blade?– Plugged Fuel Nozzle?Plugged Fuel Nozzle?– Dirt Air Filter?Dirt Air Filter?

IntroductionIntroduction

The definition of “Failure” will vary The definition of “Failure” will vary greatly and is debatable.greatly and is debatable.

Financial definition:Financial definition:– Repair cost beyond planned maintenance Repair cost beyond planned maintenance

budgetbudget– Extends downtime beyond scheduleExtends downtime beyond schedule– Exceeds Insurance deductibleExceeds Insurance deductible

No clear cut definition, but when you No clear cut definition, but when you have one you will know it!have one you will know it!

Levels of Failure Levels of Failure AnalysisAnalysis Level 1- determine Level 1- determine MechanismMechanism of of

damagedamage Level 2- determine Mechanism Level 2- determine Mechanism

and and CauseCause for the damage for the damage Level 3- determine the Level 3- determine the Root Root

CauseCause for the damage for the damage

Levels of Failure Levels of Failure Analysis- CostsAnalysis- Costs Level 1- Determine Level 1- Determine MechanismMechanism of of

damage damage Cost XCost X Level 2- Determine Mechanism Level 2- Determine Mechanism

and and CauseCause for the damage- for the damage- Cost 3-Cost 3-10 X10 X

Level 3- Determine the Level 3- Determine the Root CauseRoot Cause for the damage= for the damage= Cost 5-100 XCost 5-100 X

Managing the Owner Expectations!Managing the Owner Expectations!

Levels of Failure Levels of Failure Analysis- Certainty of Analysis- Certainty of AnalysisAnalysis Level 1- Determine Level 1- Determine MechanismMechanism of of

damage damage 80-99%80-99% Level 2- Determine Mechanism and Level 2- Determine Mechanism and

CauseCause for the damage- for the damage- 50-90 %50-90 % Level 3- Determine the Level 3- Determine the Root CauseRoot Cause

for the damage- for the damage- Always less than Always less than 100%100%

Failures never have singular causes.Failures never have singular causes.

Basic Steps in Failure Basic Steps in Failure InvestigationInvestigation Secure EquipmentSecure Equipment DocumentDocument PreservePreserve DocumentDocument Assemble RCA TeamAssemble RCA Team Plan and delegatePlan and delegate Begin DismantleBegin Dismantle

Basic Steps in Failure Basic Steps in Failure Investigation Investigation (continued)(continued) Document, DocumentDocument, Document TriageTriage Select HardwareSelect Hardware Metallurgical Analysis – Non-DestructiveMetallurgical Analysis – Non-Destructive Prepare ProtocolPrepare Protocol Metallurgical Analysis - LaboratoryMetallurgical Analysis - Laboratory Draft Findings – Review - Finalize Met.Draft Findings – Review - Finalize Met. Feed Information to RCA TeamFeed Information to RCA Team

RRoot oot CCause ause AAnalysis nalysis TeamTeam Owner/OperatorOwner/Operator OEM EngineersOEM Engineers A/E and their Subcontractors if in A/E and their Subcontractors if in

WarrantyWarranty Insurance AdjusterInsurance Adjuster Third PartiesThird Parties Repair VendorRepair Vendor

RRoot oot CCause ause AAnalysis nalysis Team- Team- CautionsCautions If RCA team is entirely OEM If RCA team is entirely OEM

Personnel- STOP!Personnel- STOP! If there is an If there is an LLong ong TTerm erm SService ervice

AAgreement in Place - Bewaregreement in Place - Beware Generally, there will Generally, there will nevernever be a be a

single root cause—best effort is a single root cause—best effort is a weighted list of contributory weighted list of contributory causes.causes.

Combustion TurbinesCombustion Turbines

Combustion Turbines are particularly Combustion Turbines are particularly susceptible to the occurrence of susceptible to the occurrence of undocumented failure mechanisms.undocumented failure mechanisms.

CTs contain many very highly alloyed, state-CTs contain many very highly alloyed, state-of-the-art materials.of-the-art materials.– These materials are usually introduced with short These materials are usually introduced with short

testing cycles 10,000 to 24,000 hours but asked to testing cycles 10,000 to 24,000 hours but asked to perform for times approaching 100,000 hoursperform for times approaching 100,000 hours

– Alloys are complex and each alloy additive can Alloys are complex and each alloy additive can react with its neighbors to produce unexpected react with its neighbors to produce unexpected consequencesconsequences

Munson’s Axiom - The more The more sophisticated the alloy the sophisticated the alloy the more insidious and more insidious and unpredictable the damage unpredictable the damage mechanism!mechanism!

Strengthening Strengthening MechanismsMechanisms

Superalloys are strengthened Superalloys are strengthened by several mechanismsby several mechanisms– Solid solution strengtheningSolid solution strengthening– Carbide or boride precipitationCarbide or boride precipitation– Gamma Prime (Gamma Prime (`) or gamma `) or gamma

double prime (double prime (``) coherent ``) coherent precipitationprecipitation

– Dispersion strengthenedDispersion strengthened

Degradation Degradation MechanismsMechanisms Superalloys are degraded in service by Superalloys are degraded in service by

many different mechanismsmany different mechanisms– Solutioning of Solutioning of ` or ` or ````– Over-aging of Over-aging of ` or ` or ` `` `– Formation of TCP (topographically close- Formation of TCP (topographically close-

packed phases- Alpha Chrome packed phases- Alpha Chrome ((- Cr)- Cr) , Delta , Delta Phase Phase (())

– Dissociation and reformation of carbidesDissociation and reformation of carbides– Growth or “ripening” of carbidesGrowth or “ripening” of carbides– Environmental deterioration such as Environmental deterioration such as Gas Gas

Phase Embrittlement, Corrosion by Phase Embrittlement, Corrosion by Combustion By-ProductsCombustion By-Products

Degradation Degradation MechanismsMechanisms Superalloys are also very sensitive to Superalloys are also very sensitive to

fabrication or processing factorsfabrication or processing factors

““The complete story of a superalloys The complete story of a superalloys manufacture and service can be read manufacture and service can be read from its microstructure” from its microstructure” JF RadavichJF Radavich– Casting segregation: frecklesCasting segregation: freckles, , ` -eutectic, ` -eutectic,

eta, script carbideseta, script carbides– Cooling rate/processingCooling rate/processing

Not all manufacturers are created equalNot all manufacturers are created equal

Physics Cannot be IgnoredPhysics Cannot be Ignored

Hot metal expandsHot metal expands Gas flows from high pressure to lower Gas flows from high pressure to lower

pressurepressure Rotating parts do not like debrisRotating parts do not like debris Compressed air gets hotCompressed air gets hot 99.7 % filtration is still 0.3 % 99.7 % filtration is still 0.3 %

contaminationcontamination Metal heated above 2600°F melts Metal heated above 2600°F melts

without coolingwithout cooling

The Metal Does Not LieThe Metal Does Not Lie

Believe the metal !!Believe the metal !!

SummarySummary

Be sure you understand the level Be sure you understand the level of failure analysis you really needof failure analysis you really need

The metallurgist has a role, but The metallurgist has a role, but only provides a piece of the puzzleonly provides a piece of the puzzle

If the data is contradictory to your If the data is contradictory to your theorytheory– Wrong theoryWrong theory– Data is wrongData is wrong

Case StudiesCase Studies

WARNING IMAGES THAT FOLLOW MAY BE UPSETTING TO WARNING IMAGES THAT FOLLOW MAY BE UPSETTING TO COMBUSTION TURBINE OWNERSCOMBUSTION TURBINE OWNERS

Gas Phase Gas Phase Embrittlement of Embrittlement of Inconel 718 and 706Inconel 718 and 706

Gas Phase Gas Phase Embrittlement of Embrittlement of Inconel 718 and 706Inconel 718 and 706 Intergranular fracture but has a distinct Intergranular fracture but has a distinct

origin and crack path discernible by optical origin and crack path discernible by optical viewing.viewing.

Fracture surface is heavily oxidized with Fracture surface is heavily oxidized with decreasing thickness as you move away from decreasing thickness as you move away from origin.origin.

No evidence of microstructural alteration no No evidence of microstructural alteration no gamma double prime degradation- i.e., no gamma double prime degradation- i.e., no long term overheating.long term overheating.

No evidence of corrosive agent.No evidence of corrosive agent.

Gas Phase Gas Phase Embrittlement of Embrittlement of Inconel 718 and 706Inconel 718 and 706 Different names- SAGBO (Different names- SAGBO (sstress tress

aaccelerated ccelerated ggrain rain bboundary oundary ooxidation), xidation), hold time crackinghold time cracking

Cracking requires stress, time, Cracking requires stress, time, temperature, and environment (oxygen)temperature, and environment (oxygen)– Addressing only stress will not alleviate the Addressing only stress will not alleviate the

problemproblem– Temperature is a factor as oxygen diffusion Temperature is a factor as oxygen diffusion

along grain boundary is necessaryalong grain boundary is necessary

Alpha Chromium Alpha Chromium Formation in Inconel Formation in Inconel 718718

Alpha Chromium Formation Alpha Chromium Formation in Inconel 718in Inconel 718

Alpha Chrome is a BCC phase caused by Alpha Chrome is a BCC phase caused by rejection of chromium from the solid rejection of chromium from the solid solution matrix usually as another TCP solution matrix usually as another TCP precipitate (Delta Phase) is formed.precipitate (Delta Phase) is formed.– Non-Coherent PrecipitateNon-Coherent Precipitate– Long Time to Form - thousands of hours at high Long Time to Form - thousands of hours at high

temperatures (1250°F)temperatures (1250°F)– Often associated with locally segregated areas in the Often associated with locally segregated areas in the

componentcomponent– Once formed, cannot be removed by heat treatmentOnce formed, cannot be removed by heat treatment

Alpha Chromium Formation Alpha Chromium Formation in Inconel 718in Inconel 718

Formation results in loss of creep resistanceFormation results in loss of creep resistance At least four high pressure CT discs have At least four high pressure CT discs have

failed (in Gas Generator Section)failed (in Gas Generator Section)– Local creep of wheel hooks resulting in blade Local creep of wheel hooks resulting in blade

liberationliberation Remediation - Retirement of hot section Remediation - Retirement of hot section

discs based upon time in service at firing discs based upon time in service at firing temperaturetemperature

Alpha Chromium Alpha Chromium Formation in Inconel 718Formation in Inconel 718

DetectionDetection– Metallography - 5% Chromic acid electrolytic and Metallography - 5% Chromic acid electrolytic and

Heppanstall Etchant Heppanstall Etchant – View in SEM - use EDAX to confirm Chromium View in SEM - use EDAX to confirm Chromium

segregationsegregation– Electro polishing is the best for preparationElectro polishing is the best for preparation– Local drop in hardness, but alpha chrome Local drop in hardness, but alpha chrome

formation can be scatteredformation can be scattered

Corrosion Fatigue Corrosion Fatigue Cracking of Precipitation Cracking of Precipitation Hardened Stainless Steels Hardened Stainless Steels (17-4PH)(17-4PH)

Corrosion Fatigue Cracking Corrosion Fatigue Cracking of PH Hardened stainless of PH Hardened stainless SteelSteel

To enhance compressor efficiency, manufacturers are using high strength stainless steels instead of the more traditional low hardness martensitic stainless steels.– Lower cross-sectional area - less inertial damping

The fatigue strength of high strength precipitation hardened stainless steel is very sensitive to corrosion damage and environment when compared to more typical martensitic steels.– Corrosion pitting causes dramatic loss in crack initiation

resistance – Pitting is likely in a compressor– Corrosive environment may increase crack propagation

rate

Corrosion Fatigue Cracking Corrosion Fatigue Cracking of PH Hardened stainless of PH Hardened stainless SteelSteel

CT compressors have both stator and rotating blades made of these steels (17-4 pH, 15-5 pH, Jethete [steam]).

On CT compressors corrosive deposits are concentrated at the evaporation zone– Moist air with deposits (small particles that pass through

filters and volatile species) enter front of compressor– Air is heated and moisture dries out leaving deposits– Load cycles and changes in ambient conditions

intermittently wet and dry deposits setting up corrosion cells

– Pitting corrosion results Stator airfoils most susceptible as rotating blades shed

deposits by centrifugal loading