Strategies for Managing the Implications of Cycle-Driven

Strategies for Managing the Implications of Cycle-Driven Operating Patterns on Turbines and Generators

Rachel SweigartLead Consulting Engineer, TG Advisers

Stephen R. Reid, P.E.President and Principal Engineer, TG Advisers

Agenda• Today’s Energy Market• Impact of Operating Pattern• Case Studies

– Rotor Shaft Cracking– HP Casing Cracking– Generator End Block Failure– Introduction to Dynamic Intervals

Confidential © TG Advisers™ Inc. All Rights Reserved 2

Today’s Energy Market• Quicker starts, even on

vintage units• Ultra minimum loads (15-

20% load on nights/weekends)

• Two shift cycling• Repowering into

combined cycles• Life extensions of LP

components• Major outage extensions


Impacts of Changes in Operations• Increased number of

start/stops– Thermal transients– Low cycle fatigue cycles

• Damage is CUMULATIVE• Static inspection intervals may

no longer accurately reflect unit needs– Often based on years rather

than today’s operating profile– May not account for age and

accumulated service on unit


CASE STUDY 1: ROTOR SHAFT CRACKING

Rotor Shaft Cracking• Common location of

cracking is J-groove• Large stresses

– Dramatic change in radius

– Large torsional load– Geometric and surface

condition stress concentrations


HP IP LP LP

Crack

Rotor Shaft Cracking• 1980s unit was retrofitted in

2000• 5 years into operation unit

shutdown due to vibration– 50+ cycles – Shaft end was found “nearly

severed”• Solution

– Weld repaired– Polished and shot peened– Returned radius to as-designed


The end?

Rotor Shaft Cracking• Second failure occurs

– Clear NDE results 1 year prior– 22 cycles after NDE, cracking found– Cracking noted due to increased unit

vibration• Root cause analysis determined

sources:– LCF stresses amplified by surface

pitting leading to crack initiation– Eventual HCF propagation – Secondary influence from

misalignment


Issue was not completely addressed after 1st failure

Options for Management• Detection of flaws:

– Eddy Current Inspections– Shifts in critical speeds

• Preventative:– Proactive machining– Removes fatigued material, pitting– Shot peen new surface

• Address found issue:– Re-radii groove– Weld build up material and re-

machine groove– Stub shaft replacement


Many options depending on rotor condition

CASE STUDY 2: STEAM CHEST CRACKING

Steam Chest Cracking• Repowered unit operating as

part of a combined cycle • Duty cycle shifted toward

heavy cycling • Found cracking around control

valve seats in steam chest– Replica testing on crack

locations concluded cracking mode was fatigue

– No creep found• Reinspection interval was set

at “3-5 years”


Reinspection interval did not account for all factors

Steam Chest Cracking Analysis• Low cycle fatigue fracture

mechanics analysis was completed– Based on as-found crack sizes– Estimated material fracture

toughness– Assumed max stress to equal yield

strength• Estimated time to reach critical

crack size (including safety factor of 2) was >1,000 cycles

• Not recommended to grind cracks– Based on past history, cracks

reappeared in fresh surface


Case Study Conclusions• Steam chest cracking was not found to be the

limiting factor for establishing future re-inspection intervals for the unit– Analysis can evaluate whether action is needed on

cracking or not

• If duty cycle is further changed, analysis can be updated


Correct solution may be to take no corrective action

CASE STUDY 3: GENERATOR END BLOCK

Generator End Block Failure• Generator slot conductors

expand and contract axially during on/off cycling

• Blocking holds end windings together as a unit

• Lost/migrated blocking allows for asymmetrical movement– Leads to mechanical forces– Leads to thermal forces

• Shorted turns• Uneven temperatures


Symptoms of Issue• High vibration on generator

– Limited base load operation– Vibration on unit correlated

with load and VARS

• Client noted movement in rotor end winding blocks and some blocks dropping from rotor


Generator End Block Failure• “Pop-rivet” type design end winding

blocking– Secures rotor end winding block to

rotor coil• RCA of end winding blocking

identified cause of failure in pop rivet flange as LCF from start/stop cycles over 1000

• Solution was modified end winding block and attachment design


Test for thermal sensitivity by varying MVars

Cracking

CASE STUDY 4: DYNAMIC INSPECTION INTERVALS

Case Study: Dynamic Inspection Intervals

• DFLP rotor • Cracking found in disc steeples

– Replications indicated fatigue – Steeples were re-radiused

• Re-inspection intervals were set based on operating hours– Re-inspection was occurring

multiple times between majors– Unit operational duty cycle

changed since hourly interval was established


Developing Dynamic Inspection Intervals• Based on unit specific analysis, not generic guidelines

– Allows for actual material properties to be used– Factors in results of prior inspection

• Correctly accounts for hours of operation, start-stop and overspeed cycles

• Appropriately credits or debits life based on actual operating conditions – Allows for impact of increased cycling and low load

operation on future maintenance costs to be assessed• Real time feedback on actual operation vs projected


Effectively & efficiently manage issues based on sound engineering analysis

Analysis DriversStress Corrosion Cracking• Operating Hours• Yield Strength• Temperature• Location of Wilson Line

Low Cycle Fatigue

• Frequency of Cycling• Stresses• Fracture Toughness


Base Loading Example

© TG Advisers™ Inc. All Rights Reserved 22

Cycles 100Hours 80,000Overspeeds 10

Inputs

Confidential

Cyclic Loading Example


Cycles 500Hours 10,000Overspeeds 10

Inputs

Confidential

LP Dynamic Inspection Intervals


NDE Minimum Detectable orCrack size found by NDE

Confidential

Case Study Conclusion

• Very different operating profiles can lead to the same cumulative effect

• Dynamic inspection intervals can efficiently manage issues with sound engineering practices

© TG Advisers™ Inc. All Rights Reserved 25Confidential

Questions?• TGA has accumulated over 200 case studies

associated with cycling and other failure modes• TGA welcomes any participant to join us at our

next training event


Feel free to contact us for advice on any turbine or generator issue (302) 691 – 330

[email protected] [email protected]

Documents

Strategies for Managing the Implications of Cycle-Driven