Case study: HP Casing LCF Life Assessment
Location: HP Casing inlet bowl
Issue: Multiple cracks were found and indications remained after grinding
Objectives: Root cause analysis and remaining life assessment
Diagnosis: LCF due to thermal cycling
Background
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Major cracks were found in the HP casing inlet annulus of a HP-IP turbine unit.
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The indications could not be completely removed after grinding away significant amount of material.
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Operators elected to pursue an advanced analyses (CFD/FEA) to assist in weighing actions that might be undertaken prior to the next outage scheduled in a few years.
Summary of Analyses
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A fairly accurate 3D model was constructed based on limited information from a longitudinal diagram and 2D drawings.
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The operating history with three different startup modes was analyzed statistically and used in subsequent aero-thermal calculations and life assessment.
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The principal source of the observed damage on the HP casing is attributed to thermal fatigue, i.e. the thermal gradient produced across the wall of casing each time the unit completes a start process.
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The analysis shows that it will take another 9-10 years before the crack reaches the critical crack depth if recent startup profiles and frequency continue.
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With sufficient margin, the finding supports "Leaving and monitoring the indication" and re-assessing the condition in the next scheduled outage.