Case study: HP Casing LCF Life Assessment
Location: HP Casing inlet bowl, diaphragm ledge key fit and packing head alignment pin hole
Issue: Multiple cracks were found
Objectives: Root cause analysis and remaining life assessment
Diagnosis: LCF due to thermal cycling
Background
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Cracks were found in multiple locations on the HP casing: Inlet bowl area, diaphragm ledge key fits and packing head alignment pin holes.
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Similar types of cracking appear to be noted by other operators.
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The purpose of the CFD/FEA analysis was to understand the risks associated with operating the unit if the cracks are left unrepaired and identify their sources and/or contributors. Fracture mechanics was employed to estimate the remaining life of cracks found in different locations on the casing and provide guidance as to their disposition.
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 it will take another 5-6 years before the crack reaches the critical crack depth if recent startup profiles and frequency continue.