Case study: Creep Analysis for a Blade with Cover Lifting
Stage: Stages 10 in a reheated IP steam turbine
Issue: Severe cover lifting found, and potentially caused by creep damage as suggested by OEM
Objectives: FEA creep analysis to assist operator to understand the underlying mechanism and evaluate the modification
proposed by OEM
Diagnosis: Creep at the juncture between cover and tenon leads to cover lifting
Solution: New cover design verified by analysis
Background
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Issue of cover lifting on 8th to 11th stages of IP turbine was encountered. The 10th stage experienced the most severe cover lifting among all stages.
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OEM believed that creep was involved.
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TTI was solicited to perform a FEA creep analysis to identify the root cause of cover lifting.
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An important goal of the study was to assess the control value or allowable limit specified by OEM.
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Another objective was to evaluate the cover band modification proposed by OEM.
Summary of Analyses
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Results from FEA analysis confirm root cause of the cover lifting in multiple stages of IP turbine is principally due to elevated temperature creep in conjunction with appreciable localized stress.
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FEA analysis establishes the relationship between creep damage at critical locations and the amount of cover lifting.
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Improvement of new cover design proposed by OEM is verified by FEA calculations in comparison against the current cover design.