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

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Background

  • 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.

  • OEM believed that creep was involved.

  • TTI was solicited to perform a FEA creep analysis to identify the root cause of cover lifting.

  • An important goal of the study was to assess the control value or allowable limit specified by OEM.

  • Another objective was to evaluate the cover band modification proposed by OEM. 

Summary of Analyses

  • 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.

  • FEA analysis establishes the relationship between creep damage at critical locations and the amount of cover lifting.

  • Improvement of new cover design proposed by OEM is verified by FEA calculations in comparison against the current cover design.