Computed Tomography Wall Thickness Inspection to Support Gas Turbine Blade Life Extension

The inclusion of Full Solution Rejuvenation (FSR®) in repairs of flight and aero-derivative gas turbine blades has shifted the primary cause for blade retirement from creep life consumption which is a function of service hours to primarily geometric limitations that are more governed by the cumulative number of repair cycles. For internally cooled components, one of the most significant causes for rejection is the remaining wall thickness of the airfoil. Operating blades with under-sized wall thickness can reduce the load-bearing capability and can increase the stresses that develop under transient thermal conditions found in operation. Typically, ultrasonic wall thickness measurement techniques are used during repair processing for determining remaining wall thickness on components but a number of limitations to obtaining accurate results with this process have been identified. Computed Tomography (CT) wall thickness inspection has addressed these limitations and become an important tool for extending the life of components beyond the typical OEM limits during repair. Entirely from the CT equipment user’s perspective, this paper explores a number of technical findings in the development of a highly accurate CT wall thickness inspection process for flight and aero-derivative gas turbine blades for utilization during repair after one or more service intervals. The importance of the accuracy of these wall thickness measurements is to ensure undersized blades are rejected and blades above the minimum wall thickness are accepted. Reducing uncertainty in the wall thickness measurements allows reconsideration of the acceptance limit and can result in more repairable blades returned for full service intervals. The target accuracy for measurements process was .002”. The findings described include aspects of equipment configuration, process parameters for the initial CT scanning, post-processing and interpretation, results validation specific to the component being measured and process limitations encountered.