A Position Paper on the Qualitative Versus Quantitative Residual Life Assessment (RLA) of Turbine Gas Path Components
A. K. Koul
A lot of confusion surrounds the residual life assessment (RLA) technology for service exposed gas turbine components because simple metallurgical analysis has often been passed on as RLA to gas turbine users by numerous independent consultants including repair and overhaul businesses. These metallurgical analysis based RLA reports provide a recommendation for further use without providing any substantial quantitative basis for life extension. It is imperative to know the exact stress and temperature at the primary fracture critical location of a component for a given engine usage profile and then use these stress and temperature values in conjunction with a damage modeling technique to predict the component life. This is the only reliable engineering approach to quantify the residual life of a component. When metallurgical analysis based experts are probed about the qualitative basis of their life extension recommendation, their past experience with similar studies is often invoked as an argument to defend their recommendations.
A close scrutiny of the two approaches thus becomes necessary to understand the differences between the two approaches including their advantages and disadvantages. The following sections provide a stepwise description of the two approaches and this is followed by a concluding remarks section.
Qualitative RLA Procedures
The conventional residual life assessment (RLA) of gas turbine blades is based on simple metallurgical testing and analysis. This involves microscopic (Optical / SEM) observation of transverse sections and coupon level testing (typically, hardness, © Life Prediction Technologies Inc. 2 impact and creep rupture tests) of the used component followed by comparative analysis with the standard microstructure and materials properties of either unused component or the cold sections such as fir-tree regions of the used blades.
Following are the standard steps prescribed for a typical turbine blade material.
- Standard metallographic specimens are prepared from the top, middle andbottom airfoil and root sections of blades.
- Specifically, the investigators look for creep cavitation in forged components and wedge type cracks or fine grain boundary cracks or oxidation induced cracks in cast blades and surface damage modes including oxidation, corrosion and erosion in each section of the airfoil.
- The investigators study and analyze any changes in the microstructural features such as the grain size, γ’ precipitate growth and coalescence, primary MC carbide degeneration and formation of any service induced continuous networks of grain boundary carbides or γ’ denuded zones adjacent to the grain boundaries in conventionally cast materials or γ’ rafting in directionally solidified or single crystal parts.
2. Temperature Assessment
- The average γ’ precipitate size in the top, middle and bottom airfoil sections is measured to assess the bulk component temperatures using standard time and temperature dependent γ’ precipitate coarsening data for the alloy.
- In some cases, the γ layer thickness in MCrAlY coated parts is also measured to assess the near-surface temperature of the blade in different locations of the airfoil.
- The work carried out in 2(a) and 2(b) on airfoil coupons that are subjected to the refurbishment treatment is repeated to demonstrate the effectiveness of the refurbishment treatment in rejuvenating the microstructure of the part.
3. Creep Rupture Testing
- Standard creep specimens are cut out of material coupons from the mid-airfoil and blade root regions.
- Standard creep-rupture tests are conducted at the same temperature and stress that is used by the original equipment manufacturer (OEM) to qualify the heat treated parts, e.g. 800˚C / 345 MPa in the case of Udimet 520 material, 760˚C / 586 MPa in the case of IN738LC material etc. Reduction in rupture life in specimens machined from the airfoil section relative to the root section stress rupture specimensis assessed and rupture life of the airfoil specimens subjected to the refurbishment process is also assessed to demonstrate the effectiveness of the refurbishment process in restoring the creep life of the part.
- Creep ductility and reduction in area of the service exposed and refurbished specimens are collated and the results are compared with the root section data to demonstrate the effectiveness of the refurbishment process in restoring the overall creep properties of the service exposed part.
- The creep-rupture life results may further be plotted as a function of service exposure time on a log-log plot as a function of rupture life of test specimen vs. service exposed component rupture life data under similar testing conditions if available.
4. Hardness and Impact Tests
- Perform Rockwell (C-Scale) hardness measurement and Charpy impact tests on specimens cut from airfoil coupons in as exposed and refurbished conditions.
- Repeat work carried out in 4(a) on root section coupons.
- Report and compare results.
- Comment on the visual observations
- Present and discuss microstructural observations in terms of operative damage mechanisms during service
- Tabulate airfoil temperature computation results
- Present and discuss creep-rupture results
- Provide guidance on remaining life
- Provide recommendations for inspections
Quantitative RLA Procedure Using XactLIFE Prognostics System
The qualitative RLA method described in the previous section relies on the results obtained from the accelerated creep or stress rupture tests to assess the creep life of the components that actually operate under considerably different stress and temperature conditions during engine operation. It is now well known that the dominant deformation mechanism in short term tests is considerably different than that operative during service. In addition, relying on γ layer thickness measurements in MCrAlY coated parts or bulk γ’ size measurements in the airfoil sections of the service exposed parts to estimate the blade operating temperature at a given location or determining the temperature profile can also yield erroneous results because such temperature estimation techniques have been shown to possess an inherent temperature estimation error of the order of 50˚C. Considering the fact that, in certain temperature regimes, a 50˚C error in temperature estimation can yield an error in life of the order of 2 to 3, the use of such RLA techniques is thus highly questionable.
The short term stress rupture tests used in qualitative RLA are actually designed to qualify a batch of material and to ensure that no mishaps have occurred during the manufacturing and processing of the new or refurbished materials. While this © Life Prediction Technologies Inc. 4 qualification test is required to ensure that the structural integrity of the parts is not compromised as a result of the refurbishment process, such short term tests often yield erroneous results from an RLA perspective. This is why quantitative RLA techniques have now evolved because qualitative RLA can compromise the machine safety. There are examples of lawsuits undertaken by the users where a refurbished set of parts was responsible for the engine failure because qualitative RLA techniques had been employed by the repair shops to establish the usable life of the refurbished parts.
The technological advancements in combustion modeling, engine modeling, finite element method based simulations in conjunction with temperature dependent deformation and fracture mechanisms and physics based materials damage modeling, a precise thermal-mechanical stress-strain-temperature profile of the parts and an accurate estimate of the design life or remaining service life can be computed for any engine component for the actual engine operating conditions and the operating environment rather than assumed operating conditions used by the original equipment manufacturers (OEMs) during design stages of the engine. Life Prediction Technologies Inc. delivers such prognostics based lifing and health management services to OEMs, third party parts manufacturers and operators of aero, marine and land based gas turbine engines. LPTi relies on physics based approaches to describe the component cracking, distortion and coating degradation processes in gas turbine components. This methodology uses homologous temperature dependent analysis and logic to choose appropriate deformation/fracture mechanism models and materials data bases depending on the microstructures, operating temperatures, environmental exposures and loading conditions of the turbine components. LPTi holds the Canadian and US patentsfor a physics based failure prediction system, XactLIFE™, for real time prognostics, analysis and residual life assessment of machine components.
Following procedures are followed in XactLIFE™ based quantitative RLA of gas turbine components.
1. Combustor Modeling
- Estimate the average combustor nozzle plane gas temperature using EGT or TIT data from the engine
- Determine the combustor nozzle plane temperature profile for the actual engine operating conditions
2. Off-design Engine Modeling
- Collect on-design and off-design engine parameters
- Generate the turbine and compressor maps and locate engine operational points on the maps
- Compute the pitch-line gas path temperature and pressure for the compressor and turbine components for the actual engine operating conditions
- Compute the 2D gas path temperature profile around the component
3. Thermodynamic Analysis
- Compute the metal surface temperature profile for the engine operating conditions using heat transfer coefficient based analysis.
- Account for component cooling effects on the component surface temperature profile
4. Finite Element Analysis
- Create a 3D solid model of the component and mesh the model.
- Compute 3D bulk material temperature profile and stress-strain profiles using surface temperature as the boundary condition through heat transfer analysis.
- Identify oxidation prone airfoil locations
5. Damage Analysis
- Locate operating thermal-mechanical loading conditions of the blade on the deformation mechanism map of the component material
- Choose appropriate physics based creep or fatigue damage models to identify the primary, secondary and tertiary fracture critical locations in the component.
- Compute safe life of the component for the primary fracture critical location
- Conduct probabilistic analysis, if necessary, to quantify the reliability of the component.
- Present gas path temperature profiles for the engine operating conditions
- Present thermal-mechanical stress-strain profiles of the component for the engine operating conditions
- Present the component displacement profile
- Identify fracture critical locations and present the life profile of the component
- Present and discuss failure modes and RLA results including microstructural degradation.
- Suggest a usable life interval as well as an inspection regiment including Boroscope inspection for the component.
The intent of this position paper is not to downplay the importance of any specific RLA methodology. Both qualitative as well as quantitative RLA procedures are required once a refurbishment process has been developed for service exposed gas path components.
A clear advantage of the qualitative RLA in the past has been its cost effectiveness and a quick response time in furnishing the results of the analysis. Typically, a qualitative RLA costs $10,000 per component in the western countries. The major disadvantage of this methodology is its highly qualitative nature and the availability of highly trained and experienced personnel to conduct the analysis. Even a trained eye can sometimes be fooled because subtle changes in the microstructure during service or the refurbishment process can have a substantial impact on the usable life of a component.
In the past, the quantitative RLA was the monopolized domain of the OEMs and major national laboratories because they possessed the necessary engineering expertise in various engineering disciplines such as computational fluid dynamics, engine modeling, structural analysis and materials engineering to be able to undertake a project of this nature. It would typically cost $250,000 to 300,000 to conduct an analysis of this nature. Most users only indulged in this type of analysis if a major mishap had occurred during service or lawsuits were involved in the event of a catastrophic failure. However, with the availability of very cheap and powerful computing power, the quantitative RLA is no more a daunting task and it is also very affordable. It has also ceased to be the monopolized domain of the OEMs and major national laboratories. With the availability of products like XactLIFE in the market, the analytical costs per component have been reduced to $40,000 to $50,000. At LPTi, we can revert back to the user with complete analysis and recommendation in 2 to 3 weeks time once the component geometry and engine operating information is made available to us. These costs are bound to reduce once processes such as those implemented in XactLIFE is further automated.