Lifing issues in hot gas path components of heavy-duty gas turbines

Abstract

Gas turbine hot-gas-path components, which include combustion liners, transition pieces, turbine nozzles and turbine buckets, are exposed to hot gases discharged from combustion systems and suffer from severe materials degradation and damage even in the early stage of operation. The severity of the damage and degradation increases with increasing inlet temperature and size of the gas turbines, which also increase the maintenance cost. ‘Lifing’ of components is, therefore, becoming a very critical issue. This paper describes several kinds of component damage and material degradation occurring in the 1,100°C- and 1,300°C-class heavy-duty gas turbines and then shows how we revised those component lives from the original design ones. Analytical-based assessment methods associated with condition-based assessment ones, some examples of assessment results, and component life extension technologies are also described.     

Functionally gradient ceramic/metallic coatings for gas turbine components by high-energy beams for high-temperature applications

Failure of turbine blades generally results from high-temperature oxidation, corrosion, erosion, or combinations of these procedures at the tip, and the leading and trailing edges of a turbine blade. To overcome these limitations, functionally gradient ceramic/metallic coatings have been produced by high-energy beams for high-temperature applications in the aerospace and turbine industries to increase the life of turbine components. Thermal spray processes have long been used to apply high-temperature thermal barrier coatings to improve the life of turbine components. However, these processes have not met the increased demand by the aerospace and turbine industries to obtain higher engine temperatures and increased life enhancement as a result of the inhomogeneous microstructure, unmelted particles, voids, and poor bonding with the substrate. High-energy beams, i.e. electron beam-physical vapour deposition (EB-PVD), laser glazing, laser surface alloying, and laser surface cladding, have been explored to enhance the life of turbine components and overcome the limitations of the thermal spray processes. EB-PVD has overcome some of the disadvantages of the thermal spray processes and has increased the life of turbine components by a factor of two as a result of the columnar microstructure in the thermal barrier coating (TBC). Laser glazing has been used to produce metastable phases, amorphous material, and a fine-grained microstructure, resulting in improved surface properties such as fatigue, wear, and corrosion resistance at elevated temperatures without changing the composition of the surface material. Laser surface alloying and laser surface cladding have shown promising results in improving the chemical, physical, and mechanical properties of the substrate's surface. Metal-matrix composite coatings have also been produced by a laser technique which resulted in increased wear and oxidation-resistant properties. The advantages and disadvantages of thermal spray processes, EB-PVD, laser glazing, laser surface alloying, and laser surface cladding will be discussed. Microstructural evolution of thermal barrier coatings, recent advancements in functionally gradient coatings, laser grooving, and multilayered textured coatings will also be discussed.     

Risk-based asset management in heat recovery steam generators

Abstract

A risk based inspection and asset management process has been implemented on a number of HRSGs for a client in Australasia. This process has been undertaken in a phased approach consisting of: data gathering on site, detailed risk assessment of all critical items of static plant, risk calculation and implementation of a risk based inspection plan based upon the results of the prior aspects of the assessment. The project has been streamlined through the use of dedicated risk based asset management software, RMS, developed specifically for the tasks described.

Following the programme of work major inspections have been undertaken, the results of which have been uploaded to the program to allow a subsequent detailed review of the new plant profile and hence risk reduction. This not only allows a clear report on progress to senior management, but ensures that the process is continually maintained and future maintenance is most efficiently deployed to ensure optimum plant efficiency. Furthermore the rigorous nature of the process and the traceability which it enforces is being used to apply for extension of mandatory inspection of pressure vessels, where appropriate, allowing longer run times for the plant such that major shutdowns can be aligned with the requirements of the gas turbine overhauls.     

Condition Assessment Study of A-286 Alloy Gas Turbine Wheel

Life extension programs for rotating and nonrotating gas turbine components are popular and widely practiced in the industry. It is often possible to extend the life of the component beyond the design life through life assessment studies. The life assessment study generally includes a combination of nondestructive and destructive tests. Considering the life assessment approach, a Frame 5002 unit gas turbine wheel was examined nondestructively. The material of the turbine wheel is A-286, which is an iron-based super alloy. This turbine wheel has accumulated more than 200,000 service hours. The objective for nondestructive testing of this wheel was to check for any material degradation and any surface cracking in order to ensure extended service hours. The nondestructive tests included eddy current test, ultrasonic flaw detection, replica metallography, and portable hardness test. As such, no significant abnormalities were detected in the nondestructive tests performed. The replica metallography revealed excessive carbide precipitation. It was recommended to retire the turbine wheel from service. The significant test result findings of replica metallography are presented and discussed in this paper.     

Risk evaluation in failure mode and effects analysis of aircraft turbine rotor blades using Dempster–Shafer evidence theory under uncertainty

Rotor blades are the major components of an aircraft turbine. Their reliability seriously affects the overall aircraft turbine security. Failure mode and effects analysis (FMEA), especially, the risk priority order of failure modes, is essential in the design process. The risk priority number (RPN) has been extensively used to determine the risk priority order of failure modes. When multiple experts give different risk evaluations to one failure mode, which may be imprecise and uncertain, the traditional RPN is not a sufficient tool for risk evaluation. In this paper, the modified Dempster–Shafer (D–S) is adopted to aggregate the different evaluation information by considering multiple experts’ evaluation opinions, failure modes and three risk factors respectively. A simplified discernment frame is proposed according to the practical application. Moreover, the mean value of the new RPN is used to determine the risk priority order of multiple failure modes. Finally, this method is used to deal with the risk priority evaluation of the failure modes of rotor blades of an aircraft turbine under multiple sources of different and uncertain evaluation information. The consequence of this method is rational and efficient.     

Multilevel approach to lifetime assessment of steam turbines

This paper presents a multilevel methodology for a steam turbine lifetime assessment based on the damage calculation, probabilistic analysis and fracture mechanics considerations. Creep-fatigue damage calculations serve as a basis for evaluating the current lifetime expenditure and for defining additional steps of analysis. The need for the use of probabilistic analysis results from the inherent uncertainty in estimating the lifetime expenditure primarily caused by scatter in material properties. Fracture mechanics considerations are helpful in determining additional safety margins for components containing cracks. This methodology has been illustrated using an example of the lifetime calculations of a high-temperature steam turbine rotor. The calculations were based on the results of 2D numerical simulations performed for steady state and transient operating conditions.     

Characterisation and quantification of defects in rotors and casings of steam turbines

The loading situation of components in modern fossil-fired power plants is characterised by higher cyclic service in comparison to older plants. This leads in combination with larger units to increased stresses in the relevant components. To evaluate the risk of failure due to natural defects, life time approaches have been developed on the basis of advanced methods of fracture mechanics. The assessment of natural defects using fracture mechanics is, however, based on the following preconditions: ? knowledge and data on the behaviour of such defects in terms of damage mechanism taking into account crack initiation and crack propagation under loading situations comparable to those in service,

? data on the real size and geometry of defects,

? data on the exact location of the defects.

To create such a database an extensive joint research programme was started in the early eighties by German turbine manufacturers, steel makers and research institutes. The paper provides an overview of the results of these efforts. Special emphasis is placed on describing the improvements in ultrasonic testing techniques. The results of feature tests are reported. They have been performed to obtain information and data on the behaviour of natural defects under creep, creep fatigue and fatigue load. Subsequent metallographic investigations determine the real size and location of the defects but also the relevant damage mechanisms. Comparison of metallographic data and NDT data prove the reliability of ultrasonic testing. Thus an improved rating of the permissible ultrasonic indications in turbines and castings was obtained.     

Integrating human health and ecological concerns in risk assessments

The interconnections between ecosystems, human health and welfare have been increasingly recognized by the US government, academia, and the public. This paper continues this theme by addressing the use of risk assessment to integrate people into a single assessment. In a broad overview of the risk assessment process we stress the need to build a conceptual model of the whole system including multiple species (humans and other ecological entities), stressors, and cumulative effects. We also propose converging landscape ecology and evaluation of ecosystem services with risk assessment to address these cumulative responses. We first look at how this integration can occur within the problem formulation step in risk assessment where the system is defined, a conceptual model created, a subset of components and functions selected, and the analytical framework decided in a context that includes the management decisions. A variety of examples of problem formulations (salmon, wild insects, hyporheic ecosystems, ultraviolet (UV) radiation, nitrogen fertilization, toxic chemicals, and oil spills) are presented to illustrate how treating humans as components of the landscape can add value to risk assessments. We conclude that the risk assessment process should help address the urgent needs of society in proportion to importance, to provide a format to communicate knowledge and understanding, and to inform policy and management decisions.     

Characterization of sea water ageing effects on mechanical properties of carbon/epoxy composites for tidal turbine blades

In recent years, many tidal turbine projects have been developed using composites blades. Tidal turbine blades are subject to ocean forces and sea water aggressions, and the reliability of these components is crucial to the profitability of ocean energy recovery systems. The majority of tidal turbine developers have preferred carbon/epoxy blades, so there is a need to understand how prolonged immersion in the ocean affects these composites. In this study the long term behaviour of different carbon/epoxy composites has been studied using accelerated ageing tests. A significant reduction of composite strengths has been observed after saturation of water in the material. For longer immersions only small further changes in these properties occur. No significant changes have been observed for moduli nor for composite toughness. The effect of sea water ageing on damage thresholds and kinetics has been studied and modelled. After saturation, the damage threshold is modified while kinetics of damage development remain the same.     

Multiaxial fatigue life prediction of a high temperature steam turbine rotor using a critical plane approach

This paper deals with the problem of multiaxial fatigue life assessment of engineering components. A computer-based procedure for multiaxial fatigue life assessment incorporating critical plane multiaxial damage models, suitable for use in design evaluations of engineering components based on finite element analysis results, is presented and applied to correlate results from tests conducted on SAE 1045 steel notched shaft specimens. The results from the two variants of critical plane models are compared vis-a-vis the results from the conventional local strain based life prediction method. Fatigue life prediction of a high temperature steam turbine rotor is also carried out using the above procedure.     

Cyclic operation of aero gas turbines – materials and component life implications

Abstract

Historically, the issues connected with the lifing of power generation gas turbine components have been very different from those associated with aero engines. Specifically, component lives in the power generation application have been dictated by creep and high cycle fatigue, whereas low cycle fatigue has been the driver for aero engines. However, developments in the design and usage of gas turbines within the respective industries have resulted in this distinction becoming increasingly blurred. This paper highlights recent advances in the materials technology, stress analysis and lifing of aero engine components, which are potentially relevant to industrial gas turbines. In particular, the development of complex constitutive equations for modelling plasticity and anisotropic creep are discussed, with particular reference to the behaviour of single crystal turbine blades. Moreover, developments in the methodologies used to estimate safe service lives for the components are considered. Specifically, a new lifing procedure, capable of accurately predicting component lives from plain specimen data alone, is discussed.     

Nondisruptive material sampling and mechanical testing

The aging and inservice degradation of industrial equipment has underscored a need for efficient and reliable evaluation of the suitability of such equipment for continued service. The structural components of traditional energy production facilities, such as fossil- and nuclear-fueled electric power plants, are prime examples of aging equipment for which integrity during extended service is of major concern. The paper describes a recently developed nondisruptive miniature material sample removal and test approach that is being applied to a range of operating electric power plant components from turbine generators to pressure vessels, and to petrochemical plant reactor vessels for inservice integrity assessment. Thein situ removal of a thin wafer-like sample (less than 25 mm in diameter and 2.5 mm in thickness) from the component surface generally has no effect on component integrity. The miniature specimen small punch (disk bend) test has been developed to mechanically test the as-removed material, and is being used to estimate the material tensile behavior and fracture properties (ductile-to-brittle transition temperature and fracture toughness) required for a reliable component integrity assessment.     

Assessment of erosion wear in low specific speed Francis turbine due to particulate flow

The erosion wear of turbine components reduces the turbine life and consequently decreases the efficiency or power generation. In the present work, the effect and cause of erosion wear in a high head Francis turbine are investigated through the measurements and numerical simulations on a prototype turbine. Localised erosion patterns are observed over the surface of the guide vane, runner, faceplate, and labyrinth seal due to existing particulate flows. Further, the physical interpretation of erosion wear mechanisms is studied through numerical simulations, and the eroded zones of different components are qualitatively validated with the actual site in-situ measurements. The erosion rate prediction of the turbine components is performed by employing a recently developed erosion model for CA6NM turbine material. The results show that the continuous interblade vortex with significant velocity and crossflow between the blade passages is found accountable for the severe erosion of the runner shroud. The dominating effect of flow instability at the off-design operation depicts higher erosion of turbine components. Moreover, the exponent value for the particle size is found to vary in the range of 1.16–1.30 for the material removal rate of turbine components.     

Gamma titanium aluminide research and applications in Germany and Austria

Research and applications of -base titanium aluminide alloys in Germany and Austria are reviewed. Special emphasis is given to the results of three major German national materials research projects: (i) gas turbine components, (ii) sheet and foil material, and (iii) automotive components. TiAl turbine blades and valves were successfully tested, and sheets are planned to be used in hypersonic applications. Powder metallurgy methods have been developed as alternative processing routes. Basic investigations on phase diagrams were carried out as a base for alloy development and high-resolution electron microscopic methods have led to a better understanding of the mechanical behaviour.     

Lebensdauervorhersage für Integralgegossene Turbinenräder unter Berücksichtigung kurzer Risse am Beispiel der Nickelbasis-Legierung IN 713C

Life Assessment for Integral Turbine Wheels in Consideration of Short Cracks on Example of the Nickel Base Alloy IN 713 C Constant and variable amplitude fatigue tests on unnotched specimens under strain controlled axial loading and on notched specimens under load controlled bending were carried out in order to investigate the transferability of unnotched specimens data to components. In this case the notched specimen represents the component, the turbine wheel. The operational conditions of a turbine wheel, high temperatures, hold times, different loading rates and the fatigue sequence Hot Turbistan, were included in the investigations. Short crack propagation was measured in a scanning electron microscope by identification of load markers on the rupture surface. Additionally CT-specimens were tested in order to determine crack propagation of long cracks. Using these test results and corresponding safety factors a better life assessment for turbine wheels is possible, if the concepts of Safe-Life-, Damage-Tolerance and Total-Life are applied.     

An assessment of the role of near-threshold crack growth in high-cycle-fatigue life prediction of aerospace titanium alloys under turbine engine spectra

Increasingly accurate life prediction models are required to utilize the full capability of current and future advanced materials in gas turbine engines. Of particular recent interest are predictions of the lifetimes of engine airfoil materials that experience significant intervals of high-frequency, high-cycle fatigue (HCF). Conventional life management practices for HCF in the turbine engine industry have been based principally on a total-life approach. There is a growing need to develop damage tolerance methods capable of predicting the evolution and growth of HCF damage in the presence of foreign object damage (FOD), low cycle fatigue (LCF), and surface fretting fatigue. To help identify key aspects of the HCF life prediction problem for turbine engine components, a review is pressented of the extensive results of an Air Force research contract with Pratt & Whitney on the high strength titanium alloy Ti-8Al-1Mo-1V. Data from this representative turbine-airfoil material are used to examine the applicability of linear elastic fracture mechanics methods for prediction of service lifetimes under load spectra that include high cycle fatigue. The roles of fatigue crack initiation and growth are examined for materials that are nominally-defect-free, as well for materials that have experienced significant prior structural damage. An assessment is presented of the potential utility of the conventional threshold stress intensity factor range, ΔK _th, defined by testing specimens containing large cracks. Although the general utility of a large-crack-ΔK _th approach is questionable due to the potentially rapid growth of small fatigue cracks, the low allowable stresses involved in turbine engine high cycle fatigue appear to limit and simplify the small-crack problem. An examination is also presented of the potential effects of high-cycle fatigue and low-cycle fatigue (HCF/LCF) interactions.     

Probabilistic Fatigue Life Prediction and Structural Reliability Evaluation of Turbine Rotors Integrating an Automated Ultrasonic Inspection System

The paper presents a general method and procedure for fatigue reliability assessment integrating automated ultrasonic non-destructive inspections. The basic structure of an automated ultrasonic inspection system is presented. Fatigue reliability assessment methodology is developed using uncertainty quantification models for detection, sizing, and fatigue model parameters. The probability of detection model is based on a classical log-linear model coupling the actual flaw size with the ultrasonic inspection reported size. Using probabilistic modeling, the distribution of the actual flaw size is derived. Reliability assessment procedure using ultrasonic inspection data is suggested. A steam turbine rotor example with realistic ultrasonic inspection data is presented to demonstrate the overall method. Calculations and interpretations of assessment results based on risk recommendations for industrial applications are given.     

Statistics of defects in one-dimensional components

Equations related to spatial statistics of defects and probability of detecting defects in one-dimensional components have been derived. The equations related to spatial statistics of defects allow to estimate the probability of existence of safe, defect-free zones between the defects in one-dimensional components. It is demonstrated that even for a moderate defect number densities, the probability of existence of clusters of two or more defects at a critically small distance is substantial and should not be neglected in calculations related to risks of failure. The formulae derived have also important application in reliability and risk assessment studies related to calculation of the probability of clustering of evens on a given time interval. It is demonstrated that while for large tested fractions from one-dimensional components, the failures are almost entirely caused by a small part of the largest defects, for small tested fractions almost all defects participate as initiators of failure. The problem of non-destructive defect inspection of one-dimensional components has also been addressed. A general equation has been derived regarding the probability of detecting at least a single defect when only a fraction of the component is examined.     

Comparative LCA of technology improvement opportunities for a 1.5-MW wind turbine in the context of an onshore wind farm

This paper presents life cycle assessment (LCA) results of design variations for a 1.5-MW wind turbine due to the potential for advances in technology to improve their performance. Five LCAs have been conducted for design variants of a 1.5-MW wind turbine. The objective is to evaluate potential environmental impacts per kilowatt hour of electricity generated for a 114-MW onshore wind farm. Results for the baseline turbine show that higher contributions to impacts were obtained in the categories of ozone depletion potential, marine aquatic eco-toxicity potential, human toxicity potential and terrestrial eco-toxicity potential compared to technology improvement opportunities (TIOs) 1–4. Compared to the baseline turbine, TIO 1 with advanced rotors and reduced tower mass showed increased impact contributions to abiotic depletion potential, acidification potential, eutrophication potential, global warming potential and photochemical ozone creation potential, and TIO 2 with a new tower concept involving improved tower height showed an increase in contributions to abiotic depletion potential, acidification potential and global warming potential. Additionally, lower contributions to all the environmental categories were observed for TIO 3 with drivetrain improvements using permanent magnet generators while increased contributions towards abiotic depletion potential and global warming potential were noted for TIO 4 which combines TIO 1, TIO 2 and TIO 3. A comparative LCA study of wind turbine design variations for a particular power rating has not been explored in the literature. This study presents new insight into the environmental implications related with projected wind turbine design advancements.     

Simulating in-plane fatigue damage in woven glass fibre-reinforced composites subject to fully reversed cyclic loading

The interest in using fibre‐reinforced composites in structural components is increasing. Some of these structural composites, such as wind turbine blades, aircraft components and torsion shafts are subject to fatigue loadings. It is widely accepted that fully reversed cyclic loading is the most adverse loading for fibre‐reinforced composites, but the modelling of the material behaviour under this loading condition is very difficult. In this paper, a damage model is presented for woven glass fibre‐reinforced composites subject to fully reversed cyclic loading. First fatigue experiments have been conducted in displacement‐controlled fully reversed bending and the stiffness degradation and damage patterns have been observed. Based on these experimental data, a damage model has been developed, which includes the in‐plane stress components and the degradation of the in‐plane elastic properties. The model has been implemented in a commercial finite‐element code and simulation of the successive stages in the fatigue life has been performed. The model has been validated for a plain woven glass fabric reinforced composite and simulated stiffness degradation, damage growth and damage distribution have been compared with experimental data.