Factors influencing gas turbine use and performance

Abstract

The reliability of industrial gas turbines can be limited by the premature degradation of critical hot gas path components. Cost effective operation of these turbines therefore requires a knowledge of component life and its limitations according to service requirements. Through this knowledge, ways to restrict the degradation of components can also be determined. Creep, corrosion, and fatigue are considered to be the principal life limiting factors with the hot gas components. The present paper discusses the influence of these three processes on component life, and examples of associated service problems are presented. Specific reference is made to the corrosion resulting from the presence of sodium sulphate deposits in offshore turbines. Control of this corrosion through the use of fuel additives and protective coatings is considered. MST/448     

Requirements for engineering ceramics in gas turbine engines

Abstract

The potential uses of ceramics in gas turbine engines are reviewed in the context of the problems arising from the brittle nature of the materials. Material properties are considered in relation to various turbine components and the themes of reliability and component design. It is concluded that substantial efforts will be required in materials and processes in achieving greater reliability and improved design before ceramics are successfully applied in gas turbines.

MST/436     

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.     

Improved creep-resistance of austenitic stainless steel for compact gas turbine recuperators

Abstract

Primary surface recuperators (PSRs) are compact heat-exchangers made from thin-foil type 347 austenitic stainless steel, which boost the efficiency of land-based gas turbine engines. Compact recuperators are also an essential technology for some new microturbines. Solar turbines uses foil folded into a unique corrugated pattern to maximize the primary surface area for efficient heat transfer between hot exhaust gas on one side, and the compressor discharge air on the other side of the foil. Allegheny-Ludlum produces 0.003–0.004 inches thick foil for a range of current turbine engines using PSRs that operate up to 660°C. One goal of this team-effort project is to modify the processing to enable improved creep resistance of such 347 stainless steel foils at 650–700°C. Laboratory-scale processing modification experiments recently have demonstrated that dramatic improvements can be achieved in the creep resistance of such typical 347 stainless steel foils. The modified processing enables fine NbC carbide precipitates to develop during creep at 650–700°C, which provides strength even with a fine grain size. Such improved creep-resistance allows greater flexibility in optimizing the cost-performance relationship as increased demands are placed on the PSR at higher operating temperatures. The next challenges are to better understand the nature of the improved creep resistance in these 347 stainless steel foil, and to achieve similar improvements with scale-up to commercial foil production.     

Corrosion in coal-fired gas turbines

The purpose of this paper is to review corrosion processes and experience related to coal-fired gas turbines. For over 40 years there have been major programmes of research directed towards burning coal in a gas turbine either directly or as a coal-derived fuel. This history is briefly reviewed, demonstrating the importance which corrosion of the hot section has had in limiting the achievements and defining the systems. The probable corrosion mechanisms are identified, and because of their synergistic interaction with corrosion, erosion and deposition are also considered. The discussion identifies pressurized fluidized bed combustion (PFBC) as the direct combustion technique most likely to be able to avoid serious corrosion problems in the immediate future. Recent results from PFBC pilot plant investigations related to turbine materials are compared, and thereby future directions for overcoming corrosion problems are proposed. MST/449     

The International Foundry and Heat Treatment Conference ‘82

Abstract

Sheet materials for hot gas path components in modern land based gas turbines demand high strength over the temperature range 650–950°C and freedom from serious in-service embrittlement. This is particularly critical where a gas turbine is subject to cycling since thermal stresses can lead to the cracking of such components. It is also highly desirable that if cracking does occur, components can be repaired safely and easily. Haynes 230, a modern alloy, is relatively immune to in-service embrittlement, particularly in comparison to some older materials, but may require a rejuvenation heat treatment to facilitate repairs after in-service exposure. A rejuvenation heat treatment at 1177°C for 0.5 hour was shown to restore stress rupture and weld ductility to those required by AMS 5878A and Section IX of the ASME Vessel and Boiler Code. Stress rupture results for the aged samples indicated that lives were in excess of those for materials in the as-received condition. This was ascribed to grain boundary precipitation.     

Erosion of thermal barrier coatings

Abstract

Thermal barrier coatings have been used within gas turbines for over 30 years to extend the life of hot section components. Thermally sprayed ceramics were the first to be introduced and are widely used to coat combustor cans, ductwork, platforms and more recently turbine aerofoils of large industrial engines. The alternative technology, electron beam physical vapour deposition,(EB-PVD) has a more strain-tolerant columnar microstructure and is the only process that can offer satisfactory levels of spall resistance, erosion resistance and surface finish retention for aero-derivative engines.

Whatever technology is used, the thermal barrier must remain intact throughout the turbine life. Erosion may lead to progressive loss of TBC thickness during operation, raising the metal surface temperatures and thus shortening component life. Ballistic damage can lead to total TBC removal.

This paper reviews the erosion behaviour of both thermally sprayed and EB-PVD TBCs relating the observed behaviour to the coating microstructure. A model for the erosion of EB-PVD ceramics is presented that permits the prediction of erosion rates. The model has been validated using a high velocity erosion gas gun rig, both on test coupons and samples removed from coated components. The implications of erosion on component life are discussed in the light of experimental results and the model predictions.     

Development of oxides at TBC – bond coat interfaces in burner rig exposures

Abstract

There is a desire to use gases derived from increasingly ‘dirty’ fuels (e.g. coal and biomass) in industrial gas turbines. The contaminants in these fuels have the potential to cause significant damage to the gas turbine hot gas path materials, many of which were developed and selected for natural gas fired conditions. This paper reports results of a study investigating the performance of thermal barrier coatings (TBCs) and bond coatings, applied to current industrial gas turbine materials, within clean and ‘dirty’ gas environments generated within a burner rig.

The materials covered by this study included: ? TBCs based on 8%Y₂O₃–ZrO₂ applied by both air plasma sprayed (APS) and electron beam – physical vapour deposition (EB–PVD) routes.

? Bond coats of the overlay and diffusion classes, applied by vacuum plasma spraying (VPS), electroplating (EP), chemical vapour deposition (CVD) and high velocity oxy-fuel (HVOF) spraying

? Base alloys of IN6203, CMSX-4 and Haynes 230

The required TBC/bond coat combinations were applied by commercial coating processes to cylindrical samples of base alloys manufactured for use in a burner rig.

The burner rig used in this study is designed to enable air-cooled probes of cylindrical samples to be exposed to a natural gas combustion environment. In this study, this enabled specific metal temperatures (~800 and ~900°C) to be targeted within a much higher temperature combustion gas stream (~1150°C). ‘Dirty’ fuel gas environments were simulated by introducing gaseous (SO₂ and HCl) and vapour phase (Na, K, Pb, Zn) contaminants into the burner rig just upstream of the edge of the gas flame. These conditions enabled continuous tests to be performed for 1,000 hours in both natural gas and ‘dirty’ fuel environments.

The relative performance of the materials was determined from cross-sections prepared after the 1000 hour exposures. These cross-sections were examined by optical and SEM/EDX to determine the thicknesses of the oxides at the TBC – bond coat interfaces, the morphologies of these interfaces and to characterise the elemental distributions in these regions.     

Mechanical design of gas turbine blading in cast superalloys

Abstract

The mechanical design of hot turbine blading has two main phases: (i) optimization, which yields the choice of material, number of blades, and general configuration, and (ii) detailed design of the particular features of the blades. The first phase makes use of the data available, which are usually limited, to provide an outline mechanical assessment of commercial viability. At this stage maximization of the lifetime to failure is considered in terms of the temperature and stress at each radial position; in the second phase failure modes are considered in more detail. The methods used in both stages are discussed, with emphasis on material and manufacture, taking as an example the blading in civil aircraft gas turbines. Particular attention is paid to the problems of designing with single-crystal cast superalloys.

MST/222     

Refurbishing procedures for blades of large stationary gas turbines

Abstract

The design life for blades of large stationary gas turbines is at least 100000 h in creep terms. Typical damage occurring within the lifetime of gas-turbine blading may be hot corrosion and/or erosion, foreign-object damage, tip rubbing, and cracking caused by low-cycle fatigue, thermocycle fatigue, high-cycle fatigue, and creep crack growth. When applying refurbishing procedures both technical and economic aspects should be considered. The repair procedures available are welding, brazing, plasma spraying, recoating, combined with special heat treatment cycles, and occasionally intermediate hot isostatic pressing. The limiting factors for repair procedures are the mechanical properties and the hot corrosion behaviour of the refurbished parts. Examples are given of results from metallographic and laboratory tests on refurbished Kraftwerk Union (KWU) test specimens and blades in relation to the material, design, and stressing of the components. Typical examples of the refurbished parts are also presented. Service experience with refurbished blades in stationary gas turbines is limited at present. Therefore, KWU can only consider these procedures in the light of the growing knowledge of refurbishing processes and the operating experience of refurbished parts as well as the economic aspects.

MST/102     

Development and application of a methodology for the measurement of corrosion and erosion damage in laboratory,burner rig and plant environments

Abstract

Fuel gases derived from solid fuels such as coal, biomass and waste and their mixes have the potential to cause both erosion and corrosion damage to components in gas turbines and diesel engines. To allow the statistically valid assessment of materials performance in short term plant runs, burner rig tests and laboratory simulated environments a methodology has been developed to collect compatible quantitative data on materials degradation. Accurate measurement techniques based on pre-exposure contact metrology and post-exposure optical microscopy/image analysis have been developed. These take into account both the low level of damage required for practical systems and the localised nature of hot corrosion damage. The data produced have been used to derive and test quantitative models for the prediction of the performance of candidate materials in such power systems. For these models to be used with confidence, similar damage morphologies must be produced in both the real and simulated conditions, as well as similar damage rates.     

High-temperature sheet materials for gas turbine applications

Abstract

Aircraft gas turbines contain various sheet metal fabrications, such as combustion chambers, exhaust units, jet pipes, and reheat liners, which operate for long periods under arduous conditions. The properties required in a sheet alloy differ considerably from those of blades and vanes, so that alloys must be developed specifically for the purpose. The performance required of sheet alloys is discussed and current alloys are reviewed in terms of their fabrication, mechanical, and oxidation properties. Some recent developments are described in the field of thermal barrier coatings, dispersion strengthened alloys, and porous laminate materials.

MST/524     

Oxidation behaviour of steels in advanced gas cooled reactors

Abstract

EDF Energy operates 14 advanced gas cooled reactors (AGRs) in the UK to generate electricity. CO₂ gas is used as the primary coolant in the AGRs, and a range of steels are used as for the structural components: e.g. 9Cr-1Mo ferritic steels and stainless steels. These steels are susceptible to both oxidation and carburisation in CO₂ dominated primary coolant gas under high pressure between 300 and 650 °C. Material degradation is a key concern for lifetime extension of the power stations, and EDF Energy and its predecessors have carried out a series of research programmes to better understand steel oxidation behaviour in AGRs and to develop more realistic lifetime prediction methodologies. These are used to secure safe and reliable operation of the AGRs. In this paper, an overview of oxidation behaviour of steels used in AGRs and some examples from the above programmes are described.     

Coupling CFD with chemical reactor network for advanced NO x prediction in gas turbine

Gas turbine pollutant emissions, especially nitric oxides (NO _x : NO and NO₂) and carbon monoxide (CO) are limited to 25 ppmvd by the European legislation for natural gas operations. To meet this objective and that of future legislation, it is crucial to have access to numerical tools that could speedily predict NO and CO emissions when operating gas turbines (fuel flexibility, tuning of the fuel distribution between burners…). In this context EDF R&D has been developing a 3D turbulent gas combustion model the past few years. Nevertheless, the introduction of complex chemical kinetics in 3D turbulent combustion code is still too CPU-time consuming for industrial use. Thus, 3D computational fluid dynamics computations, using simple chemistry, are post-treated to generate a 0D chemical reactor network (CRN), which includes a detailed chemistry mechanism. The 3D simulations are used to provide information about the mixing state, and the flow topology including the turbulence effects. The present study focuses on the impact of ambient air conditions (temperature and relative humidity) on NO production by industrial gas turbines. The detailed chemical kinetic scheme is initially validated by laboratory tests on jet-stirred reactor performed in University of Washington.     

Development of ceramic components for car gas turbines

Components for internal combustion engines are being develop ed from non-oxide and oxide ceramzc r:naterzals at the Research Division of Volkswagenwerk AG. The properties of some materials, the process technology and the design methods of the components are discussed. The advantages of the incorporation of ceramics into automotive gas turbines are deduced and th e present state of Volkswagen’s component development is reported.     

Hot corrosion failure in the first stage nozzle of a gas turbine engine

First-stage nozzles of gas turbines, which are the first elements after the combustion chamber, encounter hot gases from the combustion process and have the task of directing the fluid path and increasing the velocity of combustion products. This paper reports on the incidence and failure of the first-stage nozzles of a gas turbine in September 2013 at a seaside pump-house located in the South-West of Iran. The nozzle was made of nickel-based superalloy Nimonic105. Due to nozzle failure, the turbine was damaged severely. The cause of nozzle failure was investigated. The results of visual inspection, XRD analysis of deposits on the blade airfoil, SEM images and EDAX analysis showed the characteristics of hot corrosion. Finite-element analysis (FEM) revealed that the cause of blade trailing edge failure was thermal stress leading to thermal fatigue, which accelerated nozzle blade failure in addition to the hot corrosion.     

工业燃气轮机涡轮叶片用铸造高温合金研究及应用进展

工业燃气轮机具有热效率高、污染低等突出优点,成为未来发电机组与大型水面舰船动力的首选设备。铸造高温合金是工业燃气轮机涡轮叶片等热端部件的关键材料,其性能和制备水平在一定程度上决定了先进燃气轮机的功率、效率、寿命等性能。本文重点综述了工业燃气轮机及其涡轮叶片用铸造高温合金材料的研究及应用现状,并对工业燃气轮机涡轮叶片用铸造高温合金及涡轮叶片制造技术的发展趋势进行了展望。未来,先进定向凝固,“材料基因工程”等技术将逐渐应用到工业燃气轮机涡轮叶片用铸造高温合金的研制中;此外,先进工业燃气轮机上定向/单晶高温合金的应用将越来越广泛。     

In-depth and In-plane Thermal Diffusivity Measurements of Thermal Barrier Coatings by IR Camera: Evaluation of Ageing

Ceramic thermal barrier coatings (TBCs) are widely used for protecting hot path components from combustion gases in gas turbines for both aero- and land-based applications. TBCs undergo degradation and eventually detach from the substrate. Forecasting of the detachment of TBCs for timely maintenance is an open problem in gas turbine technology. It is known that sintering happens in the TBCs when exposed to high temperature. Sintering affects the mechanical properties of TBCs and mainly their strain compliance for which degradation causes the detachment. As sintering strongly affects the thermal diffusivity of TBCs also, the idea is to measure the latter parameter to account for the former. Pulsed thermography is the technique selected to monitor the thermal diffusivity variation due to TBC ageing. In perspective, it should be applied to monitor the gas turbine during the normal stop for maintenance. This article reports preliminary laboratory tests carried out on a set of metal samples coated with TBCs. The samples were aged during cyclic oxidation tests at various percentages of their estimated life, the end of life being the time of the TBC detachment from the substrate. The identification of the thermal diffusivity in the coating layer is carried out for the general case of anisotropic conductivity.     

Creep fatigue behaviour of Type 321 stainless steel at 650°C

Abstract

Type 321 austenitic stainless steel has been used in the UK’s advanced gas cooled reactors for a wide variety of thin section components which are within the concrete pressure vessel. These components operate at typically 650°C and experience very low primary stresses. However, temperature cycling can give rise to a creep fatigue loading and the life assessment of these cycles is calculated using the R5 procedure. In order to provide materials property models and to validate creep fatigue damage predictions, the available uniaxial creep, fatigue and creep fatigue data for Type 321 have been collated and analysed. The analyses of these data have provided evolutionary models for the cyclic stress strain and the stress relaxation behaviour of Type 321 at 650°C. In addition, different methods for predicting creep fatigue damage have been compared and it has been found that the stress modified ductility exhaustion approach for calculating creep damage gave the most reliable predictions of failure in the uniaxial creep fatigue tests. Following this, validation of the new R5 methods for calculating creep and fatigue damage in weldments has been provided using the results of reversed bend fatigue and creep fatigue tests on Type 321 welded plates at 650°C in conjunction with the materials properties that were determined from the uniaxial test data.     

A review of factors controlling the gas turbine hot section environment and their influence on hot salt corrosion test methods

Environmental and operational factors that influence the formation and deposition of corrosive species on hot section components in gas turbine engines are reviewed. In addition to air and fuel impurities, combustion gas chemistry, velocity, pressure and temperature are identified as key operational factors affecting the formation and deposition of Na₂SO₄ salt (the primary corrosive species) when trace amounts of sodium and sulphur are entrained into combustors. Test methods for ranking the resistance of different materials to hot salt corrosion are reviewed and compared in terms of the type of damage they produce. The methods considered range from a simple furnace or crucible test to burner rig and engine tests. It is shown that high velocity burner rigs, running at atmospheric pressure, allow all the relevant operational factors to be simulated, and they produce realistic hot corrosion damage similar to that observed on engine parts in service.