Materials at High Temperature Vol 18, Issue 4, 2001
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Design aspects of once through systems for heat recovery steam generators for base load and cyclic operation
M. F. Brady
Innovative Steam Technologies Ltd.
The paper describes a once-through approach to the design of heat recovery steam generators, thereby eliminating boiler drums, and many other items of equipment. Two pressure operation is possible. To enable the steam generator to start up from dry, high creep strength Incoloy type materials are used for all of the tubing. This also enables the system to cope with fluctuations in the water-to-steam transition point. Such a design is therefore highly suitable for cyclic operation. One of the features of this particular design is the ability to operate dry, that is with no water flow through the tubing, even though the gas turbine is at full output. Materials specifications for operation with dirty fuels, particularly in a corrosive marine environment, are discussed. Some details of water treatments needed and typical start up schedules are given. The paper also comments briefly on some of the limitations of conventional HRSGs.
Keywords: once-through systems, heat recovery steam generators
EPRI’s program to analyze, model, and predict costs of plant cycling
Dale Gray1, M.R. Corio2, K.E. Perf2 and J.Bellucci2
1Electric Power Research Institute, Palos Alto, CA, USA
2Applied Economic Research Co., Inc. New York, NY, USA
Quantification of the effects of cycling and varied load operations in general has become much more important in today’s industry. This change is largely a result of operational requirements proceeding from the deregulation of electricity markets and the change from total cost-based rate recovery under regulation, to competitive contractual arrangements and open market bidding where pricing of power based on economics of individual supplying units can make a significant difference in profitability. As a result, based on member requests, Electric Power Research Institute (Palos Alto, CA, USA), working with Applied Economic Research Co., Inc. (New York, NY, USA), has developed a program, the EPRl Cycling Impacts Program (CIP), aimed at investigating the various aspects and effects of cycling* to provide member companies with research into the area along with quantification, modeling, and analysis wherever possible, of information for use in financial and operational planning and forecasting. Over the last year and a half the program has surveyed generating company concerns and needs across the country relative to cycling; researched and examined various data sources and availability; conducted analyses of the data; and determined that correlations and modeling of operational data with plant expenditure data can be developed to provide valid methods for quantifying the relationships involved and determining cycling cost effects under various cycling conditions. The program is currently in the process of developing further modeling methodologies and—in the near term, during the spring/summer of this year (2001)—providing several member companies with modeling and analyses to determine cycling costs for specific generating units. The methods involved were explicitly developed to provide results in the near term in response to these companies’ immediate operational needs. Continuing cycling program plans include development of a range of more advanced analyses and planning tools in the cost area, as well as studies of the impact of cycling on reliability/availability, component-level effects, and the effectiveness of technical solutions and alternatives. This report discusses methodologies and work in progress, as well as planned project development objectives going forward.
*Note: For this work, the term ‘Cycling’ is used to cover a broadened definition, which is loosely: “operations involving various combinations of load variation, load level durations, annual starts and stops (hot, warm, cold), etc.” The effects that impact incremental cost include not only ‘cycling’ mode operating variables for current and prospective operations, but also the effects of past operations in various cycling modes, which are cumulative in their effect on current going- forward operating costs.
Keywords: cost of plant cycling
Creep-fatigue properties of high temperature turbine steels
S R Holdsworth
ALSTOM Power (UK) Ltd, Newbold Road, Rugby, CV21 2NH, UK
Cyclic/hold creep-fatigue properties for the new advanced 9–11%Cr steels are reviewed and shown to be significantly superior to those of 1%CrMoV turbine steels at 550°C. Moreover, cyclic/hold endurances for the creep resistant martensitic stainless steels at 600°C are at least as good as those for 1%CrMoV turbine steels at 550°C. An assessment of the creep-fatigue damage interaction characteristics of Grade 91 steel shows them to be no worse than those of 1%CrMoV turbine steels at their respective maximum application temperatures.
Keywords: creep-fatigue properties
Issues associated with two-shift operation of combined cycle generating plant
Michael Pearson
J. Michael Pearson & Associates Co Ltd, Toronto, Canada
Diverse experiences associated with two-shifting of conventional generating units are largely relevant to the ‘balance of plant’ in combined cycle generating units (CCGTs). Although two-shifting has adversely impacted upon reliability and availability, and incurred substantial additional maintenance costs on many conventional units, this has not been the case for all installations. Reference 1, for example, reports that four large coal-fired units, which have been extensively two-shifted for more than 15 years, have experienced no major and just a few minor problems attributable to many shutdown-startup cycles. These units are capable of more than 200,000 hours running with more than 6,000 starts; only a small additional maintenance cost is attributable to two-shifting. Startups and shutdowns impose more onerous conditions on critical parts of many components of most fossil-fired generating plant than experienced when operating continuously at maximum rating (the steady-state condition at which unit and component design verification analysis is generally performed).
The generic root cause of many of the major and the miscellany of lesser problems attributable to two-shifting has been a general omission by equipment designers, architect-engineers and purchasers to determine, during the design phase, (i) realistic transient conditions imposed on various components during unit startups and shutdowns. and (ii) the consequential life expenditure from cyclic thermal-mechanical cyclic loadings and other mechanisms of material degradation caused by mechanical load cycles. It is feasible to design CCGTs capable of very extensive two-shifting without thermally induced failures in the balance of plant [2–4]. However, CCGTs do have additional difficulties to understand and manage by appropriate design and operation. For example, CCGTs:
• have to consider the additional operational and cost implications associated with two-shifting the gas turbine (GT);
• have more complex cycles, which are additional complications to operation during shutdowns and startups;
• impose more severe conditions on the superheater and economizer of the HRSG during both the shutdown and startup process than is experienced in conventional boilers.
Therefore CCGTs require even more design attention than conventional units to enable them to twoshift without adversely impacting reliability and availability and without incurring excessive additional maintenance costs for the balance of plant. For either a CCGT or conventional coal-fired generating plant, one of the requirements for troublefree two-shift operation is that all parts subjected to significant heating and/or cooling during CCGT startups and shutdowns require good thermal flexibility. Thermal flexibility in the critical parts of the plant is achieved by a combination of:
• appropriate conceptual and detailed design of the key equipment; i.e., the GT, steam turbine (ST), and HRSG;
• appropriate design of key auxiliary systems which influence the conditions imposed on HRSG and ST;
• unit operating procedures for CCGT shutdowns and startups that minimize thermal stresses in all critical parts of GT, ST and HRSG.
The importance of having good thermal flexibility to minimize distress from thermally-induced low cycle fatigue (LCF) caused by unit shutdowns and startups has been well-publicised. There have been many failures of tubes at attachments to superheater and economizer headers most of which are attributable to transient high stresses developed during startups (in some cases in less than 200 startups). Despite this, CCGTs continue to be installed with design weaknesses that would not be difficult nor costly to correct on a new installation, and operating procedures unnecessarily harmful to the HRSG continue to be widely used. Good thermal flexibility is only one of the crucially important objectives for CCGTs intended for twoshifting. This paper discusses several other technical challenges to be overcome for a CCGT to provide high startup reliability without adverse impact on CCGT reliability and availability. It also addresses the minimum additional maintenance cost for the ‘balance of plant’ that is attributable to two-shifting.
Keywords: two-shift operation of combined cycle generating plant
Cyclic operation of aero gas turbines – materials and component life implications
D. P. Shepherd, A.Wisbey, G. F. Harrison,T. J.Ward and B.Vermeulen
Engine Materials and Lifing, Structures and Materials Centre, QinetiQ Ltd, Farnborough, Hants GU14 0LX, UK
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.
Keywords: aero gas turbines
Failure aspects of thermal barrier coatings
L. Singheiser, R. Steinbrech,W.J.Quadakkers and R. Herzog
Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany
The paper describes aspects of thermal barrier coating (TBC) microstructure and the physical and mechanical properties which they influence. The stress-strain behaviour of air plasma sprayed (APS) TBCs is discussed, including the role of residual stresses. Failure phenomena as well as the TMF behaviour of TBC coated nickel base superalloys are described. The role of bond coat oxidation on TBC life is discussed as well as some mechanical properties of vacuum plasma sprayed MCrAlY-bond coatings. Finally, life prediction methodologies are addressed and discussed in terms of a critical strain accumulation concept. From this is derived an equation which covers time dependent effects such as bond coat oxidation and sintering. The paper concludes with a brief summary of the evolution of TBCs in aero and industrial gas turbines, and the failure modes in each. In particular the increased importance of erosion, in industrial gas turbines, due to water injection is highlighted.
Keywords: thermal barrier coating
The metallurgical background to rejuvenation heat treatments and weld reparability procedures for gas turbine sheet metal components
Ian. J. Storey1, Dwaine L. Klarstrom2, Gregory L. Hoback2,Venkat R. Ishwar2 and Javaid I. Qureshi3
1Haynes International, Inc Openshaw M11 2ER UK; 2Haynes International, Inc. Kokomo, IN 46904-9013; and 3Siemens-Westinghouse Power Corporation Orlando, FL 32826-2399,USA
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.