Other than combustor-related components, the highest temperature parts in a turbine are the first-stage stator vanes. Ceramic vanes are being considered to enable the increased turbine inlet temperatures needed to meet the ATS program efficiency goals. However, ceramic vanes have not been proven for industrial turbines, even at current inlet temperatures. The Allison Phase 2 ATS program was modified to prove ceramic vanes at current industrial turbine conditions. The objectives of the task described in this paper are to design, evaluate, and demonstrate first-stage ceramic vanes in an industrial turbine operated at a current inlet temperature in the vicinity of 1100 deg C (2000 deg F). This could provide a stepping stone to the introduction of ceramic vanes into ATS turbines with very high inlet temperatures in excess of 1427 deg C (2600 deg F).
Ceramic vanes and mounting hardware will be specified and designed for retrofit into an Allison 501 turbine. For that engine, the first-stage vanes are exposed to an average combustor outlet temperature up to the vicinity of 1100 deg C (2000 deg F) with hot spots several hundred degrees higher. The intended vane life is 30,000 hr, comparable to the current design life of metallic vanes.
The initial mechanical design of the vanes and their mounting hardware will be based on Allison's extensive experience in the design and testing of smaller experimental automotove turbines that use ceramics. Computerized heat transfer and stress analyses will be used to evaluate the initial design and refine it, as needed. Typical ceramic properties of the vendor materials will be used in the initial analyses. The stress analyses will be later refined using materials data obtained from flexure and tensile test of vendor specimens formed from the same batches used to form the purchased vanes.
A probabilistic design methodology has been developed by Allison that addresses the statistical nature of a ceramic's strength distribution and the reliability requirement for the component in service. The material surface and volume strength is characterized by a two-parameter Weibull statistical treatment of the four-point bend modulus of rupture strength of test bars. The component reliability service life goal is apportioned for required reliability in customer service. Additionally, the engine operating environment is input to the sophisticated finite element modeling of the component to analytically assess the fast fracture reliability. Both steady-state and treansient (startup and shut down) thermal and mechanical loads for engine operation will be considered in design analyses.
The results of the design and analyses activities will be used to specify the ceramic vane configuration to ceramic vendors. These activities will also be used to produce mounting hardware drawings for fabrication or procurement by Allison under this task.
Proof tests will be conducted for all ceramic vanes that are expected to operate in later engine tests. The proof tests will simulate temperatures corresponding to at least one engine startup from room temperature to full load (vicinity of 1100 deg C [2000 deg F], a period of exposure at that temperature, and an abrupt drop in temperature to represent a generator trip in service which results in an immediate shutdown of fuel to a turbine. The purpose of this test is to screen out any vanes with undetected flaws that could initiate cracks and failure due to thermal shock in an operating turbine. After the proof test, each vane will be visually inspected and analyzed by nondestructive techniques such as fluorescent penetrant and microfocus X-ray.
A full set of first-stage ceramic vanes and their mounting hardware will be operated in a 501 turbine at Allison. The purpose is a proof test of both ceramic vanes and metallic mounting components in an operating test engine prior to installation at a commercial site. The test will verify that the metallic mounting hardware does not transmit excessive contact stresses or excessive mechanical loads to the ceramic vanes due to distortions caused by the combustor temperature patterns. The test will probably consist of a normal startup of the turbine, operation for up to 50 hr at load, and a normal shutdown.
Since the field demonstration depends on a final agreement with the end-user, the following tests plans are preliminary.
Vanes that had been screened in the thermal shock proof test and the engine proof test will be installed with mounting hardware in an Allison 501 turbine that has been taken out of commercial service for maintenance.
The turbine will reenter service at its commercial site for up to 8000 hr under its normal operating conditions. The commercial site will most likely be a cogeneration plant, at which operation is essentially continuous at full load, except for unanticipated shutdowns (such as generator trips) and scheduled maintenance (probably 6 month intervals). Inspection frequency for the ceramic vanes and their mounts will depend on the agreement with the end-user, since any additional inspection outages result in loss of plant revenues. At the end of the test, ceramic vanes will be removed from the engine and analyzed to assess their condition and expected additional life.
Technology advancements in metallic cooling techniques and materials will be needed if alloys are to be used for the airfoils that experience the highest gas temperatures in ATS turbines. An alternate approach is the development of structural ceramics which would need little or no cooling of the high temperature airfoils.
There are several potential benefits for ceramic airfoils over cooled metallic airfoils. Since compressed cooling air bypasses the combustor, the resulting turbine performance penalty for cooled metallic airfoils is reduced for ceramic airfoils. NOx emissions goals are more easily met if ceramic, rather than metallic, first-stage vanes are used. Since the drop in gas stream temperature between the combustor and the first rotor blades is less for ceramic vanes, a lower combustor temperature (which produces less thermal NOx) can be used to achieve a given rotor inlet temperature.
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