Qualification of New Component- Specific Inspection Techniques for Turbine & Generator Service

Transcription

1 Qualification of New Component- Specific Inspection Techniques for Turbine & Generator Service More info about this article: Hans Rauschenbach 1, Dr. Stefan Frank 1 1 Siemens AG Power Generation Services, Germany,Hans.Rauschenbach@siemens.com 2 Siemens AG Power Generation Services, Germany, Frank.Stefan@siemens.com Abstract The power generation industry is requesting to extend inspection intervals and to decrease outage time. For highly stressed turbine & generator components, advanced and reliable NDE methods are needed. Key goal is a reliable examination and evaluation of the highly stressed components of the turbine-generator train in a short period of access. To offer reliable inspection technology, a defined process of development and qualification of new inspection technologies is required. This process shall take into consideration all inspection conditions and influences, which can have an impact to the inspection. (i.e. material related influences; limitations of in- situ inspection; impact of flaw configuration, ) This paper describes the process of development of Nondestructive- Examination Techniques, which were developed and introduced by Siemens Power Generation Services in Turbine & Generator Service. Important aspects of the introduction of newly developed Nondestructive Examination Methods from definition of requirements to qualification and validation will be described in this paper. 1. Introduction Turbine and generator shafts are highly-stressed components of turbine-generator sets. Examination techniques for rotating turbine-generator set components were the focus of further development efforts following a case of severe damage in the past. Loads result for example from operating parameters, the mode of operation of the machinery, startup processes, thermal stresses, pre-stressing, residual stresses from the manufacturing process, as well as loading from the centrifugal forces acting on the rotating components. During scheduled outages, highly-stressed components are subjected to non-destructive testing designed to reliably detect any possible service-induced damage (e.g. cracking) before this can lead to failure of a component and severe consequential damage. Quite apart from the risk to personal health, damages on turbine- or generator components can lead to unscheduled outages and plant downtime, as well as unplanned costs for expensive repair and maintenance work on the turbine/generator. In comparison to these risks, the cost of inspecting such highly-stressed components is Creative Commons CC-BY-NC license

2 easily justified, as is the need for reliable and qualified techniques in the field of nondestructive testing. To be able to perform non-destructive testing in the context of servicing turbines and generators, numerous technical, organizational and personnel qualification requirements need to be fulfilled to ensure high-quality performance of testing and proper analysis and documentation of test results. EN ISO 17025/2005 is internationally recognized as the state-of-the-art standard for defining quality criteria in the context of non-destructive testing. While there are a number of commonly used non-destructive examination techniques such as visual inspection (VT), magnetic particle testing (MT), liquid penetrant testing (PT), ultrasonic testing (UT), radiographic testing (RT) and eddy current testing (ET)), modern, component-specific techniques may also be used, for example phased-array applications, and mechanized UT or ET testing. 2 Requirements for the Development of Nondestructive Test Methods The development and qualification/validation of nondestructive test methods is especially important because they are performed on highly stressed components in turbine and generator service, some of which have a high damage potential. 2.1 Description of Requirements (requirements specification) Complete identification of the requirements at the start of development is important for successful development of a a nondestructive test method. The following technical and organizational points must therefore be taken into account: - Where do the defects to be detected occur and what is their orientation? (The zone to be tested must be clearly defined) - What defect size must be reliably detected as a minimum? - Are there any material-specific aspects that can affect detection of the defects? - Are there any erection-related aspects that can affect detection of the defects? These points only represent a portion of all questions to be answered. The more comprehensively the boundary conditions for the test method to be developed are examined, the fewer unanticipated problems will have to be confronted later under practical conditions. 2.2 Specification of Procedure (product design specification) After clarification of the requirements, the way in which the test method is to be developed is specified. Further investigations may be necessary regarding the suitability of selected probes. Ultrasonic or eddy current test methods are frequently developed on test blocks. These test blocks are made of material identical to that of the future test object. In most cases, notches created by electric discharge machining serve as artificial defects, the verification of which must be documented in development of the test 2

3 method. Once a suitable method as well as the optimum test parameters for verifying the defects to be detected have been elaborated, a manipulation system can help to improve the reliability and repeatability of the test method. The requirements for manipulation systems result from the boundary conditions under which the test is to be performed later. Because the disassembly and reassembly effort must be minimized during turbine and generator overhauls, manipulation systems are needed that enable insitu nondestructive testing of components in the installed condition. This means that there is no need for elaborate disassembly of the component (whether it is a joint bolt, turbine blade or generator retaining ring) for testing. 2.3) Qualification of test method After the tasks described in 2.1 and 2.2 have been performed, the test method is qualified. Qualification in this context means the provision of verification on a suitable part (with natural or artificial defects) that the test method is suitable for detecting defects in accordance with the specifications elaborated in 2.1. Because actual parts with defined defects are not usually available for development, the test method is often qualified on test blocks with artificial defects. The most comprehensive possible documentation of the areas covered by the test method, detectability of the defects as a function of their orientation etc. is important for subsequent inquiries in the event of situations involving findings in testing under actual conditions. After identifying a suitable manipulation system (as necessary), preparation of an inspection instruction and corresponding training of the NDT personnel, the developed test method can be implemented in testing practice. 2.4) Implementation in practice Results have shown that participation of the development team in the first tests under practical conditions is essential when introducing new methods. The effect of projectspecific conditions on the successful implementation of a newly developed method should not be underestimated. Results from initial testing under practical conditions form the basis for optimization of the test method in preparation for further tests. 3

4 2.5) Validation of test method Validation is understood to be formal and documented verification that a chemical, physical or biological analysis method is suitable for its purpose and that it fulfills the specified requirements. Validation of the method is an important quality assurance tool and is required by the licensing authorities in the accreditation and approval process. (1) Technical validation is the verification that a method is suitable for providing the result for which the method was developed under practical conditions and in accordance with specifications. (Proof in practice) Validation in "nondestructive testing" can sometimes be difficult, as examined objects for which indications are detected are not always available for further (possibly destructive) analyses. If the opportunity arises to perform further nondestructive and destructive testing for verification of the detected findings in components that have been stressed in service and that have indications, investigations of this type represent an important contribution to NDT quality assurance. 3) Test method for automated UT and ET on generator retaining rings Retaining rings are provided for support of rotor winding end turns. They are shrunk on the end of the main body of the rotor and are made of a nonmagnetic alloy steel forging. In general, retaining rings are considered among the highest stressed components on the entire shaft set, including turbine, generator and exciter. Up to the mid-70s, generator retaining rings were made of 18Mn5Cr. Because this material is susceptible to stress corrosion cracking (SCC), regular nondestructive testing of the generator retaining rings was absolutely essential to ensure reliable operation of the generator. After the mid-70s, generator retaining rings were made of X 8 CrMn 18 18K. The advantage of this new material was a higher yield strength as well as resistance to SCC. After their introduction, nondestructive testing of generator retaining rings made of the new material and stressed in service was only recommended following unusual operating conditions. Fig. 1: Generator retaining rings are installed on both the exciter and turbine ends of the generator rotor 3.1 Formulation of requirements 4

5 Siemens Power Generation Services has many years' of experience in ultrasonic testing of shrink-fit generator retaining rings during inspections and overhauls. Because of the situation in which many turbine generator sets are operated beyond their projected service life, a large number of generator retaining rings have been in operation for more than 20 years without having been subjected to extensive nondestructive testing. It was necessary to develop a nondestructive test method that meets the following requirements in order to ensure reliable operation of the installed generator retaining rings: - Combined surface crack testing & ultrasonic testing of the most highly stressed areas of the generator retaining rings; - Digital data recording (UT & ET) - Use of a manipulator that also enables testing in the installed condition (i.e. the generator shaft remains in the stator for the test) - High test sensitivity of the ultrasonic test method on the inner surface of the generator retaining ring/high repeat accuracy of the manipulation system - High test sensitivity of the eddy current test method on the outer surface of the generator retaining ring - Reliable distinction between indications due to the geometry of the generator shaft and real indications (caused by defects) - Testing must also be possible through a paint coating (it is sometimes common practice to coat the generator retaining rings with corrosion protection paint before installation) 3.2 Specification of further steps After formulation of the requirements in accordance with 2.1, a development plan was elaborated with the individual requirements for a combined ultrasonic & eddy current test method for generator retaining rings. It was decided to provide a generator retaining ring with a large number of artificial defects (of different location and orientation) in the highly stressed areas (on both the outside and inside surfaces of the retaining ring) for qualification of an ultrasonic test method. This generator retaining ring served as a test block during the qualification process. An expansion disk was fabricated with the same dimensions and geometry as the generator rotor in the area of the shrink-fitted generator retaining ring in order to establish the most realistic possible test conditions. This expansion disk was then shrink-fitted into the generator retaining ring. The objective was to simulate the shrinkfit stresses and to establish the necessary conditions to simulate the phenomena occurring due to the ultrasound interactions through the shrink fit seat on ultrasonic testing of the inner surface of the generator retaining ring during outage inspections on generator retaining rings. 5

6 Fig. 2: Expansion disk during shrink-fitting into the generator retaining ring The grooves around the circumference of the expansion disk represent the grooves for accommodating the generator slot wedges around the circumference of the generator shaft. These grooves significantly compromise the ultrasonic test result on retaining ring testing, as these grooves also induce ultrasonic indications that appear in the area of expected indications. 3.3 Qualification of test method Fig. 3: Ultrasonic tests on a generator retaining ring with expansion disk After the generator retaining ring was provided with artificial defects and the expansion disk was fabricated and shrink-fitted, the practical tests could be performed to verify whether all of the artificial defects can be detected. The tests were performed by an international team of NDT specialists from Mülheim/Ruhr (Germany); Pittsburgh (USA) and Newcastle upon Tyne (UK). In parallel with the tests, a manipulation system already in use was optimized for the requirements of combined UT & ET with of the generator in the installed condition. 6

7 The outer surface of the generator retaining ring was tested with the ET array method, as this method enables testing of larger areas over a short time and achieves the required test sensitivity. Ultrasound indications caused by grooves in expansion disk Ultrasound indications of artificial defects in generator retaining ring Fig. 4: Ultrasound result of a measurement in the area of the shrink-fitted expansion disk Individual adjustment of the equipment combination/probe holders and probe equipment enabled reliable detection of all artificial defects. A catalog was prepared documenting the appearance of all indications in the ultrasound B-scan. The expansion disk proved to be a key prerequisite for developing a reliable ultrasonic test method under real conditions. After submission of the qualification report, an inspection instruction describing the test method in detail was prepared for each test method (UT and ET). Qualification of an ultrasonic test method for generator retaining rings without this technical restriction (on a generator retaining ring with artificial defects but without an expansion disk and therefore without interfering geometric indications) would indicate good ability to verify defects which might not then be achievable under real conditions. 3.4 Implementation of developed test method in practice The developed test method was first implemented in a gas power plant in Australia in January Two generator retaining rings were tested in-situ by combined ultrasonic & eddy current testing, i.e. the generator shaft remained in the stator during testing, which results in very limited spatial conditions. Because the automated test system was developed for this type of application (and, thanks to the development process, all limitations for these application were accounted for) testing was successfully performed. This included eddy current testing 7

8 Fig. 5: Automated in-situ ultrasonic testing of a generator retaining ring (generator shaft is in the stator) 4. Conclusions This document describes the procedure implemented in Siemens Power Generation Services for developing new nondestructive test methods using the example of a method for testing shrink-fitted generator retaining rings. Accounting for all relevant limitations already during development of the test method clearly pays off in subsequent testing in generator service. The implementation of an expansion disk during qualification of the ultrasonic test method was used to show how important practice-related test configurations are in the qualification of nondestructive test methods for highly stressed components. Siemens Power Generation has been an accredited test laboratory in accordance with ISO IEC since 1999 and has been successfully conducting quality-assured developments of this type for many years. References and footnotes References should be written in the order in which they appear in the text in the following format: 1. DIN EN ISO IEC 17025: General requirements for the competence of testing and calibration laboratories C. Ward ; K. Stamps, Options for management of retaining ring inspection, Insight Vol 48 No 9 September VDEW Recommendation to Avoid Stress Corrosion Induced Rotor Retaining Ring Cracking ; November Hans Rauschenbach: Advanced NDT Methods for Efficient Service Performance Power-Gen India & Central Asia; 5-7 May, 2014; New Delhi, India 8