Blade Root / Blade Attachment Inspection by Advanced UT and Phased Array TechniqueDr. Michael F. Opheys, Hans Rauschenbach, Michael Siegel; Siemens AG Power Generation, D-45466 Mülheim (Germany) Graham Goode; Siemens Power Generation, Newcastle (UK) Detlev Heinrich; Cegelec AT GmbH & Co KG, Nürnberg (Germany) Corresponding Author Contact: Email:
[email protected] 6th international Charles Parsons Turbine Conference, 16 ± 18 September 2003, Trinity College, Dublin 1. Introduction International competition in the field of power generation is increasing and customers are demanding economic and efficient power plants. In the long term, continuous power plant availability can only be guaranteed through an effective mode of operation in conjunction with a systematic maintenance and inspection concept. Apart from boiler, steam piping and valves, the rotating components of the turbine/generator (turbine and generator rotor) also belong to the most highly stressed components in a power plant. Loads result for example from operating parameters, the mode of operation of the machinery, startup processes, thermal stresses, prestressing, 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. For example, damage to a blade in the low-pressure turbine of a South African power plant (600 MW) in January 2003 resulted in the entire turbine generator unit being destroyed. Quite apart from the risk to personal health, such damage 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 easily justified, as is the need for reliable and qualified techniques in the field of non -destructive testing. The following describes two examples for non -destructive techniques used on turbine blade roots and blade attachment grooves. 2. Ultrasonic technique for inspecting turbine blade roots in situ 2.1 Description of inspection problem The blades in a steam turbine belong to the most -highly stressed components in a turbine/generator. The high turbine speed (3000 rpm) and the dead weight of the blades means that the last-stage blades in a steam turbine are subjected to enormous centrifugal forces during plant operation. The roots on such blades are designed and calculated using the most up-to-date methods to allow them to accommodate these high loads. Particularly during transient loading conditions (startup and shutdown processes) certain areas of the blade roots and blade attachment grooves are subjected to high stressing. Under unfavourable conditions unusual events occurring during operation of a turbine (e.g. loss of vacuum, overspeed) can result in damage to blading, with possible Turbine blades and their roots should be examined non-destructively at predetermined intervals to allow timely detection of any damage and the replacement of affected blades. The objective was therefore to develop a technique which allowed these highly -stressed areas of the blade root to be inspected in situ.e. 2. i. it was decided to fabricate specially-fitted pieces for each area to be scanned. In addition steam purity is also an important criterion regarding the susceptibility of a turbine blade to corrosion. Along the complex geometry of this blade root these scanning positions were also situated in radii and on other curved surfaces which required a customized inspection solution for the component in question. pressure side Fig 2: Blade root for LP rotor with reference reflectors. the necessity for non destructive testing becomes particularly apparent. providing a sui table tool for power plant inspection services. If the steam is polluted with chlorides this is one of the basic prerequisites for the occurrence of corrosion fatigue in turbine blades. Figure 1 and Figure 2 show the root for such a blade. 2 mm deep) were introduced into these calibration blocks at the most-highly-stressed areas. 6 mm long. blade roots and blade attachment areas. The blade under investigation was a last-stage blade from an LP turbine rotor. without rem oving the blade.crack initiation in the highly-stressed areas of the blade root and subsequent serviceinduced crack propagation. which would allow exact positioning of the ultrasonic search units. suction side Reference reflectors (grooves. In the light of such influences on s afe turbine blade operation. For this re ason. Fig 1: Blade root for LP rotor with reference reflectors. . The examination system had to provide reliable and reproducible results while remaining cost effective.2 Theoretical investigations Extensive theoretical investigations had to be performed before any decisions could be made regarding selection of the ultrasonic examination technique. These areas can be found on the pressure side in the vicinity of the leading/trailing edge of the blade root in the first serration of the fir-tree root as well as in the middle of the first serration on the suction side of the blade root. This inspection technique seemed to be the right solution for the inspection problem. Performance of the inspection on the blade roots of the dual flow turbine rotor required that a calibration block be fabricated for the right and left side. The task faced here was to develop an ultrasonic testing technique for a special type of blade root to allow inspection of the roots of the last -stage blades in the rotor of a lowpressure steam turbine. When installed in the rotor the most highly-stressed areas of the blade root are not accessible for standard crack testing techniques. The theoretical investigations showed that it is indeed practicable to select scanning positions at the blade root which allow reliable d etection of the reference reflectors. Three contoured . and 11 MHz) indicated only limited relia bility.Fig 3: graphic for determining possible scanning positions Fig 4: graphic for determining possible scanning positions 2. 2.3. This confirmed that all reference reflectors could be reliably detected.3 mm as well as 60° shear wave search units. Fig 5: Scanning positions at transition between root platform and airfoil for inspecting zone #2. the contoured probe holders mentioned above were fabricated.1. at least two scanning positions were determined for each reference reflector which seemed suitable for detecting the reflector. For the customized solution (using contoured probe holders with integrated search units) it was decided to use 5 MHz longitudinal wave search units with a transducer diameter of 6. in all cases. On the basis of the theoretical investigations. Owing to the fact that the different reference reflectors were able to be found using various scanning posi tions and beam angles. It was then possible to contour the Plexiglas wedges so that they could be coupled to the surface at the determined scanning positions.3 Practical investigations on the calibration block Once various suitable scanning parameters had been determined. The results of investigations with phased -array search units at various frequencies (3. 7. Figure 5 is an example showing the scanning position for a search unit at the transitio n between the blade root platform and the airfoil. Development and qualification of a practical inspection system Once the investigations on the calibration block had confirmed the suitability of the selected inspection technique for the problem in hand. These search units were equipped with Plexiglas wedges. it was decided to make use of this during the actual performance of inspections. practical tests were able to begin on the calibration blocks. All the scanning positions calculated during the theoretical investigations were checked during the practical tests on the calibration block. The probe holders are matched to the blade root contour to guarantee exact positioning of the ultrasonic search units. thus providing the possibility of verifying the presence of any indications detected.inspection system. this instrument allows the results from all the integrated search units to be evaluated at a glance. Three contoured probe holders were made for the blade root under investigation.4 Summary The manual ultrasonic inspection system described above was developed to provide a reliable and cost-effective method of inspecting turbine blade roots. each containing several ultrasonic search units.probe holders were fabricated. etc. scanning positions Fig 7: Probe holder #1 on blade rootscanning positions 2. investigations were performed using several different blades of the same type. It was shown that the dimensional differences existing between the blades inspected were able to be compensated for using a gel-type couplant and can therefore be neglected. Ultrasonic Examination of Blade Attachment Grooves of LP Turbine Shafts 3. The main considerations during development work were: y y y y y blades must be able to be inspected in situ simple handling and operation (no complex manipulators.Tomoscan) guarantees an effective inspection. Their use in conjunction with a 4 -channel ultrasonic instrument (. Fig 8: Test results when inspecting Zone #1 using probe holder #1 on a blade with reference reflectors. Once the corresponding probe holder has been brought into position. it proved to be a considerable advantage that each zone for examination was able to scanned from at least 2 scanning positions. Manufacturing tolerances were not found to have any effect on test sensitivity/defect detectability. The configuration with the contoured probe holder and a systematic inspection procedure means that only a short introduction to the equipment is required before testing can begin. Fig 9: Test results when inspecting Zone #1 using probe holder #1 on a blade without reflectors. Fig 6: Probe holder # 1 for inspecting zone # 1 in the blade root. by scanning from a second position. to verify that existing manufacturing tolerances for these large LP blades do not have any influence on the results of the examination. 3. The couplant bridges the gap between the contoured probe holder and the blade root. With respect to the reliability of testing. When testing the page 4 of 10.1 Description of the Problem .) reliable and meaningful test results possibility of verifying indications by using 2 scanning positions fast test method All these requirements are met by the inspection system. Siemens Power Generation¶s NDE laboratory received a request to perform a non-destructive examination on blade attachment grooves of a non -OEM turbine (European nuclear power plant). This meant that the advantages of the phased -array ultrasonic examination technique could be fully leveraged. The problem is discussed in greater detail below. different scanning positions and angles of incidence are required to examine the highly stressed areas for cracks. To ensure that incipient cracks exhibiting different orientations were also detected. 7. different scanning positions and angles of incidence are required to exami ne the highly stressed areas for cracks.3 Development of an Advanced Inspection Technique Following analysis of the problem. Following analysis of the problem. In December 2001. 7. Fig 6: blade attachment grooves of an LP turbine shaft 3. Given the different dimensions of the blade grooves to be inspected.Due to the world wide SCC issue there is an increasing demand for a non destructive examination of blade attachments of steam turbine rotors. the dimensions of the semi-elliptical grooves used were as follows . The requirement to ensure that all relevant areas of the blade grooves are scanned. it was decided to solve it by means of the ultrasonic phased-array technique. and that small cracks are also reliably detected. and 8). the profile of the blade attachment grooves. A separate test piece was fabricated for each blade row (row 6. test flaws in the form of grooves with a semielliptical profile were introduced at different angles in the most highly stressed areas (inspection zones 1 and 2 in Figure 10). It was known from experience of turbines of identical design at other operators¶ plants that the grooves of blade row 6. In the case in question. by way of example. necessitated qualification of the inspection technique using an identical test piece. Given the different dimensions of the blade grooves to be inspected. The grooves can run either circumferentially (in which case the blades are inserted in sequence and secured with a locking blade) or axially. To ensure detection of even the smallest incipient cracks. Each test piece reproduces the geometry of the blade groove together with the outside profile of the turbine shaft. it was decided to solve it by means of the ultrasonic phased-array technique. 3.2 Description of Requirements There are several designs of blade attachment grooves of LP turbine shafts. An advanced ultrasonic examination technique had to be developed to provide r eliable data on the condition of blade attachment grooves without deblading the rotor. Figure 6 shows. This meant that the advantages of the phased -array ultrasonic examination technique could be fully leveraged. the blade attachment grooves ran circumferentially. and 8 were particularly susceptible to crack formation. References 1. 4. qualification of the technique was carried out using a 45 EL3 phased -array search unit (natural angle of incidence: 45°. To identify the form echoes more easily the CAD drawing of the inspected blade attachment is overlaid. page 50. search unit frequency 3 MHz. 3. (inspection zone1. Due to the complex geometry of the blade grooves and the associated geometric indications. required with conventional crack inspection techniques. Fig 12: overlay of the sector scan representation with the CAD drawing of the blade attachment (row 8). The tests were carried out using a triple-axis manipulator in conjunction with the SAPHIR+ phased array system. Key examples include time savings on component disassembly and reassembly. Assessment of the TD images of all scans enabled the optimum angle of incidence to be rapidly determined. Given the requirement for virtually non-stop power plant availability and the associated reduction in plant downtimes.5 Results of Qualification and Conclusions By scanning the test pieces it was possible to demonstrate that the deployed phasedarray ultrasonic inspection technique is suitable for use in field service to examine blade grooves of LP turbine shafts for incipient cracking in highly stressed areas in the assembled condition (in other words without removing the blades). 8 x 2. International EPRI Conference on Welding & Repair Technology for Power Plants / July 20 00. 16 array elements).[journal] . Fig 13: scan results in form of a TD image showing all test flaws. Figure 12 shows the result of the vertical scanning using a sector scan presentation. 4 x 2. by way of example.(length in mm x depth in mm): 2 x 1. but elimin ated when advanced techniques are used. Fig 11: Qualification of the inspection technique on a test block. David Gandy (EPRI). row 8) 3. 4 x 1. All of the test flaws in inspection zone 1 and 2 of all three test pieces were detected (the smallest test flaw was a semi-elliptical groove 2mm x 1mm). these kinds of in -situ service techniques are playing an increasingly important role in the plann ing and performance of plant outages. a vertical scan was programmed in a range from 30° to 85° in steps of 1°. the TD image of a scan in inspection zone 1 (row 8). The grooves were positioned in inspection zones 1 and 2. Figure 13 shows. These two examples of advanced inspection techniques demonstrate that direct customer benefits can be delivered through the use of problem-focused techniques. ³Rim Attachment Cracking Promts Developement of Life Assessment Tools´.4 Qualification of the Inspection Technique using Test Pieces Following fabrication of the test pieces. Vis Viswanathan (EPRI). Bevins. RDTech. Rome. Richard Fredenberg (Wes Dyne International).al. Notario Power Generation Inc. Riccardella. Baltimore. Darryl A. MD. Andres Garcia (Technatom SA). Petru Ciorau et. A. Carlos Arrietta. (Ontario Power Generation Inc. Marta Alvaro. August 20 -24 2001 [conference] 4. P.S.: Feasibility study of ultrasonic inspection us ing paced array of turbine blade root ± Part 1. Hans Rauschenbach. N. S. 1997 7. ³Blade Attachment UT Inspection using Array ´. using phased array technology. 7 th EPRI Steam/Turbine Generator Workshop. Tang (Structural Integrity Associates San Jose. 2002 [conference] © NDT. Lamarre. USA) ³Developement of an LP Rotor Rim ± Attachment Cracking Life Assessment Code (LPRimLife)´. EPRI workshop July 29 ± August 01.2. Baltimore. Barcelona June 17 ± 21. 2000 6. Michael Opheys. August 20 -24 2001 [conference 5.) In situ examination of ABB L0 blade roots and rotor steeple of low -pressure steam turbine. ³Dovetail Blade Attachment Experience using Phased Array Ultrasonic Test Techniques´. IntelligeNDT Framatom: Advanced NDE Inspection Methods for Field Service at Power Plants. Canada. Dr.Ciorau. Dube`. Francisco Godinez. P. Peter C. Uwe Mann : Siemens Power Generation Jürgen Achtzehn. MD.net |Top| . 15 th WCNDT. Florida USA 3. CA. 7 th EPRI Steam/Turbine Generator Workshop. Rosario. Power Engineering / June 2000 Marco Island. 8 th European Conference on Non -Destructive Testing.