|NDT.net||January 2004, Vol. 9 No.01|
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)
Siemens Power Generation, Newcastle (UK)
Cegelec AT GmbH & Co KG, Nürnberg (Germany)
Corresponding Author Contact:
6th international Charles Parsons Turbine Conference, 16 18 September 2003, Trinity College, Dublin
1. IntroductionInternational 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 problemThe 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 crack initiation in the highly-stressed areas of the blade root and subsequent service-induced crack propagation. In addition steam purity is also an important criterion regarding the susceptibility of a turbine blade to corrosion. If the steam is polluted with chlorides this is one of the basic prerequisites for the occurrence of corrosion fatigue in turbine blades, blade roots and blade attachment areas.
In the light of such influences on safe turbine blade operation, the necessity for non-destructive testing becomes particularly apparent. 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 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 low-pressure 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 objective was therefore to develop a technique which allowed these highly-stressed areas of the blade root to be inspected in situ, i.e. without removing the blade. The examination system had to provide reliable and reproducible results while remaining cost effective.
2.2 Theoretical investigationsExtensive theoretical investigations had to be performed before any decisions could be made regarding selection of the ultrasonic examination technique. The blade under investigation was a last-stage blade from an LP turbine rotor. Figure 1 and Figure 2 show the root for such a blade. 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.
Reference reflectors (grooves, 6 mm long, 2 mm deep) were introduced into these calibration blocks at the most-highly-stressed areas. 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.
The theoretical investigations showed that it is indeed practicable to select scanning positions at the blade root which allow reliable detection of the reference reflectors. 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. For this reason, it was decided to fabricate specially-fitted pieces for each area to be scanned, which would allow exact positioning of the ultrasonic search units. This inspection technique seemed to be the right solution for the inspection problem, providing a suitable tool for power plant inspection services.
2.3 Practical investigations on the calibration blockOnce various suitable scanning parameters had been determined, practical tests were able to begin on the calibration blocks. The results of investigations with phased-array search units at various frequencies (3, 7, and 11 MHz) indicated only limited reliability. 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 6.3 mm as well as 60° shear wave search units. These search units were equipped with Plexiglas wedges. It was then possible to contour the Plexiglas wedges so that they could be coupled to the surface at the determined scanning positions. Figure 5 is an example showing the scanning position for a search unit at the transition between the blade root platform and the airfoil.
On the basis of the theoretical investigations, at least two scanning positions were determined for each reference reflector which seemed suitable for detecting the reflector. All the scanning positions calculated during the theoretical investigations were checked during the practical tests on the calibration block. This confirmed that all reference reflectors could be reliably detected, in all cases. Owing to the fact that the different reference reflectors were able to be found using various scanning positions and beam angles, it was decided to make use of this during the actual performance of inspections.
2.3.1. Development and qualification of a practical inspection systemOnce the investigations on the calibration block had confirmed the suitability of the selected inspection technique for the problem in hand, the contoured probe holders mentioned above were fabricated. The probe holders are matched to the blade root contour to guarantee exact positioning of the ultrasonic search units. Three contoured probe holders were fabricated, each containing several ultrasonic search units. Their use in conjunction with a 4-channel ultrasonic instrument (.Tomoscan) guarantees an effective inspection. Once the corresponding probe holder has been brought into position, 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. When testing the page 4 of 10.inspection system, investigations were performed using several different blades of the same type, to verify that existing manufacturing tolerances for these large LP blades do not have any influence on the results of the examination. 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. The couplant bridges the gap between the contoured probe holder and the blade root. Manufacturing tolerances were not found to have any effect on test sensitivity/defect detectability.
2.4 SummaryThe manual ultrasonic inspection system described above was developed to provide a reliable and cost-effective method of inspecting turbine blade roots. The main considerations during development work were:
With respect to the reliability of testing, it proved to be a considerable advantage that each zone for examination was able to scanned from at least 2 scanning positions, thus providing the possibility of verifying the presence of any indications detected, by scanning from a second position.
3. Ultrasonic Examination of Blade Attachment Grooves of LP Turbine Shafts
3.1 Description of the ProblemDue to the world wide SCC issue there is an increasing demand for a non destructive examination of blade attachments of steam turbine rotors.
In December 2001, Siemens Power Generations NDE laboratory received a request to perform a non-destructive examination on blade attachment grooves of a non-OEM turbine (European nuclear power plant). The problem is discussed in greater detail below.
3.2 Description of RequirementsThere are several designs of blade attachment grooves of LP turbine shafts. The grooves can run either circumferentially (in which case the blades are inserted in sequence and secured with a locking blade) or axially.
In the case in question, the blade attachment grooves ran circumferentially. It was known from experience of turbines of identical design at other operators plants that the grooves of blade row 6, 7, and 8 were particularly susceptible to crack formation. An advanced ultrasonic examination technique had to be developed to provide reliable data on the condition of blade attachment grooves without deblading the rotor.
3.3 Development of an Advanced Inspection TechniqueFollowing analysis of the problem, it was decided to solve it by means of the ultrasonic phased-array technique. Given the different dimensions of the blade grooves to be inspected, different scanning positions and angles of incidence are required to examine the highly stressed areas for cracks. This meant that the advantages of the phased-array ultrasonic examination technique could be fully leveraged.
Following analysis of the problem, it was decided to solve it by means of the ultrasonic phased-array technique. Given the different dimensions of the blade grooves to be inspected, different scanning positions and angles of incidence are required to examine the highly stressed areas for cracks. This meant that the advantages of the phased-array ultrasonic examination technique could be fully leveraged.
The requirement to ensure that all relevant areas of the blade grooves are scanned, and that small cracks are also reliably detected, necessitated qualification of the inspection technique using an identical test piece. Figure 6 shows, by way of example, the profile of the blade attachment grooves. A separate test piece was fabricated for each blade row (row 6, 7, and 8). Each test piece reproduces the geometry of the blade groove together with the outside profile of the turbine shaft. To ensure that incipient cracks exhibiting different orientations were also detected, test flaws in the form of grooves with a semi-elliptical profile were introduced at different angles in the most highly stressed areas (inspection zones 1 and 2 in Figure 10). To ensure detection of even the smallest incipient cracks, the dimensions of the semi-elliptical grooves used were as follows (length in mm x depth in mm): 2 x 1, 4 x 1, 4 x 2, 8 x 2. The grooves were positioned in inspection zones 1 and 2.
3.4 Qualification of the Inspection Technique using Test Pieces
Due to the complex geometry of the blade grooves and the associated geometric indications, a vertical scan was programmed in a range from 30° to 85° in steps of 1°. Figure 12 shows the result of the vertical scanning using a sector scan presentation. To identify the form echoes more easily the CAD drawing of the inspected blade attachment is overlaid.
Assessment of the TD images of all scans enabled the optimum angle of incidence to be rapidly determined. Figure 13 shows, by way of example, the TD image of a scan in inspection zone 1 (row 8).
3.5 Results of Qualification and ConclusionsBy scanning the test pieces it was possible to demonstrate that the deployed phased-array 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).
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 two examples of advanced inspection techniques demonstrate that direct customer benefits can be delivered through the use of problem-focused techniques. Key examples include time savings on component disassembly and reassembly, required with conventional crack inspection techniques, but eliminated when advanced techniques are used. Given the requirement for virtually non-stop power plant availability and the associated reduction in plant downtimes, these kinds of in-situ service techniques are playing an increasingly important role in the planning and performance of plant outages.