Technical Bulletin

Evolution and Current Status of Nondestructive Evaluation of
Steam Turbines and Generators

Nondestructive inspections of steam turbines and generators have been performed for over a half-century. The early inspections were performed during the manufacturing process by the OEM and were typically limited to the large rotating forgings, initially just the rotor forging. By current standards, these early inspections were not very sophisticated and the detection and flaw characterization capabilities were correspondingly limited. Surface inspections that included visual and magnetic particle techniques were the norm, supplemented eventually with ultrasonic inspections performed at relatively low test frequencies. Early ultrasonic practitioners did not recognize certain physical characteristics of the approach that are well established today, yet which nonetheless limited the capabilities of the inspections. For example, early inspections were conducted without any consideration for the natural decay in beam intensity with increasing propagation distance due to beam spread and the effects of this on detection of deep-seated flaws. Consequently, detection capabilities were very limited.

Starting in the early 1950s, however, extensive research began to promote a better understanding of the underlying fundamental principles of the ultrasonic inspection technology. Once the beam characteristics were understood, procedures began to recognize the importance of corrections for a number of factors affecting the sensitivity and accuracy of various techniques, including compensation for the effects of propagation distance on response strength, corrections for surface curvature, compensation for the effects of attenuation, and so on. Still, even with this improved understanding of the physical parameters involved, the inspections were performed manually, with heavy reliance on observations made by an operator to detect, characterize, classify, and size indications.

Beginning with the advent of digital data acquisition capabilities in the mid-1970s, a second evolution began to emerge, this time involving the means of acquiring, processing, and presenting the data for interpretation. The early manual inspections have given way to the completely automated inspections that are routinely performed today. Ultrasonic imaging systems are standard. With these, the operator can present different orthogonal views of the inspection volume, with flaws presented within the volume at the correct locations and relatively accurately relative to size, shape, and orientation. Annular array systems provide focused beam interrogation with electronically variable focal depth, and linear array systems provide electronic beam angulation or beam sweeping. Crack tip diffraction techniques are used routinely to define flaw boundaries and thereby provide greatly improved accuracy over early amplitude-based sizing techniques. And digital filters and processing algorithms are used to enhance data, extract useful information, filter for specific features of interest, and even perform automated processing and classification. Processing steps have even been developed to look further than the conventional distance/amplitude features of the ultrasonic response, into frequency domain characteristics that may indicate specific flaw characteristics, including reflector type.

A shift has also occurred in the types of inspections that occur annually. Where components were once typically inspected only during manufacture, today the predominant role of inspection is to assess the condition of the component after it has seen a period of service. Forging practices have evolved to the point that an initial inspection for life-limiting flaws is almost unnecessary because the presence of such flaws in a modern forging is low probability. Consequently, the role of the factory inspection is more to establish a baseline for later comparison that to identify conditions that are initially problematic in and of themselves. In addition, inspections are no longer performed exclusively by the OEM, permitting independent analysis that tends to be less conservative and less expensive than with the former. Non-OEM vendors, such as Structural Integrity Associates (SI), now conduct large numbers of service inspections and we are now even being requested to perform pre-service inspections to establish the data baseline.

In the past, most large turbine and generator rotor forgings were bored from one end to the other as a means of removing remnant ingot flaws that tend to consolidate to the center of the ingot during solidification, and which are further consolidated toward the center during the forging process. This bore surface provides a convenient surface from which an inspection can be conducted, and ultrasonic inspection from the bore surface (boresonic) has become a standard for pre-service inspection (PSI) and in-service inspection (ISI) of such rotors. Most large turbine and generator rotors currently in operation have central bores and undergo periodic boresonic inspections as a means of assessing present condition and possible flaw growth between inspections. However, the presence of a bore, in and of itself, represents a stress concentration and reduces tolerable flaw sizes in the immediate vicinity of the bore. Therefore there are certain advantages to be gained by eliminating the bore. Under the improved steel-making practices now in place, the reduction in potential flaw conditions near the rotor centerline has over-ridden the stress effect of the presence of a bore, and new or replacement rotors often have no central bores.

Without a bore, the standard for which rotor acceptance criteria have been established no longer exists. One of the real quandaries the NDE practitioner often finds himself in involves inspection of components after some period of operation but without benefit of any record of the initial condition that existed prior to service. If no indications are detected during the inspection, that is all well and good and the initial ISI can serve as the baseline to which subsequent inspections can be compared. However, when indications are detected in the initial ISI without benefit of a valid PSI baseline inspection, the NDE practitioner is faced with the additional burden of attempting to characterize the source of the indications as original discontinuities or service-induced flaws, which is often a very difficult distinction to make. Unless some positive evidence can be produced to support a conclusion that the indications were pre-existing, a conservative approach typically dictates that the indications must be treated as service-induced flaws, to be dealt with accordingly. In some cases, it may be possible to perform additional inspections and/or in-place metallography to help define the nature and source of the indications. In other cases, it may even be possible to remove material samples for detailed metallurgical analysis to define the nature of the indication sources. However, in turbine rotors this is typically not the case, or at least not a desirable option. For indications arising within the body of the forging, there is no known way to obtain additional information on indications without destroying the rotor. Even for blade attachments, where service induced damage normally initiates at free surfaces, the blades must be removed to access the surfaces. This process is undesirable because it is very expensive and provides an opportunity for damaging blades, disks, and other components. Consequently, PSI becomes a vital step in preparing such components for service.

SI’s first introduction into rotor inspection began in the 1990s with the development of techniques to inspect nonmagnetic retaining rings on generator rotors. Nearly all domestically-manufactured rings were made of the same material, which has been found to be susceptible to stress corrosion cracking (SCC) when operated in a moist environment. Within the past twenty years, a suitable replacement material has been introduced. Many utilities, however, have elected to control the rings’ operating environment, i. e., keep the rings dry at all times, and to conduct periodic inspection to assess the effectiveness of their moisture mitigation programs. SI developed an inspection program that has been very effective and is routinely used today. This program follows the recommended inspection protocol derived through an extensive R&D effort conducted at the ERPI NDE Center. The program features a multiple-inspection approach through which the overall reliability of the inspection can be maximized.

For many years now, SI has provided utilities with third party analyses as alternatives to the conservative approaches that OEMs typically follow relative to remaining life. SI has developed a number of remaining life assessment codes, including RRing-Life, developed under EPRI sponsor for probabilistic assessment of generator retaining rings. Similarly, SI has been an industry leader in probabilistic assessment of rotors based upon boresonic inspection results. More recently, SI has introduced our own boresonic inspection capabilities for ISI. The boresonic system uses the latest automation techniques and incorporates NDE techniques to detect and size both pre-service indications and service-induced flaws. The system has also been evaluated by demonstrating its capability in the EPRI boresonic demonstration program, and is the only system evaluated to date that has provided 100% detection in these trials.

To complement rotor inspection, SI is currently participating in an EPRI program to demonstrate our new ultrasonic linear phased array technology for the inspection of disk blade attachments. A linear array probe contains a series of small, individual ultrasonic transducer elements arranged in a row. Each element is supported by its own pulser/receiver and is acoustically isolated from the other elements. By controlling the timing of the pulse and reception for each element, the angle, mode, and focus of the resulting beam(s) can be accurately controlled electronically. For straddle-mount disk rim dovetails, such as those typically found on GE turbines, the linear array approach has been proven effective through EPRI research. In fact, this approach is rapidly becoming the state-of-the-art for disk dovetail inspection. In the conventional approach, a limited number of angles are used to test each hook in the dovetail independently. There can be some error in the selected angle relative to the optimum angle because of uncertainty associated with the exact dovetail geometry (i.e., choosing the correct angle), due to refracted angle error (i.e., wedge angle tolerances, velocity variances, etc.), and due to transducer position errors. The inspection is also relatively time-intensive because each different angle must be calibrated and implemented independently. The use of the phased array technology provides a more comprehensive inspection than can be accomplished with the fixed-angle inspection. By electronically varying the test angle, which is extremely rapid, all angles can be conducted in a single scan rather than in multiple scans, one per angle. In addition, prior knowledge of the attachment geometry is not needed because the attachment is reconstructed during the inspection process. Consequently, the array inspection is not only much less time-intensive, but also much more effective, accurate, and reliable.

As of December 5, 2003, SI became the first company to complete the data acquisition portion of the EPRI disk blade attachment inspection demonstration program mentioned in the associated article, “Evolution and Current Status of Nondestructive Evaluation of Steam Turbines and Generators.” Refer to that article for a technical description of the inspection process. The data is currently being evaluated and initial results look excellent. A report of this activity will be completed by year end and submitted to EPRI for comparison with the actual flaw sizes along with the results of the other participants. EPRI expects to publish their final report in 2004.

SI will also make a report detailing our findings during the inspection of the EPRI blade attachment demonstration blocks available to our clients by early 2004.

Linear phased array technology is also being developed for the inspection of solid rotors, i.e., those not having a central bore, as presented earlier. In the past, procedures for inspecting unbored rotors have varied from using a single transducer with a large beam spread to inspect the centerline region, to using a number of different beam angles to collectively cover the desired inspection volume at angles appropriate for detection of the flaw orientations of interest. Linear phased array offers an effective means of implementing multiple angle inspections for all of the inspections defined above. For the cylindrical sections of the rotor body, electronic beam angulation can be used to steer the beam from a purely radial path to interrogate away from the rotor center for radial-axial discontinuities. A much superior result is achieved because of the relatively fine angulation index, say ½-degree or 1 degree, over the range of angles needed rather than at a single angle or limited number of selected angles.

SI continues to provide innovative, state-of-the-art inspection and condition assessment technologies for critical power plant inspection applications. For additional information on our turbine and generator condition assessment capabilities, contact Larry Nottingham lnotting@structint.com at our Charlotte, North Carolina office (704-573-1369), Ron O’Hara rohara@structint.com at the Rockville, Maryland office (301-231-7746), Harold Queen hqueen@structint.com in our Florida office (954-572-2902), or Paul Sabourin psabourin@structint.com (704-957-5243) out and about in his motor home.

Boresonic system inspection of an in-service rotor.

Development of phased array inspection for straddle-mount disk blade attachment.




Phased array sector scan image overlaid on straddle-mount disk blade attachment geometry with flaws shown at points 1, 2, and 3. Actual field TOFD Image of a fatigue crack in a weldment.