Boresonic inspection methods began as early as 1959, when the General Electric Company (GE) became sensitive to the need for improvements to rotor inspections as rotor size, stresses and temperatures were rapidly increasing. By 1968, GE had taken its boresonic inspection program from the factory floor to in-service inspection of large steam turbines that focused on a class of rotors identified as “C-Grade” (4). The catastrophic burst of the TVA Gallatin Unit 2 IP-LP rotor in 1974 has been credited as being the pivotal point in time when the Electric Power industry confronted the issue of turbine reliability of in-service steam turbine and generator rotors.
Spurred by utility concerns for increased turbine reliability, The Electric Power Research Institute initiated Research Project 502, which led to development of ultrasonic inspection systems, analysis software such as SAFER and a program for performing statistical boresonic system performance evaluations on independently operated commercial systems (2).
As of this date,
EPRI has published general rotor boresonic inspection guidelines, as well as the individual system performance evaluation reports of those vendors that have voluntarily undergone an appraisal of system capabilities using the rotor bore test blocks produced by RP-502 and RP 2481-5 (1988) research projects (2). Utilities wanting to manage the risk of a turbine rotor failure often access these published reports to evaluate the performance of those vendors chosen to perform boresonic inspection services.
Past condition assessments performed on 250 rotors from various OEM’s that were placed in-service between 1941 and 1978 reveal that with the utilization of NDT testing, fatigue and fracture mechanics techniques, most vintage turbine and generator rotors can continue to be left in service with a high degree of reliability. From the 250 rotors evaluated, 9.3 percent of the sample population required overboring or bottle boring prior to being put back into service. Rotors most likely requiring bore modification were HP and HP-IP rotors placed into service between 1948 and 1961, and are rated from 30 MW-150 MW.
None of the 250 sample rotors assessed were condemned (1).
The above assessments were conducted using boresonic inspection data acquired from a period extending from 1979-1988. Since 1988, fewer rotors needing corrective bore work are encountered since most of the rotors have undergone boresonic testing, or have been retired from active service.
Today, the utilization of more advanced boresonic testing systems and improved condition assessment techniques described by others (5) ensure that the utility owner can manage the risk of turbine failure with a high degree of confidence.
In 1988, Northeast Inspection Services Inc. (NISI) started its turbine and generator rotor boresonic inspection program.
NISI participated in a formal EPRI Boresonic System Performance Evaluation as described in TR-102256, issued by the EPRI NDE Center in April 1993 (3). KEMA acquired NISI in 2008.
KEMA approach to boresonics
Boresonic inspection of steam turbine rotors performed by KEMA primarily deals with the near-bore region of the spindle forging. Forgings that are used in the fabrication of turbine or generator rotors first begin as massive steel ingots. The initial ingot will typically exhibit the presence of naturally occurring flaws that are inherent to the steel making process at the time of production. Depending on the type of material and intended rotor type, ingots are subjected to specialized heat treatments and a series of forging operations. Both the heat treatment and forging processes will result in the formation of flaws unique to these forms of processing. The ability to perform a meaningful boresonic examination of a steam turbine rotor requires a working knowledge of the types of flaws characteristic of large body forgings. Since boresonics involves the examination of the near-bore region, particular emphasis should be placed on those types of flaws that are indigenous to the central axis of the rotor forging. KEMA generally searches for flaws having a metal path distance of approximately 6? in sound path along the ultrasonic beam.