Loss of utility plant availability as a result of failure-causing cracks in steam turbine blades makes early detection of this problem critical. An Electric Power Research Institute survey, conducted as part of project RP 1266-24, indicated that 72% of turbine blade failures in fossil power plants occur in low-pressure (LP) turbines with half of all blade failures occurring in the last two blade stages (L-0 and L-1 rows). Failures are generally associated with blade tailing edges and root areas. Project RP 1266-24 also found that 79% of the blade problems in LP turbines were cracks. A turbine design of particular concern has been the Westinghouse Building Block (B.B.) 73. Reinhart and Associates has successfully inspected seven in-place B.B. 73 units for six utilities during the past 3 yr, as well as several disassembled turbines of other manufacturers and designs. These examinations consisted of visual and eddy-current examinations of the blade roots and trailing edges. The in-place inspections were performed using prototype manipulation devices to gain access to the blades through the hand holes. The only disassembly required to gain access for the examinations was the removal of the man-way covers on the main shell and the hand-hole covers on the outer cylinder covering the L-0 and L-1 blade rows.
A Level II workshop was held at the Georgia Institute of Technology in mid-July to assist the utility industry in understanding the new methodologies associated with the Westinghouse Building Block (B.B.) 73 turbine blade design. The workshop included 2 days of hands-on training focusing on the critical inspection and assessment of blade damage. The training included the new methodology and how it may be used to improve the turbine blade inspection process. The attendees included representatives from several utility companies. Also, we had two utilities who have had turbines inspected by the new methodology requesting that the methodology be used to inspect other turbines in the near future. The Westinghouse B.B. 73 turbine is a nuclear-type low pressure turbine being used in several nuclear-type power reactors. The B.B. 73 blade design, when fully developed and implemented, will replace the fifty year old Westinghouse WR-18 design. The B.B. 73 design is being developed by Westinghouse and has been in use for several years in several reactors. A baseline inspection of a number of blades was conducted in the Westinghouse facility in late 2000.
For the purposes of this paper, a crack may be defined as any discontinuity of the blade that is permanent (does not heal) or repeated. The Blade Cracks on LP Turbines are classified based on their condition. Categories are: a) No Cracks, b) Defined Cracks, c) Crack in Root Region, d) Cracks in Root Region, e) Cracks in Blade Edge, f) Cracks in Base, g) Cracks in Tail, and h) Cracks in Blade Flange.
The paper will outline a methodology for locating cracks in LP turbines that were previously undetected using eddy current technology. It will also discuss the classification of cracks and its impact on the inspection process. In addition, the paper will present an overview of the data collected, including the location of cracks, the number of cracks, and an overview of the U.S. nuclear power plants that have been inspected thus far in the project. 827ec27edc