Engineering Services for Large Steam Turbines - Related SI Project Experience
SIB-96-143

Investigation of Stator Vane Cracking at Houston Lighting & Power Company's South Texas Project, Unit 2
Extensive fatigue cracking was experienced in several rows of stator vanes of two double-flow Westinghouse BB380 LP turbines. SI conducted detailed fractography to determine the duration and amplitudes of periods of vibratory stress, and reviewed the manufacturer's coupled nodal diameter mode shape and frequency analysis. An independent assessment of the nonsynchronous aerodynamic excitation of these modes was performed under subcontract in the attempt to identify the off-design operating conditions responsible for the excitation. The results of the manufacturer's strain gage test data compared well with SI's fracture mechanics crack growth rate calculations and the measurements of vane to shroud weld defects. The vane rows were modified to eliminate the modes responsible for failure.

Review of the Fitness for Service of a Stal VAX LP Rotor with an Overmachined Blade Attachment
A design review of the static and dynamic stresses in an L-0 blade attachment was conducted for the purchaser of a new turbine rotor which had been overmachined along one axial entry attachment hook. The radius of the groove and the local nature of the deviation required a full three dimensional analysis of the blade root and disk rim. Potential failure modes investigated included low-cycle fatigue, high-cycle fatigue, and stress corrosion cracking.

Root Cause of Overspeed Failure of a Stal VAX LP Rotor
Malfunction of control and emergency stop valves was attributed to failure of a steam strainer basket, leading to operational problems that resulted in catastrophic overspeed of the turbine. An economic assessment of replacement turbines and rotors was conducted.

Root Cause of Control Blade Failures of a Stal VAX HP Turbine
Failure and cracking of several control stage blades were attributed to fretting fatigue under partial arc steam admission. Review of manufacturing procedures and root-to-disk tolerances was conducted. Assessment of the manufacturer's proposed design modification and cost-benefit analysis of repair options were performed.

Creep-Fatigue Monitoring of Steam Turbine Components
In two recently completed EPRI tailored collaboration programs for Kansas City Power and Light Company's Montrose Plant, and Western Resources Company's Gordan Evans Plant, SI has expanded the capabilities of the Creep-FatiguePro monitoring software program to include critical stationary and rotating steam turbine components. Remaining life assessments of turbine components can now be made based on actual turbine operating conditions, reducing the errors and conservatisms associated with off-line evaluations. Variability in inspection results and key material properties have been incorporated using a probabilistic approach.

LP Rotor Disk Cracking Evaluations
SI has performed deterministic/probabilistic evaluations of bore and keyway cracking in LP turbines for New York Power Authority (Fitzpatrick), Boston Edison Company (Pilgrim), Niagara Mohawk Power Company (Nine Mile Point, Unit 2), and Tennessee Valley Authority (Paradise). SI has also performed an evaluation of rim attachment cracking for Taiwan Power Company's Kuosheng LP rotors.

SI has recently completed an industry-wide survey of LP rotor rim attachment cracking. The primary objective of this survey was to collect data on LP rotor rim-attachment cracking to document the location and type of cracking, and correlate cracking experience with rotor manufacturer, rotor design, plant operating characteristics, and the run/repair/replacement options selected. The results of this survey provides utility personnel with valuable information to address the issue of rim-attachment cracking. A Phase II effort is underway to develop a life assessment computer code for use by utility personnel to evaluate rim cracking.

Turbine-Generator Health Monitoring (TGHM) System
Under EPRI sponsorship and in collaboration with Stress Technology Inc. (STI) and Southwest Research Institute (SwRI), SI is developing a health monitoring system to assess the optimum overhaul inspection interval for turbine-generators. To verify and refine these recommended intervals, the TGHM system will also provide the capability for on-line, real-time evaluations of life consumption and the cost of turbine performance degradation.

LP Turbine Weld Repair Assessment
SI recently performed an independent review and assessment of turbine rotor weld repair procedures for Taiwan Power Company's Kuosheng Nuclear Station LP rotor. The objective was to provide Taiwan Power Company with an independent assessment of the applicability of weld repair for turbine rotors, the SCC resistance of candidate repair materials and a detailed review of the proposed repair by Westinghouse.

If you would like more information, please contact SI.