Permanent Weld Repair of GE F Type IP Outer Shell With Incorporated Design Improvements

摘要

Recently an internal inspection was performed on a 30-year old 219 MW General Electric F9 design steam turbine that was forced out of service due to high vibration. During the inspection two significant cracks were discovered on the IP upper outer shell. The cracking was located in the reheat inlet bowl area on the left and right hand side. The cracking initiated at the horizontal joint bolt hole locations and propagated upwards towards the intercept valve flange. The left side crack was approximately 26＂in length and had propagated all the way through the shell wall from the ID to the OD. The right side crack was 30＂ in length and had propagated approximately halfway through the shell wall thickness in this area. This type of cracking has been observed on other General Electric turbine shells of similar design in the past. The repeatability of this cracking mode is indicative of a design weakness in this particular design. A repair vendor to determine the best geometry of the shell in the reheat bowl area performed an extensive engineering analysis utilizing finite element modeling. This was necessary since it was the owner's intent to perform a permanent weld repair because of their desire to obtain another 20 plus years of service for this unit. If design improvement could be achieved to advance this objective then the owner wanted them to incorporated into the repair. It was felt that if the geometric design issue was not addressed then the potential for future cracking in this area remained very high. This paper will detail the results obtained from the finite element modeling and analysis study. The resulting repair recommendations and procedures will be discussed with an overview of the actual permanent weld repair process that was performed on this unit. The unit was stress relieved weld repaired in the fall of 1998 and has been successfully returned to service.