American Exchanger Services, Inc (AM-EX) performed a 2012 repair of a vertical head-up High Pressure (HP) Feedwater Heater breech lock channel during a shop based re-tube for the American Electric Power (AEP) Wilkes Power Plant, Unit 1. The 48 year old channel is a carbon steel cup-forging that had 1-1/2″ deep circumferential cracks extending radially into the tight knuckle radius at the channel barrel to tubesheet junction. A small area was also found on the backside of the tubesheet which had corroded to a depth of 7/16″.
Several methods to repair these types of failures may be employed, such as grinding out the crack and weld repair, grinding or machining out the crack, leaving as is and performing regular inspections and monitoring crack propagation, or leave as is and perform a fitness for service analysis (API 579-1/ASME FFS-1). A repair was chosen that reduced the stresses in the highly stressed corner radius and was validated using modern calculations. The channel barrel was in the re-tube process so immediate access to a vertical lathe was available and this, in part, lead to the crack removal approach. The design alteration was to machine a pocket to completely remove the crack, verify tubesheet stresses are within allowable stress limits by Finite Element Analysis (FEA), and prescribe future Nondestructive Examination (NDE) to monitor potential future cracking. The size of the machined “pocket” was determined by the actual crack length “as measured” and the repair was verified using FEA. The machined pocket was Dye Penetrant Tested (PT) in September of 2013 (after approximately one year of returning to service), and no defects were noted.
This is not a new repair method; in fact it was researched by Electric Power Research Institute (EPRI) in 1988 symposium [1], and based on an EPRI 1985 [2] similar repair. In the 1985 repair similar methods of FEA were employed, however using less sophisticated forms of the calculation. Modern computing allows for 3D analysis with more complex geometries, and more conservative meshing. This analysis demonstrates that the earlier papers were correct in their understanding of the problem and the proposed solutions. The Wilkes HP feedwater heater was repaired in a similar manner with a machined radius that sufficiently reduced stresses to safe levels allowing an alteration of the vessel to be in compliance with National Board Inspection Code (NBIC) [3] and American Society of Engineers Boiler and Pressure Vessel Code (ASME B&PV CODE) [4].
