Thursday, March 7, 2013

Upcoming EPRI Presentation

I recently sent out the Abstract for my upcoming paper at the EPRI Feedwater Heater Technology Seminar and Conference August 7 - 8, 2013, in Kansas City, Kansas, USA.

The paper is titled High Pressure Feedwater Heater Cup Forging Alteration. I am co-authoring it with Bob Cashner and Sara Vestfals of American Electric Power (AEP).

American Exchanger Services, Inc 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 channel was a forged steel cup-forging with up to 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, or leaving as is and performing regular inspections and monitoring of crack propagation. Using modern calculations, a design was chosen that reduced the stresses in the highly stressed corner radius. The alteration in design was to machine a pocket to fully encompass the crack, verify tubesheet stresses are within allowable stress limits, and prescribe future NDE to monitor for future cracking. The size of the machined "pocket" was determined by the actual crack length "as measured" and the repair was verified using Finite Element Analysis (FEA).

This repair was not a new method; in fact it was researched by Electric Power Research Institute (EPRI) in 1988 symposium [1], based on an EPRI 1985 [2] similar repair. In this earlier repair similar methods of Finite Element Analysis (FEA) were employed, however using less sophisticated forms of the calculation. Modern computing allowed for 3D analysis with more complex geometries, and more conservative meshing. The analysis showed that the earlier papers had it correct in their understanding of the problem and the proposed solutions. The important information was to determine if the machined radius would reduce stresses sufficiently, which would then allow the repair be within the NBIC [3] and ASME CODE [4] requirements

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