Eileen Chant, Ph.D., P.E.; J. Robert Sims
Boiler tube failure is the leading cause of fossil plant downtime for utilities. In an effort to improve life assessment modeling tools which predict the time to failure for cracked boiler tubes, the Electric Power Research Institute commissioned Becht Engineering to develop a software tool providing a rapid remaining life estimate of tubes which have developed cracks at the return bends.
A series of 45 typical return bend designs with a range of outer diameters, wall thickness, design pressure and bend radius were selected. For each of these 45 designs, FEA analyses using Abaqus were completed for 6 ovalities to determine the maximum through-wall stress profile. The stress profiles from the FEA analyses were input into the Becht proprietary fracture mechanics spreadsheet-based software, which performs a Level 2 assessment of a growing crack subject to fatigue loading per Section 9 of API 579-1/ASME FFS-1, 2007 Edition. A series of starting crack aspect ratios (crack length to crack depth ratio) were simulated for each of the 270 FEA profiles to determine the remaining life to failure, which is defined as the crack breaching 80% of the wall thickness, or falling outside the FAD envelope.
A Windows®-based software program was developed for EPRI members which returns a remaining life estimate, based on user inputs and an interpolation of the calculated remaining life results residing in an MS-Access database. The user interface shows the simple inputs required to perform the remaining life estimate. This project was completed in December 2009.