Progress in development of acceptance criteria for local thinned areas in pipe and piping components

Over the last 30 years there has been a considerable amount of research conducted on the effect of corrosion on the burst strength of buried gas and oil transmission pipelines. The results of numerous burst tests on artificial flaws and corroded pipe removed from service were used to validate an empirical analysis that was essentially the limit–load solution for an axial crack in a pipe under pressure loading. This basic concept led to acceptance standards in ANSI B31G, and a more recent modified B31G criterion using the RSTRENG computer program developed at Battelle. This program takes into account variable flaw depths rather than the parabolic flaw shape assumed in the original B31G criterion. Since that time, more fundamental research has been conducted to develop a more accurate and theoretically based failure criterion. The Battelle/Pipeline Research Committee International PCORR computer program is an example of a special purpose shell-element based, finite element, PC criterion for the evaluation of local thinned area (LTA) flaws. This program has evolved with time from linear-elastic to elastic-plastic stress with provisions for axial as well as hoop stresses. The development and new insights into blunt flaw behavior resulting from this program will be one aspect covered in this paper. In the nuclear industry erosion-corrosion, or flow-accelerated corrosion, in single-phase liquid lines has become a major problem. Computer programs, such as the EPRI Checworks program, have been developed to assist the plant operators with deciding where to focus their inspections. However, to date no generally validated acceptance criteria have been developed for the plant piping. Plant piping, whether in nuclear power plants, fossil power plants, or petrochemical plants, have several differences from buried pipelines which need to be considered. The buried pipelines typically have low longitudinal stresses that frequently are compressive, and have no pipe fittings such as tees, elbows, and reducers except at compressor stations. Plant piping needs to consider hoop stresses and axial tension loads from the pressure, as well as, bending stresses from dead-weight loads, thermal expansion stresses, and seismic loads. In an effort to develop flaw acceptance criteria for Section XI of the ASME Boiler and Pressure Vessel Code, the criteria in Code Case N-480 have been revised and implemented into a new code case (the number has not yet been assigned). These criteria essentially use either the ANSI B31G approach for axial flaws, or the ANSI B31.1 or ASME Section III stress analysis rules to show that the residual strength of the thinned region meets the initial design stress limits. This paper presents some of the validation efforts recently undertaken to determine the inherent margins in the design stress equation approach compared with the applied safety factors in the axial and circumferential flaw limit–load solutions in: (i) the gas and oil pipeline industries; (ii) the proposed criteria in Belgium for the nuclear industry and other criteria, and (iii) the preliminary criteria from a recently proposed ASME Code Case on erosion/corrosion acceptance criteria and the ASME Appendix H criteria for flawed ferritic nuclear pipe.