The Technical Basis Document for API RP 941: Segment TECHNICAL REPORT 941 SEPTEMBER 2003 BO?i?d Industrlai Instarm? . CE Appeals I Memo" .3 No.. ADM. Fr REPORT TO API - THE TECHNICAL BASIS FOR API RP 941 17.0 DISCUSSION The topic of HTHA has been researched for over seventy years. The seemingly simple curves developed by G.A. Nelson (Appendix A) attempted to capture industry experience through the 1950's and 1960's. With the notable exception of - 0.5% Mo steel. "the Nelson Curves" depicting satisfactory and unsatisfactory pressure-temperature regions have survived as reasonable to this day. The pressure-temperature curveshe prepared were based on Nelson's judgment after considering reports about laboratory tests and'plant experience. His judgment seems to have been quite good because much of the raw "data" he obtained might not stand up to close scrutiny. The information he was given by plant operators about serviCe pressure and temperature. or the absence of damage should not be accepted at face value. This statement is made here because. even currently. we ?nd it dif?cult to obtain accurate operating data and material damage assessments. This barrier to gathering reliable data is unlikely to have gotten worse since Nelson?s time. While the data gathered by Nelson from the ?eld were certainly suspect. labor?atory work from his time on has not yielded conclusive information helpful in today's climate of interest in ?tness-for-service. risk and life assessment. One can debate whether Nelson's success has been the result of remarkable insight. his conservatism or just that his curves have not been effectively tested in service because of large operating safety margins used in.refineries. Certainly the success of API RP .941 stemsjn part from the fact the curves have been applied in regard to? design and operation. Recently. however. a growing list of problems With 0.5% Mo steel underscored-the lack of understanding of HTHA and heightened concern about lengthening exposure time and the curves for other steels. Also. now it is desired to push equipment to the limits of RP 941 for economic reasons and to perform risk-based inspection. Appendix A provides a detailed discussion of the background of RP 941 and the data gathered. It is worth pondering why seemingly little progress in understanding HTHA has been made since Nelson's times. We are still far from being able to make quantitative predictions about the behavior of steels subject to HTHA. A literature review done for this study has led to a long list of obstacles to establishing more appropriate operating limits for steels. The list in Appendix should provide someone generally familiar with HTHA with additional insight into the nature of HTHA and the current state of affairs. Also refer to Appendix A and the references cited in API RP 941 to obtain additional technical foundation. The list in Appendix explains why there is currently inadequate data about the speci?c effects of microstructure. carbon activity and hydrogen content. The inconVenient aspects of studying reaction kinetics and calculating methane pressure leaves engineers poorly equipped to consider the actual driving force for initiation and propagation of attack. This report has attempted to overcome shortcomings of prior investigations by focusing on quantifying the role of methane pressure on causing HTHA damage. To validate a model for HTHA it would be necessary to fashion a comprehensive test program and then determine for one steel the functional relations of all key variables in hydrogen attack. Such variables are stress. grain size. tensile and creep carbon content. temperature. methane pressure. microstructure. etc. Only MPC's stress rupture results for 2?1/4Cr?1Mc in high-pressure hydrogen approach. being such a database. but they fall short. The set of isochronous lines for Carbon steel satisfactory starting point. 45