Evaluation of localized thinning in elbows of standard carbon steel piping under internal pressure

Abstract

Pipings structural integrity and good safety record depend on a combination of safe design, good selection of materials, and good & reliable operating practices; nevertheless, like any engineering structure, piping systems do occasionally fail for many known and unknown causes. One of the most common causes of damage in piping is corrosion, which could eventually lead to failure and sometimes to a complete shutdown of the operating unit where the damage has occurred. Several API Inspection Codes are currently available to monitor the integrity and healthy operation of straight piping systems. These codes include an exaggerated factor of safety so they can be used by non-engineering staff. By conducting a comprehensive stress analysis on a typical piping system, we may find that the API code specified retirement thickness is not realistic and in fact the remaining life of the piping component might be much longer. This is particularly valid when dealing with elbows due to the complexity of the geometry and loading. The Scope of this thesis is focused towards examining the influence of localized wallthinning depth ratio on the outrados and internal pressure level on the remaining strength of standard schedule-40 carbon steel elbows mainly used in Petroleum and refineries and petrochemical plants. This was achieved by conducting a comprehensive parametric stress analysis using two-dimensional elastic finite element analysis. Several standard elbow sizes (NPS 4 to 14) with various wall thinning depth ratios have been considered in the study. For every NPS case a different critical intrados pressure was considered. The localized wall thinning was assumed to be uniform within the outrados region of the elbow. The final results have been summarized using tables and figures, mostly presented in Chapter 3, where the proposed basic limitation on thinning in the outrados of standard elbows subjected to internal pressure is presented as function of the original design thickness calculated using ASME/B31.3 piping construction code. Conclusions and recommendations are summarized in Chapter 4.