Abstract

Abstract

Abstract

Abstract

Abstract

Elbows are components in piping systems whose primary function is to achieve a change in flow direction. Their study is relevant, since they present higher stress levels than those observed in straight sections. Early investigations determined that this effect is caused by the ovalization effect and the presence of external moments. The capacity of a cross section to ovalize when external loads are applied results in increased flexibility of the elbows by decreasing the moment of inertia compared to a straight pipe. In this work, stress levels in elbows caused by external moments and internal pressure are studied. As a case study, a series of elbows of various diameters and thicknesses with suitable characteristics for understanding the phenomenon of ovalization and the comparison of different calculation methodologies is proposed. The stresses obtained by applying the ASME B31.1 and ASME B31.3 standards are compared with those obtained by a numerical model of finite elements, through an novel indicator called the Normative Safety Margin. This indicator considers the admissible stresses established by the standards for the materials of the accessories. Applying B31.1 Standard, higher normative safety margins are obtained than those obtained with B31.3. For the entire series under study and all types of applied loads, lower normative safety margins were obtained when the accessories are more flexible.

Abstract

Elbows and bends in piping systems show higher stress levels than those observed in straight sections, one of the main causes being the ovalization of cross sections in the presence of external moments. Codes and standards for piping systems consider the effect of ovalization of these fittings by using flexibility and stress intensification factors to approximate their stress states with respect to the stresses of straight pipe sections. This paper studies the stress states in elbows caused by the presence of external moments and internal fluid pressure, using steel with elastoplastic behavior. A series of elbows of various diameters and thicknesses is proposed as a case study, which present adequate characteristics for the analysis and understanding of the ovalization phenomenon. The results calculated by applying some of the ASME B31 codes are compared with those obtained by a numerical finite element model, applying the Normative Safety Margin, defined by the authors in a previous work. This is a control parameter that, when less than unity, indicates the presence of ultimate tensile strengths in the accessories under study that were not detected by the normative calculations. For the same load condition, the normative safety margins obtained are decreasing in magnitude with Codes B31.1, B31.3 and B31.8, respectively, so this series orders the codes from the most to the least conservative. One of the main causes of this phenomenon is the different comparison stresses established by the codes. For the entire series under study and all types of applied loads, lower regulatory safety margins were obtained when the accessories were more flexible.