In this paper cyclic test results of elbow with volumetric defect at outer surface of stretched zone are presented. Test was carried out under pulse inner pressure loading of torus welded from four elbows of the same size. Experimental durability was compared with calculated according to method of residual life estimation of straight pipeline with damage of the same type. Series of FEA calculations of torus with circular cross-section of equal thickness with model defect in form of ellipsoid part under inner pressure was performed. Obtained results confirm possibility to use for residual life assessment of elbow with volumetric surface defect remaining life estimation of straight pipeline with the same damage.
Elbows are widespread and rather often defective elements of pipelines. As a consequence of hard service conditions, stress-strain state features and deterioration of material properties during manufacture their strength is lower than straight sections of pipeline. (figure shown in paper) Fig.1. Mechanism of flow accelerated corrosion at elbow inner surface according to Shiihara (2008):
single phase flow (water);
two phase flow (water and vapor); Dn - nominal diameter; h - wall thickness.
In many cases the base load of pipeline is inner pressure that is not constant in service and changes most significantly during pumping starts-stops and in hydraulic tests. Aforementioned determines necessity of elbow cyclic strength estimation. This task gets even more significant in presence of volumetric defect at the elbow surface. As a rule it could be result of flow accelerated corrosion or corrosion process (Fig.1). Periodical investigation in technical state of KievENERGO steam pipelines with diameter Dn = 76…426mm and operation term 25685…341640 hours revealed 41 damaged elbow. The length and depth of defects amounted to (0.1–12)Dn and (0.1–0.5)h correspondingly.