Evaluation of the Limit State of a Six-Inch Carbon Steel Pipe Elbow in Base-Isolated Nuclear Power Plants

The installation of base isolation systems in nuclear power plants can improve their safety from seismic loads. However, nuclear power plants with base isolation systems experience greater displacement as they handle seismic loads. The increase in relative displacement is caused by the installed base isolation systems, which increase the seismic risk of the interface piping system. It was found that the failure mode of the interface piping system was low-cycle fatigue failure accompanied by ratcheting, and the fittings (elbows and tees) failed due to the concentration of nonlinear behavior. Therefore, in this study, the limit state was defined as leakage, and an in-plane cyclic loading test was conducted in order to quantitatively express the failure criteria for the SCH40 6-inch carbon steel pipe elbow due to low-cycle fatigue failure. The leakage line and low-cycle fatigue curves of the SCH40 6-inch carbon steel pipe elbow were presented based on the test results. In addition, the limit state was quantitatively expressed using the damage index, based on the combination of ductility and energy dissipation. The average values of the damage index for the 6-inch pipe elbow calculated using the force−displacement (P–D) and moment−relative deformation angle (M–R) relationships were found to be 10.91 and 11.27, respectively.

GRP Composite Pipe Elbows Subject to an Internal Pressure and In-Plane Bending: An Experimental Study

The use of polymer composite materials in the piping industry is increasingly gaining popularity. However, the design rules of these novel materials are relatively more complex and high safety factors are applied, requiring costly experimental validation. Experimental tests on e-glass reinforced polymer (GRP) pipe elbows is limited and in view of these challenges, this study presents a set of experimental results obtained from a series of pressurization and in-plane bending tests of polyester pipe elbows reinforced with e-glass chopped strand and woven roving mats. Details of the specimen manufacturing procedure, testing fixture and loading setup, are also given. A data acquisition system was setup to control, monitor and record the applied loads, the resulting meridional and hoop strains around the mid-plane of the elbows. A total of 3 specimens were tested where the first specimen was subject to an increasing positive pressure until global failure and was used to serve as a benchmark for the other two specimens. The latter were similarly subject to a positive pressure limited to the first-ply-failure load region established from the initial test. Both specimens were unpressurized and then subjected to a two cycle opening and unloading in-plane bending moment. Results show that the GRP pipe elbows were capable of sustaining loads beyond the identified failure initiation and successive bending cycles produced pipe elbow stiffening effect.

Asset Integrity Management and Fitness-for-Service Assessment of a Pipe Elbow With Metal Loss

The objective of this paper is to describe the inspection process and examine a piping elbow found to have metal loss within the context of Asset Integrity Management (AIM). During the last five years, Industry 4.0 technology involving mobile tablet and cloud technology has merged with the proven concepts of AIM enterprise systems. Before this shift, inspectors, engineers, and managers relied on unreliable and time-consuming methods. New technologies allow inspectors and mangers to think, react, and communicate quickly and proactively when integrity threats are discovered. A piping elbow was identified at a thickness monitoring location (TML) within a larger system due to existing internal corrosion and/or erosion. Since the metal loss is internal, an external visual examination was not sufficient to monitor the metal loss. External ultrasonic (UT) inspection was used to measure the thickness at multiple points on a grid pattern drawn on the elbow’s external surface. The thickness measurements provide the data needed for a local metal loss assessment. An AIM program was used to assess the risks, aid in inspection, and develop an assessment for ascertaining the degradation pattern. With a heightened understanding of the asset’s degradation pattern, the risk to the asset is reassessed continuously until it has been reduced to an acceptable level as defined by the stake holders.