Piercing failure mechanism of pipeline elbows in LNG terminals and its influencing rules

Abstract

[Objective] LNG has the obvious characteristics of high flammability, high explosiveness and ultra-low temperature. LNG terminals are generally built in coastal areas, and thus the pipeline elbows in an LNG terminal are easily located in the severe corrosive environment with high pressure difference due to the particularity of the process fow, facing a high risk of piercing failure. [Methods] Based on the general situation of piercing failure of pipeline elbows in an LNG terminal, the macroscopic morphology and the physical and chemical properties of the elbows, straight pipe sections and girth welds were analyzed through non-destructive testing and scanning electron microscopy. The macroscopic morphology analysis results show that the geometric size and wall thickness of the straight pipe section and the elbow meet the corresponding standards. However, obvious erosion wear is observed in the upstream straight pipe sections and the fange joint girth welds along the LNG fow direction, which is similar to the elbow defects. In the analysis of physical and chemical properties,it is found that the micro-morphology of the defective surfaces at the girth weld and the elbow are honeycomb-like, indicating that there is erosion wear. Besides, a numerical model was established with ANSYS Fluent software, and the fow state of LNG at elbows was simulated,showing that weld reinforcement affects the LNG fow in the pipeline, with throttling effect and vortex fow resulting. [Results] The failure mechanism of pipeline elbows in LNG terminal and the infuencing rules were revealed through the macroscopic morphology observation,physical and chemical properties analysis and numerical simulation in combination. Specifically, the fuid forms a high speed jet with liquid droplets in the gas phase behind the orifice plate, which erodes the wall surface of the elbow. Thereby, a low pressure area is formed on the inner side of the elbow, and a high pressure area is formed on the outer side. Then, the bubble burst produces shock waves and cavitation collapse, and finally causes the piercing of elbows. [Conclusion] The research results could provide theoretical reference for improving the fuid fow stability, optimizing the orifice design and developing elbow failure solutions in LNG terminals, which could reduce the risk of elbow piercing failure in LNG terminals, and improve the safety and reliability of pipeline system operation.

Keywords

• lng terminal
• pipeline
• elbow
• piercing failure
• physical and chemical properties test
• cavitation
• erosion