Piercing failure mechanism of pipeline elbows in LNG terminals and its influencing rules
SUN Bo1,YANG Xiaoping1,ZHANG Yunwei2,LI Zhen2,PENG Shiyao3,LIU Luoqian3

1.China Oil & Gas Pipeline Network Corporation; 2.PipeChina Beihai Liquefied Natural Gas Co. Ltd.; 3.PipeChina Institute of Science and Technology

LNG terminal, pipeline, elbow, piercing failure, physical and chemical properties test, cavitation, erosion

DOI: 10.6047/j.issn.1000-8241.2024.04.007


【目的】LNG具有易燃、易爆、超低温的显著特点,且LNG接收站通常建在沿海地带,站内管道的弯头因工艺流程特殊性易处于高腐蚀、高压差环境,具有较高的穿孔失效风险。【方法】基于某LNG接收站管道弯头穿孔失效情况,采用无损检测与扫描电镜方法,对弯头、直管段、环焊缝进行宏观形貌与理化性能分析:宏观形貌分析表明,管道直管段与弯头的几何尺寸、壁厚均满足国内外标准要求;上游直管段、法兰连接环焊缝沿LNG流动方向存在明显冲刷磨损缺陷,与弯头缺陷相似。在理化性能分析中,发现环焊缝处缺陷表面、弯头缺陷表面微观形貌均为蜂窝状形貌,表明均存在冲刷磨损。利用ANSYSFluent软件建立数值模型,模拟弯头处LNG的流动状态,发现在LNG流动情况下,焊缝余高影响管内流动,产生了节流效应及涡旋。【结果】通过宏观形貌、理化性能分析及数值模拟相结合的方法,揭示了LNG接收站管道弯头失效机理及影响规律:流体在孔板后形成气相夹带液滴的高速射流,对弯头壁面产生冲蚀。弯头内侧形成低压区、外侧为高压区,气泡破裂产生激波与空化溃灭,最终引发了弯头的穿孔失效。【结论】研究成果可为LNG接收站改善流体流动的稳定性、优化孔板设计、制定弯头失效解决方案提供理论参考,能够降低站内管道弯头穿孔失效的风险,提高管道系统运行的安全性与可靠性。(图 15表3,参[23]
[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 flow, 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 flange joint girth welds along the LNG flow 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 flow state of LNG at elbows was simulated, showing that weld reinforcement affects the LNG flow in the pipeline, with throttling effect and vortex flow resulting. [Results] The failure mechanism of pipeline elbows in LNG terminal and the influencing rules were revealed through the macroscopic morphology observation,physical and chemical properties analysis and numerical simulation in combination. Specifically, the fluid 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 fluid flow 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. (15 Figures, 3 Tables, 23 References)