Abstract

This article selects a certain gas pipeline system, the director of the line on material X60 steel and typical process pipeline structure of 20 # steel elbow, yard by the metallurgical structure observation of the typical sampling position and hardness tests, hydrogen permeation test compared the hydrogen and hydrogen content of X60 steel base, girth weld and hydrogen embrittlement sensitivity of 20 # steel elbow position size. The risk of hydrogen embrittlement at sensitive sites in hydrogen environment was studied by notch tensile test. The results show that the hydrogen diffusion coefficient of base metal and weld of X60 steel is higher, and the hydrogen concentration adsorbed on the surface is lower. The hydrogen diffusion coefficient of 20# steel is low, and the concentration of hydrogen adsorbed on the surface is high, which indicates that the hydrogen trap content in 20# steel is high, and it is easier to capture hydrogen atoms, and the possibility of hydrogen embrittlement exists. The notch tensile test of 20# steel at a sensitive position was carried out in a medium with a total pressure of 6 MPa and a volume fraction of 3% hydrogen. Compared with the similar experimental results in the air, the strength and plasticity of 20# steel have no obvious changes. The results show that the risk of hydrogen embrittlement of 20# steel is low in the mixed hydrogen transportation environment.

Introduction

With the increasing global energy demand, the transportation volume of natural gas increases rapidly, and pipeline transportation has become the most commonly used transportation mode of natural gas [1]. Hydrogen is produced as a byproduct of ethylene production from ethane. Hydrogen is flammable and explosive. If it is directly discharged into the atmosphere, there are some safety risks. As a kind of efficient and clean secondary energy, hydrogen can not only avoid energy waste, but also increase economic benefit if it is mixed into natural gas pipeline. However, considering the particularity of hydrogen, hydrogen mixed natural gas will also bring certain risks to the safety of pipeline transmission system and equipment and facilities in the station, so the influence of hydrogen on pipeline steel is attracting more and more attention [2,3]. In the process of transportation, hydrogen molecules can adsorb on the steel surface, decompose into nascent hydrogen atom, diffuse through iron lattice, trapped and concentrated at crystal defects such as dislocations and grain boundaries as well as in stress-concentrated areas, resulting in hydrogen embrittlement[4].Existing literature shows that the risk of hydrogen embrittlement of grade of X60 steel has been reduced. However, in the manufacturing and repair process of pipeline welding joints at specific positions, the properties of X60 are inevitably affected by assembly stress, heat treatment process and other factors, making material prone to local tissue hardening and formation of various defects [5,6].At the same time, the position of pipe in the station may be subjected to complex forces which could lead to hydrogen embrittlement failure when transporting natural gas mixed with hydrogen [7].At present, there is no relevant domestic and internal industry standards for hydrogen mixing proportion of natural gas pipeline for reference. Therefore, in order to have a safe transportation of natural gas mixed with hydrogen through pipelines, it is necessary to conduct a study on hydrogen mixing transportation based on the hydrogen mixing ratio of the existing pipeline transportation system.

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