Abstract

Pipelines are the "blood vessels" of chemical production, and their service conditions are often accompanied by high temperature, high pressure (difference), high flow rate and other operating conditions, including gas-liquid phase transition, gas-liquid solid three-phase local flow conditions, as well as strong corrosive environments such as hydrogen, high chlorine, high sulfur and high acid.The diversity of physical and chemical properties of conveying media, the strictness of service environments, and the complexity of damage and failure mechanisms have put forward higher requirements for chemical pipeline transportation.

This article mainly reviews the progress of chemical pipeline transportation technology at home and abroad, and analyzes the technical challenges brought by different properties of transportation media to chemical pipeline transportation based on high-temperature and high-pressure process parameters, as well as the characteristics of corrosion, hydrogenation, and solid multiphase components. From the aspects of pipeline design theory, material and monitoring technology research and development, intelligent pipeline operation and maintenance, and pipeline standard specification construction in harsh and complex environments, the key development directions for chemical pipeline transportation are proposed in order to improve the level of chemical pipeline transportation and meet the needs of chemical pipeline transportation in the new situation.

Pipelines are an indispensable component of chemical production facilities, mainly used for transporting materials and products, and also serving as the safety boundary of equipment. For chemical production, a high temperature and high pressure environment is conducive to the progress of chemical reactions.Moreover, the properties of substances undergo significant changes at high temperatures, such as a decrease in fluid viscosity, an increase in diffusion coefficient, and a decrease in dielectric constant, which is beneficial for heat and mass transfer processes. Therefore, chemical production processes are often accompanied by high temperature and high pressure (differential) environments.In addition, due to the diverse physical and chemical properties of the conveying medium, pressure pipelines are often subjected to harsh medium environments, which can cause various forms of damage and failure, posing high operational risks to chemical pipelines.With the operation of numerous large-scale projects in China, such as millions of tons of oil refining, millions of tons of ethylene, and large-scale coal chemical engineering, this issue will become increasingly prominent. For example, in petrochemical and coal chemical plants, due to the deterioration of crude oil/coal quality, pipelines are often in a highly corrosive environment with high chlorine, high sulfur, and high acid, and corrosion problems caused by chloride ions are significant;In coal gasification/direct coal liquefaction units, multiphase flow containing solids almost runs through the entire process, and high-temperature and high-pressure pipelines are significantly affected by flow corrosion such as erosion and abrasion;In catalytic reforming and hydrocracking units, chemical pipelines not only need to withstand high temperature and high pressure environments, but also have to withstand the adverse effects of a certain proportion of hydrogen components, which can cause problems such as hydrogen embrittlement and hydrogen corrosion in high-temperature hydrogen environments;In the ethylene cracking unit, the cracking gas pipeline is subjected to high temperatures above 600 ℃, resulting in significant creep failure issues.Safety accidents caused by the failure of high-temperature and high-pressure pipelines have occurred frequently. For example, the "4.23" explosion accident at Sichuan Hongtai Biochemical Co., Ltd. in 2011, the PX (P-Xylene) project explosion accident in Zhangzhou in 2015, and the explosion accident at the Philadelphia Refinery in the United States in 2019. These safety accidents are all caused by pipeline failures.

Therefore, this article focuses on the characteristics of high-temperature and high-pressure processes in chemical production processes, as well as the characteristics of media such as corrosion, hydrogenation, and solid multiphase components. Based on a summary of the current development status of chemical pipeline transportation at home and abroad, it analyzes the challenges faced by China's chemical pipeline transportation technology and proposes suggestions for future development.

1. Research Progress on Chemical Pipeline Transportation Technology

Chemical pipelines involve many fields such as petrochemicals, coal chemicals, fluorine chemicals, chlor alkali chemicals, etc., covering a wide range, with different process parameters and diverse medium characteristics.According to the process characteristics of chemical production and the characteristics of the transported media, chemical pipeline transportation can be mainly divided into four categories: high-temperature and high-pressure pipeline transportation, strongly corrosive medium pipeline transportation, gas-liquid solid multiphase flow pipeline transportation, and high-temperature hydrogen environment pipeline transportation.

1.1 High Temperature and High Pressure Pipeline Transportation Technology

In recent years, in order to reduce production costs and improve production efficiency, process industry parameters represented by advanced processes such as petrochemicals and coal chemicals have been continuously improved, and pipelines are also subjected to higher temperatures and pressures.In the ethylene cracking unit, the cracking furnace spans pipes (700℃) and cracking gas pipelines (>600℃) that are subjected to high temperatures, which can easily cause creep failure; Thermal fatigue or vibration induced fatigue problems may occur in superheated steam pipelines (>500℃) and condenser/heat exchanger pipelines in chemical plants. Regarding the problems caused by high pressure, classical strength theory can ensure the strength design of pipelines, while the problems caused by high temperature are more complex.Therefore, the transportation of high-temperature and high-pressure pipelines mainly faces the problem of failure caused by high temperatures. Under high temperature conditions, the performance of materials deteriorates over time, resulting in various complex damages such as creep, fatigue, oxidation, and carburizing, which pose higher requirements for the design and evaluation of chemical transportation pipelines.Countries such as the UK and France have successively established high-temperature structural integrity evaluation methods, such as the UK's R5 code and France's RCC-MR code. Since the 15th Five Year Plan, domestic scholars have also established high-temperature structural integrity assessment techniques and formulated JB/T 12746-2015 "Safety Assessment Methods for High Temperature Pressure Pipelines and Valves with Defects".

1.2 Pipeline Transportation Technology for Highly Corrosive Media

In the petrochemical and coal chemical industries, pipelines are often exposed to strong corrosive environments with high levels of chlorine, sulfur, and acid, such as H₂S, CO₂, and Cl^- corrosion in atmospheric and gasification units, as well as high-temperature cyclic acid corrosion in atmospheric and vacuum units.Especially in the welding area, due to the unevenness of organization and geometry, coupled with the uneven distribution of welding residual stresses, it is more likely to cause chloride stress corrosion cracking.With the increasing sulfur and acid content in crude oil and the deterioration of coal quality, this problem will become more prominent.To address this issue, on the one hand, high-precision detection technologies such as ultrasonic guided waves, ultrasonic phased arrays, and electromagnetic ultrasound have been developed for online corrosion monitoring and diagnosis, effectively controlling and reducing the risk of pipeline failure; On the other hand, the risk of stress corrosion cracking is reduced through the control of welding residual stress, and the in-situ control technology based on welding process and structural design and the post welding control technology based on external heating or mechanical/force action have been developed.Due to the randomness and concealment of pitting corrosion, traditional corrosion detection techniques and accuracy are difficult to detect pitting defects in a timely manner. Therefore, accurate prediction of pitting corrosion behavior is particularly important.

In recent years, cellular automata models, phase field models, and near-field dynamics models have been developed to quantitatively describe the pitting process, and crack propagation rate models such as slip dissolution/oxidation models and deformation/oxidation models have also been developed to predict stress corrosion cracking.Since the Ninth Five Year Plan, domestic scholars have carried out a large amount of pipeline defect assessment work to address the issue of medium corrosion. In 2018, GB/T 35013-2018 "Evaluation of the suitability of pressure equipment for use" was first promulgated in China, establishing evaluation methods for the suitability of pressure equipment for use in high-temperature and corrosive environments such as stress corrosion, corrosion fatigue, hydrogen induced cracking, and creep damage.

1.3 Gas-liquid-solid Multiphase Flow Pipeline Transportation Technology

The pipeline transportation medium in coal chemical gasification and direct coal liquefaction units contains solid particles with a total mass fraction of up to ten percent, such as coal powder, minerals, or catalysts.The black water medium caused the appearance of complex gas liquid solid three-phase flow due to the flash liquid phase change, which made the control valve in the process in the harsh working conditions of high pressure difference, high flow rate (the maximum value in some parts exceeds 100m/s), and strong erosion, as well as the extremely serious abrasion and corrosion coupling damage of the valve core, valve seat, valve liner, reducer (Venturi pipe) caused by this.In response to erosion and abrasion problems, surface hardening technology is generally used in coal chemical industry to enhance the wear resistance of materials, and surface hardening technologies such as supersonic flame spraying technology, nickel based alloy thermal spraying technology, and laser cladding technology have been developed.In addition to improving materials, establishing an erosion and abrasion failure prediction system to accurately predict the specific failure location of pipelines is of great significance for guiding pipeline optimization and in-service inspection, and effectively improving equipment reliability and safety.Early research on the single erosion and abrasion problem of multiphase flow with solid content mainly focused on experiments. Based on the summary and induction of failure mechanism characteristics, empirical or semi empirical mathematical models were established to play an important role in predicting multiphase flow erosion and abrasion.With the rapid development of computational fluid dynamics and significant advances in experimental techniques, recent research has often combined experiments with numerical simulations to explore the coupling mechanisms and models of erosion, corrosion, and cavitation in multiphase flows containing solids.Domestic research institutions have made significant progress in flash evaporation, cavitation phase change flow, liquid-solid, gas-solid, gas-liquid-solid abrasion, corrosion, cavitation models, as well as risk assessment and monitoring technologies for high-temperature and high-pressure pipelines, providing strong support for China's coal chemical pipeline transportation technology.

1.4 Pipeline Transportation Technology in High-temperature Hydrogen Environment

In chemical plants such as catalytic reforming and hydrocracking, pipelines are exposed to high temperatures (>400°C), high pressures (15-18 MPa), and hydrogen environments (such as hydrogen and hydrogen sulfide) for a long time, causing high-temperature hydrogen damage, including hydrogen embrittlement, hydrogen corrosion, hydrogen bubbling, and surface decarburization.The selection of materials for high-temperature and high-pressure hydrogen pipelines is mainly based on the Nelson curve specified in the American Petroleum Institute (API) standards.At the same time, considering the presence of H₂S in the process medium, the Couper Gorman curve is also referred to when selecting materials, focusing on its resistance to H₂S corrosion.The mechanism of high-temperature and high-pressure hydrogen embrittlement in metal materials is relatively complex, involving processes such as hydrogen atom diffusion, segregation, and damage. Although people have known about the phenomenon of hydrogen embrittlement for over a century and proposed theories such as hydrogen induced weak bond theory, hydrogen promoted dislocation emission, and hydrogen enhanced local plastic deformation, there is still no unified understanding of its mechanism of action, and there is a lack of effective means to fundamentally suppress hydrogen embrittlement.The quality of welded joints is also a key factor affecting the safety of pipelines in high-temperature hydrogen environments. High temperature and high-pressure hydrogen pipelines have the characteristics of large diameter and thick pipe wall, and with the trend of large-scale equipment development, this feature will become more prominent, posing greater challenges to the quality of welded joints, and easily causing thermal cracking, intergranular corrosion, and stress corrosion cracking. For this reason, TP321 austenitic stainless steel is generally selected for hydrogen pipelines and the welds are subjected to stabilization heat treatment.

2.Challenges faced by Chemical Pipeline Transportation Technology

With the increase of sulfur and acid content in crude oil and the deterioration of coal quality, the medium for chemical pipeline transportation is becoming increasingly stringent.At the same time, in view of the positive impact of the appropriate increase of reaction temperature and hydrogenation operating pressure on the conversion rate of middling coal and oil and gas yield of the direct coal liquefaction and heavy oil processing processes, the reaction temperature and pressure will further increase, the design temperature of the new generation of hydrogenation reactor will be as high as 500~510℃, and the temperature load of the reactor outlet pipe will also be increased accordingly.In addition, the long-term operation of chemical plants is an urgent need and development trend for enterprises to reduce costs and increase efficiency. The refining unit needs to achieve "four-year maintenance" and the chemical unit needs to achieve "six-year maintenance", and will move towards longer maintenance cycles in the future.Higher temperature and pressure, stricter media, and longer operating cycles pose higher requirements for chemical pipeline transportation, which will make corrosion, wear, erosion, creep and other problems faced by pipelines more prominent. To ensure the long-term safe and reliable operation of chemical pipelines, future chemical pipeline transportation will face the following technical challenges.