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I. Causes of failure in pressure pipelines\nThe “failure” of pressure pipelines generally refers to the situation in which they are unable to perform their intended functions, and it can be divided into natural failure and abnormal failure. Since pressure pipes operate under the influence of the internal medium and the surrounding environment, issues such as temperature and pressure fluctuations, corrosion, vibration, and changes in the material’s microstructure inevitably arise, affecting both the material’s properties and the sealing performance of the joints. Therefore, every pipe has a certain service life, and natural failure occurs when the pressure pipe reaches the end of its service life. Natural failure can be controlled in advance through regular inspections or failure analysis to prevent accidents. However, pressure pipelines can experience abnormal failures due to various issues in design, manufacturing, installation, and operation, leading to sudden catastrophic accidents. The main reasons are as follows: 1. Poor quality of employees who operate in violation of procedural guidelines, which leads to deteriorating operating conditions, including overpressure, overheating, excessive levels of corrosive substances, and abnormal fluctuations in pressure and temperature ； Pressure and temperature are the basis for the design, material selection, manufacturing, and installation of pressure pipelines. If the operating pressure and temperature exceed the specified limits, it will lead to an increase in wall stress values or a decline in the mechanical properties of the material. In particular, local stresses and peak stresses at locations where the geometry is discontinuous, such as welds, flanges, elbows, valves, reducers, and compensators, increase significantly, becoming the source of creep failure. Too low an operating temperature leads to a decrease in the material’s toughness and a reduction in the allowable critical crack size, which can result in brittle failure. Excessive temperature and pressure can also cause leaks at pipe joints. Pipelines often experience alternating loads due to the following reasons: (1) intermittent transfer of the medium, which causes repeated pressurization and depressurization, as well as heating and cooling of the pipeline ； (2) Large pressure fluctuations during operation ； (3) Periodic changes in temperature during operation cause repeated variations in thermal stress on the pipe wall ； (4) Due to alternating external forces from other equipment and supports, as well as forced vibration. Under the action of repeated alternating loads, the pipeline will suffer fatigue failure. It is mainly low-cycle fatigue of metals, characterized by high stress and a low frequency of variation. Stress concentration occurs at points where the geometric structure is discontinuous and near welds, and it is possible for this stress to reach or exceed the material’s yield limit. If these stresses are applied and removed alternately, the grains under the greatest stress will undergo plastic deformation and gradually develop into fine cracks. As the stress cycle changes, the crack also gradually expands, eventually leading to failure. Alternating loads can also cause the expansion of existing defects within pipeline components and welds, as well as leakage at pipeline joints. 2. Defects in design, manufacturing, and construction, such as inadequate pipe flexibility, improper selection or incorrect use of materials, welding or metallurgical defects exceeding allowable limits, excessive stress caused by unreasonable welding or assembly, and an irrational pipe support system, etc ； The inherent defects present in the pipeline before it is put into use can cause low-stress brittle fracture of the material. Factors such as damage from media and the environment, improper operation, and inadequate maintenance can often lead to a deterioration in the properties of materials, damage or rupture of those materials, or leakage at pipe joints, ultimately causing the pressure pipes to fail and resulting in accidents such as fires, explosions, poisoning, and suffocation. 3. Repair errors, such as failure to detect serious defects or damages in the pipes, lack of scientific evaluation, and unreasonable repair methods that result in new defects and damages ； 4. Damage caused by external forces, such as earthquakes, strong winds, floods, lightning strikes, as well as other mechanical damages and human-induced destruction. There are many types of failure modes for pressure pipelines. Based on the macroscopic deformation at the time of failure, it can be divided into two main categories: ductile failure and brittle failure. Based on the microscopic fracture mechanisms of materials during failure, it can be classified into types such as ductile dimple fracture, cleavage fracture, intergranular fracture, and fatigue fracture. Generally, on-site classification is carried out by combining macroscopic classification with fracture characteristics, including ductile failure, brittle failure, corrosion failure, fatigue failure, creep failure, etc. 5. Corrosion damage: The corrosion of pressure pipelines occurs due to chemical or electrochemical actions exerted by internal media and external environmental media. It also includes the combined effect of factors such as machinery. Unreasonable operations can cause changes in the medium concentration, exacerbating corrosion and damage. The forms of corrosion damage in pressure pipelines include general corrosion, local corrosion, stress corrosion, corrosion fatigue, and hydrogen-induced damage. Stress corrosion often occurs suddenly without any warning, which makes it even more hazardous. (1) General corrosion General corrosion is also known as uniform corrosion. It is corrosion of basically the same degree occurring over a relatively large area of the pipe. The inner surface of the pipeline is primarily subject to corrosion caused by corrosive media being transported, while the outer surface is mainly affected by atmospheric rusting. General corrosion of pipelines often intensifies due to deteriorating operating conditions. The composition of the corrosive medium, its water content, differences between gas and liquid phases, flow rate and flow pattern, as well as particle size, all affect the degree of pipe corrosion and failure. Excessive levels of corrosive agents or deterioration in the properties of raw materials can cause damage to pressure pipelines. Atmospheric corrosion can cause damage to the exterior of pipeline components, affecting their strength and sealing properties. If not maintained in a timely manner, it can also lead to accidents. (2) Local corrosion: Local corrosion is a corrosion phenomenon that occurs in specific areas of the pipeline material. a) Pitting corrosion: A type of corrosion that occurs in deep areas at individual small spots on the metal surface, also known as pocketing. Austenitic stainless steel is most susceptible to pitting corrosion when in contact with media containing chloride or bromide ions. b) Crevice corrosion: When the medium transported through the pipeline is an electrolyte solution, crevice corrosion occurs at the gaps on the inner surface of the pipeline, such as at flange gaskets and in areas where single-sided welding has not been complete. Crevice corrosion often results from a difference in oxygen concentration or metal ion concentration between the solution inside and around the crack. c) Corrosion of austenitic stainless steel welded joints: ① Intergranular corrosion: Intergranular corrosion is a type of corrosion in which the degradation occurs within and around the grain boundaries, with little corrosion occurring within the grains themselves. The corrosion mechanism is the \"chromium-deficiency theory\"; that is, because the chromium-deficient intergranular regions are in an activated state, they act as an anode and form a corrosion galvanic cell with the grains, resulting in grain detachment or a decrease in the mechanical strength of the material. ② Selective corrosion of δ-ferrite: In certain highly corrosive media, the δ-ferrite phase at the welds of austenitic stainless steels is corroded or decomposed into σ-phase, resulting in a spongy structure that damages the welded joint. ③ Edge corrosion: A knife-edge-like corrosion that occurs in austenitic stainless steels stabilized with Ni and Ti in oxidizing media. (3) Stress corrosion: The corrosion of metal materials that occurs under the combined action of tensile stress and a specific corrosive environment is known as stress corrosion. It is mainly caused by residual stresses from welding, cold working, and installation, as well as corrosive substances inside the pipes. The cracks caused by stress corrosion take on the shape of withered tree branches, developing generally in a direction perpendicular to the tensile stress. The microstructures of cracks include transgranular type, intergranular type, and a mixed type that combines both. High-strength steel pipes are highly susceptible to stress corrosion cracking in the presence of H2S at levels above a certain threshold, along with moisture. When the weld hardness exceeds HB200 or the H2S content is above the limit, it can easily lead to stress corrosion of the weld. ① Alkali embrittlement: It is the stress corrosion of metals in alkaline solutions. Alkali embrittlement can occur in carbon steel, low-alloy steel, stainless steel, and others. ② Chloride corrosion of stainless steel: Stress corrosion of stainless steel caused by chloride ions. The critical concentration of chloride ions that causes chloride corrosion decreases as temperature rises; at high temperatures, a chloride ion concentration of just 10 ppm is sufficient to cause cracking. Excessively high levels of chloride ions in materials such as gaskets at pipe flange connections, external insulation materials, and cushioning materials for supports and hangers can also lead to chloride-induced corrosion. ③ Corrosion of stainless steel by polyoxysulfuric acid: During petroleum refining, steel is corroded by hydrogen sulfide to form iron sulfide; after the process stops, oxygen and water in the air react inside the pipes to produce polyoxysulfuric acid, and stress corrosion occurs in those areas of the stainless steel pipes where there are high residual stresses. Taking the hydrodesulfurization unit as a typical example, stress corrosion failure of stainless steel in polyoxysulfuric acid has recently attracted considerable attention. ④ Sulfide stress corrosion: Stress corrosion that occurs in metals in a medium containing both hydrogen sulfide and water. Carbon steel and low-alloy steel are most sensitive to sulfuric acid in the temperature range of 20–40°C. Sulfide stress corrosion of austenitic stainless steels mostly occurs in high-temperature environments. In media containing hydrogen sulfide and water, the presence of acetic acid, or carbon dioxide and sodium chloride, or phosphine, or compounds of arsenic, selenium, or tellurium, or chloride ions, all contribute to accelerating corrosion. (4) Corrosion fatigue: Corrosion fatigue is corrosion cracking that occurs under the combined action of alternating stress and chemical agents. The fatigue sources of pressure pipelines include mechanical excitation, fluid surge, alternating thermal stress, pressure cycling, as well as wind-induced vibration and earthquakes. Corrosion fatigue cracks usually have multiple but unbranched paths, which is a difference from stress corrosion cracks. Corrosion fatigue cracks are generally transgranular. (5) Hydrogen damage: The penetration of hydrogen into the interior of a metal, resulting in a deterioration of its metallic properties, is known as hydrogen damage. These include hydrogen bubbling, hydrogen embrittlement, decarburization, and hydrogen corrosion. Hydrogen bubbling mainly occurs in media containing humid hydrogen sulfide; as hydrogen atoms diffuse into the steel, they accumulate at defects such as cracks, delaminations, voids, and inclusions to form hydrogen molecules, resulting in volume expansion. When these defects are on the surface of the steel, bubbles are formed. Hydrogen, regardless of the way it enters the steel, causes hydrogen embrittlement in the metal, leading to a significant decrease in its elongation and reduction of area. High-strength steel shows even more severe performance. At high temperatures, the cementite in steel reacts with hydrogen to produce methane. As a result of this reaction, the amount of cementite in the surface layer of the steel decreases, and carbon gradually diffuses from the adjacent, unreacted metal layers into this reaction zone. Consequently, a layer of metal of certain thickness becomes ferritic due to a lack of carbon, leading to decarburization. The result of decarburization is a decrease in the surface strength and fatigue limit of steel. The effect of high-temperature and high-pressure hydrogen on steel leads to a deterioration in its mechanical properties, with a significant reduction in strength and toughness; this phenomenon is known as hydrogen corrosion. Under the above conditions, hydrogen molecules diffuse to the surface of the steel and get adsorbed; some of these adsorbed hydrogen molecules separate into hydrogen atoms and hydrogen ions. Through chemical adsorption, these very small hydrogen atoms/ions penetrate through the surface layer and dissolve into the metal. As the dissolved hydrogen atoms diffuse into the steel through the lattice and grain boundaries, chemical reactions occur that result in the formation of methane, which accumulates in the existing microscopic voids at the grain boundaries. This reaction process reduces the carbon concentration in those areas, prompting carbon from other locations to diffuse there to compensate, thereby increasing the amount of methane and creating local pressure that eventually leads to the formation of cracks. Bubbling forms on the surface of the steel, resulting in decarburization. (6) Erosive damage: The long-term, high-speed flow of the medium inside the pipeline can cause the inner walls of the pipeline components to thin out or damage the sealing surfaces, thereby affecting their pressure resistance and sealing performance. As usage time increases, the decrease in pressure resistance caused by thinning of the inner wall, or leaks resulting from damage to the sealing surfaces, can become the cause of accidents. II. Characteristics of failure: Due to the different causes and types of pipeline failures, the characteristics of such failures also vary. 1. Ductile failure occurs when a material has no obvious defects or embrittlement, but rather fails due to overpressure. Its characteristics include: (1) it undergoes significant deformation, and generally no fragments are produced. Upon damage, the diameter increases or there is local swelling, with the wall thickness decreasing. (2) The actual blasting pressure is close to the theoretical value. (3) The fracture surface is grayish and fibrous, lacking a metallic luster, with shear lips on the fracture surface. (4) Outside the fracture fiber area, there are radial or herringbone patterns, with a tendency to point toward the initiation point. 2. Fractural failure is a type of pipe failure in which no macroscopic deformation occurs; the stress in the pipe wall at the time of failure is also far from reaching the material’s strength limit, and is even below the yield limit. It is usually caused by the brittleness of the material or severe defects, such as improper welding and heat treatment processes, defects in the welds, and cold brittleness resulting from low temperatures. Fragile failure often occurs instantly and spreads at an extremely fast speed. Since it is failure that occurs under low stress, it is also known as low-stress failure. The characteristics of brittle failure are: (1) no significant plastic deformation. (2) The stress at failure is low. (3) The brittle failure resulting from material embrittlement features a flat fracture surface and a crystalline structure with a metallic luster. (4) In brittle failure caused by material defects, the fracture surface does not exhibit a crystalline structure; instead, it shows areas of the original defects, stably expanding fibrous regions, rapidly expanding radial and zigzag patterns, as well as shear lips at the edges of the inner and outer surfaces. If the original defect is a surface crack, a dark rusted appearance will appear; if the original defect is internal pores, inclusions, lack of weld penetration, etc., these will also be visible on the fracture surface. 3. Fatigue failure is a type of failure that occurs when a material is subjected over a long period of time to alternating loads whose magnitude and direction change periodically over time; fatigue cracks form at the core of these loads, and these cracks gradually expand until fracture ultimately occurs. Its characteristics are: (1) The failure sites are concentrated at geometric discontinuities or in welds with original defects such as cracks, with no overall plastic deformation. (2) The basic forms of fatigue failure are bursting or leakage. The former tends to occur in materials with high strength but poor toughness, while the latter occurs in materials with lower strength but better toughness. (3) The fracture surface features distinct regions of crack initiation, crack propagation, and final fracture. In the expanded area, there are distinct shell-like tree patterns on a macro scale, and the fracture surface is even and shiny. The final fracture zone generally has radial patterns or herringbone patterns. (4) When observing the crack propagation zone of the fatigue fracture under an electron microscope, unique fatigue cracks can be seen. 4. Creep failure is the phenomenon of slow, continuous elongation of steel at high temperatures when the stress is below the material’s yield strength, which ultimately leads to failure. The process of material creep consists of three stages: deceleration, constant velocity, and acceleration. The constant velocity phase is the stage that controls the high-temperature service life of the material. Creep fracture is a type of intergranular fracture characterized by: (1) a macroscopic fracture surface that is rough and granular, lacking a metallic luster. (2) The surface is covered with an oxide layer or other corrosion products. (3) The pipeline exhibits macroscopic deformation in the diameter direction, along with small creep cracks in the radial direction; in some cases, surface cracking occurs or leakage results from penetration through the pipe wall. (4) The fracture plane is perpendicular to the wall surface, there is no thinning of the wall thickness, and no shear lip at the edges. III. Accident Prevention and Reporting To prevent or reduce damage accidents involving pressure pipelines, the users must take necessary measures, including: 1. The pipelines must be designed by qualified design firms in accordance with the requirements of relevant design standards ； 2. The piping system shall be equipped with safety pressure relief devices as specified, and these devices must remain sensitive and functional ； 3. Take effective measures to prevent corrosion of the pipes by the atmosphere and surrounding media ； 4. Before putting the pipeline into use, pre-service inspections and acceptance tests must be carried out; key aspects such as the pipeline structure, materials, welding, heat treatment, and pressure testing must meet the specified requirements ； 5. Operational procedures must be strictly followed during operation, process parameters must be controlled, and operation under conditions of excessive temperature or pressure must be avoided ； 6. When performing maintenance or partial replacement of pipes, avoid incorrect use or inappropriate substitution to prevent a reduction in the pipes’ ultimate stress ； 7. Strengthen the maintenance inspections and regular tests of pipelines ； 8. For pipes that have been left unused for a long time and have not received proper maintenance, resulting in extensive corrosion, reduced thickness, and weakened strength, they must undergo a comprehensive inspection in accordance with regulations before being put back into use. After a safety accident occurs in a pressure pipeline, the user entity should not only take prompt action to address the situation but also ensure strict protection of the accident scene. It is necessary to collect relevant information and materials in a timely manner, such as images recorded at the scene, the condition of the fractures on the damaged components, original operation records, and accident investigation reports, in order to provide an objective and scientific basis for analyzing the accident. When analyzing the causes of an accident, it is necessary to measure macroscopic deformations ； Test the chemical composition and mechanical properties of the material ； Technical methods such as macroscopic and microscopic analysis of the fracture surface are employed. Then, a comprehensive analysis of the accident is conducted based on relevant information and technical inspection results, including the degree of damage, the nature of the explosion, and the mode of destruction; finally, the cause of the accident is identified in order to draw lessons and prevent similar incidents from occurring.