Pressure pipeline

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This post was last edited by liuquan1100 on 2017-12-15 at 16:36. Main contents: 1. Concept of pipelines; 2. Concept of pressure pipelines; 3. Scope of safety supervision for pressure pipelines; 4. Characteristics of pressure pipelines; 5. Structural requirements for pressure pipelines; 6. Classification and grading of pressure pipelines; 7. Causes of failure in pressure pipelines; 8. Characteristics of pipeline failures; 9. Prevention and reporting of pressure pipeline accidents; 10. Safety regulations for pipeline systems. 1. Concept of pipelines: According to the **standard “Code for Design of Industrial Metal Pipelines” GB50316-2000, a pipeline consists of pipeline components, pipe supports, etc., and is used for transporting, distributing, mixing, separating, discharging, measuring, or controlling fluid flow. **The definition provided in the standard \"Code for Construction and Acceptance of Industrial Metal Piping Projects\" GB50235-97 is: an assembly consisting of piping components and piping supports, which are used to transport, distribute, mix, separate, discharge, measure, control, and stop the flow of fluids; it includes pipes, fittings, flanges, bolted connections, gaskets, valves, and other such components or pressure-bearing parts. A group of pipes connected together, classified according to the fluid and design conditions, is referred to as a \"pipe system\" or \"piping system\". The above definition has two implications: (A) The function of pipes: they are used to transport, distribute, mix, separate, discharge, measure, control, and halt the flow of fluids. 1) Fluid: referred to as a medium in some standards. Fluids can be classified by state or properties. a) By state: Gas ； liquid ； Liquefied gas: refers to a gas that exists in a liquid state at certain pressures ； Slurry: Refers to combustible, explosive, toxic, and corrosive slurry media. b) By nature: Fire hazard ； It refers to the risk of combustion caused by flammable media, and is divided into flammable gases, liquefied gases, and flammable liquids. There are three categories: A, B, and C. Explosive ； Flammable media that may explode when mixed with air, or non-flammable media that may explode under high temperature and pressure. Toxicity ； Classified according to GB5044. There are two categories and four levels: highly toxic (extremely hazardous) and toxic (highly hazardous, harmful in case of poisoning, and mildly hazardous). Corrosivity. It refers to substances that can burn human tissue and cause damage to pipeline materials. 2) Fluid transport: The fluid at the starting point of the pipeline is transported to its ending point, either by external power (through the kinetic energy provided by fluid transport machinery such as compressors and pumps) or by the driving force of the fluid itself (such as the pressure of the medium). 3) Fluid distribution: The fluid is distributed to multiple predetermined devices or users as specified in the design, through branch pipes in the piping system. 4) Mixed fluid: Mixing fluids from different branches within a piping system in the pipes, such as for dilution. 5) Fluid separation: Separating fluids in different states inside a pipeline through branch pipes, such as vapor-liquid separation and oil-water separation. 6) Discharging fluid: Discharge the fluid inside the pipeline through branch pipes, such as for overpressure relief or to discharge the separated fluid. 7) Fluid metering: The fluid being transported and distributed is metered through metering instruments installed in the piping system; this includes measuring flow rate, pressure, temperature, viscosity, etc. 8) Fluid control: The flow of fluid within the pipes is controlled through control elements installed in the piping system, such as pressure regulation, temperature reduction, fluid distribution, and shutdown. (B) Composition of the pipeline: It consists of pipeline components, pipe supports and hangers (pipeline support elements), etc.; it is an assembly that includes pipes, fittings, flanges, bolted connections, gaskets, valves, other components, as well as pressure-bearing parts and support elements. 1) Piping components: Elements used to connect or assemble pipes, including pipes, fittings, flanges, gaskets, fasteners, valves, and special piping components. So-called special pipeline components refer to those that are not part of ordinary standard components. They are pipe components manufactured under special engineering design conditions, including expansion joints, special valves, rupture disks, flame arresters, filters, flexible connectors, and hoses. 2) Pipe supports and hangers: A general term for various structures used to support pipes or restrict their movement, but does not include civil engineering structures. There are fixed supports, sliding supports, rigid hangers, guide frames, limit frames, and spring supports/hangers, etc. In the **standard GB50235-97 \"Code for Construction and Acceptance of Industrial Metal Piping Projects\", they are also referred to as piping supports, including piping mounting components and attachments. a) Pipe mounting components: Refer to the elements that transfer loads from pipes or pipe fittings to the supporting structure or equipment, including hangers, spring supports, tie rods, counterweights, set screws, support rods, chains, guide rails, anchors, saddles, shims, rollers, brackets, and sliding supports. b) Attachments: Parts that are attached to pipes by welding, bolted connections, or clamping methods, including pipe hangers, lifting/lag ears, rings, clips, lifting clamps, fastening plates, and skirt-type pipe supports. Pipeline components and supports are also collectively referred to as pressure pipeline elements in China’s current pressure pipeline regulations. II. Concept of pressure pipelines Pressure pipelines are a part of pipes. In a broad sense, so-called pressure pipelines refer to all pipes that are subjected to internal or external pressure, regardless of the medium inside them. However, since the issuance of the Regulations on the Safety Management and Supervision of Pressure Pipelines in our country, “pressure pipeline” has become a specific term for pipelines subject to supervision. Article 2 of the “Regulations on the Safety Management and Supervision of Pressure Piping” defines pressure piping as: “special equipment used in production and daily life that may pose relatively high risks, such as flammability, explosion, or toxicity.” In the “Regulations on the Safety Supervision of Special Equipment” issued and implemented by the State Council on June 1, 2003, pressure piping is further defined as: “tubular equipment utilized under certain pressure to transport gases or liquids. Its scope includes pipelines with a maximum operating pressure equal to or greater than 0.1 MPa (gauge pressure), which are used for transporting gases, liquefied gases, or steam; as well as pipelines carrying flammable, explosive, toxic, or corrosive liquid media at a maximum operating temperature equal to or higher than their standard freezing point. These pipelines must also have a nominal diameter exceeding 25 mm.” In other words, the so-called \"pressure pipelines\" referred to today not only mean those that are subjected to pressure inside or outside the pipes, but also those in which media such as \"gases, liquefied gases, and steam\" or \"liquids that may cause combustion, explosion, poisoning, or corrosion\" are transported. By combustible, explosive, toxic, or corrosive as mentioned here, the following is meant: The combustibility and explosiveness of the medium – that is, the medium is capable of burning or exploding under certain conditions, thereby causing fires and damage. These media include fire-hazardous substances such as combustible gases, liquefied hydrocarbons, and flammable liquids, as well as high-temperature and high-pressure media that are prone to explosion, such as steam, hot water at temperatures above its normal boiling point, compressed air, and other compressed gases. Among them, the fire hazard of flammable media is classified into three categories: Class A, Class B, and Class C, in accordance with the \"Code for Fire Protection Design of Petrochemical Enterprises\" GB50160 and the \"Code for Fire Protection Design of Buildings\" GBJ16. Among them, the lower explosive limits (by volume) of mixtures of Class A and Class B flammable gases with air are specified as follows: Class A flammable gases: <10% ； Class B flammable gases: ≥10%. The classification of flammable liquids in Classes A, B, and C is shown in Table 1. Table 1 Classification of fire hazard for liquefied hydrocarbons and flammable liquids
Category Name of Category Characteristics
Class A Liquefied hydrocarbons Hydrocarbon liquids and other similar liquids with a vapor pressure greater than 0.1 MPa at 15°C
Class A B Flammable liquids Flammable liquids other than those in Class A, with a flash point less than 28°C
Class B A Flammable liquids Flammable liquids with a flash point ranging from 28°C to 45°C
Class B B Flammable liquids Flammable liquids with a flash point greater than 45°C and less than 60°C
Class C A Flammable liquids Flammable liquids with a flash point ranging from 60°C to 120°C
Class C B Flammable liquids Flammable liquids with a flash point greater than 120°C
Note: Liquids with a flash point below 45°C are considered highly flammable liquids ； Liquids with a flash point lower than the ambient temperature are known as flammable liquids. According to the regulations of GBJ16, other combustible media (or production processes) that fall under Category A fire hazards include substances that can decompose on their own at room temperature or undergo oxidation in air, thereby leading to spontaneous ignition or explosion ； Substances that produce gas when exposed to water or steam at room temperature, thereby causing combustion or explosion ； A strong oxidizer that is highly prone to combustion or explosion when exposed to acids, heat, impact, friction, catalysts, or flammable inorganic substances such as organic materials or sulfur ； Substances that can cause combustion or explosion when subjected to impact, friction, or contact with oxidizers or organic materials ； and production carried out in enclosed equipment at a temperature equal to or above the substance’s own autoignition point. Media classified as Category B fire hazards mainly refer to oxidizers and chemically flammable solids that are not classified as Category A fire hazards, as well as oxidizing gases. (B) Toxicity of the medium: That is, the medium has the property of causing poisoning in humans. When these substances are inhaled or come into contact with the human body, they can cause harm to the body or even lead to death. According to the provisions of GB5044, \"Classification of Hazards from Occupational Exposure to Toxic Substances,\" toxins are classified into four categories – extremely hazardous, highly hazardous, moderately hazardous, and mildly hazardous – based on six criteria: acute toxicity, incidence of acute poisoning, incidence of chronic poisoning, consequences of chronic poisoning, carcinogenicity, and the maximum allowable concentration. Media with extreme hazard are sometimes also referred to as “highly toxic media,” while those with high, medium, and low hazard are collectively called “toxic media.” In China’s standard “Code for Construction and Acceptance of Industrial Metal Piping Engineering” GB50235-97, highly toxic substances (fluids) are defined as substances that, even if only minute amounts are released into the environment and inhaled or come into contact with the human body, can cause serious and difficult-to-treat harm to humans, even with prompt treatment. It corresponds to toxins of Grade I hazard level (extremely hazardous) as defined in the current standard **\"Classification of Hazards from Occupational Exposure to Toxic Substances\" GB5044. Based on this, highly toxic media can be understood as media that pose an extreme hazard. The definition of toxic media in the standard is: substances of this kind, when released into the environment, can be inhaled by humans or come into contact with the human body; provided that treatment is administered in a timely manner, they will not cause irreversible harm to the human body. However, even for poisons with the same level of toxicity, there are differences among various industries regarding how to deal with them. For instance, benzene is listed as a highly hazardous substance in the “Classification of the Degree of Hazard of Toxic Substances in Occupational Exposure” GB5044; it is also cited as an example of a highly hazardous substance in the interpretations of the “Regulations on the Safety Management and Supervision of Pressure Pipelines”. In the pipeline classification specified in the \"Code for Construction and Acceptance of Pipelines for Toxic and Flammable Media in Petrochemical Industries\" SH3501-2002, benzene is treated on an equal footing with highly hazardous media. It is classified in the SHB category. Conversely, four highly hazardous substances—acrylonitrile, phosgene, carbon disulfide, and hydrogen fluoride—are treated in SH3501-2002 in the same manner as extremely hazardous substances; they are classified under SHA-grade pipelines. This not only affects the construction quality standards and inspection requirements for existing pipelines, but also influences the classification of permit levels during the actual construction process. When carrying out pipeline installation work involving benzene-containing media, if benzene is considered a highly hazardous medium, the contractor must hold a GC Class 1 installation license; whereas if it is regarded as a moderately hazardous medium, the required license level depends on the pipeline’s design pressure and design temperature. The understanding of this issue can be explained based on the principles of classifying the hazards of toxic substances: **The standard “Classification of the Degree of Hazard of Toxic Substances in Occupational Exposure” GB5044-85 classifies specific toxic substances by listing the levels of hazard posed by 56 common toxic substances in various industries.** However, the standard also states that the degree of hazard for other industries exposed to the same toxin (those not listed in Table 2 of the standard) can be classified based on the concentration of the toxin in the workshop air, the incidence of poisoning, and the duration of exposure. Where the concentration of toxic substances in the workshop air frequently reaches the maximum permissible concentration specified in TJ36–79 “Hygienic Standards for Industrial Enterprise Design”, but the morbidity rate or incidence of symptoms is lower than the corresponding values in this grading standard, the grade may be lowered by one level. Therefore, for each specific substance, it is normal for **standards and professional standards to differ in classifying hazard levels. Because apart from carcinogenicity and the maximum allowable concentration in air, the other four indicators are all related to the production process and operating characteristics. In production processes such as oil, chemical, and petrochemical industries, where media are primarily transported via pipelines, toxic substances flow in a continuous and closed environment. The degree of hazard depends on the likelihood of human exposure to these substances due to accidents or of chronic occupational harm resulting from frequent leaks; this likelihood is generally lower than in open production processes. Therefore, when specifically determining the hazard level of toxic substances in the design of pressure pipelines, it should be classified primarily based on the concentration of toxic substances in the workshop air, the prevalence rate of poisoning, and the duration of exposure. Among the criteria for classifying the toxicity of toxins mentioned above, the maximum allowable concentration of toxins in workshop air is specified as follows: Highly toxic: the maximum allowable concentration is less than 0.1 mg/m3 ； Highly hazardous: The maximum allowable concentration is 0.1 mg/m3 to 1.0 mg/m3. According to the “Hygienic Standards for Industrial Enterprise Design” (TJ36–79), the maximum permissible concentrations of five toxic substances—benzene, acrylonitrile, phosgene, carbon disulfide, and hydrogen fluoride—in workshop air and in the atmosphere of residential areas are shown in Table 2:
Table 2 Maximum permissible concentrations of several toxic substances
Name of toxic substance: Benzene, Acrylonitrile, Phosgene, Carbon disulfide, Hydrogen fluoride
Permissible concentration in workshop air (mg/m³): 40.0, 2.0, 0.5, 10.0, 1.0
Permissible concentration in residential area atmosphere (daily average, mg/m³): 0.8, 0.05, —, 0.04 (for a single occurrence); 0.007 (for a single occurrence: 0.02)
As can be seen from Table 2, the maximum permissible concentration of benzene in workshop air is significantly higher than that of other highly hazardous substances. At the same time, based on the continuity and tightness characteristics of pipeline transportation in industrial production, as well as an analysis of the duration of benzene exposure to operators and the incidence of poisoning, benzene should not be classified as a highly hazardous substance. Therefore, in practical work, determining the toxicity level of a medium should be based on the properties of the toxic substance specified in the design documents or the construction acceptance standards indicated therein. Additionally, China has not yet established any standards regarding the duration of exposure. The three exposure threshold limits recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) can be used as a reference: 1) Based on the time-weighted average limit for a normal 8-hour workday or 40-hour workweek, at this concentration level, repeated exposure is unlikely to have any adverse effects on anyone ； 2) Using the time-weighted average limit for short-term exposure (no more than 15 minutes per exposure, no more than 4 exposures per day, with at least 1 hour between each exposure) as an indicator; at this concentration, brief and continuous human exposure does not cause irritation, chronic or irreversible histopathological changes. However, it may lead to increased risk of accidents, reduced ability to protect oneself, or a noticeable decline in work efficiency ； 3) The upper limit value refers to the highest concentration that must not be exceeded, even for an instant. (C) Corrosivity of the medium: refers to substances that can burn human tissue and cause damage to pipeline materials, such as acids, bases, and other fluids that can damage materials like hydrogen and hydrogen sulfide. III. Scope of safety supervision for pressure pipelines According to the Regulations on Safety Management and Supervision of Pressure Pipelines, pressure pipelines that fall under the scope of safety supervision are those that meet one of the following conditions, as well as their associated facilities and safety protection devices. 1) Media with an extreme hazard level, regardless of pressure, temperature, and state ； 2) Media with fire hazard classification of Class A or B, regardless of pressure, temperature, and state ； 3) Gases (vapors) and liquefied gas media with a maximum operating pressure greater than or equal to 0.1 MPa; their properties and temperature are not specified, except for those not falling under the scope of supervision as stipulated in the “Regulations on the Safety Management and Supervision of Pressure Piping”. 4) Flammable, explosive, toxic, or corrosive media with a maximum operating pressure greater than or equal to 0.1 MPa; or liquid media whose maximum operating temperature is higher than or equal to their standard boiling point. The Provisions on the Safety Management and Supervision of Pressure Piping specify that the following four types of piping are not subject to supervision: a) Piping that is part of the equipment itself. b) **Pipes in equipment, vehicles, and nuclear installations. c) Gas that is non-toxic, non-flammable, and non-corrosive, in pipes with a nominal diameter of less than 150 mm and a maximum operating pressure of less than 1.6 MPa. Here, the definition of the facilities associated with pressure pipelines and the safety protection devices is as follows: a) The auxiliary facilities mainly refer to pipeline equipment used for pressure pipelines, supports and hangers, cathodic protection devices, etc. b) Safety protection devices mainly refer to over-temperature and over-pressure control devices, alarm devices, etc. Note: The recently issued \"Rules for the Registration and Management of the Use of Pressure Pipelines\" (trial version) defines pressure pipelines, auxiliary facilities, and safety protection devices as follows: a) Pressure pipelines refer to systems composed of pipeline components, pipeline supports, safety protection devices, and auxiliary facilities. Tubular devices used for transporting gases or liquids ； b) Auxiliary facilities refer to cathodic protection systems, compressor stations, pump stations, valve stations, pressure regulation stations, monitoring systems, etc ； c) Safety protection devices refer to safety valves, pressure gauges, rupture discs, and emergency shut-off valves installed on pressure pipelines. IV. Characteristics of pressure pipelines A pipeline system, in order to perform functions such as transporting, distributing, mixing, separating, discharging, measuring, or controlling the flow of fluids, must be connected to corresponding power equipment, reaction equipment, storage equipment, separation equipment, heat exchange equipment, control equipment, etc., to form a complete system. This enables the fluid within the pipes to have a certain pressure, temperature, and flow rate, thus fulfilling the tasks specified in the design. At the same time, different types of pressure pipelines have distinct characteristics due to variations in materials, structure, and installation methods: (A) Characteristics of industrial pipelines 1) Large number of pipelines, large pipeline systems, with pipes arranged in a complex and compact manner within the workshop ； 2) The materials, types, and specifications of pipeline components and supports are complex, resulting in poor quality consistency ； 3) The operation process is affected by fluctuations in the production process, with many changing operating conditions such as thermal expansion and contraction, alternating loads, and fluctuations in temperature and pressure ； 4) The mechanisms of corrosion and degradation are complex, with various failure modes of the materials. (B) Characteristics of long-distance pipelines and utility pipelines: 1) The pipelines have a large length, span multiple regions, and are located in complex topographical and geological conditions ； 2) There are many buried installations, making defect detection difficult ； 3) Prone to accidental injuries. V. Structural requirements for pressure pipelines: Due to the toxic, flammable, explosive, and corrosive nature of the fluids they transport, as well as the special operating conditions such as high temperatures, high pressures, and low temperatures, pressure pipelines present a considerable level of risk. Therefore, the structure of pressure pipeline systems should meet the following requirements: Pressure resistance: The ability to withstand the pressure exerted on the pipes by the fluid inside them (internal or external pressure), as well as the stresses caused by temperature, along with the long-term, repeated effects such as creep and fatigue ； Sealability: Prevents the fluid flowing inside the pipeline from leaking into the external space or into another fluid ； Corrosion resistance: Withstanding the corrosive effect of fluids inside the pipe on the pipe material. The corrosion resistance grades of pipeline materials are divided into 4 levels, measured by the annual corrosion rate: fully corrosion-resistant ≤ 0.05 mm ； Corrosion resistance > 0.05～0.1mm ； Corrosion resistance still > 0.1～0.5mm ； Non-corrosion resistance > 0.5mm ； Flexibility: The flexibility of a pipe is a physical concept that indicates the ease with which the pipe can deform. When pipes operate under design conditions, factors such as thermal expansion and contraction, additional displacements at the ends, and improper pipe support installation can lead to excessive stress, deformation, leakage, or damage, thereby affecting their normal operation. The flexibility of a pipeline refers to its ability to absorb displacements caused by changes in temperature or other factors through its own deformation, thereby ensuring that the stresses acting on the pipeline remain within the allowable stress limits of the material. To meet the above requirements, the pipe components of the piping system must be made of materials resistant to corrosion by the medium, capable of withstanding the pressure exerted by the medium at the specified design temperatures, and must have appropriate wall thicknesses and sealing structures. At the same time, the entire piping system should have proper support. The term “severe cyclic conditions” often appears in various standard specifications. According to the interpretation of the \"Code for Design of Industrial Metal Piping\" GB50316-2000, severe cyclic conditions refer to situations where the maximum calculated displacement stress in the piping exceeds 0.8 times the allowable displacement stress, or where the equivalent number of cycles is greater than 7000, or any other conditions determined by design that result in an equivalent effect. The so-called “displacement stress range” refers to the stress calculated from the displacement caused by the thermal expansion of the pipeline. The calculated maximum displacement stress range is the stress calculated based on the full compensation value from the lowest temperature to the highest temperature. The design of pipes operating under severe cyclic conditions involves special specifications regarding the selection of pipe components, the minimum thickness of pipes and fittings, and the requirements for non-destructive testing. VI. Classification and grading of pressure pipelines Pipelines have a wide range of uses and come in many varieties. Pipes used in different fields are classified using different methods. Pipelines can generally be classified according to their usage characteristics, such as purpose, main material, installation condition, and the medium being transported. The specific details can be seen in Figure 1. In general regulations, standards, and codes, in order to facilitate design, construction inspection, as well as use management and inspection, a comprehensive classification and grading system is often adopted based on the properties of the medium and design parameters. Meanwhile, there are differences in the grading or classification of pipelines among the design codes, construction inspection standards, and maintenance/inspection procedures in various industries. For example: **In the standard ‘Code for Design of Industrial Metal Piping’ GB50316, fluids are classified into five categories – A1, A2, B, C, and D – based on their state, properties, and design parameters.** Class A1 refers to highly toxic media ； Category A2 refers to toxic media, while Category B refers to flammable media ； Classes C and D consist of non-flammable, non-toxic media; those with a design pressure of less than or equal to 1 MPa and a design temperature ranging from -29 to 186°C belong to Class D. The construction and acceptance specifications for industries such as chemicals, petrochemicals, and power generation classify pipes as follows: The industry standard for chemicals, namely the \"Specifications for Construction and Acceptance of Metal Pipes in Chemical Industries\" HG20225—95, divides pipes into four categories – A, B, C, and D – based on the properties of the fluid and design parameters. It is basically consistent with the **standards, but pipes carrying toxic media are classified as Category B pipes. The petrochemical industry standard \"Code for Construction and Acceptance of Pipelines for Toxic and Flammable Media in Petrochemical Industries\" SH3501—2002 classifies fluids into four categories: SHA, SHB, SHC, and SHD, based on their fluid properties and design parameters, as shown in Table 3. Table 3 SH3501-2001 Pipeline Classification
Pipeline Class | Application Scope
SHA: (1) Pipelines carrying media with extremely hazardous toxicity (except benzene pipelines); (2) Pipelines carrying acrylonitrile, phosgene, carbon disulfide, and hydrogen fluoride – media with highly hazardous toxicity; (3) Pipelines carrying media at a design pressure of 10.0 MPa or higher.
SHB: (1) Benzene pipelines with extremely hazardous toxicity; (2) Pipelines carrying media with highly hazardous toxicity (except acrylonitrile, phosgene, carbon disulfide, and hydrogen fluoride pipelines); (3) Pipelines carrying flammable gases of Class A and B, as well as liquefied hydrocarbons of Class A A, and flammable liquids of Class A B and B A.
SHC: (1) Pipelines carrying media with moderate or mild hazard levels; (2) Pipelines carrying flammable liquids of Class B B and Class C.
SHD: Low-temperature pipelines with a design temperature below -29°C.
VII. Causes of Pressure Pipeline Failure
“The failure” of a pressure pipeline generally refers to the situation in which the pipeline can no longer perform its intended functions. This can be categorized into natural failure and abnormal failure. Since pressure pipelines 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 metallographic structure of the material inevitably arise, affecting both the performance of the material and the sealing quality of the joints. Therefore, every pipeline has a certain service life, and natural failure occurs when the pressure pipeline 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: (A) 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 levels 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 can lead to a decrease in the toughness of the material, as well as a reduction in the allowable critical crack size, which may 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 results in 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. (B) Defects in design, manufacturing, or construction, such as insufficient flexibility of the pipes, improper selection or use of materials, welding or metallurgical defects that exceed acceptable limits, excessive stress resulting from unreasonable welding or assembly, and an inadequate pipe support system ； 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. (C) Maintenance errors, failure to detect serious defects or damages in the pipes, lack of scientific evaluation, as well as unreasonable maintenance methods that cause new defects and damages ； (D) Damage caused by external forces, such as earthquakes, strong winds, floods, lightning strikes, and other mechanical damages as well as 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 mechanism of the material at the time of failure, it can be classified into types such as 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. (E) Corrosive damage: The corrosion of pressure pipelines occurs as a result of chemical or electrochemical actions by internal media and external environmental media. It also includes the combined effects of factors such as machinery. Unreasonable operations can cause changes in the medium concentration, exacerbating corrosion and damage. The forms of corrosion-induced failure in pressure pipelines include general corrosion, local corrosion, stress corrosion, corrosion fatigue, and hydrogen 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 that occurs over a large area of the pipe to an essentially equal degree. 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 worsens 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 pipeline corrosion 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 pits in specific areas on the metal surface, also known as pitted corrosion. 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 achieved full penetration. Crevice corrosion is often caused by 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 along the grain boundaries and in the areas surrounding them, with little corrosion occurring within the grains themselves. The corrosion mechanism is the \"chromium-deficiency theory\": since the chromium-deficient intergranular regions are in an activated state, they act as an anode, forming a corrosion galvanic cell with the grains, which results in the detachment of those grains or a reduction in the mechanical strength of the material. ②Selective 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. ③Edge 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 that occurs in metal materials 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, when exposed to H2S levels above a certain threshold along with moisture, **increase the likelihood of stress corrosion cracking in the pipe wall. When the weld hardness exceeds HB200 or the H2S content is above the limit, it can easily lead to stress corrosion in the weld. ①Alkali 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. ②Chloride 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. ③Corrosion of stainless steel by polysulfuric 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 polysulfuric 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 attention. ④Sulfide stress corrosion: Stress corrosion that occurs in metals when they are exposed to 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 the corrosion cracking that occurs under the combined action of alternating stress and chemical media. 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 branches but no bifurcations, which is a difference from stress corrosion cracks. Corrosion fatigue cracks are generally transgranular. 5) Hydrogen damage: Hydrogen penetration into the interior of metals, leading to a deterioration of their properties, is referred to as hydrogen damage. Including hydrogen blistering, hydrogen embrittlement, decarburization, and hydrogen corrosion. Hydrogen blistering primarily occurs in moist hydrogen sulfide-containing media. When hydrogen atoms diffuse into 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, blisters are formed. Hydrogen, regardless of the way it enters the steel, causes hydrogen embrittlement in the steel, resulting in a significant decrease in its elongation and reduction of area. High-strength steel shows even more severe performance. At high temperatures, cementite in steel reacts with hydrogen to form methane. As a result of this reaction, the amount of cementite in the surface layer of the steel decreases; carbon then gradually diffuses from the adjacent unreacted metal layer into this reaction zone. Consequently, a certain thickness of the metal becomes ferrite due to a lack of carbon, resulting in decarburization. The result of decarburization is a reduction in the surface strength and fatigue limit of steel. The effect of high-temperature and high-pressure hydrogen on steel results in a deterioration of its mechanical properties; its strength and toughness are significantly reduced. This phenomenon is known as hydrogen corrosion. Under the aforementioned conditions, hydrogen molecules diffuse to the surface of steel and become adsorbed; some of these adsorbed hydrogen molecules dissociate into hydrogen atoms and hydrogen ions. Through chemical adsorption, the very small-diameter hydrogen atoms (ions) then permeate through the surface layer and dissolve into the metal. As the dissolved hydrogen atoms diffuse into the steel through the crystal lattice and grain boundaries, a chemical reaction occurs, resulting in the formation of methane that accumulates in the existing microscopic voids at the grain boundaries. This reaction reduces the carbon concentration in this area, prompting carbon from other regions to diffuse there to compensate for the loss. Consequently, the amount of methane continues to increase, generating localized pressure that eventually leads to crack formation. Bubbling forms on the surface of the steel, resulting in decarburization. (F) Erosion damage: The long-term, high-speed flow of the medium inside the pipeline can thin the inner walls of pipeline components or damage the sealing pairs, thereby affecting their pressure resistance and sealing performance. Over time, leaks caused by a decrease in pressure resistance due to thinning of the inner wall or by damage to the sealing pair can become the root cause of accidents. VIII. Characteristics of pressure pipeline failures: Due to the different causes and types of pipeline failures, their characteristics also vary. (A) Ductile failure occurs when there are no obvious defects or embrittlement in the material; it is caused by excessive pressure. 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. (B) Fractural failure is a type of pipe failure in which no macroscopic deformation occurs; the wall stress 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 suddenly and spreads at an extremely fast pace. 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) Fracture caused by material defects results in a fracture surface that is not crystalline; instead, it features the original defect area, a stably expanding fibrous region, rapidly expanding radial and diagonal crack 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. (C) Fatigue failure is a type of failure that occurs when a material is subjected over a long period to alternating loads whose magnitude and direction change periodically over time; fatigue cracks form at the site of these loads, and these cracks gradually expand until fracture eventually 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 burst 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 exhibits distinct regions of crack initiation, crack propagation, and final fracture. In the expanded area, there are distinct shell-like tree patterns on a macroscopic scale, and the fracture surface is flat 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 fringes can be seen. (D) Creep failure is the phenomenon in which steel undergoes slow, continuous elongation at high temperatures below its yield strength, eventually leading to failure. The process of material creep consists of three stages: deceleration, constant velocity, and acceleration. The constant velocity stage is the phase 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, without a metallic luster. 2) The surface is covered with an oxide layer or other corrosion products. 3) The pipe 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 is perpendicular to the wall surface, there is no thinning of the wall thickness, and no shear lip at the edges. IX. Prevention and Reporting of Pressure Pipeline Accidents To prevent or reduce damage accidents involving pressure pipelines, the users shall take necessary measures, including: ——The pipelines must be designed by qualified design firms and in compliance with the requirements of relevant design standards ； ——The piping system shall be equipped with safety pressure relief devices as specified, and these devices must remain sensitive and functional ； ——Take effective measures to prevent corrosion of pipelines by the atmosphere and surrounding media ； ——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 ； ——Operation procedures must be strictly followed during execution, process parameters must be controlled, and operation under conditions of excessive temperature or pressure must be avoided ； ——When maintaining or partially replacing pipes, avoid incorrect use or inappropriate substitutions that could reduce the ultimate stress of the pipes ； ——Strengthen the maintenance inspections and regular tests of pipelines ； ——Pipelines that have been left unused for a long time and are poorly maintained, due to extensive corrosion, reduced thickness, and weakened strength, must undergo comprehensive inspection in accordance with regulations before being put back into use. After a safety accident occurs in a pressure pipeline, in addition to taking prompt action to address it, the user entity should also pay close attention to properly preserving the accident scene. Relevant information and materials such as footage recorded at the scene, the condition of the fractures on the damaged parts, original operation records, and accident investigation reports should be collected promptly, so as 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, based on the relevant data and technical inspection results, Section X: Safety Regulations for Pipeline Systems. (1) Overpressure Protection: (A) Pipeline systems that may experience overpressure during operation must be equipped with pressure relief devices such as safety valves and burst discs. (B) In applications where safety valves are not suitable, rupture discs can be used. There should be a certain margin between the design burst pressure of the rupture disc and the normal maximum operating pressure. (C) Safety valves should be selected separately for discharging gases or liquids, taking into account the effect of back pressure. Unless there are special process requirements, the opening pressure (set pressure) of the safety valve is 1.1 times the normal operating pressure, with a minimum of 1.05 times, unless otherwise specified in the design codes and design documents. (D) The pressure loss in the inlet pipe of the safety valve should be less than 3% of the opening pressure, while the pressure loss in the outlet pipe should not exceed 10% of the opening pressure. (E) The maximum discharge pressure of the safety valve should not exceed 1.1 times the design pressure of the pipeline, and the maximum discharge pressure in the event of a fire should not exceed 1.21 times the design pressure. (F) Isolation valves should not be installed on the inlet and outlet pipes of safety valves or rupture disks. When special process requirements mandate its installation, a bypass valve and an on-site pressure gauge should also be provided. During normal operation, the isolation valve at the inlet or outlet of the safety valve or rupture disc should be locked in the open position. The bypass valve should be locked in the closed position. And add the specified symbols on the drawing. (G) When a three-way changeover valve is installed at the inlet and outlet of the dual safety valves, the two changeover valves shall be equipped with a reliable interlock mechanism. The pipeline between the safety valve and the selector valve should have drainage measures. (H) The manufacturer shall ensure that the product performance meets the detailed data on pressure relief devices provided in the design. (II) Installation of valves and blind flanges (A) Check valves should be installed on pipelines where backflow needs to be prevented. (B) For valves that must be strictly controlled in the open or closed position during normal operation, the design shall include requirements for locking or sealing, along with the specified codes. Such valves may only be used during maintenance under strict supervision and with the approval of the relevant authorities. (C) For pipelines that may need to continue operating outside the unit during its maintenance shutdown, in addition to installing shut-off valves at the unit’s boundaries, blind flanges should also be placed on the side of the valves that is adjacent to the unit. (D) During operation, when a device needs to be shut down for maintenance, a blind flange should be installed between the device and the valve. When a small vent valve is installed between combustible fluid pipelines, valves, and blind flanges, the pipeline downstream of the vent valve should be led to a safe location. (E) Blind plates shall be installed at the locations to be isolated for pressure testing and airtightness testing. (F) In situations where the liquid temperature is below -5°C or where atmospheric corrosion is severe, a separate blind plate, namely a plug and gasket, is recommended. Blind plates in the shape of an “8” should not be used. The insert plates and gaskets shall have identification marks, and the marked areas shall extend beyond the flange. (III) Discharge (A) Combustible fluids shall be discharged into a closed collection system; direct discharge into the sewer system is strictly prohibited. (B) Combustible gases with a density greater than that of ambient air should be discharged into a flare system, while combustible gases with a density lower than that of ambient air can be released into the atmosphere if it is permissible not to use a flare and the health standards are met. (C) Non-toxic, non-flammable, and non-vaporizing liquid streams that can be discharged directly into the sewer system, provided they meet health standards as well as the temperature requirements for use with sewer materials and do not cause corrosion. (D) The diameter of the discharge pipe shall be determined based on the discharge volume and operating pressure. The flow rate at the discharge outlet shall comply with the requirements of the design specifications. Bird-proof nets should be installed on atmospheric vent openings that are not used frequently. (IV) Other requirements: (A) In cold climate conditions, anti-freezing bypass pipes or other anti-freezing measures should be installed at the ends of the outdoor cooling water main pipes as well as on the inlet and outlet pipes of the coolers. Heating pipes should be installed when condensate is generated in gas pipes or there are dead zones in liquid pipes, as well as when the drain pipes may freeze. (B) Components that are weak points in indoor combustible fluid pipelines, such as glass level gauges and sight glasses, should be equipped with safety protection measures. (C) The static electricity generated by the piping system can be grounded through the grounding network of the equipment or civil structures. Other anti-static requirements shall comply with the provisions of the relevant standards. (D) For critical equipment where fluid interruption is not permitted, safety measures such as dual piping or a ring-shaped piping network equipped with isolation valves should be employed. (E) Flame arrestors shall be installed on the pipelines of combustible gases after pressure reduction that are connected to open-flame equipment (including flare pipes), as well as when isolating such pipelines (including vent pipes) from the equipment they are connected to. (F) Oxygen pipelines shall comply with the following requirements: 1) Pipelines for strongly oxidizing fluids (oxygen or fluorine) shall be degreased in sections or as individual units after pipeline prefabrication but before installation. All degreased pipeline components are thoroughly cleaned and sealed using nitrogen or air. It is also necessary to avoid the formation of dangerous mixtures between residual degreasing agents and oxygen. 2) The selection of components for oxygen pipelines shall comply with the provisions of the specifications. Seamless pipes and fittings should also be selected. Austenitic stainless steel pipes are recommended when the design pressure is greater than 3 MPa. When pressure regulating valves are installed on carbon steel and low-alloy steel pipelines, austenitic stainless steel pipes and fittings should be used within a range of 1.5 m before and after the valves. Valves should not be of the quick-open or quick-close type, and the gaskets and packing inside the valves should not be made of materials that tend to shed debris or fibers, nor of combustible materials. 3) TIG welding shall be used for welding. 4) The flow rate limits for oxygen pipelines, static electricity grounding, and pipeline layout shall comply with the provisions of the \"Design Code for Oxygen Stations\" GB50030 and the technical regulations for oxygen safety. Unless there are special design requirements and reliable safety measures in the process flow, it is strictly prohibited to directly connect oxygen pipelines to pipelines carrying flammable fluids. (G) Depending on the fluid’s freezing point, other changes in physical properties, and process requirements, jacketed pipes should be designed with a full jacket, partial jacket, or simple jacket structure.