Precautions for oxygen pipeline installation

Go and find a copy of the \"Power Piping Manual\" to read; it contains many guidelines regarding the installation of power transmission media. Additionally, take a look at the \"Specifications for the Construction and Acceptance of Industrial Metal Piping Projects\" and you’ll understand everything. Here is some reference: Oxygen plays a very important role in the metallurgical industry. With the adoption of advanced steelmaking measures such as oxygen-based steelmaking and oxygen enrichment in blast furnaces, the use of oxygen by steel companies has grown rapidly, and oxygen has a wide range of applications. Top and bottom oxygen injection in converters, oxygen-enriched blast firing in blast furnaces, oxygen injection in electric furnaces, automatic flame cleaning of billets, and oxygen for maintenance purposes, etc. Oxygen is used at all times and in every aspect, from production and maintenance to infrastructure construction. The metallurgy industry has strict requirements regarding oxygen use; there are uneven demands for oxygen, frequent and significant fluctuations, complex usage protocols, and a need for safe, continuous, and stable oxygen supply. In recent years, as the demand for oxygen has increased, industries that consume large amounts of oxygen have begun to use oxygen pipelines for its delivery. Due to the long lengths of these pipelines and their widespread distribution, coupled with the sudden opening or closing of valves, accidents involving fires and explosions in oxygen pipelines and valves occur from time to time. Therefore, it is crucial to conduct a comprehensive analysis of the potential hazards and risks associated with oxygen pipelines and to take corresponding measures. 1. Principles of combustion and explosion in oxygen pipelines: Analyzed based on the \"three elements\" of combustion and explosion (fuel, oxidizer, and ignition source). The material used in oxygen pipelines is usually carbon steel or stainless steel; due to its carbon content, it can burn even in pure oxygen. Moreover, when iron burns, it releases a large amount of heat, causing the temperature to rise very rapidly. The high-purity, high-pressure oxygen transported through oxygen pipelines is a highly potent oxidizing agent; the higher its purity and pressure, the stronger its oxidizing power and the greater the danger it poses. There are various sources of energy that can cause combustion and explosions in oxygen pipelines: (1) When a valve suddenly opens between the high-pressure and low-pressure sections, the oxygen in the low-pressure section is compressed rapidly; due to the high speed, there is no time for heat to be dissipated, resulting in what is known as \"adiabatic compression.\" This leads to a sharp rise in local temperature, which serves as a source of ignition ； Someone conducted an experiment: by installing a set of DN50 control valves in a DN200 oxygen pipeline to carry out an adiabatic compression test, it was possible to calculate that when the valve of the DN200 pipeline is suddenly opened while the DN50 control valves are in their fully closed position, the adiabatic compression temperature can reach as high as 637℃ ； When the valve of the DN200 pipeline is suddenly opened and the DN50 control valve is at full open condition, the adiabatic compression temperature is 358°C (calculated based on the maximum pressure of 0.3 MPa for the oxygen pipeline). ⑵ When opening and closing the valve, the impact and compression between the valve disc and the valve seat, as well as the friction between the various valve components ； ⑶ Friction between rapidly moving material particles (such as rust, dust, slag, impurity particles, etc.) and the pipe wall ； ⑷ External high temperatures such as the heating surface, flame, and radiant heat ； ⑸ Oil ignition ； ⑹ Electrostatic induction, lightning strike ； ⑺ The catalytic effect of rust, iron powder, etc. Based on an analysis of the oxygen pipeline combustion and explosion accidents that have occurred in the ice production industry over the years, and considering the factors related to combustion and explosion, there are issues with material selection, as well as improper operation and maintenance; furthermore, there are problems resulting from poor control over design and installation quality. 2. Safety considerations in the design of oxygen pipelines: Given the characteristics of the metallurgical industry, the following aspects should be taken into account when designing oxygen pipelines: (1) Oxygen pipelines should be mounted on non-combustible supports to prevent the supports from catching fire in case of oxygen leakage. ⑵Oxygen pipelines must be reliably grounded, with a grounding resistance of less than 10Ω. Wires should be used to bridge both sides of the flanges and threaded connections on the oxygen pipelines, with a bridging resistance of less than 0.03Ω. ⑶The wall temperature of oxygen pipelines should not exceed 70°C; open flames and oil contamination must be kept away from oxygen pipelines and valves. ⑷The elbows and branch fittings of oxygen pipelines should not be connected directly to the valve outlet. ⑸When oxygen pipelines are run alongside acetylene and hydrogen pipelines, they should be placed below them. ⑹When oxygen pipelines are installed alongside pipelines carrying oils, fats, or potentially corrosive substances that may leak, they should be placed above those pipelines. ⑺It is strictly prohibited to lay oxygen pipelines in the same trench as oil and grease pipelines, pipelines carrying corrosive substances, or cables. It is also strictly prohibited for the trench containing oxygen pipelines to be connected to the trenches of such other pipelines. ⑻Oxygen pipelines in the plant area should be installed overhead. Sufficient safety distances should be maintained from other pipelines, buildings, electrical wires, roads, railways, etc ； ⑼The maximum flow rate of oxygen in the oxygen pipeline should be limited. Given the characteristics of the metallurgical industry, the diameter of oxygen pipelines must be determined in such a way as to meet the maximum allowable flow rate under peak load conditions, while also leaving a margin to ensure safety. ⑽Careful selection must be made when choosing fittings for oxygen pipelines. Elbows, tees, and reducers in oxygen pipelines are all areas where oxygen flow can cause shock and intense friction; the presence of iron filings, welding slag, and similar substances can lead to serious accidents such as fires or explosions. Therefore, folded elbows are strictly prohibited for oxygen pipelines. ⑾Special oxygen valves should be used for oxygen pipelines, and these valves must be thoroughly degreased. Gaskets should be made of flame-retardant or non-flammable materials. ⑿Fire-resistant copper pipes should be installed at appropriate locations on the oxygen pipelines. In summary, when designing oxygen pipelines, the diameter of the oxygen pipes should be determined by calculating based on the requirements for flow rate and pressure, and the appropriate type of pipe material should be selected ； When arranging the pipes, the principles of simple process design, reasonable flow paths, and ease of purging should be followed. Sharp bends should be minimized, an appropriate bending radius should be chosen, and the piping system should be simplified to ensure smooth gas flow. 3 Quality control for oxygen pipeline installation 3.1 Review of the qualifications of the installation contractor and the construction plan The entity responsible for installing oxygen pipelines must possess the appropriate license for installing pressure pipelines, as well as the necessary qualification level for undertaking such projects and a sound quality management system. At the same time, the installation unit should have a complete project management organization to ensure project quality, as well as technical management systems for project quality. Before constructing the oxygen pipeline, the construction party shall prepare a detailed construction organization plan or construction scheme, and at the same time go through the notification procedures with the local special equipment safety supervision agency. The welding procedure qualification report should cover the entire welding project for the engineering work. Welders who work on oxygen pipeline welding must possess the appropriate certificates as welders of special equipment, and the qualifications of these welders must meet the requirements for welding in such projects. At the same time, flaw detectors also need to meet the requirements of regulations and standards. 3.2 Material Acceptance Control Materials are the foundation for ensuring the quality of pipeline installation; pipes, pipe fittings, valves, flanges, etc. must come with complete quality certification documents. The manufacturers of such pipe fittings, valves, and flanges must hold the appropriate safety registration certificates for pressure pipeline components, and their specifications, models, and performance test reports must comply with **technical standards and design requirements. During acceptance, its specifications, material, model, and technical requirements shall be verified in accordance with the design specifications; a visual inspection shall also be carried out in line with **current standards. Pipes and pipe fittings that have double walls, cracks, or severe scratches must not be used. Flanges, gaskets, and similar components must have smooth surfaces, free from defects such as pores, cracks, burrs, and dents. For welded elbows such as tees, care should be taken to ensure that their inner surfaces are smooth, with no weld beads or slag remaining inside the pipe. The quality of pipe fittings shall meet the following requirements: ① There shall be no cracks. ② Defects such as overburning and delamination are not allowed. ③ There shall be no wrinkles; the inner surface of elbows must be smooth, free of sharp edges, burrs, and weld spurs. For the reducer sections of oxygen pipelines, seamless or pressed-welded components should be used. When fabricated by welding, the length of the reducing section should not be less than 3 times the difference in the outer diameters of the two ends; its inner surface must be smooth, free of sharp edges, burrs, and weld beads. The valves on the oxygen pipelines should be tested individually for strength and tightness before use. The test pressure for strength testing shall be 1.5 times the nominal pressure, and the testing time shall not be less than 10 minutes; the test is considered successful if there is no leakage from the shell gasket. The tightness test is conducted at the nominal pressure, and it is considered successful if there is no leakage from the valve disc sealing surface. The liquid used for valve strength testing should be oil-free water; when testing stainless steel valves, the chloride content in such oil-free water must not exceed 25 mg/kg. The tightness test is carried out using oil-free compressed air or an inert gas such as N2, and an oil-free foam detergent is applied to check for leaks. Before the tightness test, the valve should be disassembled and degreased, and the sealing packing should be replaced with materials that are free of oil and non-flammable, such as polytetrafluoroethylene (PTFE). For valves that pass the tests, any water accumulated inside them should be drained promptly, and they should be dried using oil-free compressed air or N2. The two ends of the valve must also be sealed to prevent secondary contamination. 3.3 Rust removal and degreasing control: Since oxygen pipelines are special and hazardous pressure pipelines, the inner surfaces in contact with oxygen must have all burrs, sand deposits, rust, and other flammable substances removed completely to keep the inner walls smooth and clean. Therefore, before degreasing oxygen pipelines, their inner surfaces need to be rusted removed, and this can be accomplished using methods such as sandblasting, steel brushes, or acid washing. After pickling to remove rust, carbon steel pipes should also be passivated on their surface. Tubes and fittings made of stainless steel, copper, and aluminum alloys require only degreasing; rust removal is not necessary. Since oxygen pipelines are oil-sensitive, all parts that come into contact with oxygen, such as pipes, fittings, and valves, must be thoroughly degreased. Common degreasing agents include dichloromethane (industrial), trichloroethane (industrial), or alkaline degreasing solutions. For pipe degreasing, the pipes are placed in a rectangular tank filled with degreasing solution for immersion, or the degreasing agent is inserted inside the pipes; the pipes are then sealed at both ends and left flat for (10–15) minutes, during which time they are rotated (3–4) times. Finally, the degreasing solution is drained, and the pipes are dried using oil-free compressed air or N2. The ends of the degreased pipes are covered with previously used degreased cloth or plastic film to prevent secondary contamination. For valves, pipe fittings, instruments, gaskets, and other accessories, degreasing should be carried out using a degreaser immersion method or wiping method. When using the wiping method for degreasing, cotton yarn should not be used; instead, fabrics with fibers that do not easily fall off, such as silk or glass fiber fabrics, should be employed. After degreasing, the parts must be carefully inspected to ensure that no fibers from organic fabrics are attached to them. The inspection methods after degreasing include ultraviolet lamp irradiation, camphor ball and organic solvent analysis, etc. The ultraviolet lamp irradiation method is the simplest approach; it involves using an ultraviolet lamp with a wavelength of (3200–3800) angstroms to irradiate the surface of the degreased parts, with no purple-blue fluorescence indicating success. 3.4 Requirements during the installation of oxygen pipelines ⑴ During the installation of oxygen pipelines, effective measures should be taken to prevent oil contamination, and to stop combustibles, rust particles, welding slag, sand, and other debris from entering or remaining inside the pipelines; strict inspections must also be carried out. To prevent contamination of the pipes, the installers’ hands, tools, work clothes, etc. should be free from oil stains. ⑵When connecting pipes, forceful alignment should not be used; methods such as heating the pipes or using additional shims or multiple layers of shims shall not be employed to eliminate defects such as gaps, misalignment, or lack of centrality at the joint surfaces. When aligning the pipes, the misalignment of the inner walls must meet the specified requirements; at the same time, the straightness of the pipes during installation should also be checked. Oxygen pipelines should preferably use natural compensation; when artificial compensation is used, square compensators or wave compensators can be employed. However, wave compensators are rarely used, if at all, in steel companies. *The butt welds of the ducts should not be located at supports or hangers; the distance between the welds and the edges of such supports or hangers must be at least 100 mm. ⑷When pipes pass through walls or floors, sleeves should be installed, and these sleeves must not contain joints. ⑸When installing stainless steel pipes, they must not be struck with iron tools. The CI- content in the non-metallic gaskets used for flanges shall not exceed 25 mg/kg. ⑹The contact surface between the stainless steel pipe and the carbon steel support must be protected by a non-metallic gasket or stainless steel sheet that does not contain Cl-. ⑺The sealing material for the threaded connections of pipes and valves should be polytetrafluoroethylene tape; oily or flammable materials such as lead-based grease or hemp thread must not be used. Gaskets are in direct contact with oxygen; therefore, gaskets for oxygen pipelines must be made of non-flammable materials. Such as polytetrafluoroethylene, annealed and softened copper sheets, etc. If the gasket is installed without adjustment so that it protrudes into the inner surface of the pipe, friction can cause iron powder in contact with oxygen to catch fire, leading to combustion accidents. Therefore, the position of the gasket during installation must be adjusted properly. ⑻During pipeline installation, the flange sealing surfaces and gaskets should be inspected to ensure there are no defects such as scratches or spots that could affect the sealing performance. 3.5 Welding and Inspection To ensure the cleanliness and smoothness of the inner surface of oxygen pipelines, weld beads must not be present, and slag must be effectively prevented from entering the pipelines; for welding carbon steel pipelines, a welding process that uses TIG welding for the root layer and MIG welding for the cover layer is employed ； All stainless steel pipes are welded using the TIG welding process. After pipeline welding, the welds must be inspected visually and subjected to non-destructive testing. The weld surface must be free of defects such as cracks, pores, and slag inclusions. The root gap depth for carbon steel shall not exceed 0.5 mm, and there shall be no root gaps in stainless steel welds. After the visual inspection of the welds, non-destructive testing shall be carried out on the welds welded by each welder, at a rate specified by the design requirements or relevant standards and codes.