Installation process of long-distance vacuum-insulated liquid ammonia pipeline and connecting piece

Abstract

This invention discloses an installation process and connectors for a long-distance vacuum-insulated liquid ammonia pipeline, relating to the field of double-walled pipe technology. The installation process includes the following steps: S1, raw material preparation; S2, segmented assembly; S3, pipe system installation; S4, vacuum treatment and corrosion protection. The installation process utilizes a tapered transition connecting sleeve made of nickel-based alloy, with both ends fully welded to the carbon steel outer pipe of the land section and the stainless steel outer pipe of the salt spray environment section. By utilizing the predetermined cone angle formed by the continuous monotonic gradual change of the connecting sleeve's outer diameter from the first end to the second end, the radial misalignment difference generated at the weld due to the difference in thermal expansion coefficients between carbon steel and stainless steel is transformed into elastic bending strain uniformly distributed along the entire tapered surface. This distributes the high stress originally concentrated at a single weld point to the entire tapered surface area, significantly reducing the peak stress.

Description

Technical Field

[0001] This application relates to the field of double-walled pipe technology, and more specifically, to an installation process and connectors for a long-distance vacuum-insulated liquid ammonia transport pipeline. Background Technology

[0002] With the rapid development of the green shipping industry, liquid ammonia, with its advantages of being clean, low-carbon, and safe for storage and transportation, is gradually becoming a preferred alternative fuel for ocean-going vessels. Therefore, it is necessary to transport liquid ammonia from inland production areas over long distances to coastal ports and wharves to meet the refueling needs of ships. Liquid ammonia is a hazardous chemical, and long-distance transportation requires stringent requirements for sealing, insulation, and operational safety. Currently, the industry commonly uses long-distance vacuum-insulated pipelines to complete cross-regional transportation from land to the coast. This type of double-walled vacuum structure is also the mainstream pipeline form for such operating conditions.

[0003] For the aforementioned cross-regional vacuum-insulated liquid ammonia pipelines, the industry has developed various material selection options. If the entire pipeline is made of stainless steel, stainless steel's excellent corrosion resistance can effectively resist salt spray corrosion in coastal areas. However, stainless steel has a high unit price and high processing costs, and using it throughout the entire pipeline would significantly increase the overall project cost, making it less economically viable. If carbon steel is used uniformly throughout the pipeline, while material and construction costs can be effectively controlled, carbon steel has weak salt spray corrosion resistance and is easily corroded and damaged in the high-salt-spray environment of the coast, failing to meet long-term service requirements. Therefore, existing technologies are gradually adopting a material selection model that combines carbon steel and stainless steel in sections. Inland sections use carbon steel, while sections near the coast in salt-spray environments use stainless steel, thus balancing cost and corrosion resistance.

[0004] Currently, the joints between carbon steel and stainless steel pipe sections are generally welded directly using dissimilar steel welding wire. Due to the significant difference in the coefficients of thermal expansion between carbon steel and stainless steel, the radial expansion and contraction of the two types of pipe sections differs under changes in ambient temperature and medium operating temperature, resulting in continuous thermal stress at the weld joint. This pipeline is a vacuum-insulated structure transporting liquid ammonia; the weld joint is subjected to alternating thermal stress for extended periods, making it highly susceptible to microcracks and even open cracks. This can lead to depressurization of the inter-pipe vacuum cavity, shortening the pipeline's service life, posing safety hazards, and compromising the long-term reliable operation of the long-distance pipeline.

[0005] Therefore, it is necessary for the inventors to design a new installation process and connectors for long-distance vacuum-insulated liquid ammonia transport pipelines to overcome the above problems. Summary of the Invention

[0006] The main purpose of this application is to provide an installation process and connectors for a long-distance vacuum-insulated liquid ammonia transmission pipeline, so as to solve the problem of using a mixture of carbon steel outer tube and stainless steel outer tube for welding vacuum-insulated double-walled pipes on the market.

[0007] To achieve the above objectives, this application provides an installation process for a long-distance vacuum-insulated liquid ammonia transport pipeline. The pipeline uses carbon steel outer pipe sections in the land section far from the coast and stainless steel outer pipe sections in the salt spray environment section near the coast. The process includes the following steps: S1. Raw material preparation: Provide carbon steel outer pipe sections, stainless steel outer pipe sections, and stainless steel inner pipe sections, and perform surface pretreatment on each pipe section. S2. Segmented assembly: Insert the stainless steel inner pipe and support components into the corresponding outer pipe segment, and then conduct a quality test on the assembled segmented double-wall pipe. After the test is completed, seal both ends of the segmented double-wall pipe. S3. Piping Installation: Transport the enclosed segmented double-walled pipes to the site, and deliver the corresponding segmented double-walled pipes to the installation locations according to the division of the land section and the salt spray environment section; when welding the segmented double-walled pipes, first weld the stainless steel inner pipe of each segment, and after the inner pipe is welded, treat the weld seam, and then weld the outer pipe; at the joint where the carbon steel outer pipe segment of the land section connects to the stainless steel outer pipe segment of the salt spray environment section, install connectors; The connector is made of a nickel-based alloy with a coefficient of thermal expansion between that of carbon steel and stainless steel. Its outer diameter changes continuously and monotonically from the first end welded to the carbon steel outer tube to the second end welded to the stainless steel outer tube, forming a predetermined cone angle. The cone angle is determined by a full-condition optimization algorithm based on Taylor series approximation and finite element thermal structure coupling analysis. The two ends of the connector are fully welded to the carbon steel outer pipe section and the stainless steel outer pipe section respectively to complete the outer pipe connection; the connector converts the radial misalignment caused by the difference in thermal expansion coefficients of carbon steel and stainless steel into elastic bending strain distributed along the conical surface, thereby safely absorbing thermal stress. S4. Vacuum treatment and corrosion protection: After welding, the annular cavity between the inner tube and the outer tube is evacuated and sealed; the connector and its weld area with the carbon steel outer tube and the stainless steel outer tube are subjected to enhanced corrosion protection treatment to resist the erosion of the coastal salt spray environment.

[0008] Optionally, determining the cone angle includes the following steps: The first step is to use the lowest ambient temperature and the highest operating temperature of the area through which the pipeline passes as temperature boundary conditions; The second step is to expand the coefficients of thermal expansion of carbon steel, stainless steel and nickel-based alloys at different temperatures in the form of Taylor series, establish a finite element model of the piping system including the tapered transition connecting sleeve, and perform thermal structural coupling transient analysis. The third step is to use the intermediate temperature zero stress point design method as the initial condition, and set the annual average temperature of the area through which the pipeline passes as the zero point of thermal displacement compensation, so as to initially determine the cone angle. The fourth step involves using the minimization of the maximum von Mises stress of the connector throughout the entire temperature cycle as the objective function, the cone angle and wall thickness distribution parameters as design variables, and the material yield strength and fatigue limit as constraints. The initially determined cone angle is corrected through iterative finite element analysis to solve for the optimal cone angle value.

[0009] Optionally, in step S3, manual tungsten inert gas welding is used when welding the connector; during the welding process, a cooling jacket is fitted over the connector and a circulating cooling medium is introduced, and the cooling jacket is removed after the welding is completed.

[0010] Optionally, in step S1, the surface pretreatment includes: removing rust and degreasing the carbon steel outer pipe section and applying an anti-rust primer; pickling and passivating the stainless steel outer pipe section; and degreasing, pickling, and polishing the stainless steel inner pipe section.

[0011] Optionally, in step S4, the enhanced anti-corrosion treatment includes: first, coating the connector and weld area with a primer compatible with stainless steel, and then wrapping it with anti-corrosion tape resistant to salt spray corrosion.

[0012] A connector is used to connect a carbon steel outer tube and a stainless steel outer tube in a vacuum insulated pipeline for transporting liquid ammonia from land to the coast. The connector is a tapered transition sleeve. The tapered transition sleeve is made of a nickel-based alloy with a thermal expansion coefficient between that of carbon steel and stainless steel, and its outer surface is coated with a protective layer resistant to salt spray corrosion. The tapered transition sleeve has a first welding end for welding with a carbon steel outer tube and a second welding end for welding with a stainless steel outer tube. The tapered transition connecting sleeve has a tapered stress transition section in the middle section between the first welded end and the second welded end. The inner diameter and / or outer diameter of this tapered stress transition section changes continuously and monotonically linearly along the axial direction, thereby forming a cone angle. ; The cone angle The values ​​of and the wall thickness distribution of the conical stress transition section are determined by a full-condition optimization algorithm based on Taylor series approximation and finite element thermal-structural coupling analysis. This ensures that within the pipeline design temperature range, the peak thermal stress at any point on the connecting sleeve is lower than the allowable stress of the material, and the alternating stress amplitude caused by temperature cycling is lower than the fatigue limit of the material.

[0013] Optionally, the wall thickness of the tapered stress transition section is smoothly thinned from both ends to the middle, forming a concave wall thickness profile that is thin in the middle and thick at both ends, so as to provide greater radial flexibility without reducing the vacuum bearing capacity.

[0014] Optionally, the minimum wall thickness of the tapered stress transition section is 0.5 to 0.8 times the wall thickness of the first or second welded end, and not less than the minimum wall thickness required for vacuum external pressure stability calculation; the cone angle The value ranges from 1° to 30°.

[0015] Optionally, the full-condition optimization algorithm includes: Using the historical lowest ambient temperature and highest operating temperature of the area through which the pipeline passes as boundary conditions, the curves of the thermal expansion coefficients of each material changing with temperature are approximated by a polynomial using a Taylor series expansion, and a finite element model of the pipeline system including the connector is established for thermal structural coupling transient analysis. The optimal geometric parameters are obtained by iteratively solving a problem with the objective function of minimizing the maximum von Mises stress of the connector throughout the entire temperature cycle, the design variables being the cone angle α and the wall thickness distribution parameters, and the material yield strength and fatigue limit being the constraints.

[0016] Optionally, it also includes a detachable cooling jacket, which is composed of two semi-cylindrical bodies joined together and can be fitted over the tapered transition connecting sleeve. Its inner wall is provided with a spiral flow channel for circulating cooling medium.

[0017] The present invention provides an installation process and connectors for a long-distance vacuum-insulated liquid ammonia pipeline. Compared with existing technologies, its advantages are as follows: By using a conical transition connecting sleeve made of a nickel-based alloy with a thermal expansion coefficient between carbon steel and stainless steel, and fully welding its two ends to the carbon steel outer pipe of the land section and the stainless steel outer pipe of the salt spray environment section respectively, the predetermined cone angle formed by the continuous monotonic gradual change of the outer diameter of the connecting sleeve from the first end to the second end is used to transform the radial misalignment difference generated at the weld due to the difference in thermal expansion coefficients between carbon steel and stainless steel into elastic bending strain evenly distributed along the entire conical surface. This distributes the high stress originally concentrated at one point of the weld to the entire conical surface area. This significantly reduces peak stress, fundamentally avoiding the risk of vacuum leakage caused by weld cracking due to thermal stress concentration. Simultaneously, the cone angle is determined by a full-condition optimization algorithm based on Taylor series approximation and finite element thermal-structural coupling analysis, ensuring that the stress level of the connecting sleeve remains within the material's allowable range throughout the entire temperature cycle. This guarantees the structural integrity of the pipeline under extreme high and low temperature conditions while meeting long-life fatigue requirements. Furthermore, the enhanced anti-corrosion treatment applied to the connecting sleeve and weld area after welding enables long-term reliable service in coastal salt spray environments, achieving a safe compatibility between the economic efficiency of carbon steel and the corrosion resistance of stainless steel in long-distance vacuum-insulated liquid ammonia pipelines. Attached Figure Description

[0018] The accompanying drawings, which form part of this application, are used to provide a further understanding of the application and to make other features, objects, and advantages of the application more apparent. The illustrative embodiments and descriptions of this application are used to explain the application and do not constitute an undue limitation of the application. In the drawings: Figure 1 This is a schematic cross-sectional view of the overall structure of the connector after welding according to the present invention; Figure 2 This is a schematic cross-sectional view of the connector of the present invention after welding; Figure 3 This is a schematic cross-sectional view of the connector of the present invention after thermal expansion; Figure 4 This is a schematic cross-sectional view of the connector of the present invention after cold shrinkage.

[0019] Among them: 1. tapered transition connecting sleeve; 2. first welding end; 3. second welding end; 4. cooling jacket. Detailed Implementation

[0020] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present application.

[0021] It should be noted that the terms "first," "second," etc., used in the specification and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be used interchangeably where appropriate for the embodiments of this application described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0022] In this application, the terms "upper," "lower," "left," "right," "front," "rear," "top," "bottom," "inner," "outer," "middle," "vertical," "horizontal," "lateral," and "longitudinal" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for the purpose of better describing this application and its embodiments, and are not intended to limit the indicated device, element, or component to having a specific orientation, or to be constructed and operated in a specific orientation.

[0023] Furthermore, in addition to indicating location or positional relationship, some of the aforementioned terms may also have other meanings. For example, the term "above" may also be used in some cases to indicate a certain dependency or connection relationship. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0024] In addition, the term "multiple" should mean two or more.

[0025] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0026] like Figure 1 Figure 4 As shown, Example 1 This embodiment provides an installation process for a long-distance vacuum-insulated pipeline. The pipeline uses carbon steel outer pipe sections in inland sections far from the coast, and stainless steel outer pipe sections in salt spray environments near the coast. The inner pipe is uniformly made of stainless steel. The installation process specifically includes the following steps: S1. Raw material preparation We offer carbon steel outer pipe sections, stainless steel outer pipe sections, and stainless steel inner pipe sections. The outer diameter and wall thickness of the carbon steel outer pipe sections are the same as those of the stainless steel outer pipe sections to facilitate overall piping system design and welding connections.

[0027] The surface pretreatment of each pipe section includes: sandblasting the carbon steel outer pipe section to Sa2.5 grade, degreasing with acetone, and coating with epoxy zinc-rich anti-rust primer; pickling and passivation treatment of the stainless steel outer pipe section to form a dense oxide film on its surface to improve corrosion resistance; and sequential degreasing, pickling, and mechanical polishing of the stainless steel inner pipe section to ensure a smooth inner wall and reduce flow resistance during liquid ammonia transportation. After all pipe sections have undergone surface pretreatment, they must be inspected to confirm that the cleanliness and roughness meet the requirements for subsequent welding and vacuum use.

[0028] S2, Segmented Assembly The stainless steel inner pipe and the support components that provide support and insulation are inserted into the selected outer pipe section to form a segmented double-walled pipe. The support components include insulating support rings arranged at intervals along the axial direction of the inner pipe. These rings are made of polytetrafluoroethylene or fiberglass, which have low thermal conductivity. They not only support the inner pipe and maintain the concentricity of the inner and outer pipes, but also reduce heat conduction between the inner and outer pipes.

[0029] Each assembled segmented double-walled tube underwent quality testing, including: an airtightness test of the annular cavity between the inner and outer tubes, which was filled with 0.2 MPa nitrogen and pressurized for 24 hours, with the pressure drop observed; and helium mass spectrometry vacuum leak detection was performed on welds and other joints. After passing the tests, both ends of the segmented double-walled tube were sealed with metal caps with sealing rings to prevent impurities and moisture from entering during transportation and storage.

[0030] S3, Piping Installation The sealed, segmented double-walled pipes were transported to the installation site. According to the plans for the land section and the salt spray environment section, the corresponding marked segmented double-walled pipes were delivered to their respective installation locations.

[0031] During welding, the stainless steel inner tube of each section is welded first. Because the inner tube transports liquid ammonia, the weld quality requirements are high; therefore, manual tungsten inert gas (TIG) welding is used, with 304 stainless steel welding wire of the same material as the inner tube used as filler metal. After the inner tube weld is completed, the weld undergoes pickling and passivation treatment to remove the welding oxide layer, and 100% radiographic inspection is performed to ensure that there are no defects such as porosity or cracks inside the weld. The outer tube is then welded after this treatment.

[0032] At the joint where the carbon steel outer pipe section in the land segment meets the stainless steel outer pipe section in the salt spray environment segment, a connector is installed. This connector is an integrally formed conical transition sleeve 1. The conical transition sleeve 1 is made of a nickel-based alloy with a coefficient of thermal expansion between that of carbon steel and stainless steel. Its inner and outer diameters change continuously and monotonically linearly from the first end welded to the carbon steel outer pipe to the second end welded to the stainless steel outer pipe, thus forming a predetermined cone angle. .

[0033] When welding the tapered transition sleeve 1, manual tungsten inert gas (TIG) welding is used, and the filler metal is a nickel-based alloy welding wire of the same series as the base material of the sleeve. In order to control the welding heat input, reduce the range of the heat-affected zone and deformation, a detachable cooling jacket 4 is fitted on the outside of the tapered transition sleeve 1 during the welding process, and circulating cooling water is introduced for forced cooling. After welding is completed, the cooling jacket 4 is removed.

[0034] After the two ends of the connecting sleeve are fully welded to the carbon steel outer pipe section and the stainless steel outer pipe section respectively, the outer pipe connection is completed. Through the pre-set conical angle structure of the connecting sleeve, the radial misalignment caused by the difference in thermal expansion coefficients of carbon steel and stainless steel is transformed into elastic bending strain that is evenly distributed along the entire conical surface, thereby safely absorbing thermal stress and preventing weld cracking.

[0035] S4. Vacuum treatment and corrosion protection After all welding is completed, a vacuum pump unit is used to evacuate the annular cavity between the inner and outer tubes to achieve the designed vacuum level. Once this vacuum is reached, the evacuation port is sealed. The presence of this vacuum layer minimizes the transfer of ambient heat to the liquid ammonia, ensuring efficient transport.

[0036] Subsequently, the tapered transition connecting sleeve 1 and its weld area are subjected to enhanced anti-corrosion treatment: first, an epoxy primer compatible with stainless steel is applied to the whole body, which has good adhesion to both carbon steel and stainless steel; then, anti-corrosion tape or heat shrink sleeve resistant to salt spray corrosion is wrapped around it, with an overlap width of not less than 50mm, forming a multi-layer protection system to ensure the long-term service performance of the connector in the coastal salt spray atmospheric environment.

[0037] Further explanation regarding the determination of the cone angle In this embodiment, the cone angle determination method is implemented according to the following steps: The first step is to query meteorological data for the areas along the pipeline route to obtain the historical lowest ambient temperature, such as 15℃, and the design maximum operating temperature, such as 50℃, as the temperature boundary conditions. This temperature range covers all operating conditions from extreme low temperatures in winter to full-load operation in summer.

[0038] The second step involves expanding the curves of thermal expansion coefficients of carbon steel, stainless steel, and nickel-based alloys as a function of temperature into polynomial forms using Taylor series. For example, the transient thermal expansion coefficient of 304 stainless steel... Expandable to , where the coefficient , , The material properties were obtained by fitting the experimental material data using the least squares method. The polynomial material properties were then input into finite element analysis software such as ANSYS to create a three-dimensional finite element model containing the tapered transition connecting sleeve 1 and 6m long pipe sections at both ends, and a thermal-structural coupling transient analysis was performed.

[0039] For the coefficient of linear expansion of materials Expanding at a reference temperature T0 (taken as 20℃) results in a cubic polynomial: ; Where T is the current temperature, and a0 to a3 are the coefficients to be fitted. This application fits the experimental data using the least squares method, specifically: given m sets of experimental data points ( T i ,a i ), fitted residual sum of squares ; make The coefficients are obtained by solving the system of four linear equations. After fitting, the correlation coefficient R0 is... 2 All are greater than 0.99. Specifically, the coefficient of linear expansion of 304 stainless steel is... The fit is as follows: ; The coefficient of linear expansion of carbon steel Q345R The fit is as follows: ; The coefficient of linear expansion of the nickel-based alloy Inconel 625 The fit is as follows: ; The polynomial material properties described above were input into the finite element analysis software ANSYS Workbench to create a three-dimensional finite element model containing the conical transition connecting sleeve 1 and 6m long pipe sections at both ends for thermal-structural coupling transient analysis. During modeling, eight-node hexahedral solid elements were used for mesh generation. The mesh was refined in the conical transition region of the connecting sleeve, with the element size controlled within 2mm, and the element size controlled within 10mm in other regions. The total number of elements was approximately 1.8 million to 2.2 million. A temperature field was applied in the ANSYS Thermal module: the liquid ammonia medium temperature (-33℃ to 50℃) was applied to the inner pipe wall, and the ambient temperature (-15℃ to 40℃) was applied to the outer pipe wall. Convective heat transfer between the outer pipe surface and the air (convective heat transfer coefficient of 10~25 W / (m²·K)) and solar radiation (absorptivity of 0.6, solar radiation intensity of 600~1000 W / m²) were considered. The calculated temperature field results were imported into the Static Structural module as loads, applying axial constraints at both ends of the piping system, the pressure of the liquid ammonia medium inside the inner pipe (design pressure 2.5 MPa), and the atmospheric pressure on the outer wall of the outer pipe (0.1 MPa). A sparse matrix direct solver was used for nonlinear transient solutions, with a time step of 60 seconds. The total calculation time covered one complete temperature cycle: heating-holding-cooling-holding, totaling 24 hours. The von Mises equivalent stress at each node on the connecting sleeve was extracted, and its calculation formula is as follows: ; σ1, σ2, and σ3 are the three principal stresses. The criterion is: throughout the entire temperature cycle, the maximum von Mises stress at any node on the connecting sleeve must not exceed the allowable stress of the nickel-based alloy material at the corresponding temperature (take the yield strength σ). s The safety factor is 1.5), and the alternating stress amplitude must not exceed the fatigue limit of the material (for Inconel 625, take 10). 7 The fatigue strength under one cycle is 280 MPa.

[0040] The third step is to use the intermediate temperature zero-stress point design method as the initial condition. The average annual temperature of the region, 16℃, is taken as the zero point for thermal displacement compensation. At this temperature, the diameters of the carbon steel outer tube, the tapered transition connecting sleeve 1, and the stainless steel outer tube are all considered to be ideally matched, with zero stress. Based on this condition, the initial cold-state taper values ​​that the connecting sleeve should have at both ends when manufactured at room temperature (20℃) are calculated.

[0041] The fourth step involves using the initially determined cone angle as the initial design and further optimizing it globally through iterative finite element analysis. The objective function is to minimize the maximum von Mises stress of the connecting sleeve throughout the entire temperature cycle, with the cone angle as the limiting factor. The optimal cone angle value and the corresponding wall thickness distribution curve are obtained by iteratively solving the wall thickness distribution parameters, such as the minimum wall thickness location and the thinning rate, with the yield strength and fatigue limit of the nickel-based alloy as constraints.

[0042] Specifically, the iterative optimization employs a sequential quadratic programming algorithm, whose mathematical model is expressed as: design variables ,in Let L be the cone angle, t_min be the minimum wall thickness of the conical transition section, and L be the minimum wall thickness of the conical transition section. taper The axial length of the tapered transition section. The wall thickness reduction rate is the control parameter; the objective function is... That is, to minimize the maximum von Mises stress at all points in the volume V of the connecting sleeve within the entire temperature cycle T; the constraints include: (Static strength constraint) (Fatigue strength constraint, where Δσ_eq is the equivalent alternating stress amplitude calculated using the rainflow counting method), and (Vacuum external pressure stability constraint). The range of design variables is: 1° ≤ ≤30°, 0.5t0≤t min ≤0.8t0 (t0 is the wall thickness at the welded end), and t min Not less than the calculated value t for vacuum external pressure stability buckling , 0≤ ≤1.

[0043] The iterative solution is performed according to the following steps: Step 1: Use the preliminary cone angle value determined by the intermediate temperature zero-stress point design method as the initial design point x. （0） Set the iteration counter k=0 and the convergence tolerance ε=1×10. -4 ; Step two, at the current design point Finite element thermal-structure coupled transient analysis is performed to calculate the objective function value. and constraint function values ; Step 3: Calculate the sensitivity of the objective function and constraint functions to the design variables using the finite difference method; Step 4: Construct a quadratic programming subproblem at the current point: ; Constraints ; Where d represents the search direction. The Hessian matrix updated using the BFGS method; Step 5: Determine the step size using Armijo line search. Update design variables ; Step 6, if the convergence condition is met If the iteration stops, the optimal solution is output. Otherwise, let k = k + 1 and return to step two.

[0044] Through the above iterative optimization, the optimal cone angle was obtained for the typical working condition of an outer tube with a nominal diameter of DN200 and a temperature difference span of 65℃. At approximately 3° (with 17 iterations to converge), the maximum thermal stress on the connecting sleeve is approximately 158 MPa, which is far lower than the allowable stress of Inconel 625 (approximately 327 MPa), and the alternating stress amplitude is approximately 85 MPa, which is lower than the fatigue limit of 280 MPa.

[0045] By setting boundary conditions based on real meteorological data for all operating conditions, the design of the connectors can cover the extreme temperatures that the pipeline may encounter during its service life, thus avoiding safety hazards caused by improper selection of boundary conditions.

[0046] By using Taylor series expansion, the nonlinear characteristics of the material's thermophysical properties as a function of temperature are accurately incorporated into the finite element model, avoiding the calculation errors caused by traditional linear approximations, and making the stress analysis results more realistic and reliable.

[0047] By using the intermediate temperature as the zero-stress design point, the stress levels of the connectors are basically symmetrical under the two extreme working conditions of high temperature and low temperature, avoiding excessive stress on one side, making full use of the bidirectional load-bearing capacity of the material, and improving the rationality and economy of the design.

[0048] Through iterative optimization aimed at minimizing the maximum stress under all operating conditions, the final obtained cone angle value and wall thickness distribution can achieve the optimal stress level of the connector throughout the entire temperature cycle, satisfying both static strength requirements and fatigue life requirements, and ensuring the long-term service safety of the connector.

[0049] Example 2 The difference between this embodiment and Embodiment 1 is that in the enhanced anti-corrosion treatment in step S4, after applying the primer, a stainless steel metal protective cover is used to wrap the outside of the connectors and welds. Sealant is filled between the protective cover and the outer wall of the pipe to form a rigid seal. This solution is suitable for situations with higher requirements for mechanical protection, such as pipelines laid in areas that may be subject to falling rocks or human-caused mechanical impacts.

[0050] Metal protective covers not only provide salt spray protection but also effectively resist external mechanical impacts, preventing damage to connectors and weld areas in accidental collisions and further improving the safety and reliability of pipelines.

[0051] Example 3 This embodiment provides a tapered transition connector 1 for the above-described installation process. This connector is specifically designed for connecting carbon steel outer pipes and stainless steel outer pipes in vacuum-insulated pipelines transporting liquid ammonia from land to the coast.

[0052] The tapered transition sleeve 1 is integrally forged from a nickel-based alloy with a thermal expansion coefficient between that of carbon steel and stainless steel. Compared to casting, forging results in a denser structure, eliminates casting defects, and ensures airtightness in high-vacuum applications. The outer surface of the sleeve is coated with a salt spray corrosion resistant protective layer, which can be made of epoxy micaceous iron oxide primer combined with acrylic polyurethane topcoat.

[0053] The tapered transition sleeve 1 has a first welding end 2 for welding with a carbon steel outer tube and a second welding end 3 for welding with a stainless steel outer tube. The wall thickness of both welding ends is greater than the wall thickness of the connected outer tube, ensuring sufficient heat capacity during welding and reducing the risk of burn-through. The middle section between the first welding end 2 and the second welding end 3 is a tapered stress transition section. The inner and outer diameters of this transition section change continuously and monotonically along the axial direction, thus forming a cone angle. .

[0054] cone angle The values ​​of the and the wall thickness distribution of the conical stress transition section are determined by a full-condition optimization algorithm based on Taylor series approximation and finite element thermal-structural coupling analysis. The specific process of this algorithm includes: using the historical lowest and highest operating temperatures of the pipeline route area as boundary conditions, performing polynomial approximation of the curves of the thermal expansion coefficients of each material with temperature using Taylor series expansion, establishing a finite element model of the piping system including the connector, and performing thermal-structural coupling transient analysis; using the minimization of the maximum von Mises stress of the connector throughout the entire temperature cycle as the objective function, and using the cone angle... Using the wall thickness distribution parameters as design variables and the material yield strength and fatigue limit as constraints, the optimal geometric parameters are obtained through iterative solutions.

[0055] After optimization by the algorithm, the peak thermal stress at any point on the connecting sleeve is lower than the allowable stress of the material, and the alternating stress amplitude caused by temperature cycling is lower than the fatigue limit of the material, ensuring that it will not experience static strength failure or fatigue failure during the entire design life cycle of the pipeline.

[0056] Example 4 This embodiment, based on Embodiment 3, further defines the wall thickness profile characteristics of the conical stress transition section. Specifically, the wall thickness of the conical stress transition section is not uniform, but rather smoothly thins from both ends towards the middle, forming a concave wall thickness profile that is thinner in the middle and thicker at both ends. The change in wall thickness uses a circular arc transition curve to avoid stress concentration caused by abrupt changes in the cross-section.

[0057] For example, for a pipeline with an outer wall thickness of 6mm, the wall thickness of the first welding end 2 and the second welding end 3 can be 8mm. The wall thickness in the middle of the tapered stress transition section gradually decreases from 8mm at both ends to 5mm towards the middle, and the axial length of the thinning section is about 80mm.

[0058] The thicker walls at both ends ensure sufficient rigidity and strength at the welded ends, meeting welding process requirements and vacuum pressure bearing capacity; the thinner walls in the middle provide greater radial flexibility, allowing the connecting sleeve to more effectively absorb the radial displacement difference caused by the different thermal expansion coefficients of the two parent tubes through elastic bending. Compared with the constant wall thickness design, this variable wall thickness design significantly improves the thermal displacement compensation capability and fatigue resistance of the connecting sleeve without increasing material usage.

[0059] The minimum wall thickness has been verified through vacuum external pressure stability calculations. The calculations considered an external pressure load of one atmosphere and a safety factor, and finite element eigenvalue buckling analysis confirmed that this wall thickness would not cause instability.

[0060] Example 5 This embodiment further defines the cone angle based on embodiment 4. The value range of . For typical pipeline operating conditions of transporting liquid ammonia from land to the coast, after optimization calculations, the cone angle The optimal value usually falls within the range of 1° to 30°.

[0061] When the cone angle When the angle is less than 1°, the transition section length is too large, which not only increases manufacturing costs but also leads to excessive axial space occupation by the connectors, causing difficulties in piping layout; when the cone ... When the angle is greater than 30°, the transition of the conical surface is too abrupt, the absorption effect of radial displacement decreases, and high local bending stress will be generated in the connecting sleeve.

[0062] With optimal cone angle Taking 3° as an example, under the working conditions of an outer tube nominal diameter of DN200 and a temperature difference span of 65°, the axial length of the conical stress transition section is about 200mm, and the maximum thermal stress on the connecting sleeve can be controlled below 160MPa, which is far lower than the yield strength of nickel-based alloys, and fully meets the requirements for safe use.

[0063] Example 6 This embodiment, based on embodiment 3, further adds a detachable cooling jacket 4. The cooling jacket 4 is composed of two semi-cylindrical bodies joined together, with spiral channels machined into the inner wall for the circulation of cooling medium. In use, the two semi-cylindrical bodies are joined together and fastened to the outside of the conical transition connecting sleeve 1 with bolts. One end is connected to a cooling water pipe, and the other end is disconnected. Before welding, the cooling water circulation is turned on. The welding heat is conducted to the cooling jacket 4 through the wall thickness of the connecting sleeve and carried away by the circulating water. This device is used during the welding process and is disassembled after welding is completed.

[0064] Forced cooling during the welding process significantly reduces the heat-affected zone, decreases residual stress and thermal deformation of the connecting sleeve caused by welding, and ensures that the pre-set cone angle and wall thickness profile accuracy of the connecting sleeve after manufacturing are not affected by welding. After welding, the cooling jacket 4 can be disassembled and reused, reducing construction costs.

[0065] The above are merely preferred embodiments of this application and are not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. An installation process for a long-distance vacuum-insulated liquid ammonia transport pipeline, wherein the pipeline uses carbon steel outer pipe sections in the land section far from the coast and stainless steel outer pipe sections in the salt spray environment section near the coast, characterized in that... The installation process includes the following steps: S1. Raw material preparation: Provide carbon steel outer pipe sections, stainless steel outer pipe sections, and stainless steel inner pipe sections, and perform surface pretreatment on each pipe section. S2. Segmented assembly: Insert the stainless steel inner pipe and support components into the corresponding outer pipe segment, and then conduct a quality test on the assembled segmented double-wall pipe. After the test is completed, seal both ends of the segmented double-wall pipe. S3. Piping Installation: Transport the enclosed segmented double-walled pipes to the site, and deliver the corresponding segmented double-walled pipes to the installation locations according to the division of the land section and the salt spray environment section; when welding the segmented double-walled pipes, first weld the stainless steel inner pipe of each segment, and after the inner pipe is welded, treat the weld seam, and then weld the outer pipe; at the joint where the carbon steel outer pipe segment of the land section connects to the stainless steel outer pipe segment of the salt spray environment section, install connectors; The connector is made of a nickel-based alloy with a coefficient of thermal expansion between that of carbon steel and stainless steel. The outer diameter of the connector changes continuously and monotonically from the first end welded to the carbon steel outer tube to the second end welded to the stainless steel outer tube, forming a predetermined cone angle. The cone angle is determined by a full-condition optimization algorithm based on Taylor series approximation and finite element thermal structure coupling analysis. The two ends of the connector are fully welded to the carbon steel outer pipe section and the stainless steel outer pipe section respectively to complete the outer pipe connection; the connector converts the radial misalignment caused by the difference in thermal expansion coefficients of carbon steel and stainless steel into elastic bending strain distributed along the conical surface, thereby safely absorbing thermal stress. S4. Vacuum treatment and corrosion protection: After welding, the annular cavity between the inner tube and the outer tube is evacuated and sealed; the connector and its weld area with the carbon steel outer tube and the stainless steel outer tube are subjected to enhanced corrosion protection treatment to resist the erosion of the coastal salt spray environment.

2. The installation process of the long-distance vacuum-insulated liquid ammonia transmission pipeline according to claim 1, characterized in that, The determination of the cone angle Includes the following steps: The first step is to use the lowest ambient temperature and the highest operating temperature of the area through which the pipeline passes as temperature boundary conditions; The second step is to expand the coefficients of thermal expansion of carbon steel, stainless steel and nickel-based alloys at different temperatures in the form of Taylor series, establish a finite element model of the piping system including the connectors, and perform thermal structural coupling transient analysis. The third step is to use the intermediate temperature zero stress point design method as the initial condition, and set the annual average temperature of the area through which the pipeline passes as the zero point of thermal displacement compensation, so as to initially determine the cone angle. The fourth step involves using the minimization of the maximum von Mises stress of the connector throughout the entire temperature cycle as the objective function, the cone angle and wall thickness distribution parameters as design variables, and the material yield strength and fatigue limit as constraints. The initially determined cone angle is corrected through iterative finite element analysis to solve for the optimal cone angle value.

3. The installation process of the long-distance vacuum-insulated liquid ammonia transmission pipeline according to claim 1, characterized in that, In step S3, manual tungsten inert gas welding is used to weld the connector. During the welding process, a cooling jacket (4) is fitted over the connector and a circulating cooling medium is introduced. After the welding is completed, the cooling jacket (4) is removed.

4. The installation process of the long-distance vacuum-insulated liquid ammonia transmission pipeline according to claim 2, characterized in that, In step S1, the surface pretreatment includes: removing rust and degreasing the carbon steel outer pipe section and applying an anti-rust primer; pickling and passivating the stainless steel outer pipe section; and degreasing, pickling, and polishing the stainless steel inner pipe section.

5. The installation process of the long-distance vacuum-insulated liquid ammonia transmission pipeline according to claim 1, characterized in that, In step S4, the enhanced anti-corrosion treatment includes: first, coating the connector and weld area with a primer compatible with stainless steel, and then wrapping it with anti-corrosion tape resistant to salt spray corrosion.

6. A connector, applied to the installation process of the long-distance vacuum-insulated liquid ammonia transmission pipeline according to any one of claims 1 to 5, wherein the connector is used to connect the carbon steel outer pipe and the stainless steel outer pipe in a vacuum-insulated pipeline for transporting liquid ammonia from land to coast, characterized in that, The connector is a tapered transition connecting sleeve (1); The tapered transition sleeve (1) is integrally manufactured from a nickel-based alloy with a thermal expansion coefficient between that of carbon steel and stainless steel, and the outer surface of the tapered transition sleeve (1) is coated with a protective layer resistant to salt spray corrosion. The tapered transition sleeve (1) has a first welding end (2) for welding with a carbon steel outer tube and a second welding end (3) for welding with a stainless steel outer tube. The tapered transition connecting sleeve (1) has a tapered stress transition section in the middle section between the first welding end (2) and the second welding end (3). The outer diameter of the tapered stress transition section changes continuously and monotonously along the axial direction, thereby forming a cone angle α. The value of the cone angle α and the wall thickness distribution of the cone stress transition section are configured such that, within the pipeline design temperature range, the peak thermal stress at any point on the connecting sleeve is lower than the allowable stress of the material, and the alternating stress amplitude caused by temperature cycling is lower than the fatigue limit of the material.

7. The connector according to claim 6, characterized in that, The wall thickness of the tapered stress transition section decreases smoothly from both ends toward the middle, forming a concave wall thickness profile that is thin in the middle and thick at both ends, so as to provide greater radial flexibility without reducing the vacuum bearing capacity.

8. The connector according to claim 6, characterized in that, The minimum wall thickness of the conical stress transition section is 0.5 to 0.8 times the wall thickness of the first welding end (2) or the second welding end (3), and is not less than the minimum wall thickness required for vacuum external pressure stability calculation; the value of the cone angle α ranges from 1° to 30°.

9. The connector according to claim 6, characterized in that, It also includes a detachable cooling jacket (4), which is composed of two semi-cylindrical bodies and can be fitted outside the conical transition connecting sleeve (1). The inner wall of the cooling jacket (4) is provided with a spiral flow channel for circulating cooling medium.

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