An ultra-large diameter directly buried hot water pipeline and its design method

Abstract

The present invention discloses an ultra-large diameter direct-buried hot water pipeline and its design method, belonging to the field of steam-water pipeline system design. From the inside out, the pipeline comprises a working inner pipe that bears the pipeline medium pressure, an inner anti-corrosion layer responsible for corrosion protection of the working inner pipe, a polyurethane insulation layer responsible for heat insulation and load-bearing support of the working inner pipe, a steel outer protective pipe responsible for bearing the external forces of soil and traffic loads, and an outer anti-corrosion layer responsible for corrosion protection of the steel outer protective pipe, which are integrated into a single unit. The outer diameter of the working inner pipe is DN ≥ 1800 mm. The design method uses a local-first, then global analysis method to analyze the stress conditions of the ultra-large diameter direct-buried hot water pipeline model and verify its feasibility. This invention solves the problem of long-distance, high-flow direct-buried hot water transportation requiring dual-pipe transportation due to pipe diameter limitations, fills a technical gap in direct-buried hot water pipelines, and sets a precedent both domestically and internationally.

Description

Technical Field

[0001] The present invention relates to the field of steam-water pipeline system design, in particular to an ultra-large diameter directly buried hot water pipeline and a design method thereof, which are applied to steam-water pipelines and heating pipelines in thermal power plants. Background Art

[0002] For direct buried hot water pipes at home and abroad, mature foamed polyurethane direct buried pipes (pipe diameter ≤ 1200) are preferred, such as Figure 2 As shown. Due to limitations in technology and manufacturing, the applicable scope of relevant regulations and specifications is limited to pipes with a nominal diameter less than or equal to 1200mm. Furthermore, the maximum diameter of direct-buried pipes put into operation in actual domestic and international engineering projects is 1600mm, and these pipes are rarely used. There is still no direct-buried hot water pipe with a diameter greater than 1600mm. With the development of society and the increasing attention paid to environmental issues in recent years, the development of clean energy is particularly important. Centralized heating not only improves energy utilization and conserves energy, but also improves environmental sanitation and reduces winter heating costs. It is a key infrastructure of modern cities and towns. With the popularization of centralized heating, long-distance and high-flow heating have also placed greater demands on the diameter of direct-buried hot water pipes.

[0003] As is well known, increasing pipe diameter increases the stress generated, complicating pipe manufacturing and construction. Since there are no technical reserves for oversized direct-buried hot water pipes (DN ≥ 1800), systematic research based on existing design and regulatory specifications is required, along with the development of a unique pipe model tailored to the characteristics of oversized pipes. To meet near- and long-term needs for high-flow, long-distance heating, design research on oversized direct-buried hot water pipes is necessary, offering broad prospects for widespread adoption. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide an ultra-large diameter directly buried hot water pipeline and a design method thereof, which solves the problem that direct buried hot water transportation at long distances and large flows must use double-pipe transportation due to the limited pipe diameter. The use of the ultra-large diameter directly buried pipeline of the present invention not only saves a lot of engineering investment, but also saves floor space and shortens the construction period, which is of great significance.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] An ultra-large diameter directly buried hot water pipe integrates, from the inside to the outside, a working inner pipe that bears the pressure of the pipe medium, an inner anti-corrosion layer responsible for the corrosion protection of the working inner pipe, a polyurethane insulation layer responsible for the thermal insulation and load-bearing support of the working inner pipe, a steel outer protective pipe responsible for bearing the external forces of soil and traffic loads, and an outer anti-corrosion layer responsible for the corrosion protection of the steel outer protective pipe into a whole; the DN of the working inner pipe is ≥1800mm.

[0007] A design method for an ultra-large diameter directly buried hot water pipeline comprises the following steps:

[0008] Step 1: Based on the development requirements of direct-buried hot water pipelines gradually moving towards higher pressures and larger diameters, a model for ultra-large diameter direct-buried hot water pipelines was proposed by studying existing relevant technical standards and analyzing the mechanical properties of overhead and buried pipelines.

[0009] Step 2: Analyze the stress of the ultra-large diameter direct-buried hot water pipeline using a local-first-global analysis method and verify its feasibility;

[0010] Step 3: Build a model of the ultra-large diameter direct-buried hot water pipeline.

[0011] A further improvement of the technical solution of the present invention is that step 2 specifically includes the following steps:

[0012] 2.1, Conduct local independent analysis of pipeline stress:

[0013] 2.1.1 Determine the wall thickness of the working inner tube;

[0014] The working inner pipe bears the internal pressure of the working medium, and the effect of external soil pressure on the working inner pipe is not considered. The wall thickness of the working inner pipe is calculated to be much smaller than that of the traditional pipe model with the same parameters. It is within the production and manufacturing range of general pipe factories and meets the requirements.

[0015] 2.1.2 Determine the wall thickness of the steel outer protective pipe;

[0016] The external soil pressure including traffic load is borne by the steel outer casing, without considering the influence of the internal pressure of the working medium. The wall thickness of the steel outer casing is calculated to see whether it is within the permitted range.

[0017] 2.1.3 Determine the wall thickness and compressive strength of the polyurethane insulation layer;

[0018] A rigid polyurethane layer of a certain thickness is used as an insulation layer between the working inner pipe and the steel outer protective pipe. The rigid polyurethane also supports and waterproofs the working inner pipe. The compressive strength of the insulation layer is calculated based on the weight of the working inner pipe filled with hot water in the ultra-large diameter direct-buried hot water pipeline model. To ensure the support requirements of the working inner pipe, the thickness and compressive stress of the polyurethane insulation layer are determined.

[0019] 2.1.4, set up anti-corrosion layer;

[0020] An inner anti-corrosion layer with an anti-corrosion effect is provided outside the working inner pipe to protect the working inner pipe; an outer anti-corrosion layer with an anti-corrosion effect is provided outside the steel outer protective pipe to protect the steel outer protective pipe;

[0021] 2.2 Overall analysis of the pipeline model:

[0022] The five parts of the super-large diameter direct-buried hot water pipeline model: working inner pipe, inner anti-corrosion layer, polyurethane insulation layer, steel outer protective pipe and outer anti-corrosion layer are analyzed as a whole to ensure the safe and stable operation of the pipeline under the design parameters; the super-large diameter direct-buried hot water pipeline model is required to overcome the internal axial force and tangential shear force, and the axial force and tangential shear force values ​​are used as the verification values ​​of the super-large diameter direct-buried hot water pipeline model to ensure the safety and stability of the pipeline.

[0023] Due to the adoption of the above technical solution, the technical advancements achieved by the present invention are:

[0024] 1. The present invention integrates the five parts of the working inner pipe, inner anti-corrosion layer, polyurethane insulation layer, steel outer protective pipe and outer anti-corrosion layer into one. The diameter of the working inner pipe DN ≥ 1800mm solves the problem that direct buried hot water transportation under large flow must use double-pipe transportation due to the limited pipe diameter. The use of the ultra-large diameter direct buried hot water pipeline of the present invention not only saves a lot of engineering investment, but also saves floor space and shortens the construction period, which is of great significance.

[0025] 2. Compared with conventional direct-buried hot water pipes, the present invention not only increases the diameter of direct-buried heating pipes to DN ≥ 1800mm through technical innovation, breaking through the regulations and existing application cases in related fields, but also conducts in-depth research in the design, construction and manufacturing links, and has been successfully applied to the field of heating engineering, breaking through the technical bottleneck.

[0026] 3. This invention fills the technical gap of direct buried hot water pipes and creates a precedent at home and abroad; it is highly efficient and energy-saving, safe and reliable, and easy to construct. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. Those skilled in the art can derive other drawings based on these drawings without inventive effort.

[0028] Figure 1 This is a schematic structural diagram of the ultra-large diameter directly buried hot water pipeline provided by the present invention;

[0029] Figure 2 This is a schematic structural diagram of a mature foamed polyurethane direct buried pipeline (pipeline diameter ≤ 1200 mm) described in the background technology of the present invention;

[0030] Among them, 1. working inner pipe, 2. inner anti-corrosion layer, 3. polyurethane insulation layer, 4. steel outer protective pipe, 5. outer anti-corrosion layer, 6. polyethylene outer protective pipe. DETAILED DESCRIPTION

[0031] It should be noted that the terms "including" and "having" and any variations thereof in the specification and claims of the present invention and the above-mentioned drawings are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or are inherent to these processes, methods, products or apparatuses.

[0032] The embodiment of the present application solves the problem in the prior art that "direct-buried hot water transportation under large flow must use double-pipe transportation due to the limited pipe diameter" by providing an ultra-large diameter direct-buried hot water pipeline and a design method thereof. The general idea is: the working inner pipe 1, the inner anti-corrosion layer 2, the polyurethane insulation layer 3, the steel outer protective pipe 4 and the outer anti-corrosion layer 5 are integrated into a whole. The diameter of the working inner pipe 1 is DN ≥ 1800mm, and an ultra-large diameter direct-buried hot water pipeline is designed, which fills the technical gap of direct-buried hot water pipelines and creates a precedent at home and abroad.

[0033] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments:

[0034] like Figure 1 As shown, an ultra-large diameter direct-buried hot water pipeline addresses the limitations of long-distance, high-flow heating due to the limited diameter of directly buried hot water pipelines. Based on research into existing specifications and the operational and mechanical properties of long-distance, high-flow hot water pipelines, the pipeline integrates a working inner pipe 1, an inner anti-corrosion layer 2, a polyurethane insulation layer 3, a steel outer casing 4, and an outer anti-corrosion layer 5 into a single unit. The working inner pipe 1 bears the pipeline medium pressure, the inner anti-corrosion layer 2 protects the working inner pipe 1 from corrosion, the polyurethane insulation layer 3 provides insulation and load support, the steel outer casing 4 withstands external forces such as soil and traffic loads, and the outer anti-corrosion layer 5 provides corrosion protection. The working inner pipe 1 has a DN of 1800 mm or greater.

[0035] Example 1

[0036] Taking the parameters of a certain direct-buried hot water pipeline project for heating as an example, the specifications of the super-large diameter direct-buried hot water pipeline are calculated: the heat network circulating water volume is approximately 27,000 t / h, the design pressure is 2.15 MPa, the design maximum temperature is 95°C, the pipeline material is Q235-B, the pipeline is buried at a depth of 3.5m, and the traffic load is considered to be 45t. The calculated pipeline specifications of the present invention are as follows:

[0037] Working inner tube 1 parameters: outer diameter is 2020mm, wall thickness is 24mm.

[0038] Polyurethane insulation layer 3 parameters: thickness 100mm, compression strength ≥0.3MPa.

[0039] Steel outer protective pipe 4 parameters: outer diameter is 2258mm, wall thickness is 19mm.

[0040] A design method for an ultra-large diameter directly buried hot water pipeline comprises the following steps:

[0041] Step 1: Based on the development requirements of direct buried hot water pipelines gradually moving towards high pressure and large diameter, by studying existing relevant technical standards and analyzing the mechanical properties of overhead pipelines and buried pipelines, a super-large diameter direct buried hot water pipeline model is proposed; the super-large diameter direct buried hot water pipeline model integrates the working inner pipe 1, inner anti-corrosion layer 2, polyurethane insulation layer 3, steel outer protective pipe 4 and outer anti-corrosion layer 5 into a whole. (e.g. Figure 1 shown)

[0042] Centralized heating has grown rapidly in recent years, with the number of heat users increasing year by year. As heat loads increase, the heating capacity of thermal power plants is becoming increasingly insufficient, necessitating retrofitting. For aesthetic reasons, the main heating pipes within power plants are often buried underground. With the increasing heat load, the flow rate of directly buried hot water pipes increases, and pipe diameters can reach DN ≥ 1800mm, exceeding the range of national industry standards. Existing national standards and technical methods are no longer sufficient for the design of such large-diameter underground hot water pipes, creating a technological gap.

[0043] In the large diameter direct buried hot water pipeline model, the existing direct buried hot water pipeline model (such as Figure 2 The polyethylene outer sheath in the figure is replaced with a steel outer sheath because the maximum diameter of the direct-buried hot water pipe with a polyethylene outer sheath is 1400 mm. It is impossible to process and manufacture a larger diameter, which limits the development of the industry. Therefore, the inventors of this application redesigned the model of the direct-buried hot water pipe with an extra-large diameter and selected a steel outer sheath from many outer sheath pipe solutions.

[0044] Furthermore, in the prior art, steel sheaths are commonly used in direct-buried steam pipelines, often serving a similar function as a "tunnel." The inner pipe transporting steam is free to expand and contract within this "tunnel" of the steel sheath, utilizing a guide device. However, the present invention utilizes a steel sheath pipe 4 in a direct-buried hot water pipeline, a highly unusual practice. Furthermore, the steel sheath pipe 4 is integrated with the working inner pipe 1 transporting hot water, allowing them to expand and contract together. Although both steel sheaths are used in direct-buried steam pipelines and in this application, their applications and functions are fundamentally different.

[0045] Step 2: Analyze the stress of the ultra-large diameter direct-buried hot water pipeline using a local-first-global analysis method and verify its feasibility;

[0046] This paper combines the characteristics of overhead pipelines and buried pipelines to design ultra-large diameter (DN ≥ 1800mm) direct buried hot water pipelines. First, the stress conditions of direct buried hot water pipelines are analyzed. Compared with overhead pipelines, direct buried hot water pipelines are subject to not only the internal pressure and thermal stress of the pipeline medium, but also the external pressure from the soil (including traffic load). If we follow the conventional direct buried hot water pipeline model, such as Figure 2 As shown, the outer protective pipe is a polyethylene outer protective pipe 6 that does not bear external pressure. The inner pipe bears all internal and external pressures. Based on the design parameters of the case where the DN of the working inner pipe 1 is 2020mm (Example 1), the wall thickness of the working inner pipe 1 must reach 32mm. This wall thickness exceeds the production capacity of the welded steel pipe factory. Such a large wall thickness will also generate a large thrust on the connecting equipment during stress calculation. This shows that the conventional polyethylene outer protective pipe 6 direct-buried hot water pipeline model can no longer meet the requirements of direct-buried hot water pipelines with such large diameters (DN ≥ 1800mm).

[0047] In response to the above problems, the inventors of this application adopted a local-first-overall analysis method, that is, first conduct a local independent analysis of the pipeline stress, and then conduct an overall analysis of the pipeline model, and creatively proposed a super-large diameter (DN≥1800mm) direct-buried hot water pipeline model. The pipeline stress is analyzed locally and independently, and the working inner pipe 1 bears the internal pressure of the working medium, without considering the effect of the external soil pressure on the working inner pipe 1. Taking the DN of the working inner pipe 1 as 2020mm as an example (Example 1), after calculation, the wall thickness of the working inner pipe 1 is 24mm, which is much smaller than the 32mm wall thickness of the traditional pipeline model, and can be produced by general pipeline factories. The steel outer protective pipe 4 bears the external soil pressure (including traffic load), without considering the effect of the internal pressure of the working medium on it. Based on the pipeline burial depth and traffic load in Example 1, the wall thickness of the steel outer protective pipe 4 is calculated to be 19mm, which is also within the permitted range. Between the working inner pipe 1 and the steel outer protective pipe 4, a certain thickness of hard polyurethane is used as an insulation layer, which also bears the support and waterproofing function of the working inner pipe. Since the working inner pipe in the ultra-large diameter direct-buried hot water pipeline model weighs approximately 4236 kg / m when filled with hot water (Example 1), this places high demands on the compressive strength of the insulation layer. Rigid polyurethane, a stable and high-compressive insulation material, has a positive correlation between density and compressive strength. According to standard CJ / T114, when rigid polyurethane material is deformed by 10%, its compressive stress should be no less than 0.3 MPa. Assuming an insulation layer thickness of 100 mm, this meets the support requirements for the working inner pipe 1.

[0048] The ultra-large diameter directly buried hot water pipeline model of the present invention is also provided with an inner anti-corrosion layer 2 and an outer anti-corrosion layer 5 for protecting the working inner pipe 1 and the steel outer protective pipe 4, and has an anti-corrosion effect.

[0049] The five components of an ultra-large diameter direct-buried hot water pipeline—the working inner pipe 1, the inner anti-corrosion layer 2, the polyurethane insulation layer 3, the steel outer protective pipe 4, and the outer anti-corrosion layer 5—were analyzed as a whole to ensure safe and stable operation within the designed parameters. This requires the pipeline model of the present invention to overcome internal axial and tangential shear forces. Based on the design parameters in Example 1, the axial force is approximately 2563 kN; the tangential shear force is approximately 1262 kN. These two values ​​serve as calibration values ​​for the pipeline model of the present invention to ensure pipeline safety and stability.

[0050] Step 3: Build a model of an ultra-large diameter direct-buried hot water pipeline.

[0051] The ultra-large diameter (DN≥1800mm) directly buried hot water pipeline model designed by this invention has been successfully applied to actual projects. It has a novel design and strong innovation. It has created a precedent for ultra-large diameter directly buried hot water pipeline models with DN≥1800mm at home and abroad, and has been verified by engineering applications. It is safe and reliable, and has important significance for pipeline construction and development.

[0052] For new construction projects, the values ​​and specifications of each part of the super-large diameter direct-buried hot water pipe model can be determined according to the design parameters of different projects.

[0053] The calculation process of Example 1 is as follows:

[0054] (1) According to the formula:

[0055]

[0056] Where D i is the inner diameter of the pipe, in mm; G is the mass flow rate of the medium, in t / h; υ is the specific volume of the medium, in m 3 / kg, take 0.001; ω is the medium flow rate, the unit is m / s, select 1.5m / s~3m / s according to the specification;

[0057] Substitute the data in Example 1 into the formula to obtain D i =1955mm;

[0058] According to the above analysis method, the working inner pipe of the ultra-large diameter direct-buried hot water pipeline of the present invention only considers the internal pressure of the working medium. Referring to the pipe wall thickness formula in DL / T5054, the minimum wall thickness of the pipeline can be calculated as follows:

[0059]

[0060] Where S m is the minimum wall thickness of the pipe, in mm; p is the design pressure, in MPa; [σ] tis the allowable stress of the pipe of this material at the design temperature, in MPa, which is 123 MPa; D0 is the outer diameter of the pipe, in mm; Y is the correction coefficient, which is 0.4; η is the correction coefficient of the allowable stress, which is 0.9; C is the additional thickness required for corrosion, wear and mechanical strength, in mm;

[0061] The formula for calculating pipe wall thickness is as follows:

[0062] S c =S m +C1

[0063] Where S c is the calculated wall thickness of the pipe, in mm; C1 is the additional value of the negative deviation of the pipe wall thickness, in mm;

[0064] Substituting the design parameters and coefficients of Example 1 into the formula and appropriately taking certain additional values, the wall thickness of the final working inner tube 1 is obtained to be 24 mm, thereby determining the final outer diameter of the working inner tube 1 to be 2020 mm.

[0065] (2) In the present invention, soil loads (including traffic loads) primarily cause circumferential deformation of the steel outer protective pipe 4, generating hoop stress. Finite element analysis is used to calculate the hoop stresses due to soil and traffic loads, thereby preventing instability and deformation of the circumferential section of the pipe.

[0066] In Example 1, the parameters such as burial depth and traffic load are brought into consideration and the strength is checked based on the two-axis concentrated load.

[0067] The stress calculation caused by ground traffic load is based on the Bousinnesq equation. The two-axle load calculation formula is as follows:

[0068]

[0069] Where: P is the traffic load; h i is the buried depth of pipe top; x i is the distance between the stress calculation point and the vertical centerline of the pipe;

[0070] Substituting the data in the embodiment into the above formula, we can get σ y , and then it is concluded that the wall thickness of the steel outer protective tube 4 is 19 mm, the thickness of the rigid polyurethane insulation layer 3 is 100 mm, and finally the outer diameter of the steel outer protective tube 4 is 2258 mm and the wall thickness is 19 mm.

[0071] (3) According to the stability requirements of the Technical Specifications for Directly Buried Hot Water Pipelines for Urban Heating, the pipe wall thickness values ​​calculated above are substituted into the following local stability and radial stability formulas for verification. The local stability verification formula is as follows:

[0072]

[0073] Where D0 is the outer diameter of the working pipe, in m; δ is the nominal wall thickness of the working pipe, in m; α is the linear expansion coefficient of the steel, in m / (m·℃); E is the elastic modulus of the steel, in MPa; t1 is the maximum working cycle temperature of the pipeline, in ℃; t0 is the calculated installation temperature of the pipeline, in ℃; υ is the Poisson coefficient of the steel, which is taken as 0.3; P d The calculated pressure of the pipeline is in MPa;

[0074] The radial stability check formula is as follows:

[0075]

[0076] ΔX≤0.03D0

[0077] Where ΔX is the maximum radial deformation of the working pipe, in m; W is the total vertical load per unit area on the pipe top, in kPa; D0 is the outer diameter of the working pipe, in m; δ is the nominal wall thickness of the working pipe, in m; r is the average radius of the working pipe, in m; E is the elastic modulus of the steel, in MPa;

[0078] Substituting the data in Example 1 into the calculation of the above company proves that the wall thickness result is qualified.

[0079] In summary, the present invention is very easy to implement, has an accurate theoretical basis and practical engineering application experience, and is suitable for the currently booming direct-buried hot water pipelines for long-distance and large-flow centralized heating.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A design method for an ultra-large diameter directly buried hot water pipeline, characterized by: From the inside to the outside, the working inner pipe (1) bearing the pressure of the pipeline medium, the inner anti-corrosion layer (2) responsible for the anti-corrosion effect of the working inner pipe (1), the polyurethane insulation layer (3) responsible for the heat preservation and load-bearing effect of the working inner pipe (1), the steel outer protective pipe (4) responsible for bearing the external force of soil and traffic load, and the outer anti-corrosion layer (5) responsible for the anti-corrosion effect of the steel outer protective pipe (4) are sequentially integrated into a whole; the DN of the working inner pipe (1) is ≥1800mm; The design methodology includes the following steps: Step 1: Based on the development requirements of direct-buried hot water pipelines gradually moving towards higher pressures and larger diameters, a model for ultra-large diameter direct-buried hot water pipelines was proposed by studying existing relevant technical standards and analyzing the mechanical properties of overhead and buried pipelines. Step 2: Analyze the stress of the ultra-large diameter direct-buried hot water pipeline using a local-first-global analysis method and verify its feasibility; specifically, the following steps are included: 2.1, Conduct local independent analysis of pipeline stress: 2.1.1 Calculate the wall thickness of the working inner tube (1); The working inner tube (1) bears the internal pressure of the working medium, and the effect of the external soil pressure on the working inner tube (1) is not considered. The wall thickness of the working inner tube (1) is calculated; the calculation process is as follows: According to the formula: Where D i is the inner diameter of the pipe, in mm; G is the mass flow rate of the medium, in t / h; υ is the specific volume of the medium, in m 3 / kg, take 0.001; ω is the medium flow rate, the unit is m / s, select 1.5m / s~3m / s according to the specification; Substitute the data into the formula to obtain D i ; For the working inner pipe of an extra-large diameter direct-buried hot water pipeline, only the internal pressure of the working medium is considered. The minimum wall thickness of the pipeline is calculated using the following formula: Where S m is the minimum wall thickness of the pipe, in mm; p is the design pressure, in MPa; [σ] t is the allowable stress of the pipe of this material at the design temperature, in MPa, which is 123 MPa; D0 is the outer diameter of the pipe, in mm; Y is the correction factor, which is 0.4; η is the correction factor of the allowable stress, which is 0.9; C is the additional thickness required for corrosion, wear and mechanical strength, in mm; The formula for calculating pipe wall thickness is as follows: S c =S m +C1 Where S c is the calculated wall thickness of the pipe, in mm; C1 is the additional value of the negative deviation of the pipe wall thickness, in mm; Substitute the design parameters and coefficients into the formula, take a certain additional value, and calculate the wall thickness of the final working inner tube (1), thereby determining the outer diameter of the final working inner tube (1); 2.1.2 Calculate the wall thickness of the outer steel casing (4); The outer steel pipe (4) bears the external soil pressure including the traffic load, without considering the influence of the internal pressure of the working medium, and the wall thickness of the outer steel pipe (4) is calculated; the calculation process is as follows: Regarding the burial depth and traffic load parameters, when checking the strength, consider the concentrated load on two axes; The stress calculation caused by ground traffic load is based on the Bousinnesq equation. The two-axle load calculation formula is as follows: Where: P is the traffic load; h i is the buried depth of pipe top; x i is the distance between the stress calculation point and the vertical centerline of the pipe; Substituting the data into the above formula, we can get σ y , and then the wall thickness of the steel outer protective tube (4) is obtained, and the thickness of the hard polyurethane insulation layer (3) is added, and finally the outer diameter of the steel outer protective tube (4) is calculated; 2.1.3 Calculate the wall thickness and compressive strength of the polyurethane insulation layer (3); A rigid polyurethane of a certain thickness is used as a thermal insulation layer between the working inner pipe (1) and the steel outer protective pipe (4). The rigid polyurethane also bears the support and waterproofing functions of the working inner pipe (1). The compressive strength of the thermal insulation layer is calculated based on the weight of the working inner pipe (1) after it is filled with hot water in the ultra-large diameter direct-buried hot water pipeline model. In order to ensure the support requirements of the working inner pipe (1), the thickness and compressive stress of the polyurethane thermal insulation layer (3) are determined. 2.1.4, set up anti-corrosion layer; An inner anti-corrosion layer (2) having an anti-corrosion function is provided outside the working inner tube (1) for protecting the working inner tube (1); and an outer anti-corrosion layer (5) having an anti-corrosion function is provided outside the steel outer protective tube (4) for protecting the steel outer protective tube (4); 2.2 Overall analysis of the pipeline model: The five parts of the super-large diameter direct-buried hot water pipeline model: the working inner pipe (1), the inner anti-corrosion layer (2), the polyurethane insulation layer (3), the steel outer protective pipe (4) and the outer anti-corrosion layer (5) are analyzed as a whole to ensure the safe and stable operation of the pipeline under the design parameters; the super-large diameter direct-buried hot water pipeline model is required to overcome the internal axial force and tangential shear force, and the axial force and tangential shear force values ​​are used as the calibration values ​​of the super-large diameter direct-buried hot water pipeline model to ensure the safety and stability of the pipeline; Step 3: Build a model of the ultra-large diameter direct-buried hot water pipeline.

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