Installation method of polyurethane heat preservation prefabricated direct buried pipe with polyethylene outer protective pipe for nuclear power

Abstract

This application belongs to the field of nuclear power engineering construction technology, and relates to an installation method for prefabricated direct-buried pipelines with polyethylene outer sheaths and polyurethane insulation for nuclear power plants. The method includes trench excavation and welding pit preparation, pipeline hoisting and positioning, pipeline assembly, pipeline welding, post-weld inspection, pressure testing, insulation joint repair, and trench backfilling. By excavating a welding pit directly below the predetermined butt weld position of two adjacent prefabricated insulated pipes, a welding operation space accessible to personnel is created below the pipe, improving welding accessibility and weld quality. Furthermore, by overlapping the outer sheath with a polyethylene hot-melt sleeve, hot-melt welding, injection of polyurethane foam, and orifice sealing, a continuous sealing and insulation repair layer is formed, improving the waterproofing, corrosion resistance, and insulation reliability of the joint.

Description

Technical Field

[0001] This application belongs to the field of nuclear power engineering construction technology, and relates to the installation method of prefabricated direct-buried pipelines with polyethylene outer protective pipes and polyurethane insulation for nuclear power plants. Background Technology

[0002] In nuclear power plant heating projects, the installation quality of the circulating water pipelines in the heating network directly affects the safety, stability, and long-term service life of the entire heating system. These pipelines typically use prefabricated direct-buried pipes with polyethylene outer sheaths and polyurethane insulation (hereinafter referred to as "prefabricated insulated pipes"), which have advantages such as good insulation and strong corrosion resistance. However, in the environment of nuclear power plants, where safety and reliability requirements are extremely high, traditional installation methods suffer from the following long-standing technical bottlenecks that have not been effectively resolved: Welding quality is difficult to control: When large-diameter pipelines are laid on the ground, the welding operation space is extremely narrow, making it difficult for welders to operate. This can easily lead to defects such as welding deformation, incomplete penetration, and excessive misalignment of the inner wall, which seriously affect the structural integrity and pressure-bearing capacity of the pipeline.

[0003] Poor sealing of insulation joints: The insulation layer joints at pipe joints are weak points. Traditional methods have problems such as poor sealing and uneven foaming, which may lead to heat loss, water leakage from the outer protective pipe, and corrosion of the working steel pipe, causing the insulation effect to fail.

[0004] Lack of standardized processes: Existing construction methods are mostly based on experience and lack a standardized technical solution that integrates core aspects such as welding workspace construction, welding process control, insulation and sealing. This results in large fluctuations in construction quality and makes it difficult to meet the high reliability requirements of nuclear power plant construction.

[0005] Therefore, there is an urgent need in this field for a standardized construction method that can systematically solve the above problems and ensure high-quality, high-safety, and high-efficiency pipeline installation in the harsh environment of nuclear power plants. Summary of the Invention

[0006] The purpose of this application is to design an installation method for prefabricated direct-buried pipelines with polyethylene outer sheaths and polyurethane insulation for nuclear power plants, solving the technical problems of poor weldability and insufficient joint insulation sealing in the installation of large-diameter prefabricated direct-buried pipelines with polyethylene outer sheaths and polyurethane insulation for nuclear power plants. By constructing an underground welding operation space of specific dimensions and using precise assembly welding and foaming repair processes, an installation method is provided that can ensure weld quality and the integrity of the insulation layer.

[0007] Technical solution to achieve the purpose of this application: This application provides a method for installing prefabricated direct-buried pipelines with polyethylene outer sheaths and polyurethane insulation for nuclear power plants, including the following steps: S1. Trench excavation and welding operation pit reservation: Excavate the pipe trench according to the design location, and excavate the welding operation pit directly below the predetermined butt weld position of the two adjacent prefabricated insulated pipes to be connected, so that after the pipe is installed in place, a working space is formed between the bottom of the pipe and the bottom of the welding operation pit, which can be entered by personnel to carry out welding operations. S2. Pipe hoisting and positioning: Hoist the two prefabricated insulated pipes to be connected into the pipe trench, and adjust the position of the two prefabricated insulated pipes so that the predetermined butt weld position of the two pipe ends is above the welding operation pit. S3. Pipe assembly: Clean the bevels at the ends of the two prefabricated insulated pipes and the area adjacent to the bevels, and adjust the concentricity of the two pipe ends and the misalignment of the inner walls. S4. Pipeline welding: The working steel pipes of the two prefabricated insulated pipes are butt-welded in the welding operation pit, and the interlayer temperature is controlled during the welding process. S5. Post-weld inspection: Non-destructive testing is performed on the butt welds after welding to confirm that the weld quality is qualified. S6. Pressure test: Conduct a strength pressure test on the pipeline system to confirm that there is no leakage at the weld and no abnormal pressure drop; S7. Thermal Insulation Repair Construction: Install a polyethylene hot-melt sleeve on the outside of the weld, so that the polyethylene hot-melt sleeve covers the weld area and overlaps the outer protective pipes of the prefabricated insulation pipes on both sides; perform hot-melt welding on the polyethylene hot-melt sleeve and the outer protective pipes on both sides to form an integrated sealed structure fused with the outer protective pipes; then inject polyurethane foam into the annular cavity enclosed between the polyethylene hot-melt sleeve and the two prefabricated insulation pipes to form a thermal insulation repair layer, and seal the injection hole and vent hole. S8. Trench backfilling: After the inspection and testing are passed, the trench and the area of ​​the welding operation pit shall be backfilled and compacted.

[0008] Optionally, the welding pit is a square pit with a planar dimension of a×a, where a is 1000mm to 1500mm.

[0009] Optionally, the planar dimensions of the welding pit are 1200mm × 1200mm.

[0010] Optionally, in step S1, the depth of the welding pit is set such that after the prefabricated insulation pipe is installed, the height of the bottom of the pipe from the bottom of the welding pit is not less than 800mm.

[0011] Optionally, in step S1, the bottom of the welding pit is covered with an anti-slip mat layer.

[0012] Optionally, in step S3, the oxide scale, oil stains and impurities within a 20mm range of the pipe bevel and its inner and outer surfaces are cleaned until the metallic luster is exposed.

[0013] Optionally, in step S3, an alignment tool is used to adjust the concentricity of the ends of the two pre-insulated pipes, and the misalignment of the inner walls of the two pipe ends is controlled to be no more than 2mm.

[0014] Optionally, in step S4, tungsten inert gas welding is first used for the root pass welding, and then shielded metal arc welding is used for the fill pass welding and the cover pass welding.

[0015] Optionally, in step S4, the interpass temperature is controlled not to exceed 250°C during the welding process.

[0016] Optionally, in step S5, the weld is subjected to 100% radiographic or ultrasonic testing.

[0017] Optionally, in step S6, the strength pressure test is a hydrostatic test, and the test pressure is 1.5 times the design pressure.

[0018] Optionally, the pressure increase rate of the water pressure test shall not exceed 0.3 MPa / min, and the pressure shall be stabilized for 10 minutes after reaching the test pressure.

[0019] Optionally, in step S7, the center of the polyethylene hot melt sleeve coincides with the center of the weld, and the two ends of the polyethylene hot melt sleeve overlap the outer protective pipes of the prefabricated insulation pipes on both sides, with an overlap length of not less than 100mm.

[0020] Optionally, in step S7, after installing the polyethylene hot melt sleeve, the polyethylene hot melt sleeve is initially tightened circumferentially using rust-proof steel strapping.

[0021] The beneficial technical effects of this application are as follows: The installation method of the prefabricated direct-buried pipeline with polyurethane insulation and polyethylene outer sheath provided in this application for nuclear power plants involves using a polyethylene hot-melt sleeve to cover the weld area during insulation joint construction. The two ends of the hot-melt sleeve overlap the outer sheath of the prefabricated insulation pipes on both sides, and then hot-melt welding is used to fuse the hot-melt sleeve and the outer sheath to form an integrated sealed structure. This structure enhances the continuity and sealing of the outer sheath in the interface area, reduces the risk of external moisture seeping in from the interface, and avoids corrosion of the working steel pipe or failure of the insulation layer due to water seepage from the outer sheath, thereby improving the waterproofing, corrosion resistance, and long-term service reliability of the interface. This application injects polyurethane foaming agent into the annular cavity formed between the hot-melt sleeve and the working steel pipe, causing the weld interface area to reform a continuous insulation joint layer. Injection is done through a combination of injection holes and vent holes, and the holes are sealed after foaming. This improves the fullness, uniformity, and integrity of the insulation joint layer, reduces weak insulation areas at the interface, reduces heat loss, and allows the joint area and the prefabricated insulation pipe body to form a continuous insulation system. Detailed Implementation

[0022] To enable those skilled in the art to better understand this application, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the embodiments described below are only a part of the embodiments of this application, and not all of them. Based on the embodiments described in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0023] The installation method in this embodiment includes the following steps: S1. Trench excavation and welding operation pit reservation.

[0024] Pipe trenches are excavated according to the pipeline laying location, and welding pits are excavated directly below the predetermined butt weld positions of the two adjacent prefabricated insulated pipes to be connected. The welding pits are located below the butt weld positions of the two prefabricated insulated pipes to be connected, so that after the two prefabricated insulated pipes are installed in place, a working space is formed between the bottom of the pipes and the bottom of the welding pit, which is accessible for personnel to enter and carry out welding operations.

[0025] In one specific embodiment, the welding pit is a square pit with a planar dimension of a×a, where a is 1000mm to 1500mm. Preferably, the planar dimension of the welding pit is 1200mm×1200mm. The depth of the welding pit is set such that, after the prefabricated insulated pipe is installed, the bottom of the pipe is at least 800mm above the bottom of the pit, to meet the space requirements for construction personnel to perform operations such as beveling, pipe assembly, welding, and interlayer temperature control after entering the pit. Furthermore, an anti-slip mat can be laid at the bottom of the welding pit to improve the safety of workers during construction inside the pit.

[0026] S2, Pipeline hoisting and positioning.

[0027] Two prefabricated insulated pipes to be connected are hoisted into the pipe trench, and their positions are adjusted so that the predetermined butt weld seams at both ends are located above the welding pit. During hoisting, a truck crane with specialized lifting slings can be used to avoid damaging the polyethylene outer protective pipe and polyurethane insulation layer of the prefabricated insulated pipes.

[0028] In one specific embodiment, the prefabricated insulated pipe is a DN700 prefabricated insulated pipe, with a single pipe length of 12m and a single pipe weight of approximately 2.6t. A 50t truck crane, along with specialized lifting slings, is used to lift it into the pipe trench. After the pipe is in place, the positions of the two pipe ends are adjusted to ensure they are essentially coaxial, and the center of the weld seam aligns with the center of the welding pit.

[0029] S3, Pipe assembly.

[0030] Construction workers entered the welding pit to clean the bevels at the ends of the two prefabricated insulated pipes and the area adjacent to the bevels, and adjusted the concentricity of the two pipe ends and the misalignment of the inner walls.

[0031] Specifically, an angle grinder can be used to clean the pipe bevel and the inner and outer surfaces within a 20mm range, removing oxide scale, oil, impurities, etc., until a metallic luster is exposed. After cleaning, an alignment tool is used to adjust the concentricity of the two prefabricated insulated pipe ends, and the misalignment of the inner walls of the two pipe ends is measured. Preferably, the misalignment of the inner walls of the two pipe ends is controlled to be no more than 2mm, thereby reducing welding defects caused by excessive misalignment and improving the quality of the welded joint.

[0032] S4. Pipe welding.

[0033] The working steel pipes of two prefabricated insulation pipes are butt-welded in the welding pit, and the temperature between welding layers is controlled during the welding process.

[0034] In one specific embodiment, tungsten inert gas (TIG) welding is first used for the root pass to ensure root penetration and avoid root concavity. After the root pass is completed, shielded metal arc welding (SMAW) is used for the fill pass and cover pass. During the welding process, the interpass temperature is monitored and controlled, preferably not exceeding 250°C, to avoid overheating of the welding area, which could affect the weld toughness and overall joint performance.

[0035] Because this embodiment sets up a welding pit directly below the predetermined butt weld position, construction personnel can perform welding operations in the working space formed below the pipe bottom, thereby improving the problems of insufficient welding space and limited welding posture in traditional trenches, which is conducive to improving the weld formation quality.

[0036] S5. Post-weld inspection.

[0037] After welding is completed, wait for the welded joint to cool down to ambient temperature, then perform non-destructive testing on the buttweld after welding to confirm that the weld quality is qualified.

[0038] In a specific embodiment, 100% radiographic testing or ultrasonic testing is performed on the weld. Only after the testing results meet the requirements of the corresponding nuclear power engineering construction quality acceptance standards or equivalent standards can the subsequent pressure test and heat preservation joint coating construction steps be carried out.

[0039] S6, Pressure test.

[0040] Perform a strength pressure test on the pipeline system to confirm that there is no leakage at the weld and no abnormal pressure drop.

[0041] In a specific embodiment, the strength pressure test is a hydraulic test, and the test pressure is 1.5 times the design pressure. For example, when the design pressure is 2.5 MPa, the test pressure is 3.75 MPa. A stepped slow pressure rise method is adopted during the pressure rising process, and the pressure rising rate is controlled to be no more than 0.3 MPa / min. After reaching the test pressure, hold the pressure for 10 min, observe whether there is leakage at the weld, and observe whether there is a pressure drop on the pressure gauge; if there is no leakage at the weld and no abnormal pressure drop, the pressure test is determined to be qualified.

[0042] S7, Heat preservation joint coating construction.

[0043] After welding and testing are qualified, perform heat preservation joint coating construction on the outer side of the weld. First, remove spatter, impurities and moisture from the weld and the joint coating area, install the polyethylene heat-melting sleeve at the joint, make the polyethylene heat-melting sleeve cover the weld area, and lap it on the outer protective pipes of the prefabricated heat preservation pipes on both sides respectively.

[0044] In a specific embodiment, when installing the polyethylene heat-melting sleeve, make the center of the polyethylene heat-melting sleeve coincide with the center of the weld, and the two ends of the polyethylene heat-melting sleeve lap on the polyethylene outer protective pipes of the prefabricated heat preservation pipes on both sides respectively, with the overlapping length not less than 100 mm. After the installation is completed, use a rust-proof steel strapping to perform preliminary circumferential fastening on the polyethylene heat-melting sleeve, so as to keep the polyethylene heat-melting sleeve and the outer protective pipes on both sides closely attached.

[0045] Subsequently, connect an electrofusion welding machine, and heat-weld the polyethylene heat-melting sleeve according to the set voltage and time parameters. Preferably, before hot-melt welding, use strapping or equivalent pressure fastening measures to apply pressure and fasten on both sides of the outer edge of the polyethylene heat-melting sleeve, so as to improve the fitting stability between the heat-melting sleeve and the outer protective pipes. After the hot-melt welding is completed, the polyethylene heat-melting sleeve is fused with the outer protective pipes of the prefabricated heat preservation pipes on both sides, forming an integrated sealing structure.

[0046] After the hot-melt welding is completed, an injection hole and an vent hole are opened at the top of the polyethylene hot-melt sleeve. Component A and component B polyurethane foaming agent are mixed at a 1:1 volume ratio at the mixing head and quickly injected through the injection hole into the annular cavity formed between the polyethylene hot-melt sleeve and the two prefabricated insulation pipes. During the injection process, the discharge from the vent hole is observed; when foam overflows from the vent hole, the vent hole is sealed, and injection continues until overflow occurs at the injection hole, after which the injection hole is sealed. This allows the polyurethane foaming agent to foam within the annular cavity and form an insulation patch layer.

[0047] Furthermore, to prevent the expansion force generated by the reaction and expansion of the polyurethane foam agent from bursting or lifting the polyethylene hot-melt sleeve, uniform pressure is applied to the outside of the polyethylene hot-melt sleeve. Specifically, uniformly distributed weights can be placed outside the polyethylene hot-melt sleeve to apply pressure, and the packing straps and weights are removed after the foam has cured. Preferably, the foam curing time is 3 to 5 minutes. Finally, the injection hole and vent hole are sealed with an electrofusion sheet or a hot-melt welding gun to ensure the sealing and waterproofing of the repair area.

[0048] Through the above-mentioned insulation and patching construction, the polyethylene hot melt sleeve and the outer protective pipes on both sides form a continuous integrated sealing structure, and the polyurethane foaming agent forms a continuous insulation and patching layer in the interface area, thereby giving the weld interface area good waterproof, anti-corrosion and heat insulation performance.

[0049] S8. Trench backfilling.

[0050] After the weld inspection, pressure test and insulation repair are all qualified, the trench and welding pit area are backfilled and compacted.

[0051] In one specific implementation, medium-coarse or fine sand is preferred as the backfill material, and the backfilling is carried out in layers and by water compaction or mechanical compaction. During the backfilling process, the area of ​​the welding pit is compacted in particular to ensure that there is no gap under the pipeline, restore the bearing capacity of the foundation, and avoid uneven stress on the pipeline or settlement due to inadequate backfilling in the later stage.

[0052] Through the above steps, this embodiment can provide a usable working space for welding during the installation of prefabricated direct-buried pipelines with polyethylene outer protective pipes and polyurethane insulation, thereby improving welding working conditions. At the same time, through hot melt sleeve sealing, polyurethane injection foaming, and orifice sealing treatment, the sealing performance and insulation integrity of the joint repair area are improved, thereby improving the pipeline installation quality and long-term operational reliability.

[0053] In other embodiments, the planar dimensions of the welding pit, the specifications of the hoisting equipment, the welding process parameters, the curing time of the foaming agent, and the pressure test parameters can be adaptively adjusted according to the pipe specifications, engineering design requirements, and construction standards. However, they should still meet the technical requirements of forming a welding operation space below the weld position that is available for personnel to operate, and forming a continuous sealing and continuous heat insulation repair layer on the outside of the weld.

[0054] The present application has been described in detail above with reference to the embodiments. However, the present application is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present application. All content not described in detail in this application can be derived from existing technology.

Claims

1. An installation method for prefabricated direct-buried pipelines with polyethylene outer sheaths and polyurethane insulation for nuclear power plants, characterized in that: Includes the following steps: S1. Trench excavation and welding operation pit reservation: Excavate the pipe trench according to the design location, and excavate the welding operation pit directly below the predetermined butt weld position of the two adjacent prefabricated insulated pipes to be connected, so that after the pipe is installed in place, a working space is formed between the bottom of the pipe and the bottom of the welding operation pit, which can be entered by personnel to carry out welding operations. S2. Pipe hoisting and positioning: Hoist the two prefabricated insulated pipes to be connected into the pipe trench, and adjust the position of the two prefabricated insulated pipes so that the predetermined butt weld position of the two pipe ends is above the welding operation pit. S3. Pipe assembly: Clean the bevels at the ends of the two prefabricated insulated pipes and the area adjacent to the bevels, and adjust the concentricity of the two pipe ends and the misalignment of the inner walls. S4. Pipeline welding: The working steel pipes of the two prefabricated insulated pipes are butt-welded in the welding operation pit, and the interlayer temperature is controlled during the welding process. S5. Post-weld inspection: Non-destructive testing is performed on the butt welds after welding to confirm that the weld quality is qualified. S6. Pressure test: Conduct a strength pressure test on the pipeline system to confirm that there is no leakage at the weld and no abnormal pressure drop; S7. Thermal Insulation Repair Construction: Install a polyethylene hot-melt sleeve on the outside of the weld, so that the polyethylene hot-melt sleeve covers the weld area and overlaps the outer protective pipes of the prefabricated insulation pipes on both sides; perform hot-melt welding on the polyethylene hot-melt sleeve and the outer protective pipes on both sides to form an integrated sealed structure fused with the outer protective pipes; then inject polyurethane foam into the annular cavity enclosed between the polyethylene hot-melt sleeve and the two prefabricated insulation pipes to form a thermal insulation repair layer, and seal the injection hole and vent hole. S8. Trench backfilling: After the inspection and testing are passed, the trench and the area of ​​the welding operation pit shall be backfilled and compacted.

2. The installation method of the prefabricated direct-buried pipeline with polyethylene outer sheath and polyurethane insulation for nuclear power plants according to claim 1, characterized in that, The welding pit is a square pit with a planar dimension of a×a, where a is 1000mm to 1500mm.

3. The installation method of the prefabricated direct-buried pipeline with polyethylene outer protective pipe and polyurethane insulation for nuclear power plants according to claim 2, characterized in that, The planar dimensions of the welding pit are 1200mm × 1200mm.

4. The installation method of the prefabricated direct-buried pipeline with polyethylene outer protective pipe and polyurethane insulation for nuclear power plants according to claim 1, characterized in that, In step S1, the depth of the welding pit is set such that after the prefabricated insulation pipe is installed, the height of the bottom of the pipe from the bottom of the welding pit is not less than 800mm.

5. The installation method of the prefabricated direct-buried pipeline with polyethylene outer sheath and polyurethane insulation for nuclear power plants according to claim 1, characterized in that, In step S1, the bottom of the welding pit is covered with an anti-slip mat layer.

6. The installation method of the prefabricated direct-buried pipeline with polyethylene outer sheath and polyurethane insulation for nuclear power plants according to claim 1, characterized in that, In step S3, the oxide scale, oil stains and impurities within a 20mm range of the pipe bevel and its inner and outer surfaces are cleaned until the metallic luster is exposed.

7. The installation method of the prefabricated direct-buried pipeline with polyethylene outer sheath and polyurethane insulation for nuclear power plants according to claim 1, characterized in that, In step S3, an alignment tool is used to adjust the concentricity of the ends of the two pre-insulated pipes and to control the misalignment of the inner walls of the two pipe ends to be no more than 2mm.

8. The installation method of the prefabricated direct-buried pipeline with polyethylene outer sheath and polyurethane insulation for nuclear power plants according to claim 1, characterized in that, In step S4, tungsten inert gas welding is first used for the root pass welding, and then shielded metal arc welding is used for the fill pass welding and the cover pass welding.

9. The installation method of the prefabricated direct-buried pipeline with polyethylene outer sheath and polyurethane insulation for nuclear power plants according to claim 1 or 8, characterized in that, In step S4, the interpass temperature is controlled to not exceed 250°C during the welding process.

10. The installation method of the prefabricated direct-buried pipeline with polyethylene outer sheath and polyurethane insulation for nuclear power plants according to claim 1, characterized in that, In step S5, the weld is subjected to 100% radiographic or ultrasonic testing.

11. The installation method of the prefabricated direct-buried pipeline with polyethylene outer protective pipe and polyurethane insulation for nuclear power plants according to claim 1, characterized in that, In step S6, the strength pressure test is a hydrostatic test, and the test pressure is 1.5 times the design pressure.

12. The installation method of the prefabricated direct-buried pipeline with polyethylene outer sheath and polyurethane insulation for nuclear power plants according to claim 11, characterized in that, The pressure increase rate of the water pressure test shall not exceed 0.3 MPa / min, and the pressure shall be stabilized for 10 minutes after reaching the test pressure.

13. The installation method of the prefabricated direct-buried pipeline with polyethylene outer sheath and polyurethane insulation for nuclear power plants according to claim 1, characterized in that, In step S7, the center of the polyethylene hot melt sleeve coincides with the center of the weld, and the two ends of the polyethylene hot melt sleeve overlap the outer protective pipes of the prefabricated insulation pipes on both sides, with an overlap length of not less than 100mm.

14. The installation method of the prefabricated direct-buried pipeline with polyethylene outer sheath and polyurethane insulation for nuclear power plants according to claim 1 or 13, characterized in that, In step S7, after installing the polyethylene hot melt sleeve, the polyethylene hot melt sleeve is initially tightened circumferentially using rust-proof steel strapping.