Welding and preheating construction method for non-compensated pre-buried large-diameter heating pipeline

Abstract

The invention discloses a welding and preheating construction method for a non-compensated pre-buried large-diameter heating pipeline, which solves the problem of difficult operation of weld joint welding and pre-compensation shrinkage, realizes economical and high-quality construction of the non-compensated pre-buried pipeline, and belongs to the technical field of centralized heating. The diameter of the pre-buried heating steel pipe is 1.2 meters, the length of a single pipe is 12 meters, and the wall thickness of the steel pipe is 14 millimeters. The method comprises the following steps: excavating a pre-buried pipeline trench, digging the trench according to the designed elevation and slope, flattening and compacting the trench bottom according to the design and specification requirements, and laying a fine sand cushion layer; demagnetizing the butt welding of two steel pipes, spot welding the two butt ports together, and adjusting the central axes of the two butt steel pipes so that the two central axes completely overlap; the welding of the steel pipe weld adopts a single-sided welding and double-sided forming welding method; the welded weld is inspected by X-ray flaw detection; and the method has a guiding role in the economical and high-quality construction of the non-compensated pre-buried pipeline.

Description

Technical Field

[0001] The invention relates to a construction method for an underground heating pipeline in a chemical plant, belongs to the technical field of heating, and particularly relates to a welding and preheating construction method for a pre-buried large-diameter heating pipeline without a compensation device in the pipeline. Background Art

[0002] The hot water produced in the power workshop of a chemical plant is transported to other workshops through pipelines to meet the needs of the product production process. In order to save plant space and beautify the plant environment, the hot water delivery pipeline needs to be buried deep underground. The direct buried heating pipeline is laid in a non-compensated manner, that is, no shrinkage joints are set in the pipeline. The pipeline without compensation shrinkage is hundreds of meters long, and the connections between each section of the steel pipe are all welded. There are problems such as difficulty in welding the welds and difficulty in pre-compensation shrinkage operation. Summary of the invention

[0003] The invention provides a welding and preheating construction method for non-compensated pre-buried large-diameter heating pipelines, which solves the problems of difficult operations of weld joint welding and pre-compensation shrinkage, and realizes economical and high-quality construction of non-compensated pre-buried pipelines.

[0004] The present invention solves the above technical problems through the following technical solutions:

[0005] A welding and preheating construction method for a non-compensated pre-buried large-diameter heating pipeline, the pre-buried heating steel pipe has a diameter of 1200 mm, a single pipe length of 12 meters, and a wall thickness of 14 mm. The specific steps are as follows:

[0006] The first step is to dig the trench for pre-buried pipelines. The trench should be dug according to the designed elevation and slope. The trench bottom should be leveled and compacted according to the design and specification requirements, and a fine sand cushion layer should be laid. A welding working groove should be set at the butt welding point of the two pipelines in the trench. The depth of the welding working groove is 40 cm downward from the bottom of the pre-laid pipeline in the trench, and the width of the welding working groove is 2 meters.

[0007] The second step is to hoist the two butt-jointed steel pipes into the pre-buried pipeline trench, and set the butt joint of the two steel pipes just above the welding working groove; cover the steel pipes on both sides of the butt joint with tarpaulin, and keep the ends of the butt joint steel pipes in a dry state;

[0008] The third step is to demagnetize the butt welding of the two steel pipes. The specific steps are as follows: a flexible welding wire with a cross-section of 35-50 square millimeters is wound around the ends of the butt steel pipes to form a common demagnetization coil for the two butt steel pipes; the two ends of the common demagnetization coil are connected to the rectifier of the DC arc welding machine, so that the direction of the magnetic field formed by the common demagnetization coil is set opposite to the direction of the residual magnetic field of the steel pipe, and the number of turns of the common demagnetization coil is 16-22 turns; at the beginning of demagnetization, the current in the common demagnetization coil is controlled at 80-100 amperes. In order to smoothly reduce the magnetic flux, the current in the common demagnetization coil is gradually reduced within one minute until the current in the common demagnetization coil returns to zero, and the power supply is cut off;

[0009] Step 4: spot weld the two butt joints together and adjust the central axes of the two butt joint steel pipes so that the two central axes completely overlap;

[0010] Step 5. The welding of the steel pipe weld adopts a single-sided welding and double-sided forming welding method: the pipe docking port is processed into a V-shaped groove, the joint gap at the butt joint of the two pipes is 2-3 mm, the joint blunt edge is 1-1.5 mm, and the joint slope angle is 60-65 degrees; first use argon arc welding, the number of welding layers is three, and the welding current is 160-200 amperes; then use arc welding for three times, and polish it in time after each welding before the next welding; the weld surface formed should be complete and smoothly transition with the pipe base material, the welding undercut depth should be less than 0.5 mm, and the undercut length of each weld should not be greater than 10% of the total length of the weld; the weld surface reinforcement height should not be greater than 30% of the pipe wall thickness, and less than or equal to 5 mm; the weld should be welded 2-3 mm outside the groove edge;

[0011] Step 6: Use X-ray flaw detection on the completed welds. Ultrasonic flaw detection should be used for areas that are not suitable for X-ray flaw detection.

[0012] When the length of the welded pre-buried large-diameter heating pipeline reaches the designed construction section length, the pipeline section is electrically preheated to release the thermal expansion deformation caused by the pipeline being heated in advance; a large DC preheater is used for preheating, and the two ends of the large DC preheater are connected to the two ends of the pipeline section. The preheating operation steps are as follows:

[0013] Step 7: Set the preheating target temperature value t of the pipeline according to the following formula: m ,

[0014] t m =0.5×(t1+t2);

[0015] Where: t1 is the maximum design temperature of the pipeline, t2 is the minimum temperature of the pipeline working cycle, for pipelines that only operate in the heating season, t2 is 10℃, and for pipelines that operate all year round, t2 is 30℃;

[0016] Step 8. Calculate the preheating elongation △L of the pipeline section according to the following formula:

[0017] △L=α×(t m— t1) × L pr ;

[0018] Where: L pr is the length of the preheating pipe section, α is the average linear expansion coefficient of the preheating steel pipe;

[0019] Step 9: Install a length detection device at both ends of the pipeline to measure the elongation of the pipeline; seal both ends of the pipeline with plastic film to keep the temperature inside the pipeline stable;

[0020] Step 10: When the preheating temperature of the pipeline reaches the preheating target temperature value t m If the preheating elongation of the pipeline section has not reached the preheating elongation △L calculated in the eighth step, the preheating temperature will be increased by 5 degrees. Then, the elongation of the pipeline section is measured again. If the preheating elongation of the pipeline section has not reached the preheating elongation △L calculated in the eighth step, the preheating temperature will be increased by another 5 degrees. This cycle is repeated until the preheating elongation of the pipeline section reaches the preheating elongation △L calculated in the eighth step.

[0021] The steps for conducting a water pressure test on this section of pipeline are as follows: the strength test pressure in the water pressure test is 1.5 times the design pressure of the pipeline, and the tightness test pressure is 1.25 times the design pressure of the pipeline; during the water pressure test, first increase the pressure in the pipe to 1.5 times the design pressure, then stabilize the pressure for 10 minutes. If there is no pressure drop in the pipeline, reduce the pressure in the pipe to 1.25 times the design pressure. If the pressure drop in the pipe does not exceed 0.05 MPa within 60 minutes, the pipeline is qualified.

[0022] Weld reinforcement welding should be carried out at the elbow welds of pipeline laying.

[0023] If the designed length of the construction section is less than or equal to 500 meters, the trench backfilling of this section of the pipeline shall be completed within 10 hours after the preheating elongation of this section of the pipeline reaches the preheating elongation △L calculated in the eighth step; if the designed length of the construction section is 500-800 meters, the trench backfilling of this section of the pipeline shall be completed within 14 hours after the preheating elongation of this section of the pipeline reaches the preheating elongation △L calculated in the eighth step; if the designed length of the construction section is 800-1000 meters, the trench backfilling of this section of the pipeline shall be completed within 18 hours after the preheating elongation of this section of the pipeline reaches the preheating elongation △L calculated in the eighth step.

[0024] The pipeline of the present invention has low cost, explores a construction method for a large-diameter deep-buried heating pipeline without a contraction joint, and explores specific parameters for pipeline preheating and welding through experiments, which has a guiding role in the economical and high-quality construction of uncompensated pre-buried pipelines. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a structural schematic diagram of the welding groove at the butt joint of two pipes of the present invention. DETAILED DESCRIPTION

[0026] The present invention is described in detail below in conjunction with the accompanying drawings:

[0027] A welding and preheating construction method for a non-compensated pre-buried large-diameter heating pipeline, the pre-buried heating steel pipe has a diameter of 1200 mm, a single pipe length of 12 meters, and a wall thickness of 14 mm. The specific steps are as follows:

[0028] The first step is to dig the trench for pre-buried pipelines. The trench should be dug according to the designed elevation and slope. The trench bottom should be leveled and compacted according to the design and specification requirements, and a fine sand cushion layer should be laid. A welding working groove should be set at the butt welding point of the two pipelines in the trench. The depth of the welding working groove is 40 cm downward from the bottom of the pre-laid pipeline in the trench, and the width of the welding working groove is 2 meters.

[0029] The second step is to hoist the two butted steel pipes into the pre-buried pipeline trench, and set the butt joint of the two steel pipes just above the welding working groove; to prevent the welding slag splashed during welding from burning the insulation pipe, cover the steel pipes on both sides of the butt joint with tarpaulin, and keep the ends of the butt joint steel pipes in a dry state, make waterproof protection, and strictly prohibit the insulation layer from getting wet;

[0030] Steel pipes are all magnetic when leaving the factory. When welding magnetic steel pipes, there are problems such as arc ignition difficulty, poor arc burning stability, arc deviation from the electrode axis, and splashing of liquid metal and slag from the welding pool. In severe cases, the welding pool cannot be established, making welding impossible. In order to stabilize the welding process and improve the quality of the welded joint, the magnetized steel pipes must be demagnetized before welding. The butt welding of the two steel pipes must be demagnetized. The specific steps are as follows:

[0031] The third step is to wind a flexible welding wire with a cross-section of 35-50 square millimeters around the ends of the butt-jointed steel pipes to form a common degaussing coil for the two butt-jointed steel pipes; connect the two ends of the common degaussing coil to the rectifier of the DC arc welding machine, so that the direction of the magnetic field formed by the common degaussing coil is set opposite to the direction of the residual magnetic field of the steel pipe, and the number of turns of the common degaussing coil is 16-22 turns; at the beginning of degaussing, the current in the common degaussing coil is controlled at 80-100 amperes. In order to smoothly reduce the magnetic flux, the current in the common degaussing coil is gradually reduced within one minute until the current in the common degaussing coil returns to zero, and then the power is cut off;

[0032] Step 4: spot weld the two butt joints together and adjust the central axes of the two butt joint steel pipes so that the two central axes completely overlap;

[0033] Step 5. The welding of the steel pipe weld adopts a single-sided welding and double-sided forming welding method: the pipe butt port is processed into a V-shaped groove, the pipe wall thickness δ is 14 mm, the joint gap C at the butt of the two pipes is 2-3 mm, the joint blunt edge P is 1-1.5 mm, and the joint slope angle α is 60-65 degrees; first use argon arc welding, the number of welding layers is three, and the welding current is 160-200 amperes; then use arc welding for three times, and polish it in time after each welding before the next welding; the weld surface formed should be complete and smoothly transition with the pipe base material, the welding undercut depth should be less than 0.5 mm, and the undercut length of each weld should not be greater than 10% of the total length of the weld; the weld surface reinforcement height should not be greater than 30% of the pipe wall thickness and less than or equal to 5 mm; the weld should be welded 2-3 mm outside the groove edge;

[0034] Step 6: Use X-ray flaw detection on the completed welds. Ultrasonic flaw detection should be used for areas that are not suitable for X-ray flaw detection.

[0035] Pipeline preheating generally adopts electric preheating method, which means that before the overall backfill of the pipeline, the thermal expansion deformation caused by the pipeline is released in advance, thereby reducing the axial stress of the pipeline, reducing the displacement of the sliding section of the pipeline, and improving the safety and stability of the heating network operation; electric preheating technology is to pass appropriate current through the steel pipe, using the principle of the steel pipe's own resistance heating, so that a certain length of steel pipe can be evenly and stably heated; when the length of the welded pre-buried large-diameter heating pipeline reaches the designed construction section length, the section of the pipeline is electrically preheated to release the thermal expansion deformation caused by the pipeline after heating in advance; preheating uses a large DC preheater, and the two ends of the large DC preheater are connected to the two ends of the pipeline section. The specific steps are as follows:

[0036] Step 7: Set the preheating target temperature value t of the pipeline according to the following formula: m ,

[0037] t m =0.5×(t1+t2);

[0038] Where: t1 is the maximum design temperature of the pipeline, t2 is the minimum temperature of the pipeline working cycle, for pipelines that only operate in the heating season, t2 is 10℃, and for pipelines that operate all year round, t2 is 30℃;

[0039] Step 8. Calculate the preheating elongation △L of the pipeline section according to the following formula:

[0040] △L=α×(t m— t1) × Lpr ;

[0041] Where: L pr is the length of the preheating pipe section, α is the average linear expansion coefficient of the preheating steel pipe;

[0042] Step 9: Install a length detection device at both ends of the pipeline to measure the elongation of the pipeline; seal both ends of the pipeline with plastic film to keep the temperature inside the pipeline stable;

[0043] Step 10: When the preheating temperature of the pipeline reaches the preheating target temperature value t m If the preheating elongation of the pipeline section has not reached the preheating elongation △L calculated in the eighth step, the preheating temperature will be increased by 5 degrees. Then, the elongation of the pipeline section is measured again. If the preheating elongation of the pipeline section has not reached the preheating elongation △L calculated in the eighth step, the preheating temperature will be increased by another 5 degrees. This cycle is repeated until the preheating elongation of the pipeline section reaches the preheating elongation △L calculated in the eighth step.

[0044] A water pressure test is carried out on this section of pipeline, which is characterized by the following steps: the strength test pressure in the water pressure test is 1.5 times the design pressure of the pipeline, and the tightness test pressure is 1.25 times the design pressure of the pipeline; during the water pressure test, the pressure in the pipe is first increased to 1.5 times the design pressure, and then the pressure is stabilized for 10 minutes. If there is no pressure drop in the pipeline, the pressure in the pipe is reduced to 1.25 times the design pressure. If the pressure drop in the pipe does not exceed 0.05 MPa within 60 minutes, the pipeline is qualified.

[0045] Weld reinforcement welding should be carried out at the elbow welds of pipeline laying.

[0046] If the designed length of the construction section is less than or equal to 500 meters, the trench backfilling of this section of the pipeline shall be completed within 10 hours after the preheating elongation of this section of the pipeline reaches the preheating elongation △L calculated in the eighth step; if the designed length of the construction section is 500-800 meters, the trench backfilling of this section of the pipeline shall be completed within 14 hours after the preheating elongation of this section of the pipeline reaches the preheating elongation △L calculated in the eighth step; if the designed length of the construction section is 800-1000 meters, the trench backfilling of this section of the pipeline shall be completed within 18 hours after the preheating elongation of this section of the pipeline reaches the preheating elongation △L calculated in the eighth step.

Claims

1. A welding and preheating construction method for a non-compensated pre-buried large-diameter heating pipeline, the pre-buried heating steel pipe has a diameter of 1200 mm, a single pipe length of 12000 mm, and a wall thickness of 14 mm. Its characteristics include the following steps: The first step is to dig the trench for pre-buried pipelines. The trench should be dug according to the designed elevation and slope. The bottom of the trench should be leveled and compacted according to the design and specification requirements, and a fine sand cushion layer should be laid. A welding working groove should be set at the butt welding point of the two pipelines in the trench. The depth of the welding working groove is 400 mm downward from the bottom of the pre-laid pipeline in the trench, and the width of the welding working groove is 2000 mm. The second step is to hoist the two butt-jointed steel pipes into the pre-buried pipeline trench, and set the butt joint of the two steel pipes just above the welding working groove; cover the steel pipes on both sides of the butt joint with tarpaulin, and keep the ends of the butt joint steel pipes in a dry state; The third step is to demagnetize the butt welding of the two steel pipes. The specific steps are as follows: a flexible welding wire with a cross-section of 35-50 square millimeters is wound around the ends of the butt steel pipes to form a common demagnetization coil for the two butt steel pipes; the two ends of the common demagnetization coil are connected to the rectifier of the DC arc welding machine, so that the direction of the magnetic field formed by the common demagnetization coil is set opposite to the direction of the residual magnetic field of the steel pipe, and the number of turns of the common demagnetization coil is 16-22 turns; at the beginning of demagnetization, the current in the common demagnetization coil is controlled at 80-100 amperes. In order to smoothly reduce the magnetic flux, the current in the common demagnetization coil is gradually reduced within one minute until the current in the common demagnetization coil returns to zero, and the power supply is cut off; Step 4: spot weld the two butt joints together and adjust the central axes of the two butt joint steel pipes so that the two central axes completely overlap; Step 5. The welding of the steel pipe weld adopts a single-sided welding and double-sided forming welding method: the pipe butt port is processed into a V-shaped groove, the joint gap C at the butt of the two pipes is 2-3 mm, the joint blunt edge P is 1-1.5 mm, and the joint slope angle α is 60-65 degrees; first use argon arc welding, the number of welding layers is three, and the welding current is 160-200 amperes; then use arc welding for three times, and polish it in time after each welding before the next welding; the weld surface formed should be complete and smoothly transition with the pipe base material, the welding undercut depth should be less than 0.5 mm, and the undercut length of each weld should not be greater than 10% of the total length of the weld; the weld surface reinforcement height should not be greater than 30% of the pipe wall thickness and less than or equal to 5 mm; the weld should be welded 2-3 mm outside the groove edge; Step 6: X-ray flaw detection is used for the welds after welding. Ultrasonic flaw detection should be used for the parts that are not suitable for X-ray flaw detection. When the length of the welded pre-buried large-diameter heating pipeline reaches the designed construction section length, the pipeline section is electrically preheated to release the thermal expansion deformation caused by the pipeline being heated in advance; the electric preheating uses a large DC preheater, and the two ends of the large DC preheater are connected to the two ends of the pipeline section; the preheating operation steps are as follows: Step 7: Set the preheating target temperature value t of the pipeline according to the following formula: m , t m =0.5×(t1+t2); Where: t1 is the maximum design temperature of the pipeline, t2 is the minimum temperature of the pipeline working cycle, for pipelines that only operate in the heating season, t2 is 10℃, and for pipelines that operate all year round, t2 is 30℃; Step 8. Calculate the preheating elongation △L of the pipeline section according to the following formula: △L=α×(t m— t1)×L pr ; Where: L pr is the length of the preheating pipe section, α is the average linear expansion coefficient of the preheating steel pipe; Step 9: Install a length detection device at both ends of the pipeline to measure the elongation of the pipeline; seal both ends of the pipeline with plastic film to keep the temperature inside the pipeline stable; Step 10: When the preheating temperature of the pipeline reaches the preheating target temperature value t m If the preheating elongation of the pipeline section has not reached the preheating elongation △L calculated in the eighth step, the preheating temperature will be increased by 5 degrees. Then, the elongation of the pipeline section is measured again. If the preheating elongation of the pipeline section has not reached the preheating elongation △L calculated in the eighth step, the preheating temperature will be increased by another 5 degrees. This cycle is repeated until the preheating elongation of the pipeline section reaches the preheating elongation △L calculated in the eighth step.

2. According to claim 1, a welding and preheating construction method for non-compensated pre-buried large-diameter heating pipeline is characterized in that: A water pressure test is carried out on this section of pipeline. The strength test pressure in the water pressure test is 1.5 times the design pressure of the pipeline, and the tightness test pressure is 1.25 times the design pressure of the pipeline. During the water pressure test, the pressure in the pipe is first increased to 1.5 times the design pressure and then maintained at this pressure for 10 minutes. If there is no pressure drop in the pipeline, the pressure in the pipe is reduced to 1.25 times the design pressure. If the pressure drop in the pipe does not exceed 0.05 MPa within 60 minutes, the pipeline is qualified.

3. The welding and preheating construction method of a non-compensated pre-buried large-diameter heating pipeline according to claim 1 is characterized in that: Weld reinforcement welding should be carried out at the elbow welds of pipeline laying.

4. The welding and preheating construction method of a non-compensated pre-buried large-diameter heating pipeline according to claim 1 is characterized in that: If the designed length of the construction section is less than or equal to 500 meters, the trench backfilling of this section of the pipeline shall be completed within 10 hours after the preheating elongation of this section of the pipeline reaches the preheating elongation △L calculated in the eighth step; if the designed length of the construction section is 500-800 meters, the trench backfilling of this section of the pipeline shall be completed within 14 hours after the preheating elongation of this section of the pipeline reaches the preheating elongation △L calculated in the eighth step; if the designed length of the construction section is 800-1000 meters, the trench backfilling of this section of the pipeline shall be completed within 18 hours after the preheating elongation of this section of the pipeline reaches the preheating elongation △L calculated in the eighth step.

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