Direct Burial Insulation Pipe Manufacturing Process:

I. Construction Process Flow

II. Main Process Construction Methods

Trench Excavation

1.1 Survey and Staking

Prior to excavation, a total station is used for positioning and line setting to determine the excavation boundary line. Steel piles or marking lines are employed for positioning, noting the excavation depth and mileage, and guiding the construction process.

1.1.1 The laying of heating pipeline involves setting a central pile every 20 meters. After the pipeline alignment is leveled, a longitudinal section drawing of the pipeline should be drawn. The pipeline measurement work should have a formal measurement record book, with careful and detailed records. Illustrative diagrams should be attached when necessary. The names of the participants in the measurement, including the date of measurement, work location, content of work, and records and points should be recorded. The measurement records should be properly kept by a designated person, readily available for review, and serve as original documents for the completion of the project.

Construction Measurement Precautions

The temporary benchmark points established should be mutually checked and adjusted to prevent errors.

(2) Construction control stakes should be set at appropriate positions along the pipeline centerline and turning points, and these stakes must be properly protected.

During measurements, the instruments should be inspected and adjusted, and the original records should be thoroughly verified.

1.2 Removal of Old Pavement Structure: First, the old pavement within the trench area is removed. Prior to construction, the trench excavation boundary is marked, and concrete saws are used to cut grooves along the boundary. Asphalt concrete blocks are manually removed using pry bars, while small excavators can be used for sections more conducive to machinery operation, with manual assistance. The crushed asphalt concrete is transported to the construction unit's location using dump trucks or small agricultural vehicles.

1.3 Excavation of 1.3 Grooves

1.3.1 According to the on-site soil conditions, during the excavation process, the soil type should be measured. In areas with slope conditions, steep slopes without support should be graded according to specifications. Sections where the trench depth exceeds 5 meters should be excavated in layers. Click here to download free construction technical materials.

The soil heap at the edge of the trench must not exceed 1.5 meters in height and must be at least 0.8 meters away from the edge of the trench opening.

The bottom of the trench must be composed of natural soil layers and the sublayer should be treated according to the drawings.

1.3.4 Excavation of grooves is conducted using mechanical excavation methods. The operation surface is appropriately combined based on the number of machines and personnel, with segmented excavation. Where conditions permit, excavation is carried out in an order from shallow to deep.

Every 1.3.5 meters, clear the bottom layer and level it simultaneously, and drive horizontal control stakes on the slope.

During trench excavation on March 16th, leave a 15-20cm strip at the bottom of the trench undisturbed. Before proceeding with subsequent stages, level and excavate the area, and for any disrupted natural soil layers, first remove loose soil. Then, backfill with sand or other soil types meeting design specifications to the designated elevation and compact it.

On March 7th, when encountering waterlogging, drainage or manual pumping should ensure water is removed prior to the start of the next process.

Prior to laying the pipeline on 1.3.8, proper sizing must be conducted according to regulations. The trench bottom should be cleared to the designed elevation, and a working pit must be excavated. The interface working pit should be manually dug, ensuring that the working pit meets the interface construction process requirements.

After the trench excavation reaches the designed elevation, the trench must be carefully inspected. In case of any special engineering geological phenomena, the construction unit and supervision unit should be notified promptly to determine the treatment plan.

1.4 Temporary Bridge

At the excavation site where the trench crosses the road, a temporary bridge needs to be constructed. Based on the actual excavation conditions on site, steel plates are laid to serve as a temporary passage. Guardrails are set up on both sides of the bridge using steel scaffolding to ensure the safety of personnel and vehicle traffic.

2. Pipeline Foundation Treatment

Grooves have passed inspection and are ready for pipeline foundation construction. The pipeline foundation must be strictly constructed according to the design and specifications, with strict control over the compaction of the natural soil, lime soil, and coarse sand cushion layers. The fill material on both sides of the pipeline triangle must be compacted.

2.1 The pipeline foundation is compacted with native soil, topped with a 50cm thick 3:7 lime-sand cushion layer (with a compaction coefficient not less than 0.96). Before backfilling with 3:7 lime-sand, it must be mixed evenly, ensuring proper moisture content and loose thickness, to guarantee that each layer meets the required compaction level. After the installation of the thermal pipeline, hydrostatic testing, and joint insulation are qualified, lay a 200mm thick layer of medium sand. Click here to download free construction technical materials.

3. Pipeline Installation

3.1 Pipeline Installation Process Flowchart

      3.2 Tube material acceptance and welding rod procurement

The heating pipeline used in this project is a factory-precast, buried-insulated pipe, with the insulation layer made of polyisocyanurate foam plastic and the protective layer made of high-density polyethylene. The steel pipes are of type Q235 spiral-welded钢管, with dimensions of DN800×10mm. The buyer must verify that the pipe material's type, specification, model, quantity, and quality grade match the design requirements before picking up the materials. A complete and accurate factory certificate of conformity and anti-corrosion certificate must be provided before the goods can be collected.

The welding rods used in pipe welding are E4303 (J422), which should also comply with the following specifications: the chemical composition of the welding rods should match the mechanical strength of the base material, considering both working conditions and processability; the quality of the welding rods should meet the current national standard for "Carbon Steel Welding Rods"; the welding rods should be dried. The procurement and use of welding rods must be approved by the owner and supervising engineer before entering the site.

3.3 Transportation: Insulation pipes and fittings are suspended using a flat-bottomed hoist, ensuring gentle lifting and lowering by cranes. During transportation, they must be stabilized and securely tied down, without any collision with each other.

3.4 Storage: The storage area for insulated pipes and fittings must be flat, firm, and accessible. They must be placed securely and neatly, with no more than three layers high, and clearly marked.

Preparation Before 3.5 Pipeline Installation

(1) Clean off burrs from the pipe ends and remove any debris inside the pipes to reduce the flushing volume. Inspect if the bevels meet the welding process requirements and the quality of cleaning on both sides of the bevels. The bevel surface must not have any defects such as laminations, cracks, machining damage, burrs, or slag from flame cutting. The steel pipe welding bevels for this construction project are V-shaped, and the pipe bevels are pre-set according to the design specifications.

      (2) Welders for thermal pipeline welding must be certified individuals who have completed boiler and pressure vessel training and passed the examination. Strict checks must be conducted on the credentials held by personnel; unlicensed or expired certificate holders are prohibited from welding.

      (3) Since pipe joints are welded on the ground, when excavating the operation pit, make it as large as possible to ensure that the distance from the bottom of the pipe to the bottom of the work pit is greater than 350mm, facilitating operations and inspections.

      (4) Inspect whether the installation equipment is complete and in good working condition.

      (5) The welding rods should be pre-heated before use, with a temperature range of 150 to 200°C, and a holding time of 1.5 hours.

3.6 Welded Steel Pipe

      Before pipeline installation, each pipe section should be inspected and numbered. Quality checks of the pipe sections must be conducted before lowering them into place, and only upon passing the inspection may they be lowered.

      Installation should be carried out from low to high, with the installation direction aligning with the excavation direction. Prior to installation, the location of the manhole must be determined first, followed by the positioning of the first pipe section. Manual labor should be combined with mechanical installation, with the pipes placed according to the design specifications before installation, ensuring ease of lifting and transportation. When lowering pipe sections into the trench, avoid collision with supports and pipes beneath the trench. The transportation of pipes within the trench should not disturb the natural foundation.

      The lower pipe was hoisted into the trench using a crane to secure the position of the first pipe. Blocks were placed according to the elevation and centerline. A gate frame was set up above the pipe to determine the centerline. The centerline rope was hung on the gate frame, then controlled with a line cone to lower it to the centerline. After the pipe was hoisted into place, the elevation and centerline were adjusted, and the pipe was temporarily secured to prevent displacement of the installed pipe. Once the position and dimensions were verified, the pipe was positioned and welded.

      Before pipeline welding, each welder must perform a trial weld and only upon passing the inspection can formal welding commence. The welding is done using manual arc welding, with all welds being single-sided and double-sided shaped. During welding, pre-welding of the welding heads is conducted, with each weld joint requiring three temporary weld points, ensuring each point is secure. The welder must maintain a uniform and slow arc movement, with the arc termination position being accurate. Each weld joint is formed in multiple welding stages, with the root of the weld being sealed. The first layer of weld should be evenly焊透 without burning through. The joints of each layer should be staggered, with the thickness of each layer's weld being 0.8 to 1.2 times the diameter of the welding rod, and arcs should not be struck on the non-welding surface of the workpiece. After shaping, the weld height and width should be consistent, with the weld pattern being uniform, free from defects such as incomplete welding, bubbles, weld spatter, slag inclusions, and undercutting.

The bend radius to the interface must not be less than the pipe diameter, nor less than 100mm.Welding sections should first be repaired and cleaned at the ends. The bevel angle of the pipe end face, the burr, and the gap should comply with the specifications of the design documents. No shims should be inserted or excessive force used to reduce the gap before welding.When aligning, ensure the inner walls are flush. It is advisable to use a straight ruler measuring 300mm and place it in an orderly manner around the outer wall of the joint. The allowable deviation for misalignment should be 5% of the wall thickness but not exceed 2mm. To prevent molten slag from welding from burning the insulation pipe, cover the 500mm sections of the insulation pipe on both sides of the work point with a tarpaulin soaked in water or rubber sheet.When welding pipes, it is necessary to prevent through drafts at the welding site. The ends of the steel pipe at the welding location should be heated to approximately 15℃ within a 20mm range when the temperature of the welding piece is below 0℃ before welding can proceed.

3.6.2 Welding Intermediate Inspection

After the pipe weld seam is positioned and welded, slag should be removed for inspection. Defects found must be removed before welding can continue; for weld seams that have been interrupted due to circumstances, they should be cleaned and inspected before resuming welding, and any defects found must be eliminated before proceeding.

Post-weld Inspection

After welding is completed, the slag and spatter should be immediately removed from the weld seam, and the surface of the weld should be cleaned thoroughly before conducting an appearance inspection of the weld. Pipe weld joint inspections are conducted in accordance with the specifications of (CJJ28-2004).

When individual pipeline welds do not meet the conditions for hydrostatic testing, they must undergo 100% non-destructive testing inspection.

Pipe welds undergoing hydrostatic testing are subjected to non-destructive testing at a rate of thirty percent.

4. Water Pressure Test

      After pipeline construction is completed, a section-wise hydrostatic test of the pipeline should be conducted, with the test pressure being 1.5 times the working pressure (2.4 MPa). The hydrostatic test is carried out in the following steps:

      On April 3rd, the water points of the test pressure pipeline sections will be sealed, and water will be slowly injected while simultaneously expelling the gas inside the pipeline.

      After filling the pipeline system with water, a water-tightness check is conducted.

      Increase the system pressure to 4.5, and it is recommended to use an electric pressure pump to slowly increase the pressure, with the pressure increase time not to be less than 10 minutes.

      After the test pressure reaches the specified test value, stop increasing pressure, stabilize the pressure for 10 minutes, and then inspect the interface area. There should be no leaks in the pipeline welds, and there should be no warping or deformation in the pipe sections.

      In the event of leakage or a pressure drop exceeding the specified value during the 4.7 test, the cause should be investigated and promptly rectified, and the test should be retried until it passes.

      After the 4.8-inch steel pipe weld joint passes the inspection, the weld seam is cleaned and then coated for corrosion and heat preservation.

      4.9 Pipeline Flush

      Heating pipeline flushing should be conducted prior to trial operation. When releasing water, open the outlet valve first, then the inlet valve, and simultaneously ensure proper venting. Close both the inlet and outlet valves when the water quality appears clear. Allow the water within the pipes to stand for 24 hours, then take water samples for testing. If the requirements are not met, reflush the pipeline.

      All welds in the heating pipeline network are inspected after passing the strength test and hydrostatic test of the pipeline, and following successful trial operation.

      5. Manhole Construction

      5.1 Casing Foundation Construction

      The construction is carried out based on the dimensions specified in the drawings and standard annotations for the well chamber. Excavation of the well chamber is done in one go with the combination of manual labor and an excavator. In case of groundwater, a collection pit is used for water collection and a pump is used for water extraction.

      Well Foundation Treatment: After excavation, the original soil is compacted, and a 500mm thick 3:7 lime-soil cushion layer is laid to the bottom of the slab cushion, with a compaction factor of not less than 0.96. The treatment range extends 500mm beyond the edge of the slab.

      Casting plain concrete into the manhole sump.

      5.2 Casing Masonry

      During the construction of the well room, after cleaning the foundation with water, apply a layer of mortar first, followed by bricklaying. Ensure full coverage and tight packing, with a 1cm grout gap between bricks.

      (2) The outer surface of the well casing should be coated with 1:2 waterproof mortar (with 5% waterproofing agent) at a thickness of 20mm (up to the top). Iron climbing ladders should be installed simultaneously during the masonry of the well chamber, with accurate placement. No stepping should be allowed until the strength of the masonry mortar and concrete reaches the specified level. The iron climbing ladders and embedded parts inside the well should be rust-proofed with red lead primer and two coats of gray anti-rust paint.

      (3) After the well chamber is built or installed to the specified elevation, the well casing should be promptly cast or installed, and the well cover should be sealed.

      When pouring the manhole cover of the inspection well, use a temporary cover to seal the water-filled pit to prevent concrete from falling in.

      (5) Waterproof sleeve is installed at the pipeline crossing the well wall to prevent leakage.

      (6) When placing the lid, use a 1:2 cement砂浆mortar to set it and fill the gaps between the panels with a 1:2 cement砂浆mortar.

      (7) Asphalt corrosion treatment should be carried out after the plastering layer has dried. Before corrosion treatment, clean the plastering surface thoroughly. The asphalt corrosion treatment should be applied using a method of pouring and scraping, ensuring even application without any penetration through the base or missed spots.

      6. Excavation Backfill Points - Download Free Construction Technical Materials Here

      6.1 Construction Preparation:

      Prior to backfilling, the base and underground waterproofing layers should be inspected and accepted, and the hidden inspection procedures should be completed. Only after the project supervisor's inspection and acceptance is deemed合格 may earthwork backfilling proceed.

      Prior to backfilling, all debris such as garbage on the foundation pit bottom or ground level should be thoroughly cleaned, ensuring it is cleared down to the foundation's bottom elevation. Any loose debris, mortar, gravel, etc., that falls back should also be completely removed.

      (3) Prior to construction, the moisture content control range for fill material, as well as parameters such as the spread thickness and compaction passes, should be reasonably determined based on the project characteristics, fill material type, design compaction coefficient, and construction conditions. For critical backfilling projects, the backfill parameters should be determined through compaction tests.

      Before construction, a horizontal marker should be established to control the height or thickness of backfill soil. For instance, on the slopes of foundation pits or trench ditches, a horizontal stake should be nailed every 3 meters.

      6.2 Operation Process:

      (1) Inspect the quality of backfilled soil for debris, ensure particle size meets specifications, and verify that moisture content is within the controlled range. For excessive moisture, implement measures such as loosening, drying, or uniformly mixing in dry soil. If the moisture content is low, pre-watering to moisten the soil is recommended.

      (2) Backfill soil should be laid in layers, with the thickness of each layer determined by the soil type, compaction requirements, and equipment performance. Generally, for frog-type compactors, the thickness of soil layer should be 200-250mm; for manual compaction, it should not exceed 200mm. After each layer is laid, it should be smoothed out. Each layer of backfill soil should be compacted at least three times, with each compaction pressing half of the previous one, ensuring continuous lines and crosswise intersections.

      During backfilling and compaction, reliable protection measures should be taken for any conduits, pipelines, etc., to prevent damage to them.