Polyurethane Prefabricated Directly Buried Insulated Steel Pipe

Product Introduction

Polyurethane prefabricated directly buried insulated steel pipe, commonly known as black jacket insulated pipe, is a factory-prefabricated three-in-one composite pipe: inner working steel pipe + middle rigid polyurethane foam insulation layer + outer high-density polyethylene (HDPE) jacket pipe. After complete prefabrication, it is directly buried underground without on-site insulation work, making it a mainstream product for urban central heating and central air conditioning chilled/hot water pipe networks.

• Structural Function
  1. Working Steel Pipe: Conveys hot water media; seamless, longitudinal welded, or spiral welded steel pipes are available; internal anti-corrosion coating and external rust removal treatment can be applied.
  2. Polyurethane Insulation Layer: Rigid foam layer that provides thermal insulation, prevents heat loss, and buffers soil pressure.
  3. HDPE Jacket Pipe (Black Jacket): Black high-density polyethylene that is waterproof, resistant to soil corrosion and external forces, protecting the internal insulation layer from groundwater and mud intrusion.
• Product Advantages: Low heat loss with outstanding energy-saving performance; waterproof and moisture-proof with low water absorption due to closed-cell structure; direct burial installation for fast construction and minimal land occupation; design service life ≥30 years; leak detection wires can be pre-embedded for pipe network leakage monitoring.
• Applicable Media: High-temperature hot water, central air conditioning supply/return water, industrial circulating hot water; not applicable to steam (steam requires steel-in-steel insulated pipes).
• Application Fields: Urban central heating pipe networks, residential district heating, industrial plant heating, central air conditioning underground pipe networks, industrial hot water transmission.
• Implementation Standards
  • GB/T 29047‑2021 "Prefabricated directly buried insulating pipes and fittings with polyurethane foam and high-density polyethylene jacket pipe" (pipe-in-pipe foaming, current national standard)
  • GB/T 34611‑2017 "Prefabricated directly buried insulating pipes with rigid polyurethane spray and polyethylene winding" (large-diameter spray-winding process)
• Applicable Temperature: Long-term continuous operating temperature ≤120°C, short-term peak temperature ≤130°C
• Diameter Range: DN20‑DN1600; standard single lengths of 6m and 12m; bare pipe ends reserved at 150‑200mm for on-site welding followed by polyurethane field joint insulation.

Supplementary distinction: For steam transmission, steel-in-steel steam insulated pipes are used; polyurethane directly buried pipes are not suitable because polyurethane cannot withstand steam operating temperatures.

Core Technical Parameters of Three-Layer Structure (GB/T 29047‑2021)

1. Working Steel Pipe Layer

• Pipe material: DN150 and below mostly use seamless steel pipe GB/T 8163; DN200 and above use spiral welded pipe GB/T 9711, SY/T 5037; material grades Q235B, Q355B
• Surface preparation grade: Sa2 or above, removing oil, dirt, and rust to ensure firm bonding with polyurethane.
• Optional configurations: 3PE anti-corrosion coating on the outer wall of the steel pipe; epoxy powder anti-corrosion coating on the inner wall; pre-embedded leak detection wires.

2. Rigid Polyurethane Foam Insulation Layer (Core Insulation)

Table

Insulation layer thickness is determined by thermal design; commonly 30‑80mm, with thickened insulation for high-temperature operating conditions.

3. HDPE High-Density Polyethylene Jacket Pipe (Black Jacket)

Table

3. Selection and Construction Key Points

1. Two Production Processes
   • Pipe-in-pipe foaming: Steel pipe is inserted into the PE jacket pipe, and foam is injected into the annular gap; suitable for small to medium diameters DN20‑DN600.
   • Spray-winding process: Steel pipe is rotated while polyurethane foam is sprayed on, and PE is wound externally; suitable for large diameters DN500‑DN1600, providing more uniform foaming.
2. After welding of the bare pipe ends is completed, on-site foaming for field joints or heat-shrink sleeve + foaming is used to ensure overall waterproof sealing.
3. In buried environments, avoid direct compression of the jacket pipe by sharp stones; backfill with fine soil and prohibit direct impact from large crushed stones.
4. Working pressure: conventional PN1.0‑PN1.6 MPa, determined by the steel pipe wall thickness.

4. Simple Distinction from 3PE Anti-Corrosion Steel Pipe

• 3PE anti-corrosion steel pipe: Only external anti-corrosion coating, no insulation, used for gas and water transmission with ambient-temperature media.
• Polyurethane prefabricated directly buried insulated steel pipe: Equipped with a polyurethane insulation layer, used for hot water heating, combining insulation + external waterproof protection.