Layer 1: carrier-pipe layer. According to the design and the customer's requirements, seamless pipe (GB8163-87), spiral-welded pipe (GB9711-88; SY/T5038-92), or straight-seam welded pipe (GB3092-93) is generally selected. After the steel-pipe surface is treated by an advanced shot-blasting rust-removal process, the rust-removal grade can reach Sa2 in GB8923-1988, and surface roughness can reach R=12.5 μm in GB6060.5-88.

　　Second layer: PU insulation layer. Rigid PU foam stock solution is injected in one shot by a high-pressure foaming machine into the cavity formed between the steel pipe and the outer jacket—commonly known as the "pipe-in-pipe foaming process." Its functions are: first, waterproofing; second, thermal insulation; and third, supporting the dead weight of the heating network. When the conveyed medium temperature is -50°C–120°C, rigid polyisocyanurate foam is used as the insulation layer.

　　Layer 3: HDPE protective jacket. Prefabricated black (or yellow) plastic pipe of a specified wall thickness, serving first to protect the PU insulation from mechanical damage by hard objects, and second for anti-corrosion and waterproofing.