Prefabricated directly buried insulated pipes are composed of three layers: working steel pipe, polyurethane insulation layer, and high-density polyethylene outer protective pipe. When leaving the factory, the insulation layer and outer protective pipe at both ends of the pipe will be shorter than the steel pipe, forming a section of exposed working steel pipe, that is, the reserved section at the pipe end. The standard requires that a space of not less than 150mm±10mm should be reserved at the end of the insulation pipe for patching construction. This size is directly related to on-site welding, patching production, and joint waterproofing quality.
The primary purpose of reserving 150mm is to provide operating space for on-site welding. After the insulated pipes are transported to the construction site, the ends of the two adjacent pipes need to be aligned and butt-welded. Welding operations involve welding gun operation, bevel processing, and welding seam forming. If the end of the steel pipe is covered by the insulation layer, there will be no operating space for the welding equipment, and the quality of the welding seam cannot be guaranteed. The reserved length of 150mm ensures that the welding joint can be completely welded and meets the flaw detection requirements.
The reserved section also serves as a patch. After the pipe welding is completed, the insulation layer needs to be remade at the joint, including the installation of outer sheathing pipe or heat shrink tape, on-site pouring of polyurethane foam, etc., all of which need to be completed at the pipe end. If the reserved length is insufficient and the operating space is limited, the sealing quality and thermal insulation effect of the patching joint will be affected. This is also the reason why the standard limits the reserved length to 150mm±10mm - a shorter length will cause inconvenience in construction, and a longer length will increase material costs. 150mm is a reasonable value verified by engineering practice.
The installation of waterproof caps also relies on the cooperation of reserved sections. The standard stipulates that the end face of an insulated pipe with a transmission medium temperature not exceeding 80°C should be fitted with a radiation-crosslinked polyethylene heat-shrinkable waterproof cap. The overlap length between the waterproof cap, the anti-corrosion layer, and the protective layer should not be less than 50mm. If the end of the steel pipe is too short, the waterproof cap cannot form a sufficient overlap length with the outer protective pipe; the sealing effect cannot be guaranteed, and the risk of water intrusion at the pipe end increases.
Different pipe diameters have slightly different requirements for reserved length. The conventional practice is to leave a welding reserve section of 150mm to 250mm at both ends of the steel pipe, and the difference between the reserved lengths at the two ends should not be greater than 40mm. Some standards stipulate that if the nominal diameter of the working steel pipe is greater than or equal to DN200, a 200mm reserved section for welding without an insulation layer should be left at both ends; if the nominal diameter is less than DN200, 150mm should be left. Special-shaped parts such as elbows and tees often have longer reserved lengths due to their complex patching structures. The value of 150mm is not an arbitrary rule, but an engineering experience value based on the comprehensive welding workspace, repairing construction operability, and waterproof cap overlap sealing requirements. This size has been incorporated into a number of national and industry standards and has become one of the basic technical parameters in the production of prefabricated directly buried insulated pipes.