1. Inward folding of the steel pipe. Some manufacturers over-pursue yield and apply excessive reduction during production, causing folds during rolling. Various fold lines form on the surface and greatly reduce steel strength.

　　2. Pitted surface. When the roll grooves used to make steel pipe become severely worn after prolonged use, the manufactured pipe surface will be uneven.

　　3. Surface scabs. If the steel used to make the pipe contains many impurities, the material is inhomogeneous, and the processing machinery is too crude, impurities may be bitten into the rolls, causing scabs.

　　4. Irregular trademarks and printing on steel pipe. Quality pipe of uniform material does not show irregularities when printed, whereas pipe of uneven material easily shows skewed printing.

　　5. Inner-diameter fluctuation. When the steel is inhomogeneous, rolling temperature is unstable, or mill spring is large due to crude equipment, the inner diameter of the steel pipe can fluctuate significantly, causing uneven stress and making the pipe prone to fracture.

　　PU insulated pipe not only offers advanced technology and practical performance that traditional trench and overhead laying can hardly match, but also significant social and economic benefits, and is a powerful measure for heating energy conservation. The adoption of direct-buried heating-pipeline technology with prefabricated direct-buried insulated pipe marks a new starting point in the development of China’s heating-pipeline technology. As world energy resources dwindle and demand grows, energy conservation, emissions reduction, and environmental protection have become global development trends. The state and local governments also vigorously advocate the development, application, and industrialization of energy-saving, emissions-reducing, and environmentally friendly products. Rigid PU foam insulation materials for the construction industry are an important branch of the PU industry, characterized by multi-functionality, providing both thermal insulation and waterproofing. Such products have a history of 40 years since their application in the European construction industry in the 1960s; some countries have even legislated to designate PU as the specified insulation and waterproofing material for construction.

　　Foremost among PU insulated pipelines is steel-in-steel insulated steel pipe. It can be direct-buried underground without a concrete structure: thermal expansion of the carrier pipe takes place inside the outer pipe, thereby reducing material cost, shortening the construction period, and safeguarding the heating pipeline. It can be applied more safely across different temperature conditions and is especially suitable for high-temperature steam pipeline projects. Service temperature can reach 150°C–450°C. Steel-in-steel insulated pipe is composed of an outer steel jacket with anti-corrosion treatment, a PU foam insulation layer, and an inner carrier pipe. Steel-in-steel steam composite insulated pipe is suitable for conveying steam or other media at up to 2.5 MPa and 350°C. The product uses steel pipe as the outer protective layer and offers high strength, resistance to damage, convenient construction and maintenance, and long service life. Steel-in-steel insulated pipe structures are classified by sliding mode into internally sliding and externally sliding types. 1. Internally sliding type: The insulation structure consists of the carrier pipe, aluminum silicate, a friction-reducing layer, microporous calcium silicate, a thermal barrier, stainless-steel fastening bands, an aluminum-foil reflective layer, a PU insulation layer, an outer steel jacket, and an outer anti-corrosion coating. 2. Externally sliding type: The insulation structure consists of the carrier pipe, a glass-wool thermal insulation layer, an aluminum-foil reflective layer, stainless-steel fastening bands, sliding guide supports, an air insulation layer, an outer steel jacket, and an outer anti-corrosion coating.

　　Because PU direct-buried insulated pipe has lasting corrosion and aging resistance, it is well received in service and there is a large market demand, and therefore a large production volume is required. Production and processing of PU insulated pipe must be handled according to the specific use and structure. For insulation between an inner pipe and an outer jacket, PU foam insulated pipe is usually produced by pouring foam slurry directly between the two pipes for one-shot foaming.

　　Before pouring the slurry, the inner pipe and outer casing shall be kept in good concentricity using appropriate fixtures, and the lower seal shall be tight to prevent slurry leakage from the PU foam insulated pipe. Depending on the flow state of the slurry and the foaming rate, shorter insulation layers may be poured vertically, but for most longer insulation layers, steel-in-steel steam insulated pipe is more often poured in an inclined position to facilitate slurry flow and foaming. According to the formulation and construction requirements, injection may be from the upper side or from the bottom. For Tianjin prefabricated direct-buried insulation, when the pipe fitting is relatively long, slurry output from the mixing head may be introduced through a conduit to the lower part of the annular gap, and the conduit is gradually withdrawn as the injected volume increases. This method applies only to making insulation layers with ordinary casings of 3 mm or less.