In everyday life, steel insulated pipes—though less common among building materials than household appliances—play an important role across many categories by virtue of the characteristics of steel. The quality of the steel pipe in a project often has varying degrees of impact, so selecting high-quality Jinan insulated pipe can play a positive role in advancing the project. Poor-quality steel pipes usually differ in appearance from high-quality ones, and there are certain inspection methods. First, inward folding of the pipe: some manufacturers pursue high efficiency too aggressively, causing quality deterioration during production; folds formed during rolling of the steel pipe can appear as various creases on the surface, greatly reducing steel strength. Another issue is pitting on insulated pipes: grooves in manufactured steel pipes can suffer severe wear over long use, resulting in an uneven pipe surface. If the steel used in manufacturing contains impurities, the material is more inhomogeneous; and if processing machinery is too crude, a certain amount of impurities is introduced during pipe processing, which easily causes scars on the appearance of insulated steel pipes.
Prefabricated Insulated Pipe
Quality identification of PU insulated direct-buried steel pipes for heating shall follow the criteria below: 1. Quality standards: Density testing: Take samples 72 hours after the insulation has cured. Sampling points shall be more than 500 mm longitudinally from the end face of the insulation layer, and at least three foam samples shall be taken. Each sample shall be a cube of side T (20≤T≤30, unit mm) or a cylinder of diameter D of 30 mm (20≤D≤30, unit mm). After preparation, place the samples in a desiccator at 23±2°C for 16 hours, then measure dimensions with a micrometer to 0.1 mm and calculate volume to 0.01 cm3. Weigh the samples, determine density to 0.5% (by volume), and report the average to 0.1 kg/m3. 2. Axial eccentricity: At any position on the insulated pipe, the offset between the axes of the outer jacket and the steel pipe shall be less than 8 mm. 3. The carrier pipe of the PU insulated direct-buried pipe shall be free of cracks and end damage, and the outer jacket of the insulated pipe shall be undamaged. 4. Insulation thickness shall be measured at no fewer than 4 points, with deviation not exceeding 16 mm. In pipe-material selection, design standards shall be followed strictly and manufacturers of proven product quality shall be chosen. Because the internal pressure of PU insulated direct-buried pipe is relatively low, membrane stress caused by internal pressure is only 50% of the allowable value, so direct burst failure is generally unlikely. The more probable failure mode is plastic fatigue caused by thermal stress. Therefore, fatigue resistance of the material must be considered comprehensively when selecting pipe materials. Steels that are readily weldable and have good plasticity should be used. Grade 10 and Grade 20 steels are more commonly used in actual construction. The above are the quality identification standards for PU insulated direct-buried steel pipe. Welding quality must also be assured and shall meet the following criteria: 1. Pipe-end defects shall be repaired by welding beforehand; weld appearance and dimensions shall comply with drawings and process documents; weld reinforcement shall not be lower than the base-metal surface, and there shall be a smooth transition between the weld and the base metal. 2. Surfaces of the weld and the heat-affected zone of welded pipe shall be free of cracks, incomplete fusion, incomplete penetration, slag inclusions, arc craters, porosity, and similar defects. 3. Branch openings shall not be cut on pipe welds or their edges. After groove preparation by thermal processing, scale, slag, and surface impurities that would affect weld quality shall be removed from the groove surface, and uneven areas shall be ground smooth. 4. Before welding, inspect and clean dirt, sand, and other debris from the pipe bore, and promptly seal unfinished pipe ends during the work. 5. When connecting mains and branches in concurrent construction, avoid connecting a branch to the main with a downward bend; when installing PU insulated direct-buried pipe, there shall be no vertical “U”-shaped settlement sections, and the distance between supply and return pipes shall be not less than 15 cm. 6. When cutting pipe, the exposed insulation length shall not exceed 15 cm, and the insulation at the joint shall be tightly sealed. The foam shall be sound and firmly bonded. 7. When backfilling PU insulated direct-buried pipe trenches, protect the finished product. Backfill shall be fine-grained and free of construction debris. The above are the relevant methods for quality identification of PU insulated direct-buried steel pipes for heating. Identifying high-quality PU insulated direct-buried steel pipe is the basic insulation work in the early stage of a heating project and is essential to heating quality. At present, district heating pipelines generally use PU insulated direct-buried pipe. Also known as “pipe-in-pipe” produced by the “two-step method,” it has good mechanical properties and thermal insulation performance. It can typically withstand 120°C, and through modification or combination with other insulation materials can withstand 180°C, making it suitable for insulation of various hot- and cold-water pipelines at high and low temperatures. With good insulation performance and low heat loss (only 25% of conventional pipe), it can deliver warmth to thousands of households and make winter as mild as spring.
PU Insulated Pipe Standards
PU insulation layer: protects the outer steel pipe from corrosion and extends steel-pipe service life; outer steel pipe: protects the insulation layer from groundwater attack, supports the carrier pipe, and can bear certain external loads; PU foam layer: keeps the medium temperature and the outer-jacket surface temperature within the normal range; barrier/reflective layer: prevents organic foam from entering the inorganic rigid high-temperature layer and reflects heat from that high-temperature layer. The structure of PU insulated pipe has the following features: 1. Sliding supports fixed to the inner carrier pipe and the inner wall of the outer jacket take up friction. The insulation material moves with the carrier pipe, with no mechanical wear and no pulverization of the insulation. 2. The outer steel pipe has high strength and good sealing performance, providing effective waterproofing and anti-seepage protection. 3. The outer wall of the coated steel pipe undergoes high-quality anti-corrosion treatment, giving the coated steel pipe a service life of more than 20 years. 4. The insulation layer of the carrier pipe uses high-quality insulation materials with good thermal insulation performance. 5. A gap of about 10–20 mm exists between the insulation layer of the carrier pipe and the outer steel pipe, which can provide additional insulation. It is also a clear drainage channel for direct-buried pipelines, allowing the drain line to serve simultaneously for timely drainage and as a signal pipe. Carrier-pipe roller supports of PU insulated steel pipe use a special low-thermal-conductivity material with a coefficient of friction against steel of about 0.1, so frictional resistance during pipeline operation is relatively small.
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