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Practical Knowledge Related to Pipes I. Classification of Pipes 1. Classification by Production Method (1) Seamless pipes—hot-rolled pipes, cold-rolled pipes, cold-drawn pipes, extruded pipes, jacked pipes (2) Welded pipes (a) Classified by process: arc-welded pipes, resistance-welded pipes (high-frequency, low-frequency), gas-welded pipes, furnace-welded pipes (b) Classified by weld type: straight-seam welded pipes, spiral-welded pipes 2. Classification by Cross-Section Shape (1) Pipes with simple cross-sections—circular pipes, square pipes, oval pipes, triangular pipes, hexagonal pipes, rhombus-shaped pipes, octagonal pipes, semi-circular pipes, others (2) Pipes with complex cross-sections—irregular hexagonal pipes, five-petal plum blossom-shaped pipes, double-convex pipes, double-concave pipes, melon-seed-shaped pipes, conical pipes, corrugated pipes, watch-case-shaped pipes, others 3. Classification by Wall Thickness—thin-walled pipes, thick-walled pipes 4. Classification by Purpose—pipes for pipelines, pipes for thermal equipment, pipes for the machinery industry, pipes for oil and geological drilling, pipes for containers, pipes for the chemical industry, pipes for special purposes, others II. Seamless Steel Pipes Seamless steel pipes are long strips of steel with a hollow cross-section and no seams along their edges. Steel pipes have a hollow cross-section and are widely used as pipelines for transporting fluids, such as oil, natural gas, gas, water, and certain solid materials. Compared to solid steel materials such as steel pipes and round bars, steel tubes have a lower weight while maintaining the same bending and torsional strength; they are therefore an economical type of structural steel. They are widely used in the manufacture of structural components and mechanical parts, such as oil drill pipes, automobile drive shafts, bicycle frames, as well as steel scaffolding used in construction. Using steel pipes to manufacture ring-shaped parts improves material utilization, simplifies the manufacturing process, and saves materials and processing time; components such as rolling bearing rings and jack sleeves are currently manufactured extensively using steel pipes. Steel pipes are also an essential material for various conventional weapons; gun barrels, artillery barrels, and the like are all made from steel pipes. Steel pipes can be classified into round pipes and special-shaped pipes based on the shape of their cross-sectional area. Since a circle has the largest area for a given perimeter, circular tubes can transport more fluid. Furthermore, a circular cross-section distributes stress more evenly when subjected to internal or external radial pressures; therefore, the vast majority of steel pipes are circular. However, round tubes also have certain limitations; for example, under conditions of planar bending, their bending strength is lower than that of square or rectangular tubes. Square and rectangular tubes are therefore commonly used in the frames of agricultural machinery and steel-wood furniture. For different applications, special-shaped steel pipes with other cross-sectional shapes are also required. 1. Structural seamless steel pipes (GB/T8162) are seamless steel pipes used in general structures and mechanical structures. 2. Seamless steel tubes for fluid transportation (GB/T8163) are general seamless steel tubes used for transporting fluids such as water, oil, and gas. 3. Seamless steel tubes for low- and medium-pressure boilers (GB3087) are high-quality carbon structural steel seamless tubes, either hot-rolled or cold-drawn, used for manufacturing superheater tubes, feedwater tubes in various low- and medium-pressure boilers, as well as superheater tubes, large flue tubes, small flue tubes, and arch brick tubes in locomotive boilers. 4. Seamless steel tubes for high-pressure boilers (GB5310) are high-quality carbon steel, alloy steel, and stainless heat-resistant steel seamless tubes used to manufacture the heating surfaces of water-tube boilers operating at high pressures and above. 5. High-pressure seamless steel tubes for fertilizer equipment (GB6479) are high-quality carbon structural steel and alloy steel seamless tubes suitable for use in chemical equipment and pipelines operating at temperatures ranging from -40 to 400°C and under pressures of 10 to 30 Ma. 6. Seamless steel tubes for petroleum cracking (GB9948 applies to seamless steel tubes for furnace tubes, heat exchangers, and pipelines in petroleum refineries. 7. Steel pipes for geological drilling (YB235) are used by geological agencies for core drilling. Based on their purpose, they can be classified into drill rods, drill collars, core tubes, casings, and sedimentation tubes, among others. 8. Seamless steel pipes for diamond core drilling (GB3423) are seamless steel pipes used for drill rods, core rods, and casings in diamond core drilling. 9. Oil drilling pipes (YB528) are seamless steel pipes with thickened inner or outer walls, used for oil drilling. Steel pipes are divided into two types: those with threads and those without threads. Pipes with threads are connected using joints, while pipes without threads are joined using welding methods and tools. 10. Carbon steel seamless pipes for ships (GB5213-85) are carbon steel seamless pipes used for manufacturing Class I and Class II pressure piping systems on ships, as well as for boilers and superheaters. The operating temperature of the wall of carbon steel seamless pipes does not exceed 450°C, while that of alloy steel seamless pipes exceeds 450°C. 11. Seamless steel tubes for automobile half-shaft sleeves (GB3088) are hot-rolled seamless steel tubes made of high-quality carbon structural steel and alloy structural steel, used for manufacturing automobile half-shaft sleeves as well as the shaft tubes in drive axle housings. 12. High-pressure fuel pipes for diesel engines (GB3093) are cold-drawn seamless steel pipes used to manufacture high-pressure pipes in diesel engine injection systems. 13. Precision inner-diameter seamless steel tubes for hydraulic and pneumatic cylinder barrels (GB8713-88) are cold-drawn or cold-rolled precision seamless steel tubes with precise inner-diameter dimensions, used for manufacturing hydraulic and pneumatic cylinder barrels. 14. Cold-drawn or cold-rolled precision seamless steel tubes (GB3639) are such tubes that are used in mechanical structures and hydraulic equipment, offering high dimensional accuracy and good surface finish as a result of cold drawing or cold rolling processes. Using precision seamless steel tubes to manufacture mechanical structures or hydraulic equipment can **save machining time, improve material utilization, and also help enhance product quality. 15. Structural stainless steel seamless pipes (GB/T14975) are widely used in industries such as chemicals, petroleum, light textiles, medicine, food, and machinery. Hot-rolled (extruded, expanded) and cold-drawn (rolled) seamless steel tubes made of stainless steel for industrial corrosion-resistant pipes, structural components, and parts. 16. Stainless steel seamless pipes for fluid transport (GB/T14976) are hot-rolled (extruded, expanded) and cold-drawn (rolled) seamless pipes made of stainless steel, used for transporting fluids. 17. Special-shaped seamless steel pipes refer to seamless steel pipes with cross-sectional shapes other than circular ones. Depending on the shape and dimensions of the steel pipe cross-section, they can be further divided into seamless pipes with uniform wall thickness of special shapes (coded as D), seamless pipes with variable wall thickness of special shapes (coded as BD), and seamless pipes with changing diameter of special shapes (coded as BJ). Special-shaped seamless steel pipes are widely used in various structural components, tools, and mechanical parts. Compared to round tubes, shaped tubes generally have larger moments of inertia and section moduli, which gives them greater resistance to bending and torsion; this allows for **reducing the weight of structures and saving steel. III. Welded steel pipes Welded steel pipes, also known as welded tubes, are steel pipes made by welding steel plates or strips after they have been formed into shapes through rolling. Welded steel pipes have a simple production process, high production efficiency, a wide range of specifications, and low equipment investment, but their strength is generally lower than that of seamless steel pipes. Since the 1930s, with the rapid development of continuous rolling for high-quality strip steel and advancements in welding and inspection technologies, weld quality has continued to improve. The variety and specifications of welded steel pipes have increased, and they are replacing seamless steel pipes in an ever-growing number of applications. Welded steel pipes are classified into straight-seam welded pipes and spiral-welded pipes according to the shape of the weld. Straight-seam welded pipe has a simple production process, high production efficiency, low costs, and is developing rapidly. The strength of spiral welded pipes is generally higher than that of straight-seam welded pipes. They allow the production of pipes with larger diameters using narrower billets, and it is also possible to produce pipes with different diameters using billets of the same width. However, compared with straight-seam pipes of the same length, the weld length increases by 30–100%, and the production speed is lower. Therefore, smaller-diameter welded pipes are mostly welded by straight-seam welding, while larger-diameter welded pipes are mostly welded by spiral welding. 1. Welded steel pipes for low-pressure fluid transportation (GB/T3092), also known as ordinary welded pipes, are commonly referred to as black pipes. It is a welded steel pipe used for transporting fluids at relatively low pressures such as water, gas, air, oil, and heating steam, as well as for other purposes. Steel pipes are classified by wall thickness into ordinary steel pipes and thick-walled steel pipes ； The types of connection ends are divided into unthreaded steel pipes (smooth pipes) and threaded steel pipes. The specification of steel pipes is indicated by the nominal diameter (mm), which is an approximate value of the inner diameter. *It is commonly expressed in inches, such as 11/2, etc. In addition to being used directly for transporting fluids, welded steel pipes for low-pressure fluid transport are also widely used as the base pipes for galvanized welded steel pipes used for the same purpose. 2. Galvanized welded steel pipes for low-pressure fluid transport (GB/T3091), also known as galvanized electric welding steel pipes, are commonly referred to as white pipes. It is hot-dip galvanized welded (furnace-welded or electrically welded) steel pipe used for transporting fluids at relatively low pressures such as water, gas, air, oil, heating steam, and hot water, or for other purposes. Steel pipe wall thickness is divided into regular galvanized steel pipes and thickened galvanized steel pipes ； The types of connection ends are divided into galvanized steel pipes without threads and galvanized steel pipes with threads. The specification of steel pipes is indicated by the nominal diameter (mm), which is an approximate value of the inner diameter. *It is commonly expressed in inches, such as 11/2, etc. 3. Ordinary carbon steel wire sleeves (GB3640) are steel tubes used to protect wires in electrical installation projects such as industrial and civil buildings and the installation of machinery and equipment. 4. Straight-seam electric welded steel pipes (YB242) are steel pipes whose welds are parallel to the longitudinal direction of the pipe. They are generally classified into metric welded steel pipes, welded thin-walled pipes, transformer cooling oil pipes, and so on. 5. Spiral-welded submerged-arc welded steel pipes for transporting pressurized fluids (SY5036) are made by using hot-rolled steel strip coils as the pipe blanks, forming them in a spiral shape at room temperature, and welding them using double-sided submerged-arc welding; they are used as spiral-welded steel pipes for transporting pressurized fluids. Steel pipes have strong pressure resistance and good weldability; they have undergone various rigorous scientific inspections and tests, making them safe and reliable to use. Steel pipes have a large diameter, offering high transportation efficiency and helping to reduce investment in pipeline installation. Primarily used for pipelines transporting oil and natural gas. 6. Spiral-welded high-frequency welded steel pipes for transporting pressurized fluids (SY5038) are made by using hot-rolled steel strip coils as the pipe blanks, forming them in a spiral shape at room temperature, and welding them using a high-frequency lap welding method; they are thus used for transporting pressurized fluids. Steel pipes have strong pressure resistance, good plasticity, and are easy to weld and shape ； After various strict and scientific inspections and tests, it is safe and reliable to use; it features a large diameter for pipes, high transportation efficiency, and can save on investment in pipeline installation. It is mainly used for laying pipelines to transport oil, natural gas, etc. 7. Spiral-welded submerged-arc welded steel pipes for general low-pressure fluid transport (SY5037) are made by using hot-rolled steel strip coils as the pipe blanks, forming them through spiral shaping at room temperature, and then manufacturing them using double-sided automatic submerged-arc welding or single-sided welding methods; they are designed for transporting low-pressure fluids such as water, gas, air, and steam. 8. Spiral-welded high-frequency welded steel pipes for general low-pressure fluid transport (SY5039) are made by using hot-rolled steel strip coils as the pipe billets, forming them in a spiral shape at room temperature, and then welding them together using high-frequency lap welding methods; these pipes are intended for use in transporting general low-pressure fluids. 9. Spiral welded steel pipes for piles (SY5040) are made by using hot-rolled steel strips as the pipe billet, forming them through spiral welding at room temperature, and then manufacturing them using double-sided submerged arc welding or high-frequency welding. They are used as steel pipes for foundation piles in civil construction structures, docks, bridges, and other applications. IV. Steel-plastic composite pipes and large-diameter coated steel pipes: Steel-plastic composite pipes are made by using hot-dip galvanized steel pipes as the base, and plastic is applied to their inner surface (and sometimes the outer surface as well) through powder melting spraying technology, resulting in excellent performance. Compared to galvanized pipes, it has advantages such as corrosion resistance, no rusting, no scale formation, a smooth surface, being clean and non-toxic, as well as a long service life. Tests have shown that the service life of steel-plastic composite pipes is more than three times that of galvanized pipes. Compared to plastic pipes, it has advantages such as high mechanical strength, as well as good pressure and heat resistance. Since the substrate is a steel pipe, there are no issues with embrittlement or aging. It can be widely used in the transportation of fluids such as tap water, gas, and chemical products, as well as in heating systems; it represents an upgraded version of galvanized pipes. Due to its installation and usage methods being essentially the same as those of traditional galvanized pipes, as well as its identical fitting types, and its ability to replace aluminum-plastic composite pipes in large-diameter water supply applications, it is very popular among users and has become one of the most competitive new products in the piping market. Coated steel pipes are created by applying plastic to large-diameter spiral-welded and high-frequency welded pipes; the maximum diameter of such pipes can reach 1200 mm. Plastic coatings with various properties, such as polyvinyl chloride (PVC), polyethylene (PE), and epoxy resin (EPOXY), can be applied according to specific requirements. These coatings offer good adhesion and strong corrosion resistance, enabling them to withstand strong acids, strong bases, and other chemical agents. They are non-toxic, do not rust, are wear-resistant, impact-resistant, and have excellent resistance to penetration. The surface of these pipes is smooth and does not allow any substances to adhere to it, which helps reduce resistance during transportation, increases flow rate and efficiency, and lowers pressure losses. The coating contains no solvents or substances that can leach out, so it does not contaminate the fluid being transported, thereby ensuring the purity and cleanliness of the fluid. It can be used in cycles of heating and cooling within the range of -40 ℃ to +80 ℃, without aging or cracking, which allows it to be utilized in harsh environments such as cold regions. Large-diameter coated steel pipes are widely used in engineering fields such as tap water, natural gas, petroleum, chemicals, pharmaceuticals, communications, electricity, and the ocean industry.