A comparison between seamless pipe and welded pipe:
1, high pressure resistance. Seamless pipe has different wall thickness and wide pressure range, but the working pressure of air conditioning water system can be qualified with welded pipe, which is not the reason for selection;
2. Large caliber. There are few welded pipes above DN 100, but many seamless pipes, which is one reason for the choice;
3, easy to construct. Seamless pipes are connected by welding and flange, and it is difficult to install large-diameter pipes by threaded connection. At present, large-diameter pipes are installed in grooves, but this method is not suitable for air conditioning water systems. This is the second reason for choosing.
Second, the problem of mixing seamless pipe and galvanized welded pipe:
Due to the limitation of construction methods, seamless pipes are not galvanized before leaving the factory, and the system installation needs secondary galvanizing. In addition, the secondary installation construction cost is high (the material price itself is high) and the construction period is long. If the second galvanizing affects the system, it is not the cost of the second galvanizing.
Third, the specification does not require high welding construction, but poor welding has a great impact on the service life of the system.
The requirements of this professional specification for welding construction quality are admission strength and air tightness test, and the weld is not tested. However, the weld does not leak during the completion and warranty period, which does not mean that it does not leak during the service life of the system. Actually, it's been leaked for years. Repair welding can only be used for maintenance, which will inevitably destroy the secondary zinc coating, and the impact on the service life of the system is needless to say.
Therefore, in my opinion, seamless pipes should not be widely used in air-conditioning water system, and construction quality should be paid attention to when using large-diameter pipes, and secondary galvanizing should be done.
When it comes to thermal stress release, there are design considerations and construction practices, which have nothing to do with pipelines.
Seamless steel tube: Seamless steel tube is made of steel ingot or solid tube blank through perforation, and then hot rolling, cold rolling or cold drawing. The specification of seamless steel pipe is expressed by outside diameter * wall thickness mm. Seamless steel tubes are divided into two categories: hot-rolled and cold-rolled (drawn) seamless steel tubes.
Hot rolled seamless steel pipes are divided into ordinary steel pipes, medium and low pressure boiler steel pipes, high pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, geological steel pipes and other steel pipes. Cold rolled (drawn) seamless steel pipes include carbon thin-walled steel pipes, alloy thin-walled steel pipes, stainless steel thin-walled steel pipes and special-shaped steel pipes in addition to ordinary steel pipes, medium and low pressure boiler steel pipes, high pressure boiler steel pipes, alloy steel pipes and other steel pipes. Generally, hot-rolled seamless pipes have an outer diameter greater than 32mm and a wall thickness of 2.5-75 mm; The diameter of cold-rolled seamless pipe can reach 6mm, the wall thickness can reach 0.25mm, the outer diameter of thin-walled pipe can reach 5mm, and the wall thickness is less than 0.25mm, so the dimensional accuracy of cold-rolled seamless pipe is higher than that of hot-rolled.
Seamless steel pipes are generally made of high-quality carbon steels such as 10, 20, 30, 35 and 45, low-alloy structural steels such as 16Mn and 5MnV, or combined steels such as 40Cr, 30CrMnSi, 45Mn2 and 40MnB. Seamless pipes made of low carbon steel, such as 10 and 20, are mainly used for fluid transportation pipelines. Seamless tubes made of medium carbon steels such as 45 and 40Cr are used to manufacture mechanical parts, such as those of automobiles and tractors. Generally, seamless steel pipes should be used to ensure strength and flattening test. The hot-rolled steel pipe is delivered in hot-rolled state or heat-treated state; Cold rolling is delivered in heat treatment state.
Seamless steel pipes for medium and low pressure boilers: used to manufacture various medium and low pressure boilers, superheated steam pipes, boiling water pipes, water wall pipes and superheated steam pipes, large smoke pipes, small smoke pipes and arched brick pipes for locomotive boilers. Hot-rolled or cold-rolled (drawn) seamless steel tubes of high-quality carbon structural steel. It is mainly made of 10 and 20 steels. In addition to ensuring chemical composition and mechanical properties, water pressure test, crimping, flaring and flattening test must also be carried out. Hot rolling is delivered in hot rolling state, and cold rolling (drawing) is delivered in heat treatment state.
High-pressure boiler steel pipe: mainly used for manufacturing high-quality carbon structural steel, alloy structural steel and stainless heat-resistant steel seamless steel pipes for high-pressure and above steam boilers. These boiler tubes work under high temperature and high pressure, and the pipes will be oxidized and corroded under the action of high temperature flue gas and steam, so the steel pipes are required to have high lasting strength, high oxidation resistance and good organizational stability. The steel grades used are: high-quality carbon structural steel grades 20G, 20mg and 25MnG；; Alloy structural steel grades 15MoG, 20MoG, 12CrMoG, 15CrMoG, 12Cr2MoG, 12CrMoVG,/kloc-0 12Cr3MoVSiTiB, etc. Rusted heat-resistant steel commonly used 1Cr 18Ni9,1Cr18ni1nb high-pressure boiler tubes, in addition to ensuring chemical composition and mechanical properties, should also be subjected to water pressure test one by one, and should be subjected to flaring and flattening tests. Steel pipes are delivered in heat treatment state. In addition, there are certain requirements for the metallographic structure, grain size and decarburization layer of the finished steel pipe.
Seamless steel pipes for geological drilling and oil drilling control; In order to find out the underground rock structure, groundwater, oil, natural gas and mineral resources, a drilling rig is used to drill wells. The exploitation of oil and natural gas is inseparable from drilling. Seamless steel pipe for geological drilling is the main equipment for drilling, which mainly includes outer core pipe, inner core pipe, casing pipe and drill pipe. Because the drill pipe has to go deep into the stratum for thousands of meters, the working conditions are extremely complicated, and the drill pipe has to bear stresses such as tensile, compressive, bending, torsional and unbalanced impact loads, and is also worn by mud and rocks. Therefore, pipes are required to have sufficient strength, hardness, wear resistance and impact toughness. The yield point of steel for steel pipe is indicated by "DZ" (the Chinese phonetic prefix of geology) plus a number, and the commonly used steel grade is DZ45. 40Mn2 and 40mn2si; DZ50; 40Mn2Mo and 40MnVBDZ55; 40 mnmob of DZ60 and 27 mnmob of DZ65 are delivered in heat treatment state.
Petroleum cracking tubes: seamless tubes for furnace tubes, heat exchanger tubes and pipelines used in refineries. Commonly used are high-quality carbon steel (10,20), alloy steel (12CrMo, 15CrMo), heat-resistant steel (12Cr2Mo, 15Cr5Mo) and stainless steel (/kloc-0). In addition to the chemical composition and various mechanical properties of steel pipes, it is also necessary to ensure water pressure, flattening, flaring and other tests, as well as surface quality and nondestructive testing. The steel pipe is delivered in the heat treatment state.
Stainless steel pipe: stainless steel pipe made of all kinds of stainless steel by hot rolling and cold rolling, which is widely used in petrochemical equipment pipelines and various stainless steel structural parts. All steel pipes used to withstand fluid pressure must be qualified by water pressure test in addition to ensuring chemical composition and mechanical properties. All kinds of special steel pipes shall be guaranteed according to regulations.
Seamless steel tubes are divided into hot-rolled (extruded) seamless steel tubes and cold-drawn (rolled) seamless steel tubes due to different manufacturing processes. Cold-drawn (rolled) pipes are divided into round pipes and special-shaped pipes. Seamless steel pipes are divided into the following varieties for different purposes:
1.gb/t 8162-1999 (seamless steel pipe for structure). Mainly used in general structures and mechanical structures. Its representative materials (brands): carbon steel, No.20 and No.45 steel; Alloy steels Q345, 20Cr, 40Cr, 20CrMo, 30-35CrMo, 42CrMo, etc.
2.GB/T8163-1999 (Seamless Steel Pipe for Conveying Fluid). It is mainly used for conveying fluid pipelines in engineering and large-scale equipment. Representative materials (brands) include 20, Q345, etc.
3.GB3087- 1999 (Seamless Steel Pipe for Low and Medium Pressure Boilers). It is mainly used for conveying medium and low pressure fluids in industrial boilers and household boilers. The representative materials are 10 and 20 steels.
4.GB53 10- 1995 (Seamless Steel Pipe for High Pressure Boiler). Mainly used for high-temperature and high-pressure conveying fluid headers and pipelines on boilers of power stations and nuclear power plants. Representative materials are 20G, 12Cr 1MoVG, 15CrMoG, etc.
5.GB53 12- 1999 (Seamless Steel Pipe of Marine Carbon Steel and Carbon Manganese Steel). Mainly used for Class I and Class II pressure tubes of marine boilers and superheaters. Representative materials are 360, 4 10, 460 steel grades, etc.
6.GB 1479-2000 (Seamless Steel Pipe for High Pressure Fertilizer Equipment). It is mainly used for conveying high-temperature and high-pressure fluid on fertilizer equipment. Representative materials are 20, 16Mn, 12CrMo, 12Cr2Mo, etc.
7.GB9948- 1988 (Seamless Steel Pipe for Petroleum Cracking). It is mainly used for boilers, heat exchangers and fluid pipes in petroleum smelters. Its representative materials are 20, 12CrMo, 1Cr5Mo,1Cr19ni1nb, etc.
8.GB 18248-2000 (Seamless Steel Pipe for Gas Cylinders). Mainly used for manufacturing various gas cylinders and hydraulic cylinders. Its representative materials are 37Mn, 34Mn2V, 35CrMo and so on.
In addition, there are GB/T 17396- 1998 (hot rolled seamless steel pipe for hydraulic prop), GB3093- 1986 (high pressure seamless steel pipe for diesel engine) and GB/T3639- 1983 (cold drawn or cold rolled). T87 13- 1988 (precision inner diameter seamless steel pipe for hydraulic and pneumatic cylinders), GB13296-1991(stainless steel seamless steel pipe for boilers and heat exchangers), GB/t1. T 14976- 1994 (seamless stainless steel pipe for fluid transportation) GB/T5035- 1993 (seamless steel pipe for automobile shaft sleeve), APISPEC5CT- 1999 (specification for sleeves and pipes), etc.
The process flow is briefly described as follows:
1. Hot rolling (extrusion of seamless steel tube): round tube blank → heating → piercing → three-roll skew rolling, continuous rolling or extrusion → tube stripping → sizing (or reducing) → cooling → straightening → hydrostatic test (or flaw detection) → marking → warehousing.
The raw material for rolling seamless pipes is a round tube blank, which is cut by a cutting machine to a length of about 1 m and sent to a furnace by a conveyor belt for heating. The billet is heated in the furnace at a temperature of about 1200℃. The fuel is hydrogen or acetylene. Temperature control in the furnace is a key problem. After the round tube blank is discharged from the furnace, it should be perforated by a pressure piercer. The common piercer is the cone roller piercer, which has the advantages of high production efficiency, good product quality, large aperture expansion, and can pierce a variety of steel grades. After piercing, the round tube blank is continuously rolled by three rollers, continuously rolled or extruded. After extrusion, remove the tube and measure the size. The sizing machine rotates the cone drill bit to the billet at high speed to punch holes, thus forming the steel pipe. The inner diameter of the steel pipe is determined by the outer diameter length of the drill bit of the sizing machine. After sizing, the steel pipe enters the cooling tower and is cooled by spraying water. After cooling, the steel pipe will be straightened. The straightened steel pipe is sent to a metal flaw detector (or hydrostatic test) by a conveyor belt for internal flaw detection. If there are cracks and bubbles in the steel pipe, they will be detected. After the quality inspection, steel pipes have to undergo strict manual selection. After quality inspection, steel pipes should be painted with number, specification and production batch number. And hoisted into the warehouse with a crane.
2. Cold-drawn (rolled) seamless steel tube: round tube blank → heating → piercing → upsetting → annealing → pickling → oiling (copper plating) → multi-pass cold drawing (cold rolling) → tube blank → heat treatment → straightening → hydrostatic test (flaw detection) → marking → warehousing.
The rolling method of cold-drawn (rolled) seamless steel pipe is more complicated than hot-rolled (extruded) seamless steel pipe. The first three steps of their production process are basically the same. The difference begins with the fourth step. After the round tube blank is hollowed out, upsetting annealing should be carried out. After annealing, use special acidic liquid for pickling. After pickling, coat with oil. Followed by multi-pass cold drawing (cold rolling) and re-blank tube, as well as special heat treatment. It will straighten after heat treatment. The straightened steel pipe is sent to a metal flaw detector (or hydrostatic test) by a conveyor belt for internal flaw detection. If there are cracks and bubbles in the steel pipe, they will be detected. After the quality inspection, steel pipes have to undergo strict manual selection. After quality inspection, steel pipes should be painted with number, specification and production batch number. And hoisted into the warehouse with a crane.
Welded steel pipe: also called welded pipe, it is made of steel plate or steel strip after bending and then welding. According to the weld form, it is divided into straight welded pipe and spiral welded pipe. According to the purpose, it can be divided into ordinary welded pipes, galvanized welded pipes, oxygen-blown welded pipes, silk sleeves, metric welded pipes, idler pipes, deep well pump pipes, automobile pipes, transformer pipes, welded thin-walled pipes, welded special-shaped pipes and spiral welded pipes.
Ordinary welded pipe: Ordinary welded pipe is used for conveying low-pressure fluid. Made of q195a.q215a.q235a steel. It can also be made of other low carbon steels that are easy to weld. Steel pipes have to undergo water pressure, bending, flattening and other experiments, which have certain requirements for surface quality. Usually the delivery length is 4- 10m, and it is often required to deliver according to the length (or double the length). The specifications of welded pipes are expressed in nominal caliber (millimeters or inches). The nominal caliber is different from the actual situation. Welded pipes can be divided into ordinary steel pipes and thickened steel pipes according to the specified wall thickness, and steel pipes can be divided into threaded and non-threaded pipes according to the pipe end form.
With the improvement of welded steel pipe technology and the simplicity of processing, welded steel pipes have replaced some seamless pipes, and welded steel pipes have been able to produce more than 2 19mm(8 inches).
Galvanized steel pipe: In order to improve the corrosion resistance of steel pipes, general steel pipes (black pipes) are galvanized. Galvanized steel pipes are divided into hot dip galvanizing and electrical steel zinc. Hot-dip galvanizing has a thick zinc layer and low cost.
Oxygen-blown welded pipe: used as oxygen-blown pipe for steelmaking, small-diameter welded steel pipe is generally used, with eight specifications ranging from 3/8 inch to 2 inch. Made of 08, 10, 15, 20 or Q 195-Q235 steel strip. In order to prevent corrosion, some of them are aluminized.
Wire sleeve: it is also a common carbon steel welded steel pipe, which is used in concrete and various structural power distribution projects. The commonly used nominal diameter range is 13-76 mm, and the sleeve wall is thin, so it is mostly used after coating or galvanizing, and cold bending test is needed.
Metric welded pipe: Welded steel pipe in seamless pipe form, expressed by outside diameter * wall thickness mm, which is made of ordinary carbon steel, high-quality carbon steel or ordinary low-alloy steel by hot and cold strip welding or cold drawing after hot strip welding. Metric welded pipes can be divided into ordinary pipes and thin-walled pipes, which are often used as structural parts, such as transmission shafts, or to transport fluids. Thin-walled pipes are used to produce furniture, lamps, etc. , and shall ensure the strength and bending test of steel pipe.
Roller tube: welded steel tube used for roller of belt conveyor, generally made of Q2 15 and Q235A. B, 20 steel, with a diameter of 63.5-2 19.0 mm, has certain requirements on pipe curvature, end face perpendicular to the center line and ellipticity, and generally carries out water pressure and flattening tests.
Transformer tube: used to manufacture transformer radiator tube and other heat exchangers. It is made of ordinary carbon steel and needs flattening, flaring, bending and hydrostatic test. Steel pipes are delivered in fixed length or double length, which has certain requirements for steel pipe bending.
Special-shaped pipes: square pipes, rectangular pipes, hat pipes and steel pipes for hollow rubber steel doors and windows, which are welded by common carbon structural steel such as 16Mn and steel belts, are mainly used as agricultural machinery parts and steel doors and windows.
Welded thin-walled tube: mainly used for making furniture, toys, lamps and so on. In recent years, stainless steel thin-walled tubes are widely used in high-end furniture, decoration, fences and other fields.
Spiral welded pipe: it is made by rolling low-carbon structural steel or low-alloy structural steel strip into tube blank according to a certain spiral angle (called forming angle), and then welding the tube seam. It can produce large diameter steel pipe from narrow strip steel. Spiral welded pipe is mainly used for oil and gas pipelines, and its specification is expressed by external diameter * wall thickness. Spiral welded pipe is single-sided or double-sided, and the welded pipe shall ensure that the hydrostatic test, tensile strength and cold bending performance of the weld meet the requirements.
As for the choice, it is mainly based on the design work pressure!
They have different functions and are used in different places. Seamless steel pipe is used for high pressure, and it is generally used for pipelines with pressure above 0.8MPa. Galvanized steel pipes are used for corrosion resistance, such as water and gas pipelines. As for the price, it seems to me that galvanized steel pipes are more expensive than seamless steel pipes.
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