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Basic knowledge of chemical process pipelines: Understand pipeline technology in just one diagram! 5 types of pipes, 4 types of fittings, flanges, gaskets, and bolts – that’s actually all there is to it! Pipes: There are many ways to classify pipes. Based on material, they can be divided into metal pipes, non-metallic pipes, and steel-lined non-metallic composite pipes. Non-metallic pipes mainly include rubber pipes, plastic pipes, asbestos-cement pipes, graphite pipes, and glass fiber reinforced plastic pipes. The proportion of non-metallic pipes in use is smaller compared to metal pipes; metal pipes account for over 85 percent of all process piping installations in most petrochemical plants. Today, I will focus on introducing metal pipes to you all. Welded steel pipes, also known as seam steel pipes, are generally formed by welding steel plates or steel strips together. Based on the surface treatment of the pipe material, it is divided into galvanized and ungalvanized types. The surface that is galvanized appears whitish; it is also known as white iron pipe or galvanized steel pipe ； Those without a zinc coating on their surface are ordinary welded steel pipes, also known as black iron pipes. Galvanized welded steel pipes are commonly used in pipelines where the medium to be transported requires high cleanliness, such as domestic water, air purification, and instrument air ； Ungalvanized welded steel pipes can be used to transport steam, gas, compressed air, and condensate water, among other things. As requested by users, welded steel pipes can be available in two types upon leaving the factory: one with threaded pipe ends and the other without threaded pipe ends. Welded steel pipes with threaded ends have a length of 4 to 9 meters per pipe, while welded steel pipes without threads have a length of 4 to 12 meters per pipe. Welded steel pipes are classified into thin-walled pipes, thickened pipes, and ordinary pipes according to their wall thickness. The most commonly used material for process pipelines is ordinary steel pipes, with a test pressure of 2.0 MPa. The test pressure for the thickened steel pipe is 3.0 MPa. There are various methods for connecting welded steel pipes: threaded connection, flange connection, and welding. Flange connections are further divided into threaded flange connections and welded flange connections, while welding methods include gas welding and arc welding. The common specification range for welded steel pipes is seamless pipes with a nominal diameter of 6 to 150 mm. Seamless pipes are the most widely used type of pipe in industrial pipelines, offering the greatest variety of specifications. They are basically divided into two categories: seamless steel pipes for fluid transportation and seamless steel pipes with specialized functions. The former are steel pipes commonly used in process pipelines, while the latter include steel pipes specifically designed for boilers, cracking furnace tubes, and heat exchangers. Based on material, they can be classified into: carbon seamless steel pipes, chromium-molybdenum seamless steel pipes, and stainless and acid-resistant seamless steel pipes. Based on nominal pressure, they can be classified into three categories: low pressure (0≤1.0 MPa), medium pressure (1.0＜10 MPa), and high pressure (≥10 MPa). The commonly used seamless steel pipes for process pipelines in fluid transportation are classified as follows: ❶ Carbon seamless steel pipes; the common materials used for their manufacture are 10#, 20# and 16Mn steel. Its specification range is: hot-rolled outer diameter of φ32～630mm, cold-drawn outer diameter of φ6～200mm, single tube length of 4～12m, and allowable operating temperature of -40～450℃. It is widely used for transporting various media that are non-corrosive to steel, such as steam, oxygen, compressed air, oils, and gas-oil mixtures. ❷ Low-alloy steel seamless pipes refer to alloy steel pipes that contain a certain proportion of alloying elements. They are generally divided into two types: one is low-alloy steel pipes containing manganese, known as ordinary low-alloy steel pipes, such as 16Mn, 15MnV, etc ； Another type is low-alloy steel pipes containing elements such as chromium and molybdenum, known as chromomoly steel pipes. Commonly used grades include 12CrMo, 15CrMo, 12Cr2Mo, 1Cr5Mo, etc. The diameter range is from φ10 to φ273 mm, and the length of each tube is 4 to 12 meters. Chromium-molybdenum steel tubes can operate within a temperature range of -40 to 550°C. Low-alloy seamless steel pipes are commonly used for transporting oils and gases at relatively high temperatures, as well as saline solutions with low corrosivity and low-concentration organic acids. ❸ Stainless acid-resistant seamless steel pipes come in a wide variety, depending on the different contents of metals such as chromium, nickel, and titanium; examples include Cr13, 00Cr17Ni14Mo2, 1Cr18Ni12Mo2Ti, 1Cr18Ni9Ti, etc. Among these steel grades, 1Cr18Ni9Ti is the most widely used; it is commonly denoted by the simplified material code 18-8 on construction drawings. Its applicable temperature range is from -196 to 700°C. It is used in chemical manufacturing to transport various highly corrosive media, such as nitric acid, acetic acid, and urea. ❹ High-pressure seamless steel pipes: Their manufacturing materials are basically the same as those of the aforementioned seamless steel pipes; however, their wall thickness is greater than that of medium- and low-pressure seamless steel pipes, with the thickest walls reaching 40 mm. For fertilizer production equipment, the specifications for high-pressure seamless steel pipes are φ14×4(mm) to 273×40(mm); the length of each pipe is 4 to 12 meters, the applicable pressure range is 10 to 32 MPa, and the operating temperature range is -40 to 400°C. In petrochemical plants, the above-mentioned high-pressure seamless steel pipes are used to transport media such as raw gas, hydrogen and nitrogen gases, syngas, water vapor, and high-pressure condensate. ❺ Low-temperature steel pipes are widely used in the cold areas of imported engineering projects; their material grades are STPL-39 and STPL-46, with an operating temperature that can reach -105°C. Low-temperature steel pipes are divided into two types: seamless pipes and welded pipes. The nominal diameter of seamless low-temperature steel pipes ranges from 15 to 400 mm, and their wall thickness is the same as that of carbon steel pipes ； The nominal diameter of welded low-temperature steel pipes ranges from 400 to 1100 mm, the wall thickness is 6 to 10 mm, and the standard length of a single pipe is 6 m. It is suitable for pipelines transporting various non-corrosive cryogenic media; at present, no steel pipes that meet this standard are produced in bulk in China. Steel pipe coiled tube: A steel pipe coiled tube is formed by rolling and welding steel plates, and it is divided into two types – straight-seam rolled welded steel pipes and spiral-seam rolled welded steel pipes. Seam-welded steel pipes: Most are manufactured at construction sites or by contracted factories; specialized steel pipe manufacturers do not produce them. The steel plate materials include Q235A, Grade 10, Grade 20, 16Mn, 20g, etc. The nominal diameter range is 200–3000 mm, with the largest diameter reaching 4000 mm; the wall thickness is generally between 4 and 16 mm. The length of a single tube with a nominal diameter of 200–900 mm is 6.4 m ； Those with a nominal diameter of 1000–3000 mm have a length of 4.8 m. Applicable operating temperature: Q235A is -15～300℃, while grades 10, 20, 16Mn, and 20g are -40～450℃; all of them are suitable for low-pressure applications. Spiral-welded steel pipes: Produced by steel pipe manufacturers, with materials including Q235A and 16Mn. Its specification range is a nominal diameter of 200–700 mm, a wall thickness of 7–10 mm, and a length of 8–18 m per tube. Applicable operating temperature: Q235A is -15 to 300°C, and 16Mn is -40 to 450°C; operating pressure: Q235A is 2.5 MPa, and 16Mn is 4 MPa. Seamless rolled welded steel pipes and spiral-welded rolled welded steel pipes are commonly used for transporting media at normal temperature and low pressure with weak corrosivity, such as low-pressure steam, underground circulating water, gas, and oil and gas. Spiral-welded steel pipes have a long length per pipe, making them particularly suitable for long-distance transmission pipelines. As required by the project, there are also straight-seam stainless steel sheet coiled welded pipes, which are all rolled on-site; the manufacturing plants do not produce them. The material used is mostly 1Cr18Ni9Ti steel plate. The nominal diameter range is 200–1000 mm; for a nominal diameter of 200–400 mm, the wall thickness is 4 mm, while for a nominal diameter of 700–1000 mm, the wall thickness is 6–8 mm. The length of each tube is 4.5 m. The applicable temperature range and the types of media that can be transported are the same as those for low-pressure stainless steel pipes. Copper pipes are divided into red copper pipes and brass pipes. The material grades used for manufacturing red copper pipes include T2, T3, T4, and TUP, etc.; these materials have a high copper content, exceeding 99.7% ； The material grades used for manufacturing brass pipes include H62, H68, etc.; they are all alloys of zinc and copper. For example, in H62 brass pipes, the copper content ranges from 60.5% to 63.5%, zinc accounts for 39.6%, while other impurities make up less than 0.5%. The manufacturing methods for copper tubes are divided into drawing and extrusion: the outer diameter of drawn copper tubes ranges from φ3 to φ200 mm, while that of extruded copper tubes ranges from φ32 to φ280 mm, with a wall thickness of 1.5 to 5 mm ； The specification range for copper tubes welded from copper sheets is an outer diameter of φ155～φ505 mm, and they are available either as individual tubes or in coils. Copper pipes are suitable for operating temperatures below 250°C, and are commonly used in oil pipelines, insulation companion pipes, and oxygen pipelines in air separation plants. Titanium tubes are a type of pipe that has emerged in recent years. Thanks to their lightweight nature, high strength, excellent corrosion resistance, and tolerance to low temperatures, they are often used in applications where other types of pipes are not suitable. Titanium tubes are made from industrially pure titanium grades TA1 and TA2, and their suitable temperature range is -140 to 250°C; their mechanical properties decline when the temperature exceeds 250°C. The standard range for commonly used titanium tubes is a nominal diameter of 20 to 400 mm. Suitable for low and medium pressure; the wall thickness for low-pressure pipes is 2.8–12.7 mm, while that for medium-pressure pipes is 3.7–21.4 mm. Although titanium tubes have many advantages, they have not been widely adopted due to their high cost and the difficulty of welding them. Fittings are required to change the direction or elevation of pipes in a piping system, to alter the pipe diameter, and to branch off pipes from the main pipe. Due to the varying shapes and complexities of piping systems, there are many types of pipe fittings. Common pipe fittings include elbows, tees, reducers, pipe joints, and pipe caps. Elbow: An elbow is used to change the direction of a pipe. The common bending angles for elbows are 90°, 45°, and 180°; an 180° elbow is also known as a U-bend. Elbows with special angles exist as well, but they are very rare. ❶ Malleable iron elbow: Also known as forged cast iron elbow, it is the most common type of threaded elbow. Malleable iron elbows come in relatively small sizes, with the commonly used range being 1/2 to 4 inches. They are available in two types depending on their surface treatment: galvanized and ungalvanized. These copper pipes are mainly used for heating systems, as well as in water supply and drainage pipelines and gas pipelines. During the manufacturing process, it is rarely used on material pipelines other than the low-pressure pipelines that need to be removed frequently. ❷ Pressed elbow: A pressed elbow, also known as a stamped elbow or seamless elbow, is formed by pressing high-quality carbon steel, stainless acid-resistant steel, and low-alloy steel seamless tubes in special molds. Its bending radius is 1.5 times the nominal diameter (R=1.5DN); in special cases, elbows with a bending radius equal to the nominal diameter can also be used (R=1DN). Its size range is from DN20 to 600 mm. Its wall thickness range is consistent with the designation of seamless steel pipes. Suppressed elbows are generally manufactured by specialized factories or processing plants by stamping standard seamless steel pipes, and bevels should be prepared at both ends of the elbow at the time of delivery. ❸ Stamped and welded elbow: A stamped and welded elbow is created by using sheet metal to form half-ring elbows through stamping, and then these two half-ring elbows are joined together and welded to form the final elbow. Its bending radius is the same as that of seamless pipe elbows, with a specification range of nominal diameter 200 mm and above. ❹ Welded elbow: A welded elbow is also known as a shrimp-shaped elbow or shrimp-body elbow. There are two methods of production: one involves cutting steel plates in a processing factory, then rolling and welding them into shape, and this method is mostly used for producing components that go with steel pipe rolls. Another method involves cutting tubing and then joining and welding it together; the specifications for this approach are generally above 200 mm. The operating temperature should not exceed 200°C, and it can generally be manufactured at the construction site. ❺ High-pressure elbow: A high-pressure elbow is forged from high-quality carbon steel or low-alloy steel. Depending on the piping connection type, the ends of the elbow are machined with threads or grooves, with a high level of precision required; the threads at the pipe ends must fit tightly with those on the flange so that they can be screwed in freely without loosening. Suitable for petrochemical pipelines under pressures of 22.0 and 32.0 MPa, with a common size range of DN6 to 200 mm. Elbow: An elbow is a fitting that connects a main pipe with a branch pipe; it comes in many different types, depending on the material used in its manufacture and its intended use. Classified by specifications: they can be divided into equal-diameter tees and unequal-diameter tees, with equal-diameter tees also being known as tees with the same diameter ； A tee with the same diameter refers to a situation where the diameter of the branch pipes is identical to that of the main pipe. A tee with different diameters means that the diameter of the branch pipes is smaller than that of the main pipe; it is therefore also known as a tee with unequal diameters. Tees with different diameters are more commonly used. ❶ Cast iron tees: The manufacturing materials and specification ranges for cast iron tees are the same as those for cast iron elbows. They are not widely used in petrochemical process pipelines, but rather in systems for indoor heating, water supply and drainage, as well as gas pipelines. ❷ Steel tee: Before the 1970s, the pace of development in industrial production technology in China was relatively slow, and the engineering techniques related to process pipelines were also backward. In the design of various medium and low-pressure steel pipes, when tee fittings are required for the pipes, the hole-drilling connection method is generally used to address this need. That is, a hole is drilled in the main pipe at the location where the branch pipe is to be attached, according to the diameter of the branch pipe, and then the branch pipe is welded onto it. This method has many disadvantages: the welding quality is difficult to ensure, the pipes tend to deform after welding, and impurities can easily enter the main pipe. In the late 1970s, the introduction of large-scale modern production facilities in industries such as metallurgy and petrochemicals spurred the development of process piping engineering technology. Today, China is capable of producing medium and low-pressure pipe steel elbow fittings, with a range of products available; the specifications for seamless elbows range from DN20 to DN600 mm, while those for elbow fittings made by welding steel plates range from DN150 to DN1500 mm. The production of shaped tees involves using high-quality pipe material as raw material; it goes through processes such as cutting, drilling holes, heating, and drawing using molds, followed by mechanical processing to become the finished shaped tee. Medium and low-pressure steel fittings such as tees are welded during on-site installation. There are two scenarios regarding the tees used for rolling steel plates into tubes: in one case, the steel plates are cut at the manufacturing plant and then rolled and welded together ； Another method is to dig a socket for the pipe at the installation site. ❸High-pressure tee: There are two common types of high-pressure tees – one is a welded high-pressure tee, and the other is an integrally forged high-pressure tee. For welding high-pressure tees, high-quality high-pressure steel pipes are used as materials. The manufacturing method is similar to that of socketed joints; the hole made in the main pipe must have the same diameter as that of the connected branch pipe. Stringent requirements are placed on welding quality; usually, preheating before welding and heat treatment after welding are required. Its specification range is DN16 to 200 mm, with pressures of 22 MPa and 32 MPa. Integral forged high-pressure tees are generally connected using threaded flanges. Its specification range is table size SCH160, DN15～600mm; the table size is supplemented with “XXS”, and its pipe diameter range is DN15～300mm. Reducer pipes: The function of reducer pipes is to change the diameter of pipes. From the perspective of fluid flow direction, this usually involves a reduction in diameter, but it can also involve an increase in diameter; for example, the reducer pipes used in steam return pipelines and sewage pipes serve to increase the diameter. An eccentric pipe is commonly known as a reducer. ❶ Inconel reducer tubes: Inconel reducer tubes can be generally divided into two types; one is the reducer tube with internal threads, also known as an external fitting ； Another type is a fitting that combines internal and external threads, known as a plug. Although it is not called a reducer, it functions as one. ❷ Steel reducer pipes: Steel reducer pipes are divided into two types – seamless and welded. Seamless reducer pipes are made by pressing seamless steel tubes, while welded reducer pipes are created by cutting steel plates and then rolling and welding them together; they are also known as welded reducer pipes. Both types of reducer pipes are available in concentric and eccentric configurations. The bottom of the eccentric reducer has a straight edge, which allows the bottom of the pipe to be at a horizontal level, facilitating the discharge of material from the pipe during shutdown for maintenance. The size range of seamless reducing pipes is DN25 to 600 mm, while that of welded reducing pipes is DN200 to 1500 mm. Steel reducer pipes, as finished pipe fittings for pipelines, became available only after the 1970s; in the early days, diameter changes on pipeline systems were mostly carried out on-site. The significant change in pipe diameter occurs during strip extraction and welding at the pipe ends, where it is directly processed into a reduced-diameter pipe ； Those with little variation in diameter are formed by direct forging after heating at the pipe end. Manufacturing reducers by draw-welding requires a lot of labor, results in numerous welds, and makes it difficult to ensure welding quality. At present, this method is prohibited for manufacturing process pipelines where high welding quality is required. Other pipe fittings ❶ End caps. End caps are plugs used to seal the ends of pipes. The commonly used end caps are of oval and flat-plate types. Elliptical end caps, also known as pipe caps, have a size range of DN25 to 600 mm and are commonly used in medium and low-pressure pipelines. Flat cover end caps are divided into two types based on their installation location: one type has a flat cover end cap that is slightly larger than the outer diameter of the pipe, and it is welded outside the pipe. Another approach is to use a flat-ended head that is slightly smaller than the inner diameter of the tube, with the head plate being placed inside the tube and welded there. The commonly used size range is DN15 to 200 mm; these end caps are mostly used in pipes with lower pressure. ❷ Boss: A boss, also known as a nozzle, is a component used in process pipelines within the field of automatic control instruments. It is installed by the professionals dealing with process pipelines, which is why the boss is also classified as a pipe fitting. One-sided pipe fittings used for process pipelines fall into this category as well; one end of such fittings is welded to the main pipe, while the other end is either used to install other components or to connect another pipe. Their size range is DN15 to 200 mm, and they are used in high, medium, and low pressure pipelines. ❸ Blind flanges: These are used to isolate the medium within a pipeline. Depending on the operating pressure and the type of flange sealing surface, they can be classified as follows: 1. Smooth-surface blind flanges, which are used in conjunction with flanges featuring smooth sealing surfaces; their applicable pressure range is 1.0–2.5 MPa. 2. Convex blind plate, which has a convex surface on one side and a concave surface on the other, and is used in conjunction with flanges featuring convex and concave sealing surfaces. The operating pressure is 4.0 MPa, with a size range of DN25 to 400 mm. 3. Trapezoidal groove face blind flange, used in conjunction with flanges featuring trapezoidal groove sealing surfaces; the operating pressure range is 6.4–16.0 MPa. The size range is DN25 to 300mm. 4. The “8”-shaped blind flange is also available in three types: smooth surface, ribbed surface, and trapezoidal groove surface. The operating pressure for this type of blind flange is the same as that for the other three types. What sets the “8”-shaped blind flange apart is that it combines two functions in a single component, that is, it integrates the blind flange with the gasket and fixes them together. When a blind plate is inserted into the flange, the washer that remains exposed on the outside serves as an intuitive indicator of whether the pipeline has been disconnected. 5. There are various materials available for manufacturing \"8\"-shaped blind flanges, and the choice is determined by the temperature and pressure of the medium being transported. For ordinary low-pressure pipelines where the temperature does not exceed 450°C, the materials used include Q235A, steel grade 20, and steel grade 25 ； At temperatures between 450 and 550°C, the materials used are 15CrMo and 1Cr5Mo. Stainless steel should be used when the pressure is between 4.0 and 16.0 MPa and the temperature is above 550°C. Flanges, gaskets, and bolts: Flanges are components that serve to connect elements in process pipelines. This type of connection is widely used, such as for connecting pipes to process equipment, as well as for connecting flanged valves and accessories on pipes. Flange connections offer both the flexibility for installation and removal as well as reliable sealing. The media transported through process pipelines are diverse in type, and their temperatures and pressures vary as well; therefore, different requirements are placed on the strength and sealing performance of flanges. To meet the requirements of process pipeline installation projects, a wide variety of flanges with different structures and pressures have been developed. The following is a brief introduction to various flanges: ❶ Butt-weld flanges. Butt-weld steel flanges are the most commonly used type in medium and low-pressure process pipelines. The method of fixing a flange to a pipe involves placing the flange over the pipe end and welding the inner and outer edges of the flange to secure it; this approach is suitable for nominal pressures not exceeding 2.5 MPa. Flat welding flanges used for connecting carbon steel pipes are generally made from Q235A and No. 20 steel plates ； Flat welding flanges for stainless acid-resistant steel pipes are manufactured from stainless acid-resistant steel plates of the same material as the pipes. The sealing surfaces of flat-welded steel flanges are generally smooth, with shallow grooves machined on them. Usually referred to as the water line, as shown in the figure: The specification range for butt-welded steel flanges is as follows: For nominal pressures of PN0.25 and 0.6 MPa, the diameter ranges from DN10 to 2000 mm ； Those with PN1.0～2.5MPa are DN10～600mm.