What is elbow in a pipe fitting?

Elbow is a commonly used pipe fitting in the pipeline system, which is used to connect two pipes and change the direction of the pipe (common angles are 90°, 45°, 180°, etc.). It makes the fluid turn from the original straight direction to a new direction by setting a certain bending curvature in the pipe. The existence of elbows will inevitably introduce additional resistance and pressure loss: the larger the angle, the greater the resistance and vortex generated when the fluid passes through. Therefore, when designing the pipeline system, it is necessary to reasonably select the elbow angle according to the fluid characteristics, flow rate and system requirements to take into account the flow demand and energy consumption control.

In addition to changing the direction, elbows can also be used to achieve a gradual change in pipe diameter (reducing elbow) or connect pipes of different sizes. In piping design, special elbows with wall thickness and material compensation functions are also commonly used. The manufacturing method and structural form of the elbow determine its basic functional principle: to ensure the smooth flow of fluid, reduce vibration, and reduce pipeline stress by controlling the bending radius and angle. For example, when it is necessary to reduce the pressure loss of the pipeline elbow, a long radius elbow (radius of curvature ≥ 1.5D) can be used, and when the space is limited or the system rigidity is increased, a short radius elbow (radius of curvature ≈ 1D) can be used.

Classification of elbows

Elbows can be classified from multiple dimensions, including angle, radius of curvature, material, manufacturing process and connection method. Common classifications are as follows:

● By bending angle: the most commonly used are 90° elbows (right-angle elbows) and 45° elbows, and there are also 180° (return elbows), 60° and other special angle elbows; even arbitrary angle elbows can be customized in terms of technology. Elbows of different angles have different effects on fluid resistance. Generally, 90° and 45° are often used in design.

● By curvature radius: usually defined as the ratio of the centerline radius of the elbow to the outer diameter of the pipe. The curvature radius of the long radius elbow (LR) is 1.5 times the pipe diameter (R=1.5D), and the curvature radius of the short radius elbow (SR) is 1 times the pipe diameter (R=1.0D). In actual projects, the 1.5D long radius elbow is used by default; when the space is limited or there is no strict requirement for resistance, the short radius 1D elbow can be selected. In addition, there are large radius elbows such as 3D and 5D, which are often used to compensate for pipe expansion or vibration reduction. Compared with short radius, long radius elbows have a longer center distance under the same pipe diameter, smaller fluid resistance and higher pressure bearing capacity; short radius elbows are smaller in size and have greater material strength per unit volume, but the local loss of fluid is slightly larger.

● According to the material: There are many types of elbow materials, including carbon steel, stainless steel, alloy steel, malleable cast iron, non-ferrous metals (such as copper, brass) and plastics. Common carbon steel elbows (such as 20#, 20G, Q235, etc.) are used for general working conditions; stainless steel (such as 304/316L) or alloy steel (such as chrome-molybdenum steel) is used when corrosion resistance is high; copper elbows or galvanized steel elbows are commonly used for building water supply or decorative pipelines; ductile iron or PVC elbows are commonly used for water supply and drainage pipelines. These material classifications are listed.

● According to the manufacturing process: including cold and hot processing elbows and welding, casting elbows, etc. Common processes include stamping elbows, push-forged elbows, hot-pressed rolled (hot-pushing) elbows, casting elbows and butt-welded elbows. For example, large-diameter or complex alloy elbows often use hot bending (heating bending) technology; small-diameter thin-walled elbows can use cold stamping or extrusion technology; standard parts are mostly butt-welded forging.

● According to the connection method: including butt welding connection (common in high-pressure systems), socket welding, threaded connection, flange connection, clamp/loop connection (mostly seen in plastic pipes), etc. Butt-welded elbows can withstand higher pressures; socket-welded and threaded elbows are easy to install but have limited pressure ratings; flange elbows are easy to disassemble and maintain; hot-melt butt or electric-melt socket connections are commonly used for plastic pipes.

Main standards and typical specifications

There are complete standards and specifications for pipe elbows at home and abroad. Common standards include:

● Chinese national standards: mainly GB/T 12459 (steel butt-welded seamless pipe fittings, applicable to DN10~DN500), GB/T 13401 (steel plate butt-welded pipe fittings), GB/T 14383 (forged steel socket-welded pipe fittings), GB/T 17185 (steel flange pipe fittings), etc. These standards specify the size, nominal diameter, wall thickness, pressure rating, marking and inspection requirements of elbows. GB/T12459-2017 implemented in 2017 merged the original GB/T12459 and GB/T13401, expanded the size range, and added calculation instructions for R=3D elbows and special angle elbows. Industrial pipelines also have industry standards such as HGJ series for petrochemicals, SH series for petrochemicals (such as SH3408, SH3065-94, etc.) and DL/T 515-2004 "Power Station Bends" for the power industry.

● American ASME/ANSI standards: mainly ASME B16.9 (factory-made forged butt-weld pipe fittings, including elbows, tees, reducers, etc.), ASME B16.28 (steel butt-weld short radius elbows and return bends), ASME B16.11 (socket welding and threaded forged pipe fittings), etc. These standards cover the size, wall thickness, end form (groove, thread) and inspection requirements of forged steel elbows. For flange connection elbows, ASME B16.5 (pipe flange) and ASME B16.47 (large size flange) must also be matched.

● European DIN/EN standards: For example, the German standard DIN 2605 series specifies the size and nominal pressure of butt-weld elbows (ordinary elbows and pressure reducing elbows), and EN 10253-2 also gives welding pipe fittings standards. In terms of ISO international standards, EN 10253-2 (or equivalent ISO standards) involves steel butt-weld pipe fittings, including long radius and short radius elbows. Others such as Japan's JIS B2311/B2312/B2313 series also correspond to similar requirements.

● Common specifications: elbow specifications are generally selected according to the nominal diameter of the pipe (DN/DNPS) and the angle. For example, conventional 90° long radius elbows, 90° short radius elbows, and 45° long radius elbows are the most widely used types in engineering piping. The standard specifies key dimensions such as "A (center to end face distance)" and "B" under different pipe diameters and angles, as well as the corresponding wall thickness and tolerance (as shown in the ASME B16.9 table). In special occasions, reducing elbows or flanged elbows are used, and the specifications must meet the matching of the interface pipe diameter and pressure level at the same time.

Comparison between elbows and bends

The main difference between elbows and bends lies in manufacturing and use occasions:

● Production process: elbows usually refer to factory accessories manufactured according to certain standards, which can be made by butt welding or forging pipe fittings; they can also be obtained by cold drawing or hot bending of pipes and then cutting. Bends are bent pipe fittings (such as hot press bends) that are formed in one step after heating or cold bending the pipe. In contrast, the manufacturing tolerance and shape of elbows are more regular and repeatable; bends are more flexible and can be customized on site, or they can be formed by continuous bending with a large radius.

● Curvature radius: Bends usually use a larger curvature radius (such as R=2D, 3D and above), and the pipe diameter transitions smoothly; elbows are often R=1D (short radius) or 1.5D (long radius). As mentioned above, the larger the R, the smaller the internal resistance.

● Mechanical properties: Due to different thicknesses and processing methods, the strength of elbows of the same diameter per unit bending area is usually higher than that of elbows. For example, to improve wear resistance and erosion resistance, thick-walled short-radius elbows can be used; and when conditions permit, large-radius elbows can be used to reduce scouring forces.

● Cost and application: Generally speaking, the production cost of elbows is slightly lower than that of elbows of the same specification, but elbows need to be subsequently treated with anti-corrosion (such as painting or coating), otherwise they are prone to corrosion. Elbows are often directly sprayed or treated with corrosion resistance, and the quality is reliable. In pipeline construction, compact elbows can be used first in narrow spaces, and lower-cost continuous elbows can be used in places with ample space. In addition, the elbows commonly used in power stations and other industries (such as DL/T 515 standard) are forged elbows designed for large steam pipelines.

Analysis of typical industry applications

Each industry has its own characteristics in terms of the selection and technical requirements of elbows according to the different media, pressure, temperature and safety requirements:

● Petrochemical industry

The pipeline pressure level in this industry is high, the temperature range is wide, and the media is variable (crude oil, gas, acid and alkali liquid, etc.), so the elbow materials and process requirements are strict. Commonly used carbon steel (such as 20#, 15CrMo alloy steel), stainless steel or corrosion-resistant alloy elbows are used to adapt to high temperature and high pressure corrosive media. Elbows are usually forged butt welded, and non-destructive inspections such as radiographic flaw detection are performed. 

When designing, long radius elbows are preferred to reduce pressure drop and pipe stress, and thick-walled short radius elbows are also used in high medium flow rate pipelines to improve wear resistance. Petrochemical enterprises have formulated special standards (such as SH3065-94 Technical Standard for Tubular Furnace Bending in Petrochemical Industry) to guide the selection of elbows. The general process requirements stipulate the use scenarios of optional R=1D, 1.5D, and even 3D elbows to take into account both economy and safety.

● Water supply and drainage (municipal and construction)

This type of pipeline has low pressure and focuses on durability and economy. Galvanized steel pipes or plastic pipes such as PPR and PE are commonly used in water supply pipelines. The elbows are corresponding galvanized elbows or plastic hot-melt elbows according to the material. Drinking water pipe elbows must use materials that meet sanitary standards (such as stainless steel or food-grade plastics) and be treated with anti-corrosion and anti-scaling; drainage pipes (sewage) are often made of hard plastic (PVC, HDPE) or ductile iron elbows. 

When the pipe diameter is large, on-site hot bending is sometimes used to reduce costs; in buildings with limited space, compact elbows prepared by the factory are preferred. During construction, the interface compatibility and installation convenience with other pipe fittings (valves, tees) must also be considered.

● Power industry

Steam and water supply pipelines in thermal power, nuclear power and other power plants use high-temperature and high-pressure pipelines, and elbows generally use large-diameter forged products for butt welding. The power industry standard DL/T 515 "Power Station Elbows" stipulates the manufacturing and inspection requirements for main steam, reheat steam and main feed water pipeline elbows. In terms of selection, large-radius (1.5D or larger) elbows are often used for main steam pipelines to reduce pressure drop and thermal stress. 

Short radius can be selected for auxiliary pipes or low-temperature pipelines when space is cramped; welds and materials (such as 20G, 12Cr1MoV steel, etc.) must meet the requirements of long-term stable operation. In addition, power stations also need to consider equipment installation. For example, the inner casing system often uses a combination elbow with an inner pipe of 1.5D and an outer pipe of 1D to simplify casing construction.

● Shipbuilding industry

Marine pipelines are often in marine corrosive environments, and materials are required to be resistant to salt spray corrosion. Ship elbows are mostly made of stainless steel (316L) or corrosion-resistant alloy parts, and anti-corrosion spraying is performed. Relevant standards such as GB/T 10752-1995 (Butt-welded steel pipes for ships) and international classification society specifications specify the size and performance of marine pipe fittings. 

Common applications include domestic water, fuel oil, condensate, air conditioning circulating water and other pipe systems, which have higher requirements for sealing, welding quality and corrosion resistance.

● Food and pharmaceutical industry

Food, beverage and pharmaceutical pipelines require strict sanitary conditions. Elbows are generally made of 304/316L stainless steel, and the inner and outer surfaces are mirror polished or electrolytically polished to ensure that there are no cracks and dead corners on the surface, and the finish meets Ra0.8μm or even higher to prevent the growth of microorganisms. 

In addition, welding elbows are often combined with quick-release clamps to facilitate cleaning and maintenance. The ASME BPE standard or 3A and EHEDG certifications commonly used in the industry regulate the design of sanitary pipe fittings. Sanitary elbows also require that the welds undergo non-destructive testing and that all materials and lubricants comply with food contact safety regulations.

As an indispensable key component in the pipeline system, the design, manufacture and selection of elbows and bends are directly related to the safety, efficiency and economy of the engineering system. With the advancement of industrial technology and the continuous expansion of application fields, various standards and new materials are constantly being improved and developed, so that elbow products can better meet the diverse and demanding working conditions.