Radius Elbow

Note that the elbow radius (the corner of the L-shape) is 1 in.

From: e-Design, 2015

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Erosion and Sand Management

Yong Bai, Qiang Bai, in Subsea Engineering Handbook (Second Edition), 2019

18.4.5 Erosion in Long Radius Elbows

In this model, the erosion condition in a long radius elbow has been studied on the basis of a standard elbow mechanistic model. To extend the mechanistic model to be able to predict the penetration rate in long radius elbows, a new term called the elbow radius factor is introduced [13]. The elbow radius factor (ERFr/d) is defined as follows:


where PnL is the maximum penetration rate in the long radius elbow, and Pnstd is the maximum penetration rate in a standard elbow. The introduction of elbow radius factor preserves the accuracy of the mechanistic model for standard elbows and extends it to predict penetration rates in long radius elbows.



ERFr/d: elbow radius factors for long radius elbows;

Cstd: r/d of a standard elbow; Cstd is set equal to 1.5;

ρf: fluid density;

μf: fluid viscosity;

dp: particle diameter;

r: radius of curvature of the elbow.

This equation accounts for the elbow radius curvature effect in different carrier fluids and sand particle size. Note that this equation is based on a sand particle density of 165.41b/ft3.

The model did not investigate the effects of turbulent fluctuation on the erosion predictions because direct impingement is the dominant erosion mechanism for elbows. However, as the radius of curvature increases significantly, the long radius elbow becomes closer to a straight section of pipe and the random impingement mechanism can become important.

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URL: https://www.sciencedirect.com/science/article/pii/B978012812622600018X

Pipe Fittings

Roy A. Parisher, Robert A. Rhea, in Pipe Drafting and Design (Fourth Edition), 2022

Drawing the 90° Long-Radius Elbow

Two “Step-by-Step” methods will be presented for constructing the 90° long-radius elbow. Figure. 3.7 depicts steps to draw a double-line symbol of a 14” elbow using AutoCAD commands. Figure 3.8 provides the steps required to draw a single-line 12” elbow symbol.

Figure 3.7. 14″-90° elbow. AutoCAD step-by-step drafting procedure.

Step 1.

Create a 21″ radius ARC using the Center, Start, End option. (14″ NPS × 1½ = 21″).

Step 2.

Develop the elbow by OFFSETing the centerline arc 7″ (one-half the pipe’s OD) above and below.

Step 3.

Change the middle arc to the “Center” linetype and add the weld lines.

Step 4.

Create a BLOCK of the elbow. Use the names assigned to the various symbols found in Figure 3.68. INSERT the symbol as needed.

Figure 3.8. Single-line 12″-90° elbow. AutoCAD step-by-step drafting procedure.

Step 1.

Construct an 18″ radius CIRCLE (12″ × 1½ = 18″). Change the circle’s lineweight to match the pipe’s (0.53 mm).

Step 2.

ERASE, BREAK, or TRIM the circle from the top quadrant to the left quadrant. All that should remain is a 90° arc.

Step 3.

Add the elbow’s weld dots. Create with weld dots with the DONUT command. The donut will have an inside radius of 0.0″ and an outside radius of 1.75″.

Step 4.

Create a BLOCK of the elbow. Use the names assigned to the various symbols found in Figure 3.68. INSERT the symbol as needed.

NOTE: The step-by-step instructional procedures presented using computer-aided drafting techniques presume each student has a comprehensive knowledge of basic AutoCAD commands. These instructional steps provide a simple method to create each fitting. They are not intended to restrict the student to any particular series of commands. Each student is encouraged to experiment with various commands which may achieve the same result in a more efficient manner.

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Pipe Fittings

Roy A. Parisher, Robert A. Rhea, in Pipe Drafting and Design (Third Edition), 2012

Short-Radius Elbow

Another elbow that may be used under certain circumstances and with permission from the customer is the 90° short-radius elbow. The 90° short-radius ell makes a much sharper turn than does the long-radius ell (see Figure 3.10). Conversely, the short-radius ell also creates a rather large pressure drop inside the line and does not have the smooth flow characteristics the long-radius ell has. For these reasons, the short-radius ell is seldom used.

Figure 3.10. Long-radius and short-radius elbows.

A simple formula can be used to calculate the center-to-end dimension of a 90° short-radius ell: Fitting length equals 1 times NPS (nominal pipe size). Or, even simpler, fitting length equals nominal pipe size (Figure 3.11).

Figure 3.11. Center-to-end dimension of a 90° short-radius elbow.

Example: The length of an 8″ 90° long-radius elbow is


NOTE: Use this formula for butt-weld fittings only.

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Safety Relive Valves Design

Alireza Bahadori Ph.D., in Natural Gas Processing, 2014

7.31 Discharge piping support


If guided laterally, a maximum of 3 m of piping may be supported by the welding neck flange and long radius elbow on safety valve outlet. If moved,then 3 m of piping is used or if high pressures are involved, mass and reaction forces should be supported as close to valve discharge as possible using a free support


Long piping should be anchored to the equipment as close to the valve discharge as possible, providing the expansion between the connection and anchor is taken care of and guided from that point up a tower, steel structure, or into the closed header piping


Stops and guides shall be provided to support piping independent of the safety valve when the safety valve is removed


Discharge piping shall be designed to prevent undesirable temperature and mechanical stresses being placed on the valve body and inlet connection


Supports shall be provided for discharge piping to compensate for the reaction force or load caused by discharging of the valve.

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ByPETER SMITH, in Piping Materials Guide, 2005


The most commonly used piping components and the dimensional standards are as follows:

Type of Component Function Butt-Weld Ends Threaded-Socket-Weld Ends Held between Flanges
90°long radius (LR) elbow Change direction ASME B16.9 (½–48 in.) ASME B16.11 (½-4 in.) Not applicable
90° short radius (SR) elbow Change direction ASME B16.28 (½–48 in.) Not applicable Not applicable
45° Elbow Change direction ASME B16.9 (½-48 in.) ASME B16.11 (½-4 in.) Not applicable
180° return Change direction ASME B16.9 (½-48 in.) Not applicable Not applicable
Equal tee Change direction ASME B16.9 (½-48 in.) ASME B16.11 (½-4 in.) Not applicable
Reducing tee Change direction and size ASME B16.9 (½-48 in.) ASME B16.11 (½-4 in.) Not applicable
Reinforced branch (O'let) Change direction and size Manufacturer's standard Manufacturer's standard Not applicable
Eccentric reducer Change size ASME B16.9 (½-48 in.) ASME B16.11 (½-4 in.) Not applicable
Concentric reducer Change size ASME B16.9 (½-48 in.) ASME B16.11 (½-4 in.) Not applicable
Flanges Join pipe and components ASME B16.5 (½-48 in.) ASME B16.5 (½-24 in.) Not applicable
Flanges Join pipe and components ASME B16.47 (26-60 in.) Not applicable Not applicable
Couplings Join pipe and components Not applicable ASME B16.11 (½-4 in.) Not applicable
Unions Join pipe and components Not applicable BS 3799 Not applicable
Spectacle blinds, spades and spacers Isolation Not applicable Not applicable API 590 or company's standards

Each piping component type also has one or more methods of being connected to pipe or another component. The end connection chosen can be selected from one of the flowing commonly used alternatives:

Butt weld.

Plain end or socket weld.



Other, less commonly used methods include hubbed connections and SAE flanges, however the preceding four types cover a vast majority of end connections and on certain projects, all requirements.

Dimensional Standards Covering End Connections of Components

The most commonly used dimensional standards for end connections are as follows:

End Connection Joint Type ASME Standard Size
Weld end (WE) Butt weld ASME B16.25 All sizes
Plain end (PE) Socket weld ASME B16.11 4 in. and below
Threaded (Thd) Screwed ASME B1.20.1 4 in. and below
Flanged (Flg) Flanged ASME B16.5 ½–24 in.
Flanged (Flg) Flanged ASME B16.47 26–60 in.

Generally, a piping component has the same connection at both ends. However, it is possible to have a mixture, especially with valves; for example, flanged by threaded, flanged by socket weld, or threaded by socket weld. This is acceptable as long as both end connections satisfy the design conditions of the fluid being transported in the piping system.

As mentioned previously, numerous other national standards cover the dimensional standards for piping components, however, differences in the dimensions and tolerances, in a vast majority of cases, could make the components incompatible.

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Pipeline pigging

Maurice Stewart, in Surface Production Operations, 2016

13.3.4 Bends Forged bends

Bends with a relatively short radius must be factory made and are generally forged to a number of standard radii. Radius of a bend is measured to its centerline as shown in Figure 13.9. Forged bends have an increased wall thickness, with the additional material added on the inside diameter. This should be avoided.

Figure 13.9. Dimensions of common factory bends.

Short radius (D) bends should not be used in pipelines if pigs are to be run. Long radius (1.5D) elbows are only suited for spheres. Pigs can be designed to pass long radius elbows but they are generally less effective than those that require a longer radius bend. For pigging, bends should have a minimum radius as follows:

10D for pipelines 4 in. and smaller

5D for pipelines 6-12 in.

3D for pipelines larger than 12 in.

Ideally, bends should not be installed adjacent to one another. At least three diameters of straight pipe should be installed between any two bends. Field bends

When laying a pipeline, the pipe must be bent to the contour of the land through which it is passing. Bending machines are normally used. The pipe's allowable yield stress dictates the minimum allowable bend radius. When bending the pipe, its allowable yield stress should not be exceeded. When bends are made without a bending machine, the bends may not be of a uniform radius and could result in a series of sharper bends that might not be acceptable for pigging. Bends made without a bending machine are susceptible to ovality, resulting in a reduction in diameter, usually in the place of the bend. This should be avoided. Localized deformation should be limited to no more than 2% of the pipeline diameter. Miter bends

Miter bends are made by cutting the end of the pipe at an angle to achieve a change in direction of the pipe as shown in Figure 13.10. Miter bends should be avoided; however, small angles, not exceeding 3°, may be necessary to achieve proper fit up at a field joint of mating pipes. A bend made of a series of miter joints is unacceptable in all pipelines. If a miter bend exists in a pipeline, the pig supplier must be notified of the dimensions to make sure that a suitable pig is provided.

Figure 13.10. Typical miter bend.

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Compressed air systems

Alireza Bahadori PhD, in Essentials of Oil and Gas Utilities, 2016

2.8.4 Discharge piping

Unnecessary pockets should be avoided. The discharge pipe should be the full size of the compressor outlet or larger, and it should run directly to the aftercooler if one is used.

The discharge piping is considered to be the piping between the compressor and the aftercooler, the aftercooler separator, and the air receiver.

The discharge pipe should be as short and direct as possible, with long-radius elbows where bends are necessary, and should have as few fittings as possible. If the design cannot avoid pockets between the compressor and the aftercooler or receiver, it should be provided with a drain valve or automatic trap to avoid accumulation of oil and water mixture in the pipe itself.

For a centrifugal compressor, if it is necessary to increase the pipe diameter just beyond the compressor discharge flange, this transition should be gradual.

The installation of a safety valve between the aftercooler and the compressor discharge piping should be considered.

If an aftercooler is not used, the discharge pipe should run directly to the receiver, the latter should be set outdoors, and as close to the compressor as is practical.

Piping after the air receiver will have accessories dictated by the application (dryers for oil-free air) preseparators, afterseparators for the dryers, and so on.

If the discharge line is more than 30m long, pipe of the next size up diameter than that calculated should be used throughout. If the designer wants to install a shut off valve between the separator and the receiver, installation of a suitable safety valve between the compressor and the valve shut off point is mandatory.

A method of bleeding the air pressure from the system between the shutoff valves and the compressor discharge is required. This may consist of a simple plug valve located in the piping between these two points.

To detect possible clogging of aftercooler tubes, a means of monitoring the discharge pressure between the aftercooler and compressor discharge should be provided.

The main header size for plant air should not be less than DN 80 (3 in.).

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Bulk piping items

Karan Sotoodeh, in A Practical Guide to Piping and Valves for the Oil and Gas Industry, 2021


An elbow is used to change the piping direction. Piping systems commonly contain 45° (Fig. 14.8) or 90° elbows. The end connection of an elbow can be socket weld, butt weld, or thread end, same as the piping. The radius of an elbow can be long or short. Fig. 14.9 compares short and long radius elbows. A long radius elbow provides a smoother flow than the short radius, but it requires more space. A short radius elbow creates a relatively large pressure drop and fluid does not flow as smoothly as it does in a long radius elbow. For this reason, short radius elbows are not popular.

Fig. 14.8. 45° Elbow.

Fig. 14.9. Comparison between long radius and short radius.

The radius of a long radius elbow is 1.5 times of the elbow nominal size (see Fig. 14.10). With a short radius elbow, the radius is equal to the elbow nominal size (see Fig. 14.11). Both Figs. 14.10 and 14.11 show 90° elbows.

Fig. 14.10. Long radius elbow.

Fig. 14.11. Short radius elbow.

Bends provide a longer radius, 3–5 times longer than the 1.5 X nominal size. Fig. 14.12 shows a three-dimensional (3D) bend and 90° rotation. The elbows in sizes 2″ and below are usually forged and larger sizes are made of wrought pipe. Fig. 14.13 shows small size forged elbows, one with a threaded end and the other with a socket weld end. ASME B16.11 covers wrought carbon steel and alloy steel factory-made butt welding fittings from ½″ to 48″. ASME B16.11 covers socket and threaded fittings designed as per class 2000, 3000, and 6000 for threaded and 3000, 6000, and 9000 for socket weld fitting.

Fig. 14.12. Sketch of a 16″, R = 3D and 90° rotation.

Fig. 14.13. Forged elbows.

An elbow can be made as miter joint or miter, which is a joint made by beveling each of two parts to be joined (see Fig. 14.14). A miter has a high-pressure drop, so it is not recommended for making elbows.

Fig. 14.14. Miter bends.

A pipe return provides 180° pipe rotation (Fig. 14.15). Pipe return is like welding two 90° elbows together, but it is one singular piece piping connection.

Fig. 14.15. Pipe return.

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Piping Maintenance and Repairs

A. Keith Escoe, in Piping and Pipelines Assessment Guide, 2006

Full Encirclement Repair with End Plates at an Elbow

Unlike the bolted elbow clamp discussed previously, this repair involves a welded enclosure with two flat end plates enclosing an elbow. Instead of serrated edges on the end plates or the use of shear pins along the clamp body, the end plates are fusion welded to the enclosed pipe. Depending on the size and actual location of the leak, thickness of the elbow, and other parameters, the enclosure may be sized to enclose the entire elbow or only a portion thereof. The following procedure is followed in designing such an enclosure:


Select the material, diameter, and corrosion allowance in the same manner as for a straight section of pipe. Note that if a short radius elbow is used for the enclosure, the diameter may have to be somewhat larger than would otherwise be required, as noted later.


Calculate the required thickness for the elbow following the requirements of ASME B31.3 Paragraph 304.2.1 for Pipe Bends. The equation for the required wall thickness is as follows:

Eq. (6-34)trell=PD2[(SEI)+PY]
where, P, D, E, and Y have already been defined.

For the intrados (inside bend radius),


For the extrados (outside bend radius),

where I = 1.0 at the sidewall on the bend centerline radius

R1 = bend radius of welding elbow or pipe bend, in. (mm)


Determine if a short radius elbow may be used to enclose a leaking long radius elbow. This may be possible if the enclosure diameter is made large enough and the amount of welding required is decreased.

Note that if a short radius elbow is used, the distance between the OD of the pipe elbow and the ID of the enclosure elbow is not constant along their perimeters. This should be a consideration in sizing and positioning the enclosure, especially if there is an obstruction on the elbow, such as epoxy wrap.


Find where the enclosure must terminate to ensure that it encloses the area of concern (the leak), permits welding to sound metal, and is an adequate distance from the pipe-to-elbow welds.

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Heat recovery steam generators: performance management and improvement

V. Ganapathy, in Power Plant Life Management and Performance Improvement, 2011

16.5.2 Feedwater chemistry

Flow accelerated corrosion (FAC) is a recently recognized problem with HRSGs. FAC is a process whereby a normally protective magnetite (Fe3O4) layer on carbon or low alloy steel dissolves into a stream of flowing water or water/steam mixture. Both the PH and temperature and level of dissolved oxygen in the stream influence the stability and solubility of the magnetite oxide layer.

Oxidation-reduction potential (ORP) monitoring techniques show good promise as a method to analyze and control feedwater chemistry to minimize FAC. Another promising technique for FAC control in future HRSGs is to fabricate sharp radius elbows out of low chromium alloys, such as the 21/4 Cr–1 Mo variety. This prevents FAC without the need for complex chemistry control methods.

Past industry water chemistry practices believe that all of the dissolved oxygen must be eliminated from feedwater to control corrosion. To deoxygenate the feedwater, oxygen was mechanically removed by the condenser/deaerator with supplemental additions of an oxygen scavenger. (e.g. hydrazine) being applied to maintain a 40–100 ppb hydrazine residual. Maintaining an oxygen scavenger residual causes the feedwater to reduce more and more and has for all ferrous systems produced the opposite desired effect of producing a protective oxide film to one where erosioncorrosion of iron based materials is increased. This mechanism is active in condenser shells, feedwater and wet steam piping, feedwater heaters and economizers. FAC occurs in many materials but more in carbon steel piping in the 100 to 250°C range and hence economizers and low pressure evaporators are affected by this process. Sharp bends where the fluid velocity can erode the protective layer should be avoided.

Some European standards require that the maximum O2 in feedwater be increased from 0.02 to 0.1 mg/kg. Flow velocity also has to be lower to reduce erosion of the protective magnetite layer.

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