Piping and Pipeline Bends

1 Overview of the Article

The article covers bends used in the oil and gas industry. The bends described in the article are for steel piping and pipelines. The article will cover bend radius, bend specifications etc.

To present a holistic picture of bend requirements, requirements such as thickness, dimensions etc., have been extracted from various codes and standards, including ASME B31.3, 31.4, 31.8, 16.49, 16.9 etc.

Also, the basis for selecting the type of bends, bend radius, calculations related to bends etc., are presented in this article.

2 Introduction

Pipelines traverse a long distance. In some cases, they run for a thousand kilometres. The distance covered by the pipeline is not a straight distance; the pipeline changes direction as it traverses the distance covered. The introduction of bends achieves a change in direction. The bends might utilise the pipeline flexibility or an introduced manufactured or fabricated bend.

In oil and gas facilities, piping connects equipment and other piping etc. The connection is not achieved by straight piping; bends are required to achieve the desired alignment and connection.

The type of bend or elbow is a function of the angular directional change desired, the pipeline design characteristics, the available space to make the direction change etc.

2.1 Bends and Elbows

The terms bend and elbow are frequently used interchangeably; however, there is a difference between the two.

A pipe bend is a term used to describe an “offset” or change in the direction of a piping or pipeline, usually for specific reasons. It is a vague term that also includes elbows. The word bend is often used in pipelines. It describes a change in the direction of a pipeline achieved by not utilising the standard elbows.

Elbow is an engineering term used to describe a bend with a radius of curvature mostly 1.5D and 1D. Dimensions of 1.5D and 1D elbows are given in ASME B16.9; 2001 standard; the 2007 version included dimensions of 3D elbows; The inclusion of 3D elbow dimensions implies the definition of an elbow can be extended to bends with radius 3D (3 x nominal pipe size).

Below is a summary of the comparison between bends and elbows.

• Elbows bend angles are 45, 90 and 180 degrees return, but bends can have any angle such as 15, 17, 30, 75 degrees etc.
• Bends are fabricated per the piping need; however, elbows are prefabricated and standard and available off the shelf.
• Elbow is a standard fitting while the bend is a custom fabricated fitting
• Bends are never sharp corners, but elbows are.
• All elbows are bends, but all bends are not elbows.
• Due to the larger radius of bends, the flow through bends is smoother compared to elbows
• Bends are utilized in piggable pipelines, while elbows are utilized in piping where pigging is not applicable

As per ASME B31.3: The minimum required thickness, t_m, of a bend, after bending, in its finished form, shall be determined per the equation below given in section 304.2

t_m = Minimum required thickness, including mechanical, corrosion, and erosion allowances

t   = Pressure design wall thickness

c = Sum of all tolerances (Mechanical and corrosion allowance)

Where at the intrados (inside bend radius)

At the extrados

And at the sidewall on the bend centreline radius, I = 1.0

P = Internal design gage pressure

D= Outside diameter of pipe as listed in tables of standards or specifications or as measured

E = Quality factor from Table A-1A or Table A-1B of ASME B31.3

t = Pressure design thickness

S = stress value for material from Table A-1 or Table A-1M of ASME B31.2

T = Pipe wall thickness (measured or minimum in accordance with the purchase specification)

W = Weld joint strength reduction factor in accordance with para. 302.3.5(e) of ASME B31.3.

Y = Coefficient from Table 304.1.1 of ASME B31.3.

R₁ = bend radius of welding elbow or pipe bend

3 Classification of ELBOWs

Bends can be classified based on bend radius or angle of turn. See subsections for details

3.1 90 Degrees Elbow

Of all the types of elbows, this is the most often used. It is used when a pipe changes direction to either left, right, up or down. It can also be installed and rolled in any direction to achieve the desired change.

90 degrees elbow can as well be classified as:

3.1.1 Long Radius Elbow

This is the most often used elbow in oil and gas facilities. It does not provide a sharp bend like the short radius. As the fluid flows through the elbow, there is less frictional resistance compared to the short radius elbow; hence the pressure drop across this elbow is lesser. The bend radius or radius of curvature is calculated using the formula below

Bend Radius = 1.5 x Nominal Pipe Size.

For NPS 12 Pipe

The Bend Radius = 1.5 x 12 = 18 inches

Dimensions of long radius elbow (90 and 45 degrees) are shown in table 1 of ASME B16.09; 2007

3.1.2 Short radius Elbow

This type of elbow is used when there is space constraint and sometimes requires client approval. It provides a quick sharp bend which results in a higher pressure drop than the long radius elbow. This type of elbow should not be used where there is a high likelihood of pipe erosion.

The bend radius or radius of curvature is calculated using the formula below.

Bend radius = 1 x Nominal Pipe size

Refer to Table 4 of ASME B16.9; 2007 for the dimension of short radius elbows.

NPS 18 Pipe radius of curvature is calculated below

Radius = 18 x 1 = 18 inches

3.1.3 Reducing Elbow

This type of elbow is used to join two pipes of different sizes while achieving the desired directional change. This fitting is not very common in oil and gas process piping.

As stipulated in section 8.4.1 of SHELL DEP 31.38.01.11-Gen, client approval is required before using this type of elbow.

Dimensions of long radius reducing elbow are given in ASME B16.9; 2007 table 2.

3.2 45 Degrees Elbow

The 45 degrees elbow is another important fitting used to achieve directional change. It is shorter than 90 degrees because it is half of the elbow. Just like the 90 degrees elbow, the elbow can be manufactured as a long or short radius. Table 1 of ASME B16.9; 2007 gives the dimension of 45 degrees elbow.

3.3 180 Degrees return

The 180 degrees return elbow is a combination of 2 (two) 90 degrees elbows to form the 180 degrees change in direction. Refer to tables 3 and 5 of ASME B61.9; 2007 for dimensions of long and short radius 180 degrees return.

4 Classification of BENDS

Bends are classified either based on the manufacturing process or the bend radius. As previously stated, the bend radius might range from 3D and above; therefore, classification based on bend radius will be discussed in the manufacturing process as applicable.

4.1 Induction Bend

Induction bends or hot bends are manufactured as per ASME B16.49 (Factory-Made, Wrought Steel, Buttwelding Induction Bends for Transportation and Distribution Systems)

The bends are manufactured by heating the pipe and forming the bend under controlled conditions. The thickness of the mother pipe to be bent shall be calculated per section 403.2.1 of ASME B31.4. Kindly refer to other applicable codes and standards.

The formed bend shall be free from buckling, cracks, or other evidence of mechanical damage.

Section 404.2.3 of ASME B31.4 stipulates that the pipe diameter shall not be reduced at any point by more than 2.5% of the nominal diameter, and the completed bend shall pass the specified pig.

Section 2.2 of ASME B16.49 stipulates that the minimum thickness of the bend at the intrados shall satisfy the equation below. Also, the thickness at the neutral axis (see Fig. below) and on the extrados (outer radius) of the bend shall be no less than the mating pipe design thickness or the customer-specified minimum wall thickness

The image below shows bend dimensions and illustrations extracted from ASME B16.49

4.2 Field Bends or Cold Bends

As defined in section 404.2.2 of ASME B31.4, Field bends are cold bends made in the field during pipeline construction to allow the pipe to conform to the contour of the ditch. Field bends include horizontal bends to accommodate changes in direction, vertical bends (sags and overbends), and combination bends. Because the bends are not manufactured in a controlled environment, there shall be adequate precautions to ensure bends are free from buckling, cracks, or other evidence of mechanical damage. In the following paragraphs, I have summarised bends requirements from ASME B31.4 and B31.8.

Section 434.7.1 of ASME B31.4 stipulates that when longitudinal welded pipes are bent on the field, the longitudinal weld should be located on or near the neutral axis of the bend.

Approximately 6 ft. (2 m) tangent lengths are preferred on both ends of cold bends. The tangent length is the length on both sides that should not be bent.

For other requirements to fulfil in making field bends refer to section 841.2.3 of ASME B31.8 and section 404.2.2 of ASME B31.4

Below is an illustration of calculating the total bend angle for a given pipe length utilising the bend radius stated in the table above.

Estimate the maximum directional change that can be achieved by bending a 12m length of 20 NPS pipe.

Solution

The bend angle can be calculated utilising the relationship between the length of an arc and the angle subtended.

L = the diameter length of the pipe (the diameter is the length of the arc formed by the pipe as defined in the table).

L =20 inches

R = 600 inches

Recommended length on both side of the pipe as stated in ASME B31.4 section 434.7.1 is 2m this implies the total available length for bending is 8m (314.96 inches).

This implies the maximum achievable bend

Alternatively, the total length available for bending can be utilised to compute the angle.

In this case L = 8m = 314.96 inches

4.3 Elastic Bends

Elastic bending means the pipe is bent without exceeding its yield strength. If you lift a 12m pipe at the centre, the ends will drop. Lowering it back to the ground will regain its straight shape once on the ground without applying any force to it. The bend formed by the ability of the pipe to form a curve and return to its original shape is elastic.

These types of bends are mostly utilised in buried pipelines to achieve small directional changes vertically due to the change in the elevation of the pipe trench. They are also used to achieve horizontal change in direction to fit the pipe into the ditch.

The elastic bending radius of a pipeline can be calculated using the formula below.

Where

E = Pipe Material Modulus of Elasticity

D = Pipe Diameter

S_b =Bending Stress

4.4 Miter Bend

A Miter bend is not a factory-manufactured fitting but only fabricated to provide the required directional change. It is made by cutting straight run of pipes and welding them together to achieve the desired angular directional change.

Miter bend may be classified as one, two, three, or four weld miter. The higher the number of welds, the smoother and the lesser the turbulence as the fluid flows through the fitting. There are limitations to the use of miter bends.

Shell DEP 31.38.01.11-Gen.stipulates that miter bends may only be used in ASME Class rating 150 and with the Principal’s approval.

Section 404.2.4 of ASME B31.4 prohibits miter bends for systems intended to operate at hoop stress of more than 20% of the specified minimum yield strength of the pipe. Kindly refer to the referenced section 404.2.4 for other restrictions on miter bends.

5 Fittings/Elbow Material Specification

Elbows or bends can be made of carbon steel, stainless steel, duplex stainless etc.

The material specifications covering elbows also cover other fittings such as Tee, Cap, Reducer etc.

Some standards covering fittings, including elbows are listed below:

ASTM A234/ A234M (Piping Fittings of Wrought Carbon Steel and Alloy Steel for Moderate and High Temperature Service¹).

Elbow for low temperature application are manufactured to ASTM A420/ A420M (Standard Specification for Piping Fittings of Wrought Carbon Steel and Alloy Steel for Low-Temperature Service¹)

Other standards include:

A403/A 403M: (Standard Specification for Wrought Austenitic Stainless Steel Piping Fittings¹)

A815/A 815M: (Standard Specification for Wrought Ferritic, Ferritic/Austenitic, and Martensitic Stainless Steel Piping Fittings¹)

6 Basis for Selecting Type of Bend

The type of bend utilised depends on many factors including those below

6.1 Space Available

One of the major problems experienced in piping is space availability, which is even more complicated in offshore platform piping. As previously explained, the larger the bend radius, the larger the space required to make a bend. Therefore elbows utilised in offshore piping are mostly long radius and short radius elbows. Pipelines usually have more space than piping; therefore, space constraint is usually less critical.

6.2 Piggability

Short and long radius pipelines are usually not recommended for piggable pipelines, especially when intelligent pigging is performed. Though intelligent pig tools can be utilised for 1.5D and 3D bends, such as the ROSEN pig tool, a minimum bore is required for the pig to navigate the bend easily. A bend radius of 3D and above is usually recommended for easy intelligent pigging. Before selecting the type of bends, especially in piggable pipelines, kindly consult the pig tool vendor for the minimum required bend radius.

6.3 Client Requirements

Client and project requirements vary. Some clients do not accept the use of miter bends, while some specifically require 3D and above bend radius for a pipeline. Some companies also require an increased cold bend radius compared to Table 1 above.

6.4 Cost

Cost is a key factor in every project. In pipeline projects, cold bends may be utilised for bend angles less than 30 degrees to eliminate the cost of procuring hot bends.

7 References

• ASME B16.49: Factory-Made, Wrought Steel, Buttwelding Induction Bends for Transportation and Distribution Systems
• ASME B16.9: Factory – Made Wrought Buttwelding Fittings
• ASME B31.4: Pipeline Transportation Systems for Liquids and Slurries
• ASME B31.8: Gas Transmission and Distribution Piping Systems
• SHELL DEP 31.38.01.11-Gen: Piping – General requirements