welding details for welded steel wye branches

Welding Details For Welded Steel Wye Branches

Chris Sundberg, S.E., SCWI (Senior Certified Welding Inspector) Principal Structural Technologist CH2M HILL, Inc., 1100 112th Avenue NE, Suite 500, Bellevue, WA 98004 Phone: (425) 453-5000; Fax: (425) 468-3100; Email: [email protected]

Abstract Welded steel wye branches are a key component of most water projects where flows must be divided. They are also a specialty item that requires many engineering considerations. The design of wye branches has been studied by many agencies over the years and good design guidance can be obtained from several sources including the American Water Works Association (AWWA). (2004). Manual M11, Steel Water Pipe: A Guide for Design and Installation, where the focus is determination of proportions of steel reinforcement necessary to resist internal pressure. However, welding details for wye branches is often left to the fabricator and little guidance is offered. Selection of proper welding details requires a high degree of skill on the part of the fabricator to complete the design and to make it function reliably as a safe, integral part of a conveyance system, as intended by the designer.

Welding details for complex pipe fittings such as wye branches are not published elsewhere and fabricators are on their own to make these structures work; there is no common methodology for analysis and design of groove welds that are key structural components of wye branch crotch plates. Many times welding details are left to the poor welder actually doing the work, but most welders are not engineers so owners of wye branches end up with pot-luck without adequate engineering oversight. The author is aware of failures attributable to faulty welding details.

Figure 1-4300 mm x 4300 mm x 90 Degree Tees, Singapore

848Pipelines 2014: From Underground to the Forefront of Innovation and Sustainability ASCE 2014

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This paper will address several areas of wye branch fabrication that are not well covered by other sources including local dihedral angle and groove weld bevel determination and weld joint detailing. Weld joint geometry is complex and graphs for determining local dihedral angles for common 90 degree and 45 degree pipe intersections are presented as an aid to the designer of wye branches in Figures 12 and 13 respectively.

Geometry Understanding of the geometry of welded steel wye branches is essential to accurate design and fabrication for dependable service. However, somewhat rigorous calculations are necessary to accurately design and detail wye branches. Analytic geometry and vector algebra play a significant role in determining templates for cutting and bending the steel plates and for developing groove welding details. The author has previously presented the equations and an example problem illustrating the geometry of intersecting cylinders in another ASCE Pipelines publication Fabrication of Welded Steel Wye Branches C. Sundberg; Atlanta, GA 2008.

Wye branches are typically reinforced with external crotch plates that are designed to replace the loss of hoop strength that occurs when the main pipe wall is removed to accommodate the branch pipe. The crotch plates, fabricated from pressure vessel steel, act as indeterminate curved beams to carry internal pressure loads around the perimeter of the intersecting pipes. The type of crotch plate reinforcement depends upon the relative sizes of the main and branch pipes as follows;

Equal Diameters; Where the branch and main pipes are equal diameters, the pipe intersection is geometrically straight sided and planar crotch plates are assembled to follow the intersection as indicated in Figure 1 that shows several very large diameter, 4300 mm x 4300 mm x 90 degree, 3-plate tees located in Singapore.

Unequal Diameters; Where the branch and main pipes are unequal diameters, the intersection is curved and a single curved crotch plate is provided as indicated in Figure 2 that shows a 2400 mm x 1200 mm x 60 degree wye branch design for a major water project located near Toronto, Ontario. Curved crotch plate wye branches adds a level of complexity to design, fabrication and welding details when compared to planar crotch plate wye branches because additional calculations are necessary to determine the dihedral angles between the pipes and crotch plate and their respective weld joint bevels.

Local Dihedral Angle The angle between intersecting pipes is called the local dihedral angle and is defined in AWS D1.1/1.1M:2010, Structural Welding Code-Steel, Annex K, Terms and Definitions, local dihedral angle, (tubular structures). The angle, measured in a plane perpendicular to the line of the weld, between tangents to the outside surfaces of the tubes being joined at the weld. The exterior dihedral angle, where one looks at a localized section of the connection, such that the intersecting surfaces may be treated as planes.

In addition AWS D1.1, Annex P, Local Dihedral Angle presents 7 graphs for pipe with intersecting angles that vary from 20 degrees to 90 degrees in 10 degree increments. However a graph covering a very common 45 degree pipe intersection is not presented and the reader must


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interpolate local dihedral values using the 40 degree and 50 degree graphs that are provided. Figure 13, Local Dihedral Angle for 45 degree Intersection, herein, provides a handy tool for designers of 45 degree wye branches not covered in AWS D1.1, Annex P.

Figure 8-Example Problem for Determining Local Dihedral Angles for 60 Degree Wye Branch indicates the relationship between branch end rotation angle, rho () and local dihedral angle psi (). Once a branch rotation angle, rho has been selected, Figure 8 can be used to determine the dihedral angle psi for any value of beta, a series of curves, where beta=d/D, d is equal to the diameter of the branch pipe, and D is equal to the diameter of the main pipe.

Welding Details Welding details for wye branches must provide adequate strength to meet design loads, yet permit easy welder access to achieve complete joint penetration and promote good weld quality. Constructability is best if pipe shells are groove welded to sides of crotch plates as indicated in Figures 3, 4 and 5. For pipe diameters 30 inches and greater, welder access is assumed to be primarily from inside the main pipe for application of single bevel groove welds that are double welded from the exterior to provide best quality and CJP (complete joint penetration). In addition, exterior welds provide a transition, balance weld metal shrinkage and control distortion.

The local dihedral angle is constantly changing around the welded joint, however the groove angle should remain constant to avoid welding difficulties. For example, a groove angle that becomes too narrow may result in inability of the welder to adequately clean weld passes, thus trapping slag. Too wide a groove angle results in excessive weld metal, greater weld shrinkage and distortion. Geometry requires that weld bevels must vary if groove angles are constant and local dihedral angle varies. Figure 6 Dihedral and Beveling Table for 2400x1200x60 Degree Wye Branch and Figure 7 Dihedral and Beveling Diagram for 2400x1200x60 Degree Wye Branch provide a detailed geometric summary of information necessary to prepare base metal for welding including dihedral angles (pipe to pipe and pipe to crotch plates) and respective bevels needed for 45 degree single bevel groove welds.

Welding symbols alone are not sufficient to describe welding details indicated in Figures 2, 3, 4 and 5. The author believes that a typical wye branch design should include several full scale joint welding details similar to that shown in addition to beveling diagrams to convey detailed design information to the shop fabricators and welders.

Detailed Procedure A detailed 12-step procedure for calculation of wye branch welding details is presented in Figure 10-Procedure for Developing Wye Branch Groove Weld Bevels. As previously mentioned, in a previous paper, Fabrication of Welded Steel Wye Branches, the author presented the procedure for calculation of the local dihedral angle measured from pipe to pipe in steps 1 through 7 of Figure 10. Figure 9 Local Dihedral Angle Vectors illustrates the vectors N1 through N5 needed to perform detailed calculations for determination of the local dihedral angle from pipe to pipe if graphs of Figures 12 or 13 are not used. However, additional calculations are necessary to determine the dihedral angles between the pipes and crotch plates.


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Equal Diameters; for the case of equal diameter pipes the crotch plates equally divide the local dihedral angle from pipe to pipe into two identical half dihedral angles and additional vector calculations for determination of weld bevels are unnecessary.

Unequal Diameters; for the case of unequal diameter pipes resulting in a curved crotch plate, the dihedral angles, pipe to crotch plate, and their respective weld joint bevels, cannot be easily determined and additional vector calculations are indicated in steps 8 through 12 of Figure 10.

Figure 11 Radius of Curvature of Curved Crotch Plates shows additional vector N6 that is normal to the plane of curvature of the curved crotch plate and Weld axis Vector N1, in the plane of the crotch plate. The cross vector product, N7, as calculated from N1xN6, results in a vector that is perpendicular to both N1 and N6, located in a plane within the crotch plate. The calculation of the dihedral psi1 measured from the vector N7 in the plane of the crotch plate normal to the weld axis vector, N1, to the vector N5 tangent to the plane of the branch pipe and normal to the weld axis, can be obtained from the arc cosine of the vector dot products and absolute values of N7 and N5 as indicated by the equation of Figure 10, Step 10 The dihedral psi2can be obtained by subtracting psi1 from the local dihedral psi calculated in step 7. Once all dihedral angles, psi, psi1 and psi2 have been determined and the weld groove angle (a single bevel weld groove angle, 45 degrees is assumed), the weld groove bevels can be calculated from other joint geometry.

Summary Design and fabrication of wye branches requires focus on welding details to make these structures function reliably as a safe, integral part of a conveyance system as intended by the designer. Competent welding detailed design can only occur by employing skills that require understanding of engineering and fabrication. The examples and tools presented in this paper are intended to make both the designer and fabricators job easier for establishing proper welding details.

References Sundberg, C., (2008). Fabrication of Welded Steel Wye Branches, ASCE Pipelines, Atlanta, GA

American Water Works Association (AWWA). (2004). Steel Water Pipe: A Guide for Design and Installation (M11), Fourth Edition.

American Welding Society (AWS). (2010). Structural Welding Code Steel D1.1/D1.1M-2010


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Figure 2-2400x1200x60 Degree Wye Branch

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Figure 3-Welding Detail at Crown of Branch for 2400x1200x60 Degree Wye Branch


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Figure 4-Welding Detail for Acute Crotch for 2400x1200x60 Degree Wye Branch


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Figure 5-Welding Detail at Obtuse Crotch for 2400x1200x60 Degree Wye Branch


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Figure 6-Dihedral and Beveling Table for 2400x1200x60 Degree Wye Branch

Figure 7-Local Dihedral and Beveling Diagram for 2400x1200x60 Degree Wye Branch


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Figure 8-Example Problem for Determining Local Dihedral Angles for 60 Degree Wye Branch


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Figure 9-Local Dihedral Angle Vectors

Figure 10-Procedure for Developing Wye Branch Groove Weld Bevels


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Figure 11-Radius of Curvature for Curved Crotch Plates


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Figure 12-Local Dihedral Angle for 90 degree Intersection


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Figure 13-Local Dihedral Angle for 45 Degree Intersection


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welding details for welded steel wye branches

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