Welded and Brazed Joints Method for Joining Piping Components

Welded and brazed joints are the most commonly used methods for joining piping components because these joints are stronger and more leak-tight than threaded and flanged joints. Furthermore, they do not add weight to the piping system as flanges do, and they do not require an increase in pipe wall thickness to compensate for threading, as threaded joints do. Today, Piping & Fabrication continuing inside the Piping Fundamentals.

Pipe-Weld Joint Preparation and Design
Butt Welds. The most common type of joint employed in the fabrication of welded pipe systems is the circumferential butt joint. It is the most satisfactory joint from the standpoint of stress distribution. Its general field of application is pipe to pipe, pipe to flange, pipe to valve, and pipe to fitting joints. Butt joints may be used for all sizes, but fillet-welded joints can often be used to advantage for pipe NPS 2 (DN 50) and smaller.
The profile of the weld edge preparations for butt welds may be any configuration the welding organization deems suitable for making an acceptable weld. However, to standardize the weld edge preparation on butt-welded commercial piping components, standard weld edge preparation profiles have been established in ASME B16.25. These weld edge preparation requirements are also incorporated into the standards governing the specific components (e.g., B16.9, B16.5, B16.34). Figures A2.22, A2.23, and A2.24 illustrate the various standard weld edge profiles for different wall thickness.

FIGURE A2.22 Basic welding bevel for all components

FIGURE A2.23 Typical end preparations for pipe which is to be welded by the
inert-gas tungsten-arc welding process

FIGURE A2.24 End preparation and backing-ring requirements for critical-service applications
employing flat or taper-machined solid backing rings. See Table A2.21 for dimensional data

On piping, the end preparation is normally done by machining or grinding. On pipe of heavier wall thicknesses, machining is generally done on post mills. On carbon and low-alloy steels, oxygen cutting and beveling are also used, particularly on pipe of wall thicknesses below 1/2 in (12 mm). However, the slag should be removed by grinding prior to welding.

Because of fairly broad permissible eccentricity and size tolerances of pipe and fittings, considerable mismatch may be encountered on the inside of the piping. Limitations on fit-up tolerances are included in several piping codes. For severe service applications, internal machining may be required to yield proper fit-up. When one is machining the inside diameter, care should be taken to ensure that minimum wall requirements are not violated. Table A2.21 lists the counter bore dimensions typically specified.

When piping components of unequal wall thickness are to be welded, care should be taken to provide a smooth taper toward the edge of the thicker member. The length of the taper desirable is normally 3 times the offset between the components, as outlined in ASME Boiler and Pressure Vessel Code Sections-I and III, and ASME B31.1, Power Piping Code. The two methods of alignment which are recommended are shown in Fig. A2.25.

The wall thickness of cast-steel fittings and valve bodies is normally greater than that of the pipe to which they are joined. To provide a gradual transition between piping and components, the ASME Boiler and Pressure Vessel Code and the ASME Code for Pressure Piping permit the machining of the cylindrical ends of fittings and valve bodies to the nominal wall thickness of the adjoining pipe. However, in no case is the thickness of a valve permitted to be less than 0.77tmin at a distance of 1.33tmin from the weld end, where t min is the minimum valve thickness required by ASME B16.34. The machined ends may be extended back in any manner, provided that the longitudinal section comes within the maximum slope line indicated in Fig. A2.25. The transition from the pipe to the fitting or valve end at the joint must be such as to avoid sharp reentrant angles and abrupt changes in slope.

TABLE A2.21 Dimensions for Internal Machining and Backing Rings for Heavy-Wall Pipe in
Critical Applications

(Download the picture first to see the detail of those table)

End Preparation for Inert-Gas Tungsten Arc Root-Pass Welding. 

The pipe end bevel preparations shown in Fig. A2.24 are considered adequate for shielded metal arc welding, but they pose some problems in inert-gas tungsten-arc welding. When this process is used, extended U or flat-land bevel preparations are considered more suitable since the extended land reduces the heat sink, thereby affording better weld penetration. The end preparations apply to inert-gas tungsten-arc welding of carbon- and low-alloy steel piping, stainless-steel piping, and most nonferrous piping materials. On aluminum piping, the flat-land bevel preparations are preferred by some fabricators.