Where welded branch connections are made to pipe in the form of a single connection, or in manifolds as a series of connections (see Clause 4.3.20), the design shall be adequate to control the stress levels within safe limits. The following Items a) to k) provide design rules based upon the stress intensification created by the existence of a hole in an otherwise symmetrical pipe, but do not take into account stresses induced by other loads such as those due to thermal effects, mass, and vibration; the overall design shall take such stresses into account:
a) The reinforcement required for welded branch connections shall be determined by the rule that the metal area available as reinforcement in the reinforcement zone [see Item d)] shall be equal to or greater than the minimum required reinforcement area specified in Item b) and shown in Figure 4.4. b) The minimum required reinforcement area shall be determined by using the following formula:
AR = Do × tr
AR = minimum required reinforcement area, mm^2
Do = length of the finished opening in the run pipe wall, measured parallel to the axis of the run
tr = design wall thickness of the run pipe, mm
c) Except as limited by Item e), the area in the reinforcement zone available as reinforcement shall be the sum of the cross-sectional area resulting from any excess thickness available in the run pipe wall thickness, the cross-sectional area resulting from any excess thickness available in the branch pipe wall thickness, and the cross-sectional area of any added reinforcing metal, including weld metal. d) The reinforcement zone shall be a rectangle with dimensions as shown in Figure 4.4. e) Where the reinforcement materials are of lower strength than the run pipe, the applicable areas available as reinforcement shall be proportionately decreased; however, where the reinforcement materials are of higher strength than the run pipe, such applicable areas shall not be altered. f) The use of pad- and saddle-type reinforcements having strengths differing from those of the run pipe shall be limited to those reinforcing materials having welding qualities comparable to those of the run pipe. g) Where reinforcements are used that cover the weld joining the branch and run pipes, a vent hole shall be provided in the reinforcement to reveal leakage and to provide venting during welding and heat-treating operations. Vent holes shall be plugged during service to prevent crevice corrosion between the pipe and reinforcements; such plugging materials shall not be capable of sustaining pressure within the crevice. h) Ribs and gussets shall not be considered as contributing to the reinforcement of branch connections. i) Branch pipes shall be attached by welds for the full thickness of the branch or run pipe wall plus a fillet weld (W1), as shown in Figure 7.5. The use of concave fillet welds is preferred to minimize corner stress concentrations. Pad and saddle reinforcements shall be attached as shown in Figure 7.4. Where full fillets are not used, the edges of the reinforcement shall be relieved or chamfered at approximately 45° to merge with the edge of the fillets. j) Reinforcement pads and saddles shall be accurately fitted to the parts to which they are attached. Figure 7.4 illustrates some acceptable forms of reinforcement. k) Branch pipe connections attached at angles less than 85° to the run pipe are progressively weaker as the angle is decreased. Such designs shall be given individual study and sufficient reinforcement shall be provided to compensate for the inherent weakness. Where encircling ribs are used to support the flat or re-entrant surfaces, their strengthening effects may be taken into account.
Note: Partial ribs, straps, and gussets should not be used since stress concentrations near their ends can defeat their reinforcing value.