The weakest points in a piping system are the connecting joints. Leak prevention by design begins with the relative ranking of the piping and valves joints used in a piping system. Only a full penetration butt-weld is considered as strong as the parent pipe; the use of other types of piping joints, including longitudinal seam welds and circumferential welds, can weaken the piping system. The reduction in strength due to a longitudinal seam weld in the pipe must be accounted for when performing minimum wall thickness calculations using the weld quality factor. The reduction in pipe strength due to circumferential welds should be accounted for in the weld joint strength reduction factor. Other joints, like flanges, valve body, bonnet and stem seal types, also add to the weakness of piping, and can cause accidental leakage. Evaluating these joints when subjected to fire may require changes in the joint design to achieve the required safety. For example, metal seated valves will perform better in a fire, so a change of an elastomeric gasket to a metallic gasket that can survive under fire will create a fire-safe flanged joint.
When performing stress analysis calculations to comply with ASME B31.3 or B31.1, a stress intensification factor (SIF) should be included in the calculation as a measure of weakness associated with various connecting joints to fittings. A full penetration butt-weld should have an SIF of 1, as it is considered as strong as the parent pipe. Fillet welds on slip-on and socket-weld flanges should have an SIF of 1.2 and 2.1, respectively. The threaded connection should have an SIF of 2.3. For a typical stub-in branch connection (as an SIF depends upon size of run pipe, its thickness and pad thickness), an in-plane SIF should be 3.9 and an out-of-plane SIF should be 4.9, including a reinforcing pad with a thickness the same as the run pipe, which will strengthen the connection and reduce the SIF to half the value. A welding tee may still be a better choice, considering the extra effort needed to weld the pad.
