NB-3600 provides methods for assessing the structural integrity of piping systems. One of the major loading types in piping systems is the moments generated within the pipework to react relative movement of components, thermal expansion and external loads such as gravity or seismic loading.
When assessing a pipe tee or branch, there are three sources of moments that must be considered: from the branch and from the run on both sides of the junction; each of these sets of moments has three components.
The three components of the branch moment can be easily combined using the square root sum of the squares approach to determine a single resultant moment. For the run, the process is more complex as the moments from either side of the junction need to be converted into a single set of component moments before the resultant run moment is obtained. The method for doing this is given in Subparagraph NB-3683.1 of ASME BPVC Section III. Once the three components run moments have been obtained, they are also combined to determine the resultant. The resultant run and branch moments are then multiplied by geometry-dependent stress indices and summed to give a stress range.
For fatigue and shakedown assessments, it is unclear whether the “sign rule” should be applied to the individual load sets before ranging or to the range of each individual component. This potentially results in uncertainty in calculations, leading to unnecessary conservatism and/or repeat calculations.
For this study, 10 randomly generated sets of moments have been applied in a 3D finite element model of a typical pipe tee. The stress intensity ranges obtained from the FE results have been compared with the stress range obtained using the NB-3600 stress indices with the sign rule implemented in three different ways.
The study shows that the sign rule should be applied after ranging the individual moment components; a change to the wording of the code is recommended to confirm this. However, given the levels of conservatism in other parts of the calculation, it is unlikely that any calculations carried out using the sign rule first are non-conservative.