Assessment of Local Decreases in Wall Thickness at the Connection Straight-Pipe to Bend

Welds are ground during manufacturing to free them from offset edges and notches and thus to obtain a more favorable stress distribution. Apart from the above, welds are also ground to prepare them for and improve the conditions of in-service testing and inspection. The grinding of welds may result in a local decrease in wall thickness, so that there may be local deviations from the required minimum wall thickness. In order to fulfill the task of evaluating the strength of such material-loss regions, we have determined appropriate stress concentration factors for typical wall-thickness deviations and various wall-thickness/diameter ratios, which enable us to assess quickly and, if necessary, directly after the on-site measurement of wall thickness, whether a detected deviation from the minimum value is permissible. To be able to evaluate deviations from minimum wall thickness, especially in welds that form a connection to bends, we have determined stress indices for the beginning and the end of bends for common pipe bend dimensions and various bend angles. Compared with the maximum stress indices commonly used in piping calculations for the crown of the bend, the stress indices at the end of the bend are lower than those at the crown and can help to reduce unnecessary conservatism. In the paper, stress indices for various grinding geometries and for the beginning and the end of common bend shapes will be presented, as well as the method used to evaluate strength and the criteria pertaining to the tolerability of decreases in wall thickness.