Use of NC3658 Flange Design Rules for B16.47 Flanges: An Analytical and Finite Element Based Study

The Design of flanges can be approached through different routes some of which [1] involve use of plate and shell theory and some [2] use concepts like “Equivalent Pressure”. A rigorous finite element analysis is a solution, but it is not always practical to do so considering the complexity of the problem and the man-hour requirement for the same. Particularly for piping systems where the number of flanges are many, an engineer always looks for a robust and easy to apply method and the method outlined in the American Boiler and Pressure Vessel Code ASME SEC III (for nuclear plants) [2], paragraph NC3658.3 provides one such method. It simply involves checking if the applied bending moment is within an allowable limit. Theoretically this method addresses the design from the standpoint of checking the bolt stress and also if the applied loading will overstress the flange. This method can easily be developed in simple spreadsheet form and is an integral part of almost every commercially available pipe stress program. The difficulty of using this method is that its applicability has been recommended in [2] as for ASME B16.5 flanges only i.e. for a maximum pipe size of 24″. Frequently an engineer encounters a pipe size which is greater than 24″ and the applicability of this method for such flanges becomes a question mark. In this paper, applicability of the NC3658.3 method for flanges >24″ has been investigated based on the standpoint of computing operating stress in bolts which is the basis of this method and also the results have been checked against finite element analysis.