It is common to rate a piping system to its weakest component to maximize flexibility for future operations. In many situations, the bolted flange joint is the lowest rated component. Rating a system for its full flange rating reduces the flange’s capacity to carry external bending moments. In the past, moments on flanged joints have been evaluated by using the concept of equivalent pressure, first presented in the Kellogg Design of Piping Systems. Operating moments are converted to an equivalent pressure. This equivalent pressure is added to the design pressure and compared against a limit. According to conventional practices, the design pressure plus the equivalent pressure must not exceed the rating pressure. Consequently, designing up to the flange rating pressure presents an issue, since no margin is left for the effects of external moments on flange joints. Depending on the circumstances, many designers have compensated by permitting the combined design pressure and equivalent pressure to be as high as twice the flange rating. In this paper, the authors demonstrate a robust methodology to define an appropriate limit for operating moments on bolted flange joints. Using the calculation methodologies of EN-1591-1, the authors calculate the maximum external moment that various classes of standard ASME B16.5 flanges (for Group 1.1 materials) can tolerate over a range of temperatures and present a representative sample. Conclusions are drawn about appropriate limits for moments on flanges and are compared to results using the equivalent pressure method.
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