Reduction of the fatigue risk presented by acoustically induced vibration in flare header systems using mitigations that either reduce dynamic stress concentration effects or the level of vibration are of considerable interest to designers and plant operators. Assessments of the relative performance of different types of pipe fittings in reducing dynamic stress levels are presented based on the evaluation of data from full-scale laboratory tests of a pressure-relief system. A modal-analysis based finite-element methodology is also developed so that predictions may be extended to other piping arrangements that vary in thickness, size or connection type. The pipe fittings considered in the test are Pipet®, fabricated tee (Stub-on arrangement), sockolet (small-bore branch connections only), full-wrap reinforced fabricated Tee and Sweepolet®. For the finite-element method reducing tee connection is considered in addition. The test system produced significant levels of both turbulent-induced vibration (FIV) and acoustically induced vibration (AIV), which required differentiation of stress evaluations for the low-frequency FIV region and the mid-to-high frequency AIV region. The relative performance of mitigations (through selection of the type of pipe fitting) was found to be particularly relevant in the low-frequency FIV region. The reductions in dynamic stress and vibration of small-bore branch connections from installation of clamped bracing are also presented. The results show that the use of reducing Tees and full-wrap reinforcements for Stub-on connections for tailpipe and sub-header branch connections provide significant mitigation of dynamic stress and improvement of fatigue life over the use of Pipet® and Stub-on fittings. However, for the Sweepolet® connection which was expected to provide similar improvement the benefits are not fully realized in the 10S configuration.

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