Abstract
During testing to support a program to improve acoustic induced vibration (AIV) assessments, it became clear that flow induced vibration (FIV) is excited at the same time; both mechanisms excite the shell flexural modes of the piping system, although their frequency characteristics are different but overlap. Typically, AIV and FIV are evaluated separately. However, review of some of the existing AIV assessment methods, particularly those that rely on empirical correlations with known failure cases, suggests that they effectively include an FIV element. In the case of a PSV tailpipe joining a subheader at a tee, the FIV is primarily present near the tee while AIV also present away from the tee. Therefore, combining the AIV and FIV assessments unwittingly into one assessment can result in significant conservatism away from the tee. Formulations for the FIV prediction and combining AIV and FIV into a pipe fatigue life assessment are presented. Data from a flare header test system is used to demonstrate the formulations and justify simplifications. The flare header test system consists of an NPS6x8 pressure relief valve, an NPS12 tailpipe with a tee connection to an NPS20 header.
