Bracing for Acoustic Induced Vibration Applications

Abstract Bracing is one of the mitigations applied for acoustic induced vibration (AIV) of fittings between a branch and header. The general guidance is to ensure that at least two-plane bracing is used. In some cases, bracing from two gussets is considered sufficient and in other cases bracing from four gussets is considered a requirement. Much of the information about bracing is designed for low frequencies where bending modes or low order shell modes in the branch and header are the primary vibration modes. With bending modes, the primary design objective is to ensure that the branch vibrates with the header movement, so that there is not high stress at the fitting caused by different motion of the branch and header. However, the consideration for shell modes needs to be different, depending on the order of the mode. For low order shell modes, the distance between nodes in the shell mode can be large enough, that the local movement of the header is like a bending mode relative to the branch. However, at the frequency range of AIV, the header can be vibrating with high order shell modes and as the frequency increases, the distance between anti-nodes in the shell mode decreases so that the point on the header where a gusset is attached can have very different vibration than the branch and there can be modes of the bracing components. In addition to creating locally high stress regions on the branch to header fitting, modes in the brace can create high stress in the brace parts and high stress in the branch and header where the brace is attached. The distance of the bracing on the header relative to the branch can in some cases be large enough that the modes of the bracing can be especially problematic. There are many field examples of fatigue induced cracks in bracing exposed to high levels of AIV excitation. Examples include both welded and bolted bracing. This paper will show the consequence of the shell mode shapes and that the design of the bracing can be different for AIV dominated vibration compared to low frequency bending mode and low order shell mode vibration. This paper will compare how bracing designs considered acceptable for a bending mode vibration impact an application where AIV is present.