Methods for evaluating allowable vibration velocity levels in power plant piping systems

Abstract

Vibrations in piping systems can have detrimental effects on the structural integrity, it is therefore of great importance to quantify the source of vibrations and to assess the impact of vibration induced fatigue. This thesis is limited to describe vibrations in a vibrating structure and not to investigate sources of excitation. The finite element method was used to perform modal synthesis and synthesis on three computational models with different levels of complexity. Modal analysis was performed by mode superposition which is an effective method used to approximate the dynamic response of a structure by superposition a small number of the structures eigenmodes. The main objectives in this thesis are to investigate allowable vibration velocity levels for different methods. Allowable vibration velocity levels are determined by fatigue stress data obtained from fatigue curves. The endurance limit is taken as the fatigue stress at infinite lifetime, i.e. the stress where the material does not undergo fatigue. The endurance stress is thus set as the allowable limit for vibration velocities. Natural frequencies, modal displacement and modal stresses are obtained from the dynamic response from FE-analysis. Modal analysis was performed on all models and frequency response analysis with a prescribed acceleration amplitude has been performed on the most complex computational model to obtain vibration displacement and stress amplitudes, as well as forcing frequencies. The results show that all methods show the same trend for allowable vibration velocity with the only difference being allowable vibration velocity is shifted in magnitude for different methods. The first model exhibits a clear linear frequency dependence for allowable vibration velocity while model 2 and model 3 exhibit a more non-linear behaviour with respect to allowable vibration velocity due to combinations of vibrating modes.