Abstract
Current know-how in the field of piping system analysis remains incomplete with room for progress. Indeed, piping vibration can come from several sources: direct or indirect machine excitations, cavitation and acoustic excitation due to differential pressure devices or excitations due to fluid flow. This last topic has long been identified as essential and the Energy Institute (EI) Guidelines for the Avoidance of Fatigue Failure that are more and more often applied in both design and operation confirm the importance of this subject.
The use of such guidelines presents certain technical shortcomings: to date, there is limited information available for predicting vibration levels caused by turbulent flow. If piping responses are known, their excitation sources remain unknown, leading to suggesting solutions that tend to be more complex, potentially over-dimensioned and less cost-efficient than truly required.
The aim of this paper is to present FIV sources for several discontinuities that are present in piping systems.
In order to better understand the parameters that govern turbulent excitations, a series of measurements (pressure and vibration) was performed in the vicinity of several singularities.
Three common potential irregularities usually met in pipelines were selected for the study. Only restricted orifices (RO) are presented in this paper.
Two different test loops which have different characteristics in terms of size and flow were used for the measurements, making it possible to validate dimensionless analyses in order to compare results obtained from both loops.
The characteristics of the two loops place the acoustic modes of their systems in the frequency range of interest. Considering that the turbulent phenomenon is coherent on small distances whereas acoustic plane waves are coherent over long distances, coherence was used to separate the acoustic part from the total measured signals.
A dimensionless analysis was then performed, enabling the comparison of measurements from both test loops, determining if specific behavior was associated with an acoustic or a turbulent phenomenon and validating the hypotheses such as the homogeneity of the turbulent flow.
These results on a straight pipe (similar for different rates and pipe sizes) demonstrate the consistence of the dimensionless analysis and were used as a reference pressure level for further comparisons with levels obtained in the presence of a singularity.
The evolution of pressure fluctuations in function of the distance was extracted in this new configuration. Location and amplitude of these fluctuations were compared for different sizes.
The measurements and techniques used during this study make it possible to predict the amplitude and location of maximum of pressure fluctuations downstream of most common piping discontinuities such as restricted orifices.
