Physical Properties and Boundary Influence of Singularity in Fluid Pipelines Based on Vibration Wave’s Transmission Characteristics

The pipeline, as the fifth largest transportation tool after railway, highway, waterway, and air transportation, has been widely used in industrial production and life. However, in the process of pipeline operations, rapid opening and closing of valves or pump stopping can lead to water hammer. The periodic pressure wave caused by water hammer can be much greater than the normal operating pressure of the pipeline. Because of water hammer, pipelines are susceptible to aging and damage that may cause local wall thickness reduction or local pipe property change. In this paper, singularity is defined as an abnormal state area during the use of the pipeline. It includes the corrosion thinning area of the pipeline wall, changes in the supporting environment around the pipeline, and the uneven stress area applied to the pipe wall because of water hammer vibrations. Different types of singular points have different characteristics. We focus on two types of singular points’ characteristics in this paper: (1) nonpenetrating damage on the outer surface of pipe; and (2) local material change. Specifically, if the pipe wall thickness decreases locally due to pipeline damage, the ratio of wall thickness reduction to wall thickness corresponds to the depth of singularity, and the decrease of local wall thickness corresponds to the increase of the depth of singularity. The axial expansion of damage corresponds to the increase of the axial length of singularity. Similarly, when the local material of the pipe changes, the singularity corresponds to the part of the pipe where the local material changes. This part of the singularity material can be other material types that are inconsistent with the pipe material, such as the connecting part of a PVC pipe in two sections of steel pipe. Note that most detection methods of the pipeline leakage are post-leakage detection, which brings risks such as the wasting of resources. In this paper, the harmful energy of water hammer is taken as the excitation source of the pipeline, and the vibration characteristics of the pipeline are studied. The singularity characteristics of the pipeline relate to the vibration of the pipeline excited by water hammer, which provides a basis for the detection before the singularity becomes the leakage point. Based on COMSOL software, the fluid-free pipeline model and the water hammer excited flow pipeline model are constructed. Detection points are set to detect different characteristics (depth characteristics, axial length characteristics, and material characteristics) of pipeline singularity, and the influence of the singularity on the pipeline stress, pressure, displacement, and other parameters is studied. When the depth of singularity is 20%, 40%, 60%, and 80% of the wall thickness, the pipeline’s maximum pressure increases with the increase of the depth of singularity; when the water hammer is applied to the pipeline, the depth of singularity increases, the pipeline’s maximum stress increases, and the displacement at the singular point of the pipeline decreases. Compared with the depth of the singularity, the axial length of the singularity is 0.02, 0.025, 0.03, 0.035, and 0.04 m, respectively. It has a lesser influence on the pipeline characteristics than the depth of singularity. The material change of the singularity will also have an impact on the pipeline vibration, a factor which can be applied to the seismic design of pipelines. Detection before the singularity becomes a leak is the objective. These phenomena show that, based on vibration wave transmission characteristics, it is feasible in mechanism to achieve this objective.