If a stable through-wall defect (leak) occurs in a pipeline, the leak rate is an important factor for both safety (consequence) and environmental assessments as well as for determining the performance requirements for a leak detection system. For a “large” leak,  estimates of the leak rate can be based on simple idealization as an orifice based on the area of the leak and a discharge factor. For crack-like defects where the opening is much less than the pipe wall thickness this is not appropriate. The flow through a crack is dependent on geometric factors such as the surface roughness, the crack length and opening together with fluid properties such as the viscosity. These issues have been extensively studied for nuclear pipework where the fluid is either pressurized water, steam or CO~2~, and guidance is given in Annex F of BS 7910. However, there is little published work for pipeline geometries and single-phase liquids such as refined hydrocarbons.

This paper presents the results of experiments measuring the leak rates through a natural tight axial through-wall crack in an NPS 8 refined-products pipeline. The crack had originated at a gouge and then propagated through the wall. Leak-rate measurements were made using water (for safety reasons) over a range of pressures. The data was fitted to a model for leakage through a tight crack that takes account of the interaction of asperities in the surface roughness. The model gives an approximate square law relationship between the leak rate and the local pressure. The fitted equation was then adjusted to take account of the different densities and viscosities of the pipeline products. It was concluded that the model was able to give a good prediction of the measured leak rates and that in this case the adjustments for the product properties were small.


Leak Rate Testing of a Natural Pipeline Defect

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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