If a stable through wall defect (leak) were to occur in a pipeline, the leak rate is an important factor for both safety and environmental assessments, and also to determine the performance requirements for a leak detection system. For a “large” leak, an assessment can be based on simple idealization as an orifice, but for narrow crack like defects this is not appropriate. The flow through a crack is dependent on factors such as the surface roughness and the crack opening. These issues have been extensively studied for nuclear pipework where the fluid is either pressurized water, steam or CO2, 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 tight axial through wall crack in a NPS 8 refined products pipeline. Leak rate measurements were made using water (for safety reasons) over a range of pressures. The data were fitted to a model for leakage through a tight crack that takes account of interaction of asperities in surface roughness. The fitted equation was then adjusted to take account of the different density and viscosity of the pipeline products. It was concluded that the model was able to give a good prediction of the measured leak rates and that the adjustments for the product properties were small.

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