Liquid transmission pipeline systems passing through geological areas with significant elevation changes along their route may possibly run at “slack flow” conditions if improperly designed. Slack flow is a phenomenon in which a pipeline transporting a liquid product develops vapor bubbles at points where the pipeline pressure falls below the vapor pressure of that liquid. Slack flow operating conditions will cause the occurrence of fluid column separation (a portion of the liquid is vaporized), which can result in leak detection system inaccuracy, pipeline vibration (when bubbles collapse) and excessive PIG speed, which makes the pipe difficult to inspect with intelligent PIGs.

To eliminate the occurrence of slack flow as well as to reduce project costs, optimization methods could be used, including smaller diameter pipe, thicker walled (and/or higher-grade) pipe, and the use of a pressure control station (PCS).

The subject pipeline is a proposed liquid transmission pipeline with nominal diameters of 762 mm/914 mm/1067 mm and a length of approximately 1200 km. The pipeline passes through a significant elevation change (1326 m elevation change within 50 km, between mountain top and river valley bottom) where the static pressure can reach as high as 12,200 kPa (at a crude density of 938 kg/m3). Slack flow or overpressure may occur due to this situation if the system is not designed properly.

This paper showcases the pipeline design considerations and methodologies used to solve the slack flow operation condition while avoiding all possible overpressure threats. The considerations include maximum and minimum operating conditions for flow rates and pressures. The system optimization methods include optimizing the installation length of thicker wall pipe (higher design pressure) in combination with designing a pressure control station (PCS). Furthermore, future expansion of the pipeline system was included into design consideration.

With the optimized design, slack flow conditions can be avoided, and the pipeline will be operated in a safe, leak detectable, inspectable (intelligent pigging) and cost-effective manner.

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