Avoidance of Slack Flow in Liquid Pipelines

Abstract

There are many existing liquid transmission pipelines that have significant changes in elevation along their route, making them susceptible to operating at “slack flow” conditions. Slack flow occurs in a pipeline when the pipeline pressure falls below the vapor pressure of that liquid (i.e. the pressure head decreases below the elevation at a certain point and causes the gauge pressure at that point to drop below zero atmosphere). Separation of the fluid column occurs, which can result in leak detection system inaccuracy and poor pressure/flow control during pigging operations. The designs of older pipelines typically did not address the slack flow issue. In order to eliminate the occurrence of slack flow, some method of pressure control is necessary, such as the installation of a pressure regulator station (PRS). In this paper a case study is used to demonstrate how a detailed hydraulic analysis was utilized in the design of an effective PRS, to eliminate slack flow. The subject pipeline system was approximately 800 km in length; with six pump stations and one terminal tank farm. One section of the pipeline contained an elevation difference of more than 1000 m (between mountain top and river valley bottom), creating slack flow operating conditions. A decision was made by the pipeline operator to prevent (potential) over pressurization at the lowest point on the pipeline. A secondary goal was to upgrade the leak detection system by eradicating the slack flow operation. Designing and installing a PRS and an over-pressure safety valve station achieved both of these goals. The PRS design, operation philosophy and safety philosophy development utilized information derived from a transient hydraulic simulation of the pipeline, using a hydraulic pipeline simulator (HPS). By using transient hydraulic analysis, an optimized solution to slack flow and over-pressuring on a liquids pipeline with large elevation differences, was achieved. By installing a PRS in an optimized location the pipeline operator has increased the reliability of leak detection and reduced the risks of over-pressuring, in a safe, cost effective manner.