A pressure surge in a pipeline occurs following a sudden valve closure, compressor start-up or trip. Pipeline simulations can be used to model the physical surge as it travels down the line. As it travels, the amplitude of the pressure wave decays, and the slope of its leading edge may decrease due to friction or to effects related to compressibility. These surges can be problematic for automatic emergency shutdown (ESD) valves. ESD valves are configured to close automatically during a pipeline rupture to minimize spillage. These valves can also trigger on the pressure surge from a leak. ESD valves must be calibrated to trigger from a rupture or large leak, but not from the surges that arise during normal operations.

Determining how to do this presents a challenging problem for pipeline simulation. The simulator must give an accurate picture of the surge, but the true physical dispersion of the surge due to friction, if any, can easily be saturated by the numerical dispersion inherent in the pipeline model. This paper investigates the numerical dispersion of pressure waves in a gas pipeline, and what can be done to minimize that dispersion. We examine both small-amplitude surges such as those coming from compressor trips and large-amplitude surges such as those that might be generated from a pipe rupture. In order to distinguish the true physical behavior of pressure surges from numerical effects, we compare the results of several types of simulator. The Preissman box scheme [1], a widely used implicit method which was first developed for simulating open-channel flow, will be compared with the MacCormack predictor-corrector method [2] and a higher-order extension of the box scheme. The advantages and deficiencies of each of the methods will be discussed, as well as ways to achieve accurate simulation of this sort of surge with simulation methods commonly found in commercial pipeline simulators.

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