For simulation of transient gas-liquid two phase flow, the continuity, momentum and energy equations for two fluids should be solved, which requires complex calculations. In oil and gas pipelines, it is possible to perform some simplifications on continuity and momentum equations. This will be done by using quasi steady state assumption for gas continuity equation and also local equilibrium momentum balance for both phases in some flow patterns such as stratified, annular, slug and bubbly flows. In this paper, simplified transient simulation with assumption of isothermal flow was utilized together with flow patterns transition criterion and tested against experimental data for verification of the results. For this purpose, a computer code was written and implemented as transient flow simulator. In this code, the one dimensional differential equations were discretized by semi- implicit finite difference method and solved by an iterative manner. Also, a model for pigging operation was developed to analyze the flow parameters in pipeline during and after pigging. The numerical results were compared with the experimental data and it is observed that agreement with experimental data is satisfactory from practical engineering stand point.
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