Summary This study presents an approach to determine the dimensions of three-phase separators. First, we designed different vessel configurations depending on the fluid properties of an Iranian gas/condensate field. We then devised a comprehensive computational-fluid-dynamics (CFD) method for analyzing the phenomena of three-phase separation. The results in terms of separation efficiency and behavior of secondary-phase particles were reviewed to choose the optimal configuration. Only a slight difference in the length of this vessel and the existing separator was found. In addition, simulation data were compared with industrial data pertaining to a similar existing separator. The results of this work showed that the CFD model used is capable of investigating the performance of three-phase separators.