Summary In this work, the performance of two pilot–scale separators was investigated using computational–fluid–dynamics (CFD) simulation with one operating at low gas volumetric quality comprising a bucket–and–weir configuration, and the other operated at high gas volumetric quality with a weir configuration. The pilot–scale separators were selected for this work because of their availability and the lack of data on industrial separators. The effects of the liquid (oil and water) flow rate and weir height on separation performance have been investigated for the separator operating at low gas volumetric quality. For this separator, the design of experiments (DOE) and a preliminary run of the separator were used to select the number of experiments and simulations to conduct and the levels (values) of the three variables investigated. For the second separator, the effects of the inlet flow rate on separation performance have been investigated. Eulerian and volume–of–fluid (VOF) multiphase–flow models in ANSYS® Fluent (Fluent 2019), combined with a k–ε turbulence model, were used to simulate the fluid–flow pattern and phase behavior inside each of the separators. The numerical solutions were initialized with a water level set at 50% of the weir height using a patching tool. A mesh–independence test was carried out to ensure that the results are not dependent on the mesh quality. The separation efficiencies from both models were compared with that from the experimental data. The results indicated that the two multiphase models, namely, Eulerian and VOF, predict the experimental results within 30% error. However, different separation performances were obtained for the same flow conditions. For the separator operating at low gas volumetric quality, the results from the Eulerian multiphase model produced a maximum deviation of 8%, while results from the VOF multiphase model produced a maximum deviation of 23% of the experimental data. For this separator, the oil flow rate was found to have the greatest effect on the separation efficiency. This is followed by the water flow rate and weir height. For the separator operating at high gas volumetric quality, a maximum percentage error of 30% for the Eulerian model and 21% for the VOF was obtained.