This paper presents a comparative study of two commonly used three-phase separator design procedures; Arnold and Stewart and Svrcek and Monnery. The procedures were developed based on droplet settling and retention time theories for the separation of gas, oil and water and are known to predict different separator geometries for the same operating conditions. These procedures were constrained to allow optimisation of the design but details of the constraints applied on the rational for their application are not available. To better understand these constraints, the two procedures for sizing a three - phase horizontal separator equipped with weir plate were investigated. Each procedure was used to calculate the geometries for three different sets of flowrates namely; fixed oil with varying gas and water, fixed water with varying gas and oil and finally fixed gas with varying oil and water. The calculated geometries determined from each procedure were then investigated using ANSYS Fluent to determine the separation achieved. To ensure that the ANSYS Fluent simulations accurately defines the separation process, a small scale industrial separator was modelled. Simulation results in terms of the separator outlet quality predicted that the separator designed using Arnold and Stewart procedure has a greater separation efficiency than that designed by Svrcek and Monnery.