Abstract A three-phase separator in the oil and gas industry integrates two separate parts: a gas-liquid section and an oil-water section. The gas-liquid separation section is determined by the maximum removal droplet size using the Sauders-Brown equation with an appropriate K factor. The oil-water separation section is held for a retention time that is provided by laboratory test data, pilot plant operating procedure, or operating experience. In the case where the retention time is not available, the recommended retention time for three phase separator in API 12J is used. The sizing methods by K factor and retention time give proper separator sizes, but sometimes engineers need further information for the design conditions of downstream equipment, i.e., liquid loading for the mist extractor, water content for the crude dehydrator/desalter or oil content for the water treatment. The scope of this paper is limited to sizing a three-phases separator using droplet size distributions in the inlet of the separator. The droplet size distributions are estimated in the downstream of flowline choke and they are reestimated in the production manifold by considering droplet coalescence. These droplet size distributions can assess liquids in the gas stream, water in the oil stream and oil in the water stream so that the feed conditions of the equipment downstream of the separator can be defined. The basis of this study is annular two-phase flow in the pipe segment downstream of flowline choke. For liquid entrainment fraction, the correlation found in Sawant (2008) is used. For liquid droplet size distributions for gas phase, the correlation found in Kataoka (1983) is employed. For the liquid-liquid phase, the correlation found in Brauner (2001) is applied for maximum droplet size and the Rosin-Rammler distribution is used for the droplet size distribution. 1.0 Introduction Well fluids, usually two phases, generate droplets when the fluids flow through flowlines and pressure letdown valves. These generated droplets are sheared and coalescenced into a specific droplet size distribution depending upon the flowrates, physical properties and the pipe geometry. Typical offshore platform consists of subsea flowlines, choke valve, and production manifold at the upstream of a production separator. The droplet size distributions in two phases in the inlet of the separator will be characterized by the sequence of these pipe segments, but it is assumed, in this work, that the final droplet size distributions at the inlet of the separator are determined by the last two segments: the pipe segment downstream of the choke and the production manifold. In offshore platform, annular flow is of particular interest as it occurs in vertical upward pipe. If the flow pattern is annular, an accurate prediction of the mean droplet size and droplet size distribution is possible. Many investigators have carried out tests and developed formulas to predict the droplet size distribution for calculating pressure drop in pipe. The droplet size distribution in two-phase annular flow can be calculated by estimating entrainment fraction into gas core stream. Two separated droplet size distributions are calculated in this work: one for the droplet size distribution in the gas core stream and one for the droplet size distribution in the presence of the liquid film on the pipe wall. The entrainment fraction is calculated by the Sawant (2008) entrainment correlation. The entrainment fraction is defined as a fraction of the total liquid flow in the form of droplets through the central gas core stream. Knowing the entrained liquid amount laden in gas phase, the liquid remained in the pipe wall can be obtained.