De-oiling hydrocyclones are a promising choice for produced water treatment in the oil and gas industry. The compact nature of hydrocylones makes them suitable for offshore and subsea installations. The commitment toward reduction in environmental footprint makes it a necessity to maintain the efficiency of the produced water treatment system (hydrocyclones) under all plant scenarios. A mathematical model for de-oiling hydrocyclones can help to develop robust control algorithms to handle the uncertainties and to maintain high efficiency. In this paper, a simple first-principle model is developed for hydrocyclones. A pressure-flow relationship is derived using Bernoulli's equation, a droplet trajectory analysis is done for calculating the separation, and a dynamic mass balance is used for control purposes. A simple PI control algorithm tuned using the SIMC tuning rules is used to verify the model and its control properties.