A Numerical Simulation of Rapid Depressurization in Pressure Vessels Incorporating Nucleate Boiling of Hydrocarbon Mixture

In offshore operations, overpressure of pressure vessels can arise in case of emergencies like fire or malfunction of valves. This situation can cause physical damages of the vessel and, operation break. Thus, managing overpressure is important in terms of safety of offshore facilities. To handle the overpressure problems, the rapid depressurization, so-called ‘Blowdown’, is used. During depressurization, temperature of internal fluids in a vessel get decreases by the expansion of the fluids. Predicting decrease of the temperature is critical to choose the material of a pressure vessel. Overdesign without the prediction leads to the rapidly decreasing profit margins. For these reasons, the analyzing dynamic behavior of thermodynamics properties like temperature is required for material selection and design verification during depressurization. In this study, a dynamic model for depressurization was developed to simulate thermodynamics behavior in a vessel during depressurization including low temperature phenomena. The model contains non-equilibrium zone between phases, heat transfer between walls and fluids in the vessels. The heat transfer coefficient between internal vapor and wall was calculated from a combined convection that includes the both natural and forced convection. This study includes the calculation of liquid/wall heat transfer coefficient. During depressurization, liquid in the vessel becomes boiling closed to surface of the wall because the temperature of the wall is higher than the boiling point of the liquid. This phenomenon can be described as ‘nucleate boiling’, causes decreasing convective heat transfer coefficient from inner wall to the liquid in the vessel. Using the proper correlations about this phenomenon, the calculated coefficient made this study get closer to reality. The results were compared to experimental and simulation data from literature and it shows this model can properly estimate the thermodynamic property change in a vessel.