Process Safety and Environmental Protection 1 1 3 ( 2 0 1 8 ) 343–356 Contents lists available at ScienceDirect Process Safety and Environmental Protection journal h om ep age: www.elsevier.com/locate/ps ep Numerical modeling of rapid depressurization of a pressure vessel containing two-phase hydrocarbon mixture Ahmin Park a,1 , Yoonae Ko a,1 , Sijin Ryu b , Youngsub Lim a,c,* a Department of Naval Architecture and Ocean Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea b Samsung Heavy Industries, 23, Pangyo-ro 227 beon-gil, Bundang-du, Seongnal-si, Gyeonggi-do 13486, Republic of Korea c Research Institute of Marine Systems Engineering, Seoul National University, Seoul 08826, South Korea a r t i c l e i n f o Article history: Received 3 April 2017 Received in revised form 25 September 2017 Accepted 20 October 2017 Available online 31 October 2017 Keywords: Blowdown Depressurization Non-equilibrium Numerical modeling Simulation Flare system a b s t r a c t Blowdown or rapid depressurization of pressure vessels is a well-known safety process that removes overpressure at an emergency situation. Since the thermodynamic and transport properties in a vessel change remarkably during depressurization, rigorous estimation of the properties with respect to time is essential. Particularly, the temperature drop due to the expansion would cause the wall of the vessel to become brittle, and hence, it should be evaluated in an early stage of the design process. This study developed a numerical model to simulate the phenomenon of the rapid depressurization and estimate the non-equilibrium temperature changes of the vapor, liquid and vessel wall during the depressurization process, considering combined convection, nucleate boiling and transient multilayer con- duction through the vessel wall. The results of this study were compared with experiment, numerical models from literature and several commercial software and showed good agree- ment with experimental results. © 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. 1. Introduction The term blowdown can be defined as a rapid depressurizing process of equipment or facilities such as pressure vessels. In an emergency, over- pressure causes mechanical failure of equipment or rupture of a vessel, and the flammable fluid from the rupture causes the fire or explosion. Blowdown valves are emergency safety valves that depressurize equip- ment by discharging fluid for safe operation, often installed in a parallel configuration with pressure relief valves. The discharged gas is sent to the flare network or other suitable disposal process to prevent accidents from overpressure. The API Standard 521 (API, 2007) recommends that efficient depressurization condition is to reduce the operating pressure below 50% of design pressure or 690 kPa in 15 min. ∗ Corresponding author at: Department of Naval Architecture and Ocean Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea. E-mail address: s98thesb@snu.ac.kr (Y. Lim). 1 Both authors equally contributed. During the depressurization, two aspects are important. The first one is to remove the pressure as soon as it is required, and the sec- ond one is to confirm whether the changes in fluid properties harm the pressure vessel itself. During depressurization, the fluid inside the vessel expands quickly due to the decrease in pressure, and the expan- sion of the fluid in the vessel causes a sudden drop of the temperature. If the wall temperature reaches the ductile-brittle transition temper- ature (DBTT) of the vessel wall material, the vessels can be ruptured and the leakage of toxic and flammable fluids causes severe problems such as explosion (Cui et al., 2010; Khattak et al., 2016). Therefore, the estimation of a reliable minimum temperature is critical, and it should be evaluated in the early stage of design (Moss and Basic, 2012). https://doi.org/10.1016/j.psep.2017.10.017 0957-5820/© 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.