A Study on Design Procedure for Efficient Depressurization System in High Pressure Hydrocarbon Process

Abstract

High pressure hydrocarbon processing facilities accompanied by exothermic reactions such as hydrocracker plant usually handle flammable, combustible, or toxic fluids which pose severe risks with respect to fire, explosions, equipment ruptures and toxic gas releases. Loss potential from such incidents can be prevented and/or limited efficiently by providing inventory isolation and removal systems which are commonly referred to as ESD (emergency shutdown) system and emergency depressurization system respectively. Many papers established a variety of models to predict gas flow rate through emergency depressurization system and temperature experienced by equipment and piping during depressurization, but they were all under the premise of that emergency depressurization system had already been installed in the plant. Besides, the applications of emergency depressurization system in industries are mainly empirical, dependent on licensors design which includes design conditions for emergency depressurization system, not based on exact operating conditions, nonstandard and just considering consequences of equipment ruptures due to fire or other over-temperature scenarios, even many literatures provide lots of guidelines. To overcome these shortcomings, an integrated design procedure was proposed covering judgment of emergency depressurization system installation in this study. First of all, carry out the qualitative risk assessment-HAZOP Study to find out hazards related to fire, exothermic runaway reactions or other over-temperature scenarios. In this study, the HAZOP Study was performed by using computer program TechmasNavi® series which can reduce time and efforts required, and build database for HAZOP report. Through HAZOP Study, the necessity of emergency depressurization system installation can be discussed qualitatively. Secondly, by calculating equipment rupture stress and comparing with selected rupture criterion to judge whether equipment ruptures or not, thereby the necessity of emergency depressurization system installation can be confirmed quantitatively. Thirdly, if the emergency depressurization system installation is confirmed, estimate and decide depressurization rate which will be used for emergency depressurization system design including depressurization valve sizing, restriction orifice sizing, pipe sizing, and flare capacity confirmation, and check the metal wall temperature variation which will support equipment and pipe material selection. Herein, dynamic simulator HYSYS v7.3 with rigorous dynamic model was used to reduce the efforts required. This study proposed an integrated design procedure for efficient depressurization system in high pressure hydrocarbon processing facilities. Following this procedure, not only the emergency depressurization system can be designed efficiently, but also the plant safety can be improved. Otherwise, this study is expected to be more beneficial research from the point of view of CBA (costbenefit analysis).