The design of Delayed Coking Unit of a refinery is a challenging project. This study highlights some of the design considerations in the delayed coking unit with an emphasis on process point of view and more specifically on the relief conditions of coke drums operating under an upset event which results in overpressure of coke drums. The results provided a practical and useful approach to control release of hot and heavy hydrocarbons to minimize their environmental impact. Blocked outlet conditions of normally open path of coke drum to the main fractionator is a critical issue in a refinery. Pressure safety valves calculations were carried out to address their dynamic behavior. It was assumed that the operating team will recognize the problem with sudden abnormal pressure fluctuations and intervene the upset condition in less than 15 min. A HYSYS simulation helped us to analyze the problem in more details. (C) 2011 Elsevier Ltd. All rights reserved.