Abstract A pressure relief valve (PSV) cannot depressurize a vessel or system; it can only limit its maximum pressure during upset or emergency conditions. A blowdown system is usually specified to accomplish the removal of the pressurized, volatile, hazardous, and flammable compounds such as hydrocarbon or ammonia vapor in vessels or systems. The pressure is removed from the vessel using a blowdown system; however, some hydrocarbon liquid will remain in the vessel. This paper explains different criteria for designing a blowdown system. Examples are presented to show how the blowdown time is affected by several factors. The Joule–Thomson effect and its effect on the vessel's inner wall temperature are illustrated using charts and tables. Several applications of the blowdown system are provided that relate to actual offshore and refining operations. Different mechanisms by which the blowdown system is activated are discussed, and a simplified design procedure to design a blowdown system is also provided.