(126b) Understanding the Depressurization System Design

A relief valve (PSV) cannot depressure a vessel or system. It can only limit its maximum pressure during upset or emergency conditions. A Blowdown system is usually specified to accomplish removal of the pressurized hydrocarbon vapor in vessels or systems. The pressure is removed from the vessel, but some hydrocarbon liquid will remain in the vessel. Different design criteria's for designing a blowdown system is explained. How the blowdown time is affected by several factors is shown by several examples. Joule–Thomson effect and it's effect on vessel inner-wall temperature is illustrated by charts and tables. Several application of the blowdown system is provided and related to the actual offshore and refining operations. Different activation mechanism of blowdown and simplified design procedure to design a blowdown system is explained.