The most common criteria for the emergency blowdown and depressurizing against fire case in facility safety design is API Std 521, which indicates that it is necessary to reduce pressure to 690 kPa or half of design pressure in 15min for ASTM A515 Grade 70 system with wall thickness equal or greater than 25.4mm only, while faster depressurization rate is required for wall thickness less than that. In order to strengthen the recognition of survivability in fire, the method and steps of survivability quantitative analysis and calculation for fire case especially jet fire are studied and summarized according to API Std 521 and guidelines of Scandpower and methods applied in oversea project, which provides reference for facility safety design under jet fire.