Abstract: This paper presents a dynamic simulation and analysis of the blowdown system for a first-stage three-phase separator in a floating production storage and offloading (FPSO) project. Using the HYSYS Blowdown module, a dynamic simulation model incorporating vessels, pipelines, valves, restriction orifices, and other components was developed to simulate the blowdown processes under both fire and cold scenarios. The transient behaviors of system pressure, temperature, and blowdown rate were investigated, and the key system design parameters were evaluated accordingly. The results show that under both pool fire and jet fire scenarios, system pressure decreases continuously with blowdown time, while system temperature first declines and then rises. The gas phase fraction of the blowdown medium remains above 99.8%. The optimal diameter of the restriction orifice is determined to be 66 mm, with a corresponding maximum blowdown rate of 49.23 t/h. The inner diameters of the blowdown valve and its inlet pipeline are 202.7 mm, and the tailpipe inner diameter is 307.0 mm. Under cold blowdown scenario, the minimum metal temperature of the vessel is -0.81 ℃, the lowest medium temperature downstream of the restriction orifice is -44.3 ℃, and the minimum metal temperature of the tailpipe is -16.67 ℃. Based on these results, the minimum design metal temperature of the system is established as -20 ℃. The HYSYS Blowdown module effectively captures the dynamic coupling of multiple parameters throughout the blowdown process, addressing the limitations of conventional simulation approaches and providing reliable support for the design of blowdown systems.

Key words: blowdown system, fire blowdown, cold blowdown, dynamic simulation model, FPSO