Managing overpressure of a pressure vessel is critical to the safety of a hydrocarbon producing facility. As a result of rapid depressurisation called blowdown, temperature of the pressure vessel wall can be severely decreased. For these reasons, dynamic behavior of properties (e.g., temperature in pressure vessels) during depressurisation has to be analyzed for material selection and design process. In this work, a dynamic model for depressurisation was developed to simulate the low temperature phenomena inside a pressure vessel under depressurzation. Thermodynamic and transport property calculation theories were used to describe hydrocarbon systems. Heat tranfers between internal fluid and inner wall surface and between surroundings and outer vessel wall surfaces were considered. Temperature of each vessel wall surface was calculated using transient conduction temperature profile. The result showed good fitting to reported experimental data.