Process modeling is a fundamental tool for control and estimation design. Pressure control is a basic process used for academic and even research purposes. This article addresses a modeling methodology to achieve a phenomenological based semi-physical model describing the pressure dynamic behavior in the buffer tank of a laboratory-scale plant. The used constitutive equations are based on critical flow conditions and from the representation of the inherent flow characteristics in valves. Finally, the model was validated using experimental data, showing that the model behavior is highly dependent to the pressure input.