Theoretical and experimental modelling of a blowdown from a partially full vessel with a vent pipe pressurized with Freon 12 has been performed. The effect of nucleation and slip at the liquid-vapour-vapour interface have been introduced and assessed. A simple theoretical model utilizing the centred expansion fan equations was used to predict the average velocity of the diaphragm fragments after a blowdown. Good agreement was provided by the homogeneous model (no velocity ratio, thermal equilibrium).