In order to predict the consequences of a leak or rupture of a pipeline or vessel, or to design a controlled pressure release system, a method of predicting the mass efflux is required. Although a large body of literature exists on blowdown simulation for vessels or short pipes, relatively little analysis and experimentation has been performed for the depressurization behavior of pipelines. In addition, the impact of the thermodynamics of multicomponent hydrocarbon fluids has been relatively unexplored. A mechanistic homogeneous equilibrium model applicable to both vessels and pipelines has been written. Since fluid thermodynamics are evaluated external to the model, single or multicomponent fluids can be simulated as well as single or multiphase flows. In addition, a series of small scale blowdown experiments has been made using both gas bottles and coiled tUbing. The coiled tubing has a length to diameter ratio of SO,OOO, comparable to that encountered in field piping. Air, carbon dioxide, and carbonated water have been used in the experiments.Excellent agreement between the model and experiments has been obtained, along with excellent agreement with at least one field data set.