The storage and processing of fluids in a two-phase state at pressure requires the consequences of a failure of the vessel wall or a pipe break due to corrosion or third party interference, be considered as part of a safety analysis. The present paper describes a mathematical model for predicting outflow rates from top venting high-pressure vessels undergoing level swell. A key aspect of the models development is there are no free parameters that require adjustment to predict the correct outflow from a top venting vessel. The mathematical model is validated by comparing predicted flow parameters such as vessel pressure and mass flow rate with measurements and existing mathematical models. The new model agrees more closely with the experimental measurements than previous approaches over the range of experimental conditions considered. (C) 2002 Elsevier Science B.V. All rights reserved.