Rigorous Simulation of Accidental Leaks from High-Pressure Storage Vessels

Abstract

Several major industrial disasters involve accidental releases of hazardous chemicals from ruptured vessels or pipelines as consequence of equipment failures, maintenance errors, operational errors, cracks, corrosion, ruptures, or also by acts of nature. The released chemical can form and disperse as vapor cloud leading to fire, explosion, or toxic exposure. The resulting leak could be single phase or multiphase release, choked or non-choked. These releases could result in liquid spills, vapor cloud formation, explosion, toxic dispersion and flashing liquids. The impact of the release depends on the properties of the fluid and the exit conditions. Often the leak goes unnoticed and also it becomes hard to estimate the leak flow rate. When a leak occurs, it is very important to have an idea of the leak flow rate and the fluid properties. This helps in assessing the hazards posed by the leak and also predict the consequences.

To assess the consequences of a leak, it is important to estimate its flow rate and properties of the discharged fluid, but both change with time during the leaking process. Several models and programs exist to simulate accidental releases, often based on assumptions that increase the uncertainty of their predictions when applied to high-pressure vessels. This thesis takes a different approach by using rigorous calculation procedures and a cubic equation of state to: (1) find the state of fluid within the vessel using a flash algorithm for systems of specified internal energy (U), volume (V), and mole numbers of each component (N); (2) track phase appearance and disappearance in the vessel; (3) find the state of fluid as it exits the vessel, assuming the leaking point is the throat of an adiabatic, converging nozzle that operates isentropically; (4) compute sound speeds in multiphase systems to establish whether the leak flow is choked. The code that implements these steps has been validated against ideal gas state for single component data and against experimental data for leaks from vessels containing mixtures.