Numerical Simulation of Pressure Recovery Phenomenon in Liquid Ammonia Tank

A phase transition develops when a pressurised ammonia vessel is vented through a relieve valve or as a result of shell cracking. Significant pressure recovery in the vessel can occur as a consequence of this phase transition following initial depressurisation and may lead to complete vessel failure. It is critical for safety engineering to predict the flash boiling behaviour and pressure dynamics during the depressurization of liquid ammonia tank. This research aims to develop and compare against available experimental data a CFD model that can predict two-phase behaviour of ammonia and resulting pressure dynamics in the storage tank during its venting to the atmosphere. The CFD model is based on the Volume-of-Fluid (VOF) method and Lee evaporation/condensation approach. The numerical simulation demonstrated that liquid ammonia, which is initially at equilibrium state, begins to boil throughout due to the decrease of its saturation temperature with the pressure drop during tank venting. In order to understand phenomena underlying the pressure recovery, this paper analyses dynamics of superheated ammonia formation, its swelling, vaporisation, contribution to gaseous ammonia mass and volume in ullage space and gaseous ammonia venting. Performed in the study quantitative analysis demonstrated that the flash boiling, and gaseous ammonia produced by this phase change were the major reasons behind the pressure recovery. The simulation results of flash boiling delay accurately matched the analytical calculation of bubble rise time. The developed CFD model can be used as a contemporary tool for inherently safer design of ammonia tanks and their depressurisation process.