Modelling and Validation of Atmospheric Expansion and Near-Field Dispersion for Pressurised Vapour or Two-Phase Releases
Abstract
The consequence modelling package Phast includes steady-state and time-varying discharge models for vessel orifice releases of toxic or flammable materials. These models first calculate the depressurisation between the stagnation and orifice conditions and subsequently impose the ‘ATmospheric EXpansion model’ ATEX for modelling the expansion from orifice conditions to the final conditions at atmospheric pressure. The latter post-expansion conditions are used as the source term for the Phast ‘Unified Dispersion Model’ UDM. The ATEX mathematical model determines the unknown post-expansion variables (diameter, velocity, temperature, liquid fraction, density and enthalpy) by imposing conservation of mass and energy, and equations of state for density and enthalpy. In addition, conservation of either momentum or entropy is imposed; by default the conservation option which results in the minimum change in temperature and/or liquid fraction is used. Finally a maximum is imposed for the post-expansion velocity. The current paper summarises the results of a literature review on atmospheric expansion modelling, and provides recommendations on selection of ATEX model equations to ensure a most accurate prediction for the near-field UDM jet dispersion against available experimental data. First, the correctness of the numerical solution to the ATEX equations has been verified against an analytical solution of ideal-gas releases for both cases of isentropic and conservation-of- momentum assumptions, including comparison against published data in the literature. Also the importance of non-ideal gas effects is investigated. Secondly, both ATEX expansion options have been applied to known available experimental data for orifice releases. This includes gas jets (natural gas and ethylene – British gas experiments, hydrogen - Shell/HSL experiments) and flashing liquid jets (ammonia – Desert Tortoise, Fladis; propane – EEC; HF – Goldfish; CO2 – CO2PIPETRANS). For these experimental data it was confirmed that the ATEX conservation-of-momentum option without a velocity cap provides overall more accurate concentration predictions than the isentropic assumption. However the existing default ‘minimum thermodynamic change’ option was found to mostly impose conservation of entropy (velocity cap not applicable) for two-phase releases and conservation of momentum (velocity cap applicable) for the sonic gas jets. Rainout calculations for flashing two-phase releases are currently always based on the isentropic assumption, which is inconsistent with the recommended conservation of momentum; a further
Description
PresentationSubject
dispersionCollections
Citation
Witlox, Henk W.M.; Fernandez, Maria; Harper, Mike; Stene, Jan (2016). Modelling and Validation of Atmospheric Expansion and Near-Field Dispersion for Pressurised Vapour or Two-Phase Releases. Mary Kay O'Connor Process Safety Center; Texas &M University. Libraries. Available electronically from https : / /hdl .handle .net /1969 .1 /193673.