The Non-Steady Flow of Gases and Vapours from Pressure Vessels

Abstract

The flow of gases and vapours from pressure vessels to the atmosphere is investigated. Two major points are addressed, first, the non-steady mass flow rates and the non-steady pressure and temperature variations, due to the finite size of the pressure vessel, and second the effect on the blow-down process if the vapour is not assumed to be a perfect gas, as is suggested in current pressure vessel safety codes, but if a more realistic equation of state is selected to describe the thermodynamic behaviour of the vapour. For the non-steady outflow of perfect gases analytically derived correction functions are given with which the complete blow-down phenomena can be calculated from the well known steady flow equations. For the non-steady outflow of real gases and vapours a calculation method is described and the computer algorithm is given. Both calculation methods are discussed and compared in examples using propane vapour as the flowing medium and the dimensions of a rail tank car for the pressure vessel.