The Stratification Process During Pressurisation and Zero Loss

8 The Stratification Process During Pressurisation and Zero Loss 

Under isochoric (constant volume) conditions for the whole tank contents, about 98-99 % of the heat-flow energy, entering the liquid through the tank-wall insulation, appears to be absorbed by increasing the enthalpy of the upper layer of liquid. A small amount, of the order of 1-2 % of the heat flow, is absorbed by the latent heat of vaporisation of a small amount of liquid to provide the vapour needed to increase the pressure. 

Thereby, there is an equilibrium between the rising saturation pressure in the ullage (vapour) space, together with the rising surface temperature of the evaporating liquid. As a consequence, the heat energy is retained within the tank as the pressure rises slowly towards the specified maximum pressure ( 8-10-24 bar), determined by the relief valve setting. 

Looking more closely at stratification in a VI tank, we note that A 1 heat fluxes entering the upper layer of liquid are carried by convection to the liquid/vapour surface by superheated boundary layer flows up the walls of the tank. A small part of the energy (1-2 %) is absorbed by evaporation, while the bulk of the energy remains in the upper liquid layer of superheated (with respect to Tl) liquid, forming a stable upper layer at a higher temperature T2. 

The B heat fluxes into the unwetted upper section of the tank enter the ullage vapour helping to raise its temperature and pressure. 

---