Determination of the pressurising period for stored cryogenic fluids Original Research Article

It is a normal operating condition for a lot of cryogenic storage vessels that no boil-off gas is vented over long periods, which leads to a simultaneous pressure increase of the stored fluid. One main reason therefore is to avoid product losses during transport or between withdrawals. At transport conditions the mixing of the fluid can be assumed to be ideally, which results in a maximum reachable pressurising period. At stationary conditions the pressurising period is expected to be shorter, because a stratification is rising up, so that the heat capacity of the stored fluid cannot be used completely. In a thermodynamic view, an isochore change of state takes place and the heat flux into the vessel rises the internal energy of the fluid. For the representation of the isochore change of state a new developed Δu/v–v-diagram with the 1 × 105 Pa reference as a basis line is introduced. The basis line is linear for the filling rate and functionally connected with the specific volume. For the fluids He and H2 e.g., Δu/v–v-diagrams are pointed out, using the u- and v-values on the saturation lines for the two phase region and those of some isobar lines for the region above the critical pressure.