A common scenario that is encountered in pressure relief systems design centers around the calculation of vapor generation rates from liquids under external heating, internal heating, or fire exposure. Read this white paper for the most practical and appropriate method for determining the relief requirement for vapor venting for a multi-component mixture of liquid. While the other methods can establish a reasonable vent size, they fall short in providing reasonable estimates for relief exit conditions.
Pressure relief design is all about a volume balance. As the heating increases the liquid temperature and generates more vapor (volume) in a vessel, the pressure increases to fit the additional vapor generation (volume created) within the confines of the vessel. Relieving the vapor at a specific pressure removes the additional vapor volume and keeps the pressure in the vessel in check.
This relief design scenario shares some commonalities with batch distillation when the liquid being heated is a chemical mixture. Similar to batch distillation, mixture light ends are preferentially depleted first. The resulting volumetric vapor generation rate depends on the vapor composition. Initially, the vapor composition will be rich in light components.
As the light components are preferentially depleted, the vapor composition will become rich in heavier components. Thermodynamic, physical, and transport properties change as the mixture is fractionated for both the liquid and the vapor. The maximum relief requirement may occur anywhere along the fractionation curve.
For reactive mixtures where all vapor venting occurs, special care must be taken to ensure that materials that are preferentially concentrated do not spontaneously decompose or deflagrate.
Download white paper
