Release limited aperture

Release mechanisms are classified into wide and limited aperture releases. In the wide aperture case a large hole develops in the process unit, releasing a substantial amount of material in a short time. An excellent example is the overpressuring and explosion of a storage tank. For the limited aperture case material is released at a slow enough rate that upstream conditions are not immediately affected the assumption of constant upstream pressure is frequently valid. 

Limited aperture releases are conceptualized in Figure 4-2. For these releases material is ejected from holes and cracks in tanks and pipes, leaks in flanges, valves, and pumps, and severed or ruptured pipes. Relief systems, designed to prevent the overpressuring of tanks and process vessels, are also potential sources of released material. 

The released fluid may be a gas or a vapour, a liquid, a two-phase mixture, a mixture of several components or a fluid in supercritical state (vid. Example 10.19). If the release is from a vessel storing a liquefied gas, liquid is released if the leak is below the liquid level. If the leak is above the level either vapour or a mixture of liquid and vapour is released. For a given difference of pressures across the leak usually higher mass flow rates are reached if a liquid or liquid/vapour mixture is released than in case of a gas or vapour. The equipment from which the release takes place can be a vessel or a heat exchanger, a pump or a pipe. The maximum released quantity depends on the material inventory and the possibility of isolating the leak. The size of the leak may vary between a large portion of the vessel surface and a limited aperture as that of a hole. 

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