Discharge of a Two-Phase Mixture from Vessels

Releases of superheated hquids have to be considered in safety analyses for process plants. Such liquids vaporize on depressurization, which may occur, for example, following leaks or pressure relief of reactors. It is a difficult issue, which is not yet fuUy clarified. Most work on two-phase flow concerns mixtures of water and steam, which play an important role for accidents in nuclear reactors. The more complex task of treating flow processes of mixtures of several components and phases, which are characteristic of process plants, still requires intense research. 

A basic problem for modelling two-phase flow is whether equilibrium exists between the two phases. This is generally not the case. However, equilibrium is usually assumed because it facilitates the analytical treatment of the problem. 

In the lower portion of a vessel containing a superheated liquid we have the liquid phase and in the upper portion the vapour phase. The storage pressure is the vapour 

As was mentioned, if the discharge takes place above the liquid surface, but not far enough for pure vapour release, the possibility of two-phase flow exists. Whether this actually takes place or not, can be decided by using the DIERS criterion [16]. This is presented here based on [15]. The following steps are carried out  

Determine the discharge rate for gas according to the procedures of Sects. 10.2.3 and 7.4.3 (Assumption the quality of the discharged fluid is Xa = 1, i.e. only vapour) 

---