Determination of vessel flow regime

The relief sizing methods described in Chapters 6, 7 and 8 make worst case assumptions about the vessel flow regime (see (1.) below) in terms of the,extent to which it causes two-phase flow to enter the relief system. It is therefore not necessary to know the vessel flow regime in order,to safety use these sizing methods. However, it may sometimes be desirable to determine it and calculate whether, or how much, two-phase relief would occur, because . ,  

The worst case assumptions for relief sizing, regarding vent flow type, are described in (1.) below. If required, (2.) to (4.) describe aspects of the procedure to determine what the vessel flow regime would actually be. 

If required, the following method may be used to assess whether single or two-phase venting would actually be expected to occur  

Inherent foaminess can be caused by trace quantities of certain materials, so tests should normally be.done on the mixture undergoing runaway reaction. Levdl swell does not scale up directly, so that any small-scaje blowdown test must seek to reproduce the same superficial velocity (volumetricflow divided by vessel cross-sectional area) /as on the fuli-scale. This will be much higher than the superficial velocity otherwise produced by the runaway reaction at test-scale. Techniques for carrying out small-scale tests are discussed in Annex 2. 

It may occasionally be possible to deduce from normal operation that the fluid is not inherently foamy, for example if the normal process boils the mixture (when cooling by reflux condenser) and a stable foam is not produced. However, if runaway might cause surface-active agents to be produced, then there is no substitute for testing under runaway conditions. t. . . 

---

Further information on level swell calculations and the determination of vessel flow regimes is given in Annex 3. 

---