Vessel flow regime classification

For tempered systems, the pressure relief system will almost always need to be bigger if two-phase flow occurs, and DIERS[1] recommended that two-phase relief should normally be assumed for vent sizing purposes using the type of hand calculation methods given in this Workbook. This is explained in 4.3.2, Subsection 

For untempered systems, it is generally conservative to assume initial single-phase gas relief followed by two-phase relief at the peak reaction rate. However, for the hand calculation method given in Chapter 7, it is safe to assume two-phase relief. This is also explained in 4.3.2, Subsection (1)  

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