As additional comment, the DIERS Project Manual (copyright 1992) is introduced as being a record of the DIERS research project. In that respect, it covers a wide range of aspects of the various methodologies that were investigated for evaluating two-phase vapor-liquid relief and will be somewhat overwhelming for a novice.
More recently, API RP-520 Part 1, 7th edition, January 2000, includes procedures for sizing relief devices for two-phase flow using the Leung "omega" method, Joseph Leung having been involved in the DIERS project and the International Symposium on Runaway Reactions. The API procedures give a step-by-step approach for sizing a relief valve for various two-phase flow conditions but do not address how to determine if two-phase flow should be considered for the required relief rate from a vessel.
Another excellent reference is the AICHE - CCPS publication "Guidelines for Pressure Relief and Effluent Handling Systems", copyright 1998. As stated in the objective of this publication, it was intended to provide a more user-friendly coverage of two-phase flow calculation technology (as compared to the DIERS Project Manual). I've only recently had a copy and have not reviewed its treatment on the subject of two-phase flow for valve sizing but I have found this to be a very useful reference in many aspects of relief system design. It also comes with a computer disc which includes programs for sizing relief devices and associated piping for either liquid, vapor or two-phase flow.