you may use PRODE PROPERTIES (see prode.com) which includes specific rigorous methods for
both critical and two-phases (vapor-liquid) relieving,
the procedure can model

a) critical flow with rigorous procedure (described in API and superior to standard formulation)

b) two phase (vapor + liquid) with rigorous HEM (Homogeneous Equilibrium Model) based on evaluation of critical speed in vena contracta

c) two phase (vapor + liquid) with rigorous HNE (Homogeneous Non-Equilibrium Model) which allow to specify different non-equilibria parameters

d) two phase (vapor + liquid) with HNE-DS (Homogeneous Non-Equilibrium Model) , this is the procedure part of ISO standard

etc...