If two-phase flow is likely then we will use DIERS methodology to determine the two-phase venting requirement based on using the two-phase homogeneous sizing method describe by J.C. Leung from the article, "Simplified Vent Sizing Equations for Emergency Relief Requirements in Reactors and Storage Vessels". Download
Select the "Results" tab on the bottom left and click the "Find Convergence" button, to converge the DIERS two-phase Fire modeling equation below. Notice the two-phase relieving rate is 262,888.3 lbs./hr., compared to the all vapor venting rate of 8, 858.4 lbs./ hr.
Also notice the conditions at "Peak Pressure", showing time for pressure to rise to the peak, the mass in vessel at peak, the weight fraction in vessel at peak, and vessel volume fraction at peak conditions.