Although two-phase flow is frequently encountered in various location of the process plants, there is no a general accepted and verified two-phase flow model that may be used to size lines for such conditions. An obvious example is condensate water return lines. The API method that used in this study is based on the homogeneous equilibrium flow assumption, that is, equal velocity and equal temperature in both liquid and vapor phases. Also, DIERS method has used to verify and clarify the HEM approach to calculate the pressure drop in two phase regimes. The objective of this study is to introduce a solution for process lines design during different flashing scenarios. In this work by applying API method we can find the two-phase line pressure drop and upstream pressure while by using DIERS method we would be able to realize that for a specified length of pipe how much two phase flow could pass through when the pressure drop is just same as the API model.