How to model a two phase pipe system under vacuum pressure?

I'm trying to model a two phase flow through a pipe network with an inital pressure (vacuum) of 3.5kPa. Two receiver-accumulator blocks are also used to model evaporation and condensation. However, even after setting the initial conditions to 3.5kPa and a intial temperature of 300K, the internal pressure goes up to about 6MPa. Because of this increase there is no phase change in the system as well. I'm not sure if an external vacuum pressure needs to be introduced to the system to model a vacuum sealed pipe system. I would be really grateful if someone could point out any alternatives or mistakes I've made.

Please see the subsystem below

Many thanks