Minimum Temperature of Upstream Piping Blowdown Utility HYSYS

Dear all,

Now I am working on the project which involves blow down calculation using HYSYS utility.

We need to blow down a small inventory (piping spool with volume of 0.3 m. cub) and concerned about the minimum temperature upstream of the restriction orifice because the design temperature of upstream piping is only -20°C. HYSYS provides us with two temperatures - vessel fluid (-26C) and inner wall temperature (-12C).

Could you please advise which of these temperatures do I need to use as a basis to protect upstream piping?

Thanks in advance.