Many times, the process engineers will take the main header design temperature and will assign to all the lines downstream the PSV regardsless the design temperature of the lines upstream PSV. The nature of gases is that after PSV the temperature will drop because of gas expansion. Some will say that if the main header is designed for fire condition (somewhere at 400C) and if this temperature is real then all the branches will see this temperature. Well....maybe a small part, let say 1000 mm (for convenience).

Since the design temperature is not the same with flexibility temperature, you may want to take advantage of this, regardless the "official" design temperature. So as a minimum you should take the upstream temperature for part of the downstream line, then for the last meter the header temperature.

Do not forget to have a operating scenario with upstream lines hot, downstream lines cold (as this is normal operating mode)

Regards,