While calculating Hydrotest Pressure follow the followings:
As per Pressure Temp ratings specified in B16.5, you will get the pressure at Atm Temp 38degreeC. That will be the design pressure for the flange. You take 1.5 times of it as Max allowable hydrotest pressure for the line.
If you are going to conduct test by using hot water, take corresponding pressure of that temp (water temp) and take hdrotest pressure as 1.5 times of it.