Which tool is to be used depends on your objective. I think there are two scenarios & I am elaborating it because of lack of knowledge of your objective.
1) If you are going to do cold venting and intent of your study is to find out minimum design temperature of metal, then first use Hysys depressurisation utility. In this utility, assumed backpressure of 2 psig will provide for maximum differential pressure across RO and thus it will give peak flow and corresponding minimum temperature. Further to this build model (as per isometrics) in flarenet and use peak flow rate (from Hysys depressurisation utility) to analyse vent header. In this case flarenet will calculate actual back pressure and corresponding minimum temperature. you can use this temperature to finalise your material.
Note that no need to go back to Hysys depressurisation utility with backpressure given in flarenet.
2) If you have flare and you are analysing flare network, then go for Flarenet. In this case first use Hysys depressurisation utility with Flare tip pressure drop (data to be taken from flare tip vendor, but for preliminary study you can assume same from literature) as backpressure and find out peak flow rate. Then build model (as per isometrics) in flarenet and use peak flow rate (from Hysys depressurisation utility) to analyse flare model. In this you case, you need to study various parameters like Mach no., rho V2 criteria, noise limits prior to freezing your flare network configuration. you can use minimum temperature from this study for metal selection.
Please note that if there are more sources releasing simultaneously into flare header, then consideration of conservative scenario with operational knowledge is important in this case.
On a practical note, if you don't have time or enough resources ( like isometrics, vendor data etc.) then use minimum temperature given by Hysys depressurisation utility (by assuming 2 psig backpressure) as minimum metal design temperature.
Hope this helps.