hcWow……complicated answers to a simple question. It did get answered in amongst those posts, then it got weird so I will clarify.
Basically because the tank is a higher rating (300#) than the piping system (150#) this is an easy fix. The flange rating needs to match the nozzle…..so 300#. The 300# flange wall thickness/schedule needs to match the piping schedule as specified by the 150# spec. So simply if your piping is sch. 80 you would require a 300# sch.80 flange at the nozzle. Not to confused you but, technically you are creating a spec break at the weld between the flange and the pipe but, because its just at the flange a spec break isn’t required. Its built as part of your 150# piping spool with an odd ball flange. Hopefully that makes sense.
Now if that tank had 150# nozzles and your piping system was 300# then the above doesn’t apply. You can’t run a higher rated system into a lower rated tank. You would then need ask questions.