[rblogan]

these people are making it more complicated than it is. It’s Very simple.
Connect a 300 rating flange to the 300 rating nozzle. Then weld 150 rating pipe to the 300 rating flange. If the schedules don’t match, change the schedule of the 300 rating flange to be the same as the schedule of the pipe (why? becuase the schedules of a weld should match on both sides of the weld for welding easablitiy).