I would be interested to hear how others calculate the nozzle loads on a flat header box in accordance with API 661 / ASME VIII - apart from of course using FEA.
Is there a "simple" method for doing this?
Simple? No! I have spreadsheets for box headers, but always relayed on vendors for nozzle loads. Codecalc did not use to have a feature for this, maybe now they do. Check around.