1- Which code is referred for Boiler External Piping in Section IV? as Section I does refer to B31.1 for BEP.
2- If a boiler produces steam at less than 15 Psi from the exhaust gases of an engine, it is neither fall in Section IV because it is not directly fired nor it falls under Section I because pressure is less than Section I boundary. Then is Section VIII will be the right choice? taking this vessel as unfired steam boiler.

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Thanks & Regards,
Nasir
Welding Engineer
DESCON Engineering Limited.
Lahore, Pakistan.
Mobile # 0092-336-4145402