Sharikahamed

Heater tube design is done with API 530 now. But i came across a heater designed with ASME Section I. The section I specify tube size of 5" and below. But the one i came across is 6" size. How the design will be checked in this case.

The same heater was designed for 25 kgf/ cm sq. and psv was set at 21 kgf/ cm sq considering static head. This is not specified in Section I or API 530. Can anyone share the design philosophy in this case?