If you will only design the steel structure, you should follow other codes such as AISC, but, you need to know the loads, restrictions, thermal growth, and some impact load due to vibration, water hammer, rotary equipment, etc. Then you need to have experience in boiler structure design.
About non-pressure parts, you need to be more specific, there are non-pressure parts having nothing to do with pressure-parts and need to be designed according to codes different than ASME B&PV, but, some non pressure parts are attached to pressure-parts to support, align, protect, etc. Those parts have to be designed according their specific code, but you also need to comply with some ASME B&PV articles.
Javier Guevara E.
Projects, Mechanical Engineer
TERMOGUAJIRA - GECELCA S.A. E.S.P.