In a nutshell, Section I deals with "construction of power boilers, electric boilers, miniature boilers, and high temperature water boilers to be used in stationary service and includes those power boilers used in locomotive, portable and traction service". Please read the Introduction for specific definitions for power boilers....
For ASME Section VIII, read the Scope U-1 (a)(2) "for pressure vessels that are containers for the containment of pressure, either internal or external. This pressure may be obtained from an external source, or by the application of heat from a direct or indirect source, or combination thereof."
