Select ID for shell sections based on the inside diameter. Select OD for shell sections based on the outside diameter. Normally, for a flanged & dished torispherical head, the inside crown or radius is equal to the vessel outside diameter.
For flat heads, this value is ignored. Refer to Fig. UG-34 for equivalent diameter of the head. For example, in case of most welded heads this is the diameter over which the pressure acts. For bolted heads with narrow faced gasket this is the diameter of the gasket reaction.
For cones, the program expects the diameter of the cone at the point where the nozzle intersects the shell.