One of the main differences between Divisions 1 and 2 is that Division 2 uses lower design margins often resulting in higher material allowable stresses. Design margins are reduction factors applied to the material’s ultimate tensile strength (UTS) for the purpose of setting material allowable stresses in ASME II-D. The design margins are currently 3.5 for Division 1, 3.0 for Division 2, Class 1 and 2.4 for Division 2, Class 2. In Division 1, hydrotest stresses are not specifically limited and partial penetration nozzle welds are permitted. In Division 2, hydrotest stresses are limited so hydrotest stress calculations are mandatory and full penetration nozzle welds are required.
Another major difference is the theory of failure assumed and therefore the design equations used. Specifically, Division 1 uses the maximum principle stress theory while, starting with the 2007 Edition, Division 2 uses Von Mises. As a result, Division 1 uses two sets of design equations one for “thin” and another for “thick” vessels while Division 2 uses one set of equations for all vessel thicknesses. Of particular note are the more accurate nozzle design and allowable compressive stress (external pressure) rules in Division 2 both of which can provide additional savings.
In general, thinner Division 2 vessels retain safety factors that are comparable to thicker Division 1 vessels by incorporating more extensive engineering analysis and design requirements.