The Major Differences Between Divisions 1 and 2
One major difference between Div. 1 and Div. 2 is that Div. 2 uses a lower design margin for determining the allowable stresses in ASME Section IID. The design margin is a reduction factor applied to the material’s ultimate tensile strength (UTS) and is 3.5 for Div. 1 and 2.4 for Div. 2. The reduced design margin results in higher allowable stresses for higher temperatures where the UTS, not the material yield strength, governs. Div. 2 retains a comparable safety factor to Div. 1 by incorporating more rigorous engineering and manufacturing requirements.
Another major difference between Divisions 1 and 2 is the theory of failure assumed and therefore the design equations used. Specifically, Div. 1 uses the maximum principle stress theory while, starting with the 2007 Edition, Div. 2 uses Von Mises. Of particular note are the more accurate nozzle design and allowable compressive stress (external pressure) rules both of which can provide additional savings.
Hydrotest Stress Calculations
In Division 1, hydrotest stress calculations are optional and partial penetration nozzle welds are permitted. In Division 2, hydrotest stress calculations are mandatory and full penetration nozzle welds are required.
The overall cost reduction between Division 1 and Division 2 depends on answering the following question. Do the material and labor savings exceed the additional engineering, quality control and administrative costs? To a first approximation, large, high pressure\temperature (thick), vessels are often good candidates to be built to Division 2. The COMPRESS ASME VIII-2 option used with COSTER can help you quickly determine which Division is the more cost efficient option. With the COMPRESS Division 2 Option you can switch between Divisions at any time so it is easy to calculate and cost the vessel twice, once in Division 1 and again in Division 2.