hi buytree. I see you have a parallel thread in engtips.
http://www.engtips.com/viewthread.c...=307182&page=1
That's a good place for advice. As mentioned:
 You can't simply use the internal pressure calculations to determine external collapse pressure.
 The ASME BPV code is fairly complex for externally pressurized structures. You'll need to dig into UG28 through UG30 yourself to do this analysis.
Quote by buytree
3. I used the ASME Sec VIII Div 1 mentioned in ASME B 31.3 Sec 304.1.3 to find the thickness due to external pressure. I calculated the factor A and B, but I have a lot of confusion when finding factor B. My duct working temperature is 120 deg c and carbon steel A 515 grade B material. Which chart/table should I use to find factor B?

Good question, which figure to use for your material? Use CS2 since if you check ASME B31.3 in the materials section you’ll find A 515 Grade 60 (presume you mean 60, not grade B) has a yield of 32 ksi.
4. According to step 8 in Sec VIII Div 1, the pressure obtained is much higher than the actual pressure I used as internal pressure. I think I am safe. Comment pl?

Correct. If you go through the steps in UG28 and get to step 8 and find your actual pressure is less than the allowable pressure (P
_{a}) then you’re pipe wall thickness is sufficient.
5. When comparing the minimum thickness formula for internal pressure in ASME B31.3 and the ASME Sec VIII. Div 1 there is a slight variation in formula right?

Yes, but don’t worry about ASME Sec VIII. Div 1 for internal pressure. Use B31.3 for internal pressure.
6. How can I determine the stiffener required for the particulaer duct?

UG29 covers stiffeners.