Dear Friends,

I am analyzing a 36” vapor line with a design pressure of 45 psig. The standard schedule of 36” Pipe thickness is 9.52 mm, but at present the measured pipe thickness is 4.5 mm (60 deg) at 6 o’ clock position and 9.52mm (300 deg) at 3 o’ clock, 12 o’ clock and 9 o’ clock positions. I run the model with 9.52 mm thickness and noticed the stresses are well within the limit, after that I reduced the thickness gradually and checked the system then I found failure at the nearby branch joint, when the thickness is below 8 mm. Therefore I took the average of the thickness at bottom portion of pipe to the top portion of pipe (4.5mm (60 deg) +9.52mm (300 deg) = 8.7mm) and did the analysis. Our Client asks us to check the system how far to the minimum thickness it can withstand. Required thickness as per ASME B31.3 for the given pressure is 1.08 mm without corrosion allowance and mill tolerance.

Questions:

1. Is it possible to model the cross section of pipe with different thickness in CAESAR II? If yes, please advise me how to model it?

2. Average of pipe thickness used is correct or incorrect? Please correct me if i am wrong.

3. Keeping the thickness as 4.5 mm and increasing the remaining portion (9.52-4.5 mm = 300 degree) weight in pipe insulation and fluid density produces different value compared to the average thickness.

4. Keeping the thickness as 4.5 mm and increasing the remaining portion (9.52-4.5 mm = 300 degree) weight in pipe dead weight produces almost the same result compared to average weight thickness. In this case I observed same sustained case values and slightly different expansion case values compared to the average thickness results.

Please help me and give your valuable suggestions.