thin wall under external pressure

Hello every one,
As per B31.3 304.1.3, D/t > 100, I am following the procedure outlined in the BPVC-VIII-1, UG28,UG29,UG30 I am trying to determine the minimum wall thickness required under external pressure, and the required stiffening ring. "A" factor that I found falls to the left material/temperature line chart.
the design pressure in my case is 0.7 bar (internal pressure). does it mean that external pressure is just atmospherique pressure?
Anyone who has performed or familiar with such calculation, his/he help would be very appreciated.
NB: 1- I couldn't find appendix L ,that has examples illustrating the use of rthe precedure.
2- I made a seach in this forum, I didnot find answers to my questions.
Thanks in advance
Yani.

Yani,

When you use Fig. G in ASME II D with a L/Do ratio > 50 then you use L/Do = 50.

How has the external pressure been specified? If your component is single skin (no jacket) then the maximum external pressure that can be achieved is when the inside is fully evacuated. In that instance your external pressure will be, as you have said, atmospheric pressure (1 bar extenal).

You may find more info if you search in the Pressure Vessels Forum instead. If one of your colleagues uses PVElite then they will probably be able to do the calculation for you very quickly.

Thanks for your reply,
I am checking a 66" and 70" pipe under vacuum condition ( design pressure -0.21 barg), internal design pressure is 0.7 barg. to prevent from buckling and ovalization, i am calculating the required minimum thickness under external pressure and the required stiffening rings, using BPVC section VIII div 1, UG28,UG29 and UG30.
My concern is the external pressure, I assume is 0.79 barg, am I correct?
thanks again for your help.
Yani

Yani,

If your design pressure conditions include -0.21 barg then that would appear to be the negative gauge pressure range (i.e. 0 to -1 barg). In that case your design external pressure for the purposes of calculation would be 0.21 barg.

By comparison, if the design pressure was specified as 0.21 bar absolute
(the atmospheric pressure outside would be 1 bar absolute) this would result in approx 0.79 bar external pressure.

Thanks Stan,
That s the purpose of avoiding buckling, when the line is under vacuum condition, the minimum wall thickness required is designed when the vacuum pressure(-0.21 barg) is equal to the external pressure (0.21 barg).That s the worst case for buckling.
The line is designed for 0.7 barg @240 C.
Please correct me if I am wrong.
Kind regards
Yani

Yani,

That sounds like you have the correct external pressure for calculation. Can I just add one other point. Designing for external pressure using ASME VIII Div 1 (by comparison to other codes) can result in a small margin of safety in avoiding collapse. UG-28 (f) (see footnote) warns about designing for partial vacuum conditions and suggests that an additional margin should be used so you do not come close to the collapse pressure. Some codes recommend designing for full vacuum even if you have a partial vacuum condition. If you are designing for a partial vacuum condition you need to be certain that 0.21 barg external is a maximum or else you must use a higher value.

Stan,
Isn't it too conservative?
I could not find the warning footnote UG-28(f), may be you meant UG-99(f)where pneumatic test is required.
Thank you very much for your time and help.
Kind Regards.
Yani

Yani,

It should be noted that Section VIII-1 calculations for external pressure assume that all other loads are negligable. In the case of piping which is goverend by external pressure design bending, axial load or torsion may have a large impact on the stability of the pipe. The vessel code does not give any guidance as to how to include these loads in the calculations. That is typically the reason that full vacuum design (1 bar external) is called for.

Regards,
EJL

Small issue, which most probably doesn't effect this design. We are at abot 1700 m above sea level. As a result vessel designers tend to use less than 1 bar as full vacuum. Big saving sometimes. Best to check always the standard pressure where the equipment is going.

Guys, i would really need help on how do you model stiffening rings in the CAESAR, any reply is truly appreciated

What effect of stiffening rings are you attempting to model in Caesar ?? Remember that the Caesar uses beam elements and stiffening rings would only have a real effect on piping modelled using plate / shell elements.