External Pressure Design of Piping Systems Including 90 deg Ells 3

[spayette]

Does anyone have any guidelines or calculation methods for external pressure or vacuum design of piping system? The concern is not straight lengths of pipe which can be analayzed using the methods of Sec. VIII, Div. 1, UG-28 but rather the concern is a typical piping system which includes 90 deg. ells and perhaps tees.

Are the ells considered as lines of support or should the lenght of bends be added to the length of the straight sections and the piping analyzed accordingly? Or perhaps should another approach be taken?

The piping alluded to in this thread would be larger diameter, thin walled systems.

 

[SnTMan]

spayette, in my industry we use UG-28 all the time to analyze u-bends of tubing. I take the total length of tube from baffle to baffle, using baffles as lines of support. I think UG-28 is very conservative for bends, but don't know of any other "design rule" type analysis.

I have seen a heat exchanger book that had some theory on torus under ext. pr. that seems like it would be applicable for ells, unfortunately, I don't have it and don't recall title or author. All the best reference books for this industry are out of print.

Not familiar with pipe code reinforcement rules for branches, tees, etc, but UG-37 may be usable for tees.

Anyway, if nothing else, I think UG-28 is usuable for ells, but it may give thicker wall than you like. Having said that, it doesn't take much wall for vacuum pressure (15 psig).

Not a lot of help I'm afraid.

 

[spayette]

SnTMan
Thanks for your reply. What you mentioned is the approach that we are inclined to take. In this particular case vacuum is very significant because the piping is 16" NPS, Schedule 10S, with a 1/16" corrosion allowance.

 

[be22323]

I suggest.
Go to

http://www.paulin.com

and download the FE/pipe demo.
In the user manual (which is a pdf file) do a search for "full vacuum". Move to the beginning of the section.
Print out the Eigenvalue buckling section and the next Non-linear analysis section. (pgs 240-252)
Read these 2 sections.
Your next search should be "collapse". This is found 96 times. Review and and distinguish between collapsed fe model elements (mesh problems) and collapse due to loading of physical structure.
Reread the printout.
Try the demo. Note the caution that it is for demo ONLY. Several properties are fixed.
Buy the Fe/pipe software.
You will have the best overall method IMHO. IF you know little about ASME endurance curves and allowables this manual and software is where you will make a very comforting leap.
The Paulin Research Group support staff are very helpful.

 

[spayette]

be22323:

Thanks. I have the demo version of FEPipe and wrote a justification for purchase of the program some months ago. I appreciate your "plug" for the program.

 

[bvi]

You did not mention which piping code you were using.

The typical approach is to simply use the total length of pipe between ring type restraints (e.g. flange, rings designed for external pressure, heads, etc.). This length is used to compute D/l and the normal cylinder external pressure rules are used. While I believe the elbows actually may act as a point of restraint, this is not considered in the piping codes I am familiar with.

 

[spayette]

bvi
B31.3 is the applicable code. We have proceeded as you and others have suggested, which was also our inclination. Thanks very much.