Large Diameter Piping - Min Wall Thickness
Large Diameter Piping - Min Wall Thickness
(OP)
Hello,
This is my first post and my question is trivial. I am trying to determine the minimum practical thickness that can be used for a 96" diameter pipe (longitudinal weld seam) with a very low pipe pressure(23 psig) and the fluid is just air.
B31.1 min thickness to meet the low pressure requirements is about .06 wall thickness or so, very thin. Let's assume there is no corrosion allowance required for now. Furthermore, even if bending stresses are to be kept to a maximum of 2.3ksi between supports and deflection limited to .1 inches, only a very thin wall is required to support its own weight at spans above 40 feet.
How is the pipe likely fail if a .06 wall is chosen? Would such a thin wall pipe be impractical? Would it be too likely to be damaged or to sag to an out of round shape under its own weight?
It seems the code leads me to a wall that is probably too thin when only burst pressure and self weight are considered. Is there a minimum practical thickness that must be used even if pressure and bending stresses only indicate the need for a 1/16" thick wall?
Do certain other loadings (wind, impact strength) make a very thin wall thickness impractical for the real world?
This is my first post and my question is trivial. I am trying to determine the minimum practical thickness that can be used for a 96" diameter pipe (longitudinal weld seam) with a very low pipe pressure(23 psig) and the fluid is just air.
B31.1 min thickness to meet the low pressure requirements is about .06 wall thickness or so, very thin. Let's assume there is no corrosion allowance required for now. Furthermore, even if bending stresses are to be kept to a maximum of 2.3ksi between supports and deflection limited to .1 inches, only a very thin wall is required to support its own weight at spans above 40 feet.
How is the pipe likely fail if a .06 wall is chosen? Would such a thin wall pipe be impractical? Would it be too likely to be damaged or to sag to an out of round shape under its own weight?
It seems the code leads me to a wall that is probably too thin when only burst pressure and self weight are considered. Is there a minimum practical thickness that must be used even if pressure and bending stresses only indicate the need for a 1/16" thick wall?
Do certain other loadings (wind, impact strength) make a very thin wall thickness impractical for the real world?





RE: Large Diameter Piping - Min Wall Thickness
Even with six times the wall, I think your pipe will buckle as soon as it's allowed to do so.
Mike Halloran
Pembroke Pines, FL, USA
RE: Large Diameter Piping - Min Wall Thickness
Hoping for some insight on this!
RE: Large Diameter Piping - Min Wall Thickness
Where I was going, without saying so directly, was that I think you need to re-check your calculations. The numbers you've posted just smell like you're missing a big unit conversion, or have a wrong exponent somewhere.
Mike Halloran
Pembroke Pines, FL, USA
RE: Large Diameter Piping - Min Wall Thickness
Here are the numbers used in ACIPCO's design guide for spiral welded pipe.
http://www.acipco.com/aswp/pdfs/ASWP2-Design.pdf
RE: Large Diameter Piping - Min Wall Thickness
I think what you're looking for are local buckling.
Search for "Global and local buckling" or some variation thereof and you'll get a lot of hits, and have a look at this too,
http
It seems like you have only considered internal pressure so far. You must consider loading factors other than just internal pressure, of which the following may help,
There are practicle limits to transportation and installation. Will the pipe need to be stacked when shipped? Will the pipes be lifted on to or off of a flatbed truck using standard spreader bars and end hooks, slings, etc. or will you require special handling with multiple support points during lifting?
Installation method.
Will the pipe be welded above the surface and lowered in, as in pipeline construction, or will the pipe sections be lined up and welded in place, as would be the case when installed in a pipe rack or on sleepers.
Will the pipe be subject to lateral external pressures due to backfill in an underground trench? Spanning, either intentional between supports, or by loss of support from underlying soil from washout or liquifaction during earthquakes.
"Less than 1% of the energy moving a car goes towards the driver."
Amory Lovins - The Oil End Game http://virtualpipeline.spaces.live.com/
RE: Large Diameter Piping - Min Wall Thickness
The pressure is low and does give this very thin wall thickness.
the pipe is not buried. there are probably handling forces that should be considered, but what calculations are applicable? I'm trying to get to a thickness where I can be comfortable that handling will not pose a problem. What min thickness would be required to cover handling?
RE: Large Diameter Piping - Min Wall Thickness
If you wish to get into these matters in more depth you may wish to also obtain and read the technical article, "Minimum Thickness for Handling Pipes, Water Power and Dam Construction" by J. Parmakian 1982, that I beleive is also referred to by AWWA M11.
While your application may not be exactly the same, I assume your pipe must still be handled and supported, by many folks and in many means, in manufacture, shipping, and also may be exposed to other conceivable loadings and impacts in any meaningful service life.
RE: Large Diameter Piping - Min Wall Thickness
I find it curious and somewhat unfortunate that the piping code B31.1 doesn't point to formulas such as this one to cover the low pressure case and guard against inadequate design.
RE: Large Diameter Piping - Min Wall Thickness
However, they also have general statements that the designer is responsible for considering other aspects that affect the mechanical integrity of the line, such as externally applied forces/moments to the system, as well as loadings that create localized stress issues, such as at a support crossing. There are a number of ways to address these, from FEA to simplified formulas. The design work is left to the engineer and it what pays the bills for us.
mechglitch - I give you credit that it is a very good sign that, while you didn't know quite how to approach the problem, you did have a good enough understanding to be concerned about such a thin wall result.
Edward L. Klein
Pipe Stress Engineer
Houston, Texas
"All the world is a Spring"
All opinions expressed here are my own and not my company's.
RE: Large Diameter Piping - Min Wall Thickness
Again, I don't normally design pipes, and am not qualified. I persume that this application has no risk and that you are taking everything I'm saying for what it's worth (not much).
-- MechEng2005