Large Diameter Piping - Min Wall Thickness

[mechglitch]

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?

 

[MikeHalloran]

Once upon a time, I saw a rocket tank of roughly the diameter you mention. It was made of something that looked like polished stainless steel, was a little over a diameter long, and had a wall thickness of just .010". It had to be kept pressurized or under mechanical axial tension in a fixture throughout its life in order to prevent collapse.

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

 

[mechglitch]

This pipe will not always be under pressure. Is there a calculation that can demonstrate the need for a thicker wall? My question is genuine, and I'm looking for a formula based calculation/approximation that will show the wall needs to be thicker than .06 inches.

Hoping for some insight on this!

 

[MikeHalloran]

Recognize that I have never even seen B31.1. Nevertheless, I'll offer an opinion.

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

 

[unclesyd]

Handling
For handling shop applied cement mortar lined pipe the minimum required wall thickness should be limited by a D/t (outside diameter/thickness) ratio of 240. For handling pipe with a spray-applied flexible lining or no lining at all, that is also hydrostatically tested, the minimum required wall thickness should be limited by a D/t ratio of 288. Cement mortar lined pipe: D/t = 240
Sprayed-applied flexible lined or bare pipe: D/t = 288

Here are the numbers used in ACIPCO's design guide for spiral welded pipe.

http://www.acipco.com/aswp/pdfs/ASWP2-Design.pdf

 

[BigInch]

Very thin walls from the pressure calculation sometimes indicate that other materials could be more suitable for your application, such as plastic piping materials, or concrete. Might want to look into that possibility.

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://www.msm.cam.ac.uk/phase-trans/2005/LINK/192.pdf

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/

 

[mechglitch]

Thanks all,

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?

 

[rconner]

You can of course do any kind of "calculations" or finite element analyses for any kind of hypothetical handling and loading/support stresses and deflections you wish. However, I wouldn't be surprised however if deflections, stresses, and also risks of buckling instability etc. come out in some cases quite high, as the recommendations of ACIPCO are basically as recommended by AWWA Manual M11 for large diameter steel water pipe, and that I suspect based at least in part on more than a hundred years experience in manufacturing and handling of large diameter steel pipes. AWWA Manual M11 incidetnally contains the additionally guidance, "In no case shall the shell thickness be less than 14 gauge [0.0747 in. (1.9 mm)].
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.

 

[mechglitch]

Thanks rconner, this seems a good answer to the question I had. A min thickness of t=(D+20)/400 (pipes over 54" dia) recommended in the Steel Water Pipe AWWA M11 gives a formula based approach for handling.

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.

 

[StressGuy]

Piping codes such as B31.1 and B31.3 make is clear that they are not design manuals. They do give some basic equations, such as wall thickness requirements for pressure.

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.

 

[MechEng2005]

I typically don't design pipes. However, if I were in your shoes and was looking for a reasonable prediction for the loading when handling, I would assume that somebody is going to stand on it. Figure a 300 pound person, plus the fact that they are moving and shifting around... Maybe do a calculation for two 300 pound point loads approximately 2 feet apart in the center of the span.

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