Buried Double Wall Pipe Interaction (Bearing)
Buried Double Wall Pipe Interaction (Bearing)
(OP)
Let's start with a buried double wall pipe. Let's say the carrier pipe is hot and the encasement pipe is cold. The former expands, the latter contracts. Thermal expansion loops are integrated in the system at calculated pipe span lengths to avoid interaction between inner and outer pipes (clashing) near changes in direction. Using AutoPIPE (or similar piping software), fictitious restraints (with gaps) can be added between the inner and outer pipe at coincident nodes near these changes in direction to verify that no load is getting transferred between the bends of the inner and outer pipes (and hence, no inner pipes will virtually pass through their encasement pipe).
Here is the problem: With a contracting outer pipe and an expanding inner pipe, a large number of expansion loops are needed (in areas where they will not fit, in some cases) due to the limited span length calculated based on the gaps between the pipe and the amount of thermal expansion.
So I pose the question: What if we allowed the carrier pipe to bear on the inside of the encasement pipe? Is there any guidance for this in B31 codes? I imagine that concentrated bearing loads will develop unwanted stress concentrations, but at what point of loading is this deemed an unacceptable level? Is this a bad practice? Is this left to the designer/engineer/owner to determine?
Here is the problem: With a contracting outer pipe and an expanding inner pipe, a large number of expansion loops are needed (in areas where they will not fit, in some cases) due to the limited span length calculated based on the gaps between the pipe and the amount of thermal expansion.
So I pose the question: What if we allowed the carrier pipe to bear on the inside of the encasement pipe? Is there any guidance for this in B31 codes? I imagine that concentrated bearing loads will develop unwanted stress concentrations, but at what point of loading is this deemed an unacceptable level? Is this a bad practice? Is this left to the designer/engineer/owner to determine?
RE: Buried Double Wall Pipe Interaction (Bearing)
There is no code for this stuff.
You need to talk to these guys.
https://www.itp-interpipe.com/copy-of-solutions-1
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RE: Buried Double Wall Pipe Interaction (Bearing)
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RE: Buried Double Wall Pipe Interaction (Bearing)
If you have a straight line then you only need to do this when you come to a bend.
What's in the bit between inner and outer pipe?
Is this a real system or just thinking about it?
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RE: Buried Double Wall Pipe Interaction (Bearing)
This is a real system that has a 3" inner pipe carrying hot water (~200deg F) and vertically supported (with supports connected to outer pipe) in a 6" outer pipe. Owner requires a low installation temperature of 60deg F for pipe stress analyses, so we're looking at a 140-degree thermal delta. Only air between inner and outer pipe.
My thought regarding the inner pipe bearing on the outer pipe is that it seems to me that stresses in the inner pipe would be relaxed (a certain amount) since the outer pipe is bearing against the soil which has some flexibility (compared to a concrete anchor). The fabricators of this system will not be allowed to hot-spring weld the pipes to each other.
It sounds like (from LittleInch) that the code is silent on this topic. Thank you for your responses.
RE: Buried Double Wall Pipe Interaction (Bearing)
Then it will also be possible that the this will be overstressed during operation, or you may buckle the inside pipe and get your bearing on the inside of the carrier pipe, whether you want it or not. Or the 3" will have a lot of lateral spacers to prevent buckling, or your 3" will have extremely thick wall.
What pipe material and wall thicknesses are you planning on using?
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RE: Buried Double Wall Pipe Interaction (Bearing)
If you used a proper spacer / spider at suitable intervals (3m?) then you should be able to run very long distances in a straight line with support in all directions.
At the bends you might need to go to a bigger pipe size just for the bends to allow it to move axially, but is that a big problem?
Unless you're doing this as a double containment type of thing, I'm puzzled why you're going to the expense of a pipe in pipe system and not just insulate the thing??
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Also: If you get a response it's polite to respond to it.
RE: Buried Double Wall Pipe Interaction (Bearing)
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RE: Buried Double Wall Pipe Interaction (Bearing)
RE: Buried Double Wall Pipe Interaction (Bearing)
What you need to decide next will depend on how you intend this to work. I get the feeling that the carrier pipe is intended to slide inside the casing pipe. Am I to understand that the carrier pipe will not be welded to the casing pipe at any point? So it is just like a hot pipe sliding inside a cold "tunnel" pipe. How far between expansion loops? Will you pull the carrier pipe through the tunnel pipe in segments equal to the distance between expansion loops?
What diameters?
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RE: Buried Double Wall Pipe Interaction (Bearing)
You are right on with your understanding of the hot guided pipe sliding inside a cold "tunnel" pipe. We need expansion loops approximately every 41 feet (according to our calculations so that the inner pipe does not bear on the outer pipe). This is for a 3" STD carrier pipe and a 6" STD encasement pipe.
RE: Buried Double Wall Pipe Interaction (Bearing)
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RE: Buried Double Wall Pipe Interaction (Bearing)
Geothermal wells have steel casing cemented in the ground and operate at 300C and are OK most of the time. Then again never let drillers design piping.
RE: Buried Double Wall Pipe Interaction (Bearing)
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RE: Buried Double Wall Pipe Interaction (Bearing)
Care to elaborate? I get a 140F expansion on 41 ft length of pipe to be max 0.5" total. At either end you have 1.5" min before you hit the outer bend at each end so 3" in total.
If you know your issue is expansion then don't centre the inner bend in the loop so can get another 1-2" easily. I would model a very long length of straight pipe ( say 2000m) and see what happens if you fix one end and let the other end float. My guess is that the pipe will eventually stop expanding and "lock up". Then look to see how you can deal with the end expansion or at any bends.
Still not sure why you're doing this though??
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Buried Double Wall Pipe Interaction (Bearing)
My quick look finds that the coefficient of expansion and contraction for carbon steel is about .0000065, and that for stainless steel is about .0000099 ("/"/deg. F). Given 140̊F temp. delta, and 41' pipe length: carbon stl. implies about (.0000065)(140)(41' x 12) = .45" length change, or 1.09" per 100' and delta T of 140̊F; the stainless stl. implies about (.0000099)(140)(41' x 12) = .68" length change, or 1.66" per 100' and delta T of 140̊F. That expansion will obviously act differently at a tight 90̊ bend, than it will at a nice sweeping (large radius) bend. And, it will also act differently if all of the expansion can happen, at one bend from both directions. Mightn’t it be wise to fix the carrier pipe to the casing pipe at about the mid point of each long straight run, to assure that all of the expansion doesn’t happen at one bend?
Why is this pipe cased, how long is it, and what are its plan and elevation views, in terms of bends, types of bends, straight lengths, etc? Is the pipe cases for protection from the outside, or is it cased for safety against an internal, carrier pipe, failure/rupture? These questions would seem to me to enter into the design considerations. You said...., “Owner requires a low installation temperature of 60deg F for pipe stress analyses, so we're looking at a 140-degree thermal delta. Only air between inner and outer pipe.” But, obviously, the pipe stress analysis has to consider both extreme conditions.
RE: Buried Double Wall Pipe Interaction (Bearing)
There is a kickout tendency at the top of the loop which gives the bends both X and Y displacements, or if kickout is restrained, can increase friction on spacers near the bends. Could depend on how tightly the carrier pipe is held within the casing.
"He's declaring war on the planet itself."- Vicente Fox