Pipeline bends in buried pit to relieve stress

[FilippoT]

Hi,
we are installing an high WT 12" pipeline for gas injection (600 bar) with a differential temperature of around 80 degree celsius. To guarantee the compliance with allowed stress values, our engineering contractor is proposing to install the most critical bends into buried concrete pit in order to let the pipe free to move.
They said it's quite a common solution in cold place where thermal expansion is big and created stress are huge.
Any reference for this kind of application?

 

[racookpe1978]

So, he proposing to let the pipe lay in an open pit (maybe on a smooth concrete floor ?) unrestrained so there is no stress imposed by thermal growth?

OK, if the flat concrete floor extended the length of the pipeline, and there were no bends in the pipeline. Real world, the length of pipe buried in the soil will restrain the pipe. Even if there were thermal expansion Loops (U-bends or double-L bends in each pit), his plan only substitutes underground open pits with a sliding floor for standard (and cheaper and more easily maintained above-ground bends.

So a sketch of dwg or photo of his proposed plan. The concept appears wrong in principle and in economic terms.

 

[GD2]

So it will be snaky pipeline in the buried pit. Quite interesting. What is the arrangement for above ground transition? What are the end connected equipment(s)? What is the depth of bury? Will there be any axle load/road crossing over the pipe?? Did you consult your stress engineer??

Ganga D. Deka, P. Eng
Canada

 

[LittleInch]

At 600 bar and 80c diff temp you're at the high end of pipeline design. So there aren't many similar systems.

Use of pits to relieve stress by allowing movement is a solution but is costly and complex to construct but I have installed one before now where no other solution worked reliably.

What do you mean by reference? Ask your engineering contractor and see what alternatives have been considered.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[KevinNZ]

The stress in the 80C steel restrained in soil is not all that high. I know of buried pipes with much higher design temperature. There are geothermal wells with 300C casing concreted in to the ground. (not recommended in this forum but drillers are world on their own)

 

[LittleInch]

KevinNZ,

It's not really the stress in the pipe that's an issue, but either stress in the bends or also the amount of movement. If you get a lot of movement then you can get ratcheting where the soil fills in behind and then when you stop flowing it doesn't contract to the original shape and then next time expands some more and on and on.

Whilst looking for something else I came across this product / company who may be able to provide more information.

https://www.norseman.ca/en/node/52

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.