Confusion in Stress result

[MediumInch]

Dear Seniors,

Our consultant did an study on buried pipe bend analysis and found when you bend buried pipe with lower angle higher stress and higher angle lower stress.

How is it possible if I bend my buried pipe by 90 degree lower stress but if bend my pipe by 45 degree I have higher stress? Please enlighten me I could not understand.

Pipeline design
30 inch X70
61degree C
Line will be pigged.

Thanks.

 

[BigInch]

A 90 degree bend is more flexible. It will have more deformation, but less stress.

Another inch???

 

[MediumInch]

Thanks BigInch for your insight.
But still I couldn't understand the concept behind it, If I bend my pipe by 90 degree it will have more stress than 45 degree because when my angle decrease it cause more compression and tension on the down and up of a pipe.

Can you please explain me the mechanics behind it.

for the name MdiumInch inspired from biginch and little inch.

Thanks for your time and support

 

[BigInch]

You said this is a buried pipe bend analysis. Not a bending force-stress analysis.
But unfortunately even when bending a pipe you are still not correct. Bending moment and the resultant bending stress is dependent on radius of curvature, not the number of degrees to which it is bent. A pipe bent to 45 or 90, or 180 degrees will require the same amount of bending moment to do each bend, as long as the bending radius remains constant. If you bent the 45 degree bend using a radius of 12" it would have far more stress than if you bent the same pipe using a 24" radius. When the bending forces are removed after the pipe is bent, there is no more stress in the pipe (actually some remains as residual stress, but that is relatively small compared to the stress it had when it was bending).

Now the buried analysis. With pipe, the more the bend, the more flex you have at the joint and consequently in the system. If a joint is more flexible, when exposed to a load the pipe system is able to move more and to deform more to alleviate stresses. A pipe with a full 90deg dogleg is much more flexible than with a bend of only 45deg. As a pipe flexes laterally (vertical or horizontal) it tends to convert axial load to bending moment, but in the process total stress decreases due to the increased strain movements.

 

[MediumInch]

Thanks a lot BigInch for sharing your knowledge and time.

Is there any reference study material or book I can go through in order advance my knowledge in buried piping stress analysis.

Thank you once again for your time.

 

[BigInch]

Lots of references around.
First start by reading the appendix in ASME B31.1
Then move on to the Corp of Engineer Manual Conduits, Culverts and Pipes, here

http://aquageo.es/wp-content/uploads/2012/10/CONDUITS-CULVERTS-AND-PIPES.pdf

Not so much on thermal expansion, but a lot of general installation stresses and surface loads.
Peng has a good article on underground thermal pipe stress.

https://www.google.com/search?q=Peng+underground+pipe&ie=utf-8&oe=utf-8

It's just not all that complicated.
Uplift buckling is about as difficult as it gets, so we'll save that for later.

 

[MediumInch]

Thanks you so much. I will start my reading from ASME B31.1

 

[LittleInch]

MI,

It might seem a bit odd, but as usual I agree with BI. you often find that maximum stress peaks between 25 and 40 degree bend angle. Why? Essentially if you break down the forces and stresses making up the combined stress, whilst bending forces might not be as high as say 90 degrees, because the pipe can't mive as much or as easily, the compressive forces coming into the bend from either side are still there. The larger the bend angle above approx. 40 degrees, the more the bend actually physically moves. This in essence relieves a lot of the compressive forces and reduces the compressive stress more than any increase in bending stress.

Each situation, soil type etc is different as are the pipe sizes, properties etc, so this is a generalisation, but might help you understand?

I'm now waiting for someone to join as micro inch....

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

 

[BigInch]

Maybe there's a Milli-Inch, or an Inch-Lb lurking around as well.

 

[MediumInch]

Sorry for the late reply.

Thank you so much littleinch for sharing your knowledge. can I ask one more silly question how does 40 degree bend moves and relives compression load?


Thanks in advance.

 

[BigInch]

A pipe bend will move outward towards a new position given by the expanded dimensions of each leg of pipe on either side of the bend.