"Natural Bend" of a piping system. 2

[gbratis]

Hi everybody,
Here's a good one for you.
We are trying to estimate the bending radious of an API 5L X 56 pipe in order to achieve the what is called " Natural
Bend". This piping terminology is not familiar to me that is why I'm in search of a mature answer from an experienced
piping expert.
We have recieved a study that introdused the term "Natural Bend" as the "Tensile bending stress allowed in the outermost fibre of the pipe." Since we use a class 300 piping with a design pressure of 5100 Kpa the study read that by using the 72% of the above mentioned SMYS as maximum hoop stress, subtracted by the longitudinal stress we can calculate the Minimum Bending Radius obtained by the formula : R(bending radious)= E*Y /Fb where E is the Modulus of Elasticity, Y is pipe radius, Fb is the bending stress in the outermost fibre as stipulated above (72% SMYS (Hoop stress) - Longitudinal stress).
I got a feeling that this is not a good approach and I sense ambiguity. I'm I right? Could someone verify or invalidate the above mentioned study taking into account that our main concern is to avoid the use of pipe fitting by bending the pipe as ...." naturally"...as we can.

Thanks in advance!!!

 

[MikeHalloran]

No expertise with API claimed here. Okay, no expertise in anything.

The formula as given allows you to compute a _minimum_ bending radius.

I can see no reason to use that minimum as a design value.

If you use an arbitrary radius much larger than the computed minimum, you will use less pipe, and you may be able to make the bends with simpler tooling.



Mike Halloran
Pembroke Pines, FL, USA

 

[BigInch]

Something is a bit confussing here,

B31.4 and 8 limit the sum of the combined stresses to 0.72 S as the hoop stress, not hoop stress - longitudinal.

833 Combined Stress Calculations.

SE = (Sb^2+4*St^2)^0.5 < SE

where SE <= 0.72 SMYS para 833.3

833.4
The total of the following shall not exceed the specified minimum yield strength, S:
(1) the combined stress due to expansion, SE
(b) the longitudinal pressure stress (see para. 841.11, SFT)
(c) the longitudinal bending stress due to external loads, such as weight of pipe and contents, wind, etc.
The sum of paras. 833.4(b) and (c) shall not exceed 0.75S.

You may want to refer to 841.23 Bends, Elbows and Miters in Steel Pipelines and Mains, specifically 841.231 (b) and (c) where B31.8 specifies the maximum degree of bending on a field cold bend. I believe it sets the limitations you are looking for. You say you are trying to avoid fittings, and cold field bends are the way to avoid fittings, not "Natural bend" stress limitations. Then get a good bending machine and a good operator.


Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[gbratis]

Thank you all for your fast reply, especially you BigInch for your straight, right to the core, answers. You seem omniscient to me. I assume that you' ve spent a lot of your precious time struggling with codes, specs e.t.c .
Bye

 

[BigInch]

...and the pipe that created them.

Definately not omniscient, but thanks for the thought. Actually, the more I learn, the more I know I don't know. It seems to be one of those [&infin;] loops.

I only use store-bought fittings when I can't bend one tight enough myself... or if they need a hole in the side. The natural bend idea works offshore, where "S" or "J" laying a cold bend is impossible. With the tension required for laying, you'd straighten it out again if you tried.

Cheers.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com