Internal Pressure and flexibility 2

[Gabichou]

In the B31.3 code, Table D300 Note (7), it is said that internal pressure can significantly affect the pipeline flexibility for large diameter thin wall elbows and bends. The code gives a formulation to take into account this flexibility loss. I would like to understand this formulation. I did finite element model to verify the influence of the internal pressure on the flexibility but the results aren’t significant. Has someone an idea to help me ?
Thanks

 

[DSB123]

Gabichou,
In a piping system you have bends which have flexibility factors associated with them. When you pressurise the system the internal pressure reduces the flexibility factors of the bends hence the system is less flexible. The greater the pressure the less flexible are the bends. This reduction in system flexibility is most noticeable for large diameter thin walled piping systems.

 

[StevenHPerry]

Lay a thin-walled hose out in a snake pattern. Pressurize it. It will straighten out to some degree.

[I'm thinking of a fire hose or something else that will collapse on itself if not pressurized. An old rubber garden hose may show this behavior too, but not the new ones that are "anti-kink".]

Or better yet, think of those birthday party noisemakers that are coiled up and extend when you blow into them.

- Steve Perry

 

[Gabichou]

Thank you very much for your responses.
I well understand the phenomenon, but I would to translate it into a mathematic formulation. What is the mathematic link between the pressure and rigidity (who depends generally on material pipe and geometry)??


And why it only concern bends or elbows? In practice pressure has also an influence on the flexiblity of straight pipe, isn't it ?

 

[BigInch]

The amount of flexibility, or inversely rigidity, of a bend is in relation to that of a straight pipe already under internal pressure. Bends under internal pressure tend to straighten, hence are less rigid than a straight pipe, which is already as straight as it can get.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[Gabichou]

Big Inch,

Imagine a straight pipe under internal pressure with an anchor at the first extremity and an external force applyed to the other extremity.The external force is normal the pipe.The rigidity of the pipe to the flexion depends on internal pressure. In other words :In the case where you tend to bend a straight pipe, the internal pressure have an influency on the rigidity (in flexion) of your straight pipe. did I a mistake ?

 

[BigInch]

No mistake. The reference case is a straight pipe WITH internal pressure. All other things must have less rigidity. Imagine a bend next to a straight pipe, both with internal pressure. Which one has the rigidity factor of 1.00 and which has less. The bend, right. Why? Because the straight pipe can't possibly get any straighter (rigidity = 1) and the bend will, so the bend is more flexible (rigidity < 1).

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[StevenHPerry]

Trying to rephrase the question...

What if a run of straight pipe is subject to first some thermal displacements and then second pressure? Would the bent "straight" pipe be subject to pressure straightening?

Yes, in theory.

But how much? Not much. Even for 90° LR elbows it isn't much of an effect except large bore thin wall. Unless you're dealing with high pressure, thin wall, large displacement piping it's probably not worth considering.

For an equation, see Section III Case N-319-2 which gives flexibility factors as a function of pressure (P), bend radius (r), and bend thickness (t). For a deflected straight pipe, r is going to be very very large to the point where I believe k approaches 1.0.

Rodabaugh & H.H. George wrote a paper on it that you might find interesting if you can get your hands on it. "Effect of Internal Pressure on Flexibility and Stress-Intensification Factors of Curved Pipe or Welding Elbows", ASME, May 1957. I have a blurb here (courtesy Ron Haupt) that references that paper but I'm not sure it is quoting or paraphrasing Rodabaugh & George:

"Although the flexibility factors and SIFs seem to change significantly, for thick wall pipe the combined effect has been thought to be relatively insignificant since thick wall pipe is often the pipe of primary concern as it is typically high-pressure, high-temperature pipe."



- Steve Perry

 

[BigInch]

I would suspect that internal pressure has more of an effect in preventing local ovalization and subsequent local buckling leading to full length buckling, in which the amount of initial ovalization is quite important. Until buckling occurs, a straight pipe is pretty damn rigid in relation to an elbow, even if there is considerable elastic bend.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/