Force to Collapse A Pipe 1

[ziptron]

Hi All,

I have a pipe (a long hollow cylinder open on both ends) which I would like to flatten into something that will look like two flat sheets (theoretically). I want to find a formula which I can use to calculate how much force it would take to flatten my pipe by applying an external load or pressure to the pipe. I think this would be analogous to asking how much of a vacuum would I need within the pipe to have it totally collapse on itself.
I keep finding buckling equations, however I feel that my pipe is not just buckling in one small spot, the entire length of the pipe is buckling as it is entirely flattening out.
Does anyone know any good sources for this?

Thanks in advance!

 

[BigInch]

You're probably finding longitudinal buckling equations and I think you're looking for ring buckling equations. Google "Ring buckling", and "pipe collapse pressure".

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[SNORGY]

If I understand your question correctly, Timoshenko's equation for vacuum collapse ought to work for your needs. If you Google it you will find the equation and a pretty good paper on the subject.

Regards,

SNORGY.

 

[blacksmith37]

There is flatttening (like between plates in a press) and there is collapse (as under hydraulic pressure. I am sure "vaccum" , 14.7 psi, will not collapse much metal. Real pipe does not become "flat" under hydraulic collapse.
I believe API bul 5C3 has collapse calcs (or some other API Com 5 document.)

 

[BigInch]

If the diameter is large and the wt thin, it can get really, really flat, even under just a half vacuum. put it at the bottom of the ocean and see what it does.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[racookpe1978]

Textbook? It might "flatten".

Real world? Buckle. Irregularly buckle actually. Someplaces will differ from others due to slightly varying wall thickness and steel strength variations. Ain't gonna be pretty. And it won't be evenlly bent exactly on both sides: the crease will wander back and forth.

There's no reason for this but a textbook problem. Just buy plate.

 

[blacksmith37]

I would consider ocean depth stresses due primarily to hydrostatic pressure , not vaccum.

 

[ziptron]

I think the collapse equations would work for me, thanks for the suggestion!

I happened to stumble upon some collapse equations:

http://www.roscoemoss.com/waterwell-calculations.html

However, what boggles my mind now is that there is a collapse equation for steel pipe and it is different than the collapse equation of a PVC pipe. Why would those two quations be different?

 

[prex]

The two formulae are equivalent, as they are for elastic buckling of relatively thin pipes, so the factor (D_o/t-1)=D_avg/t is nearly equal to D_o/t.
However this is for very long pipes with uniform lateral external pressure applied over the full length (or a very long portion of it), and:
-I can't see how you could apply such a pressure on a pipe with open ends
-with the ends capped as needed, only the center portion of the pipe will buckle
If you are trying to flatten out the pipe by applying a load like in a press, then it's not a matter of buckling, but of bending and energy of deformation.

prex

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[saplanti]

I guess "flatten my pipe by applying an external load or pressure to the pipe" are different concepts.

I suggest you to look at the Roark's Formulas to select the applicable load case sceneario for your loading type to calculate the critical load.

External pressure application will not provide you the flattening effect, the pipe will buckle inwards starting from the weak locations (pipe not 100% uniform in thiskness due to manufacturing tolerances). If you are after flattening you need to use the Roark's Formula for the rings with top and bottom uniform force applications. I can not remember the table number right now, and additionally the table number changes with the version of the book. Second, you can calculate the minium theorical loads for the uniform thickness ring under uniform line forces, but your applicable force should be a lot higher than this calculated with the formulas. The reason is that the formulas are giving you the critical loads to start the buckling. When the shape start changing it will go through different loading scenarios and geometries. When somewhere before the top and bottom surface start touching to each other the required force will increase to put steel into plasticity, after thouching you need a lot higher forces to flatten it. These are not given in the formulas. Be aware of it.

Hope it helps,

Ibrahim Demir

 

[chicopee]

B.I.,if you put the pipe with both of its ends per the OP in the bottom of the deepest ocean, it will not collapse. The W.T. may reduce a little bit though.

 

[amorrison]

GregLocock commented on this several years ago.
Some of the problem was that as the pipe collapses it changes from a circle - to an ellipse - to an "???" - to two "flat" sheets connected at the ends by a hemicircle.
The maximum force required is going to be squishing the hemicircles such that the "inside diameter is zero length. This will be a very non-linear problem. There is also the problem of "bounceback".
If is is desired to stop fluid flow in the pipe maybe the answer is to "the best you can" and then fill the remaining opening with solder or weld fill.

 

[ziptron]

I am definitely starting to realise that this problem is not as easy as I initially thought and not as linear as I initially conceptualized..

I think I should just compress my pipe with something that has a force gauge and just read off the force required. That will be bang on with no assumptions at all

 

[saplanti]

I think that is the best approach. However, if you are using a short sample to flatten it may give you less load than required.

The distance between open ends has significant effect on the load. So, you may need to take additional factors to simulate longer length.

Regards,

Ibrahim Demir

 

[BigInch]

chico, Why would anybody do that?

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[chicopee]

Beats me!!