Local buckling of a presurized pipe 1

[vroegop]

Hi,

I would like to verify the following statement with someone:
The effect of internal pressure on the buckling of a pipe is only due to the introduced axial tension due to the pressure on the end-caps.

My company would like to presurize a pipe (150ft lenght, 12inch diameter) in order to reduce the buckling/deflection of the pipe when positioned horizontally.
I can't find any other argument that this would work accept for the above. However I might overlook something. If so can someone please let me know!

Thanks for the help!

Ing. SA Vroegop Msc.
Rotterdam, The Netherlands
sa_vroegop@hotmail.com

 

[Bjegovic]

vertical pipe is exposed to buckling, and horizontal is exposed to deflection - bending.

yes, increasing the pressure will help with buckling. but the stress in pipe wall (p*D / 2*wall thickness) has also the influence.

imagine that fluid is flowing through the pipe, the pressure drop in axial direction is negligible, but I guess that you will have less buckling.

this is just my feeling, let me know if i'm wrong.

 

[vroegop]

Hi, thanks for the reply.

You mention that increasing the pressure will help buckling, does it also help for deflection?

Could you explain, how presurizing the pipe it can help? Do you have any formulas to support this? Does the increased hoop-stress influences the deflection?

With respect to my theory:
You've got to imagine a pipe with end-caps, like a very long pressure vessel. The pressure inside the pipe is working on the pipe and the caps. Presure on the caps will cause tension in the walls. This tension will need to be overcome before compression (due to bending) will occur.

Ing. SA Vroegop Msc.
Rotterdam, The Netherlands
sa_vroegop@hotmail.com

 

[CESSNA1]

VOREGOP: Pressuizing the pipe with closed ends is the same as applying a tension force at each end. The force is equal to (internal pressure/cap area). It will tend to reduce the deflection and lengthen the pipe. Also do not forget the wall stress.

Your buckling statement is a unclear. Usually when we talk of bucking we usually speak of a vertical pipe that collapses by buckling. If the pipe is horizontal and fully supported, buckling due to bending will not occur. If it is only partly supported say, at each end, as in crossing a stream or ditch then it is possible for buckling to occur due to the bending of the pipe by its own weight and the weight of the material in it and/or on it.

Regards
Dave

 

[jte]

Vroegop-

Yes, the longitudinal stress induced by internal pressure will have a stiffening effect on a pipe or vessel. What concerns me is the operability of the scheme. Have you considered startup and shutdown conditions (pipe full of liquid but not fully pressurized)? How about a hot shutdown (emergency) where the pipe is hot, full of liquid, and pressure is lost?

I would not accept a pipe at my plant which depends on internal pressure for structural stability. If the operators are already dealing with some event (loss of power, loss of instrument air, loss of steam, runaway reaction...) the last thing they need is to worry about a pipe which will buckle if it depressurizes.

Don't forget that valves leak. Unless you are desperately hoping that it'll leak...

jt

 

[stanier]

The re rounding effect of presure does assist in reducing the propensity to buckle. This is a factor applied in buried pipeline design. ref AS 2566.1 Flexible Buried Pipelines - Design

http://www.standards.org.au.

I am unsure if this rerounding coefficient appears in AWWA, ISO or EN standards.

One would expect the re rounding to occur in aboveground pipe and therefore be a contributing factor. However unlike a soil/pipe structure an aboveground pipe is standalone.

 

[vroegop]

Guys,

Thanks a million for your input. I appriciate the concern, however it is not all that critical.

My company is investigating how to keep a pipe, a polymer liner, of 150ft. straight while a composite layer is applied using filament winding. Presurizing the liner was one of the options mentioned. The idea got stuck with one of the managers.

Thanks once again for all your input in this case!