Gas pipes support stresses 1

[LJ_]

Hello everyone I have a question about the design forces used for gas pipes supports.

The pipes undergo equal tensile forces Which they cancel out at the support F and -F. However, these forces get transmitted to the support, even when they also cancel out, the support sees the stresses. Is it better to design for the force F as it didn’t cancel out. Or what design procedure would you use?


Thank you!


Thank you.

 

[r13]

Do you meant the pipe produces +F at one side of the support, and -F at the other? The forces cancel out at the center of the support, but the support faces still feel the stress due to +F at one side, and due to -F on the other side. Draw a free body diagram to find the answers.

 

[AskTooMuch]

The braced bay of the piperack and the foundations are designed that only a certain percent goes to it, as you said pipe friction cancels out from the support/anchor. Beam is locally designed without any of the cancellation effect.

 

[HTURKAK]

You did not mention the reason for tensile forces .. I assume tensile force developing due to thermal contraction.. Yes.. thermal forces will develop and tension for contraction and compression for expansion.. The friction force developing at supports and eventually will be transfered to braced bay.

The vertical reactions of pipe supports are calculated and 10 % of calculated reactions are imposed on pipe rack as friction load (if we assume that friction factor between pipe and pipe rack is 10%) .

Regarding anchor and guide forces and locations, shall be calculated with the piping stress analysis and decided with piping isometric..

I think , this subject should be digged too many times.. try to look previous threads..

 

[1503-44]

Hard to tell, but those are guide supports, which will only transfer friction. Some force lateral to pipes may be resisted.

The support should be designed for weak axis bending using pipe weights x friction coefficient, acting in the direction of pipe axis and of course pipe weights acting vertically and creating the strong axis bending moment. It would be prudent to consider all 3 pipe's friction forces acting in the same direction, unless you have a pipe stress analysis that says otherwise.

 

[LittleInch]

LJ,

I'm afraid this

"The pipes undergo equal tensile forces Which they cancel out at the support F and -F. However, these forces get transmitted to the support, even when they also cancel out, the support sees the stresses. Is it better to design for the force F as it didn’t cancel out. "

is complete gobbledegook. You mix force with stress and don't say in which direction these forces (F) apply.

The pipe stress program will give you the forces in x,y and Z depending on the type of support you have.

Vertical force is simply the weight, but axial forces come from the weight and friction factor.

Note that piping designers often use FF of 0.3, but in reality and for structural purpose you should ask them to use something like 0.7 or 0.8 to account for when the steel surfaces are old and corroded a bit.

Sometimes they specifiy low slip supports using e.g. teflon or similar to allow the pipes to move more easily, which reduces the horizontal forces on your supports.

But sometimes the supports have lateral or vertical guides to stop the pipes moving too much. These result in greater forces on the support.

So ask you piping designer for the forces and ask questions about friction factor and whether those forces are the worst case for the support.

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

 

[LJ_]

Hello, everyone. Thank you for you answers. I will get back with more [ignore][/ignore]info tomorrow

The pipe designer has asked us to design the support system using only F as if it acted in one direction and didn’t cancel out. But I am not sure this procedure is prudent.

 

[LJ_]

I found my answer in other threads thank you everyone

 

[r13]

If the pipe system is capable of producing equal and opposite pressures in the straight run, the pipe is subjected to tension, and the support in the middle shall be designed for the prevailing pressure, if required, abs(F).

 

[LittleInch]

Care to let us know what the "answer" was?

There is only a force F as shown when the pipe is actually expanding or contracting. Once it stops moving then the lateral force on the support become zero.

The same F will act in opposite directions depending on the movement of the pipe.

Also make sure for a gas line you use the weight as the hydrotest weight when it is full of water.... Or provide some temporary supports.

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

 

[1503-44]

Friction force doesn't disappear when movement stops. It simply turns into a permanent static load.
Friction developed before movement occurs is W x static friction factor.
During sliding, friction force = W x dynamic friction factor.
When sliding stops, that force is still present. It might be the reason that whatever moved stopped sliding.

I dont believe f=0.7 Can you find any reference anywhere for that high a number?

 

[LittleInch]

https://www.hse.gov.uk/research/rrpdf/rr071.pdf

Table 2



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

 

[1503-44]

Thank you. Quite a useful reference.

 

[LittleInch]

As we've discussed before, if the pipe gets any sort of upwards movement (jump) or vibration the support can effectively reset itself even if the pipe is now longer or shorter than it was when it was installed.

But LJ seems to have gone...

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

 

[1503-44]

Well it's an open web and someone else may read this some day.

Since it is biaxial bending with both gravity weight and sliding friction happening at the same time, that's the worst design case anyway. As long one realises that week axis bending is possible in either direction in case of a pipe jump, that should be sufficient. Pipe stress programs might not take that into consideration. There is only so much theory that you can reliably assume is 100% safely transferable to being practical, especially with pipe stress. And that is a cantilever without any kind of longitudinal bracing, so no second chance to get it right.