Can any body guide me how i can know allowable displacment for any pipe line.
like if i have 6in line of ASTMA-grd B Material what will be its allowable displacement.
i tried to use search option to find related topic. but cound not find any.

Hi Sami
In general allowable displacement of pipe at any point restricted to 25 mm in lateral direction & 100 to 150 mm(company specific standard) in axial direction.
In some specific case if there is no choice we can allow even more displacement. In that case please ensure stress & in layout that there will not be any clash with other piping or any other items.

Thanks shr.
does this displacement limit not depend on piping material and pipe dia?

No as long stress is not a problem.

ok thanks shr for guidness

Remember with large displacements to check to ensure your pipe shoes are long enough to handle the expected growth and stay on the supports. Also make sure that any ajacent piping has enough room so that when one of the pipes is hot and the other pipe is cold the lines will not interfere with eachother. Don't forget to take into account insulation thickness if present.

hi Sami,
for me as a beginner in this PSA job, you should check your pipeline arrangement. is it allow the pipe to move 10mm or 100 mm. otherwise your pipes will interfere (clash) with other.
also as Greg said that you should check the pipe shoe details as well. is it long enough to handle the pipes movement.

Thanks sir i have these two points in my mind and i am also seeing my arrangment that whether it is allowing pipe to move or not?
Thanks for suggestion

Dear all,
In one of our plants, we have 24" line with design temperature of -45 °C. The line has cold insulation of 90mm and has clamped shoes on the top of insulation. Due to line movement, support shoes have displaced . In my opinion, it is not cause of concern. The length of the shoe is 400mm and beam width is 300mm. The displacement is such that the centre of the shoe is still on the beam. More of the shoe length that is around 280mm is on the structural beam in most cases. Only 120mm has moved out of structural beam which looks odd to many operators. I have modeled the line in CAESAR II and the stresses of the line are will within limits. Displacements are as is the actual scenario in the field. The line has moved in such a way that it is almost touching the nearby lines at the expansion loops. But it is not pushing them.



So:

1) Is this displacement any cause of concern. Should I ask to lengthen the shoe?

2) Any criterion for displacement that I can refer and include in my report.

3) Any other idea?

My only cause for concern is that the shoe appearently moved in an unexpected way. If you can reason out why it moved as it did and everything is still OK with the piping, then there is not as much concern.

Extending the shoe is never a problem, I had a bunch of those happen on a job site where the shoes and pipe rack did not quite line up after installation. As more of the shoe is on the steel than off it, I don't think there is a concern. But if the field guys would feel better, I would not prevent them from doing it.

Basically, the only criteria for sliding displacement is that you need to be able to accommodate it in your design. I once worked on a main steam line at a power plant with a section that grew 14". You should have seen how big our slide plate was.

So my suggestion is make sure you understand why they pipe moved as it did and have a good idea of how you expect it to move in the future and then review the support.

Good Luck,
Duncan

Dear Duncan,
Thanks for your opinion. My reply:
1) No! Shoes did not move in unexpected way. Actually the movement of the supports is same as what I am getting in CAESAR II analysis (at those nodes). And that is why my conclusion is that this movement should not be of ANY concern.

2) Yes, right extending the shoes is never a problem as I have also suggested on a modification done recently and there IT WAS NEEDED. Main thing here is that though it is not needed, but it will add to the peace of mind of operating team.

3) Hmmm! No criterion! Ok, it would have made my life easier (up to XXX mm, do not do anything, above XXX mm, do that.) I have to figure out something else now. Line anyhow is safe with this much movement (maximum 100 mm) and is NOT hitting anything else.

4) Thanks for your reply and suggestions again. Appreciated!

Friction may cause hysteresis (I hope I'musing that word properly). The pipe support may not return to the cold position and continue to incrementally "walk" in the direction of thermal growth as cycling continues. Keep an eye on "hot" position over time.

Adding limits that seem otherwise unnecessary help guard against this effect. I just call this macroscopic entropy.

I think hysteresis is similar, but refers mostly to the dampened time-response to an input to a system... E.G. friction can cause a time-delay response to a temperature input to a system by having to first overcome static friction and slip.

Thanks.

Dave/Michael,
I had doubt about this "hysteresis or macroscopic entropy". Though the installation is not old, but this phenomenon can be there to gradually throw the clamp off its original location. Incremental advancement in the direction of forced movement and "denial" to comeback fully to original position due to resistance or whatever may become reason over time to displace supports in the piping. But at the moment system is under operation. Line is cold and shrunk. So we can check this only during next shutdown. I shall consider axial stop.
Below is another view of the condition:

I wish more plants would consider a pipe support review part of normal shutdown maintenance. In my opinion anytime a thermally active system shuts down someone should go walk the pipe and make sure the supports look good. Of course it seems like most operating personnel pretty much ignore supports since they pretty much just sit there.

Thanks,
Duncan

Dear All,

I am a beginner in learning stress. I have some basic questions i hope you will help me to resolve my confusions.
My first question is when the software calculate the stress for thermal expansion & compare it with code equation f(1.25sc+0.25sh) then the calculated stress is for the complete cycle or from ambient/installation to operating point. If it is for cycle then it seems to be ok as per texts but if it is the other way then the basic idea that stress should not exceed the yield in hot or cold condition is not valid.

My second question is are theories of failure such as rankine von n tresca are applicable for thermal failure of pipe. or above theories are for primary stress only.

stresslearner,

Rather than adding your question to existing threads, start your own.

It looks like you are referring to the B31.1/B31.3 sort of stress evaluation.

Since expansion stress range is strain-based, actual stress does not exceed yield - even with excessive strain. The calculated expansion stress range (assuming elastic response) can then be higher than yield.

In terms of stress, keep that categories separate - yielding is the mode of failure for force-based loads and fatigue is the mode of failure for strain-based loads.

The Tresca (Maximum Shear Stress) Theory of failure is used for both force-based load and strain-based load evaluation.

Thanks for your reply sir as you said tresca theory stands for both force based load & strain based load. In that case whether these loads can be evaluated seperately. I mean to say that since the direction of stresses i.e sustained & expansion both is same & also the conservative equation tells that both are additive (1.25sc +1.25 sh-sl). So as a total i have to apply tresca criterion. Means the above equation can be directly related to tresca if yes then as per tresca the material will fail when max shear stress is half the yield only.

The B31.1/B31.3 Codes do not necessarily calculate and evaluate stress as you have learned from principles. Except for the use of SL in calculating SA, these Codes evaluate yield and fatigue loads independent of one another.
Max shear is the radius of Mohr's Circle. Stress Intensity is the diameter of Mohr's Circle. The diameter is easier to come by (S1-S3), so use that number and compare to double the allowable (using Sy rather than Sy/2 - another easy number - Sy). Simplification (for a slide rule).

Dear Sirs,

Thanks for the reply sir. But my question how come tresca theory is applied for thermal & longitudinal stress both independently as dave sir quoted

"The diameter is easier to come by (S1-S3), so use that number and compare to double the allowable (using Sy rather than Sy/2 - another easy number - Sy). Simplification (for a slide rule)." I agree to the above statement.

However S1 is hoop & S3 is longitudinal stress so where comes the thermal stress.

If i have to evaluate combined three stress thermal hoop & longitudinal then is there any equation for tresca which i can apply.

As i can understand that calculated stress in thermal stress is much higher than actual but it is still at yield point. So what happens to shear thermal stress when both longitudinal & thermal or displacement stress is applied simultaneously. If tresca would have predicted theory of failure as it is applicable for primary stress only then there would have been no confusion. But i understood it is applicable to both independently but then what is tresca significance.