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#41945 - 03/29/11 12:32 AM What Do Lift Off can Cause Most Badly?
Nalibsyah Offline
Member

Registered: 01/24/11
Posts: 59
Loc: Abu Dhabi
Dear ll i need to raise this question.
"What do Lift Off can Cause Most Badly?
i raise this question since now i am working at the project that set and modelling the system as "Linier" system, so therewill be no single direction restraint and gap, but friction is account here.
Yesterday i am in discussed with my collegues, when i see in one calculation that he has done and he is just ignored lift off case.
the pipe w/ pipe shoe is lift off from the beam support of 3.5 mm, and he is just ignored it by saying that it is better like this because if he give a hold down guide then it will cause large force. Yes indeed he is true, the force is becoming so big to handle by the pipe shoe, and also he said also that this lift off is happened in maximum case not operation case, and the distribution load to the support next to it is still can be handle by the next support after all. I closed the discussion there, because i don't want to argue to long with him, but there is something wrong i said to my self, if Lift Off support is fall into secondary load offcourse it will cause a fatigue problem if this condition is continues happened? beside that, so what if the force is become to big to handle by the pipe shoe if we are using hold down guide, then we can add some repad, or if it is still failed we can change it to trunnion, as long as future problem can be afford.
So i think it will be more satisfied to me if you guys that have more experience shed your opinion here about what is the effect of lift off that can cause the most badly in piping system? is it true if it is fatigue? or is it the other effect?
Thanks

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#41972 - 03/29/11 09:56 PM Re: What Do Lift Off can Cause Most Badly? [Re: Nalibsyah]
Borzki Offline
Member

Registered: 09/16/04
Posts: 759
Loc: Traz

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#41978 - 03/30/11 06:52 AM Re: What Do Lift Off can Cause Most Badly? [Re: Borzki]
Nalibsyah Offline
Member

Registered: 01/24/11
Posts: 59
Loc: Abu Dhabi
Thanks Borzki,
But i have read all the topic about Lift Off and Hot Sustained in this forum. And as you see there is still two different opinion about this, also some are said that lift off is Primary Load, and others is secondary load. I also haave read all the Mech. Newspaper and CAESAR II Manual.
I'm not asking about the way to analysis this case (because place where i working has it own procedures to handle this that quiet different than the way Coade do), rather than i only ask about If i let this pipe support lift off by 3.5 mm, what will the worst effect that will happened? will fatigue failure take over?, if it is not a secondary load, rather than primary load, then it is ok to let the pipe lift off by 3.5 mm (in my case, the redistributed load to the other support next to the lift off support is still below the pipe support alloawable load, it is in Pipe Rack Modules). In operation case the support still on its place, but in max. operating case (my operating case is including all the effect of seismic, wind, etc) and transportation case (sway, heave, surge, etc) the support will be lift off.

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#42062 - 03/31/11 03:13 PM Re: What Do Lift Off can Cause Most Badly? [Re: Nalibsyah]
danb Offline
Member

Registered: 04/22/05
Posts: 1453
Loc: ...
Under the code provisions your pipe will "suffer" a collapse but who will be there to witness the reality? It will sag back into a fraction of second. So really will fail?

Regards,
_________________________
Dan

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#42303 - 04/08/11 09:50 AM Re: What Do Lift Off can Cause Most Badly? [Re: danb]
Nalibsyah Offline
Member

Registered: 01/24/11
Posts: 59
Loc: Abu Dhabi
Moorthi,

If you remove the support showing lift off what are the stresses and displacements? If these are acceptable then you don't need to worry about the lift off, except for maybe a phone call from site asking about it during operation.

Removing the support from the Caesar model treats the lift-off condition as a primary/sustained stress. There's always good debate about wether this is a primary or secondary load condition. Treating it as sustained is generally conservative however.

Second, if the support lifts off, more load is transfered to adjacent supports. You may need to check local stresses in the pipe wall from the support. Roarks or FEA works well for this. Span charts don't always consider local stresses.

Sometimes removing the support at the bottom and allowing it to grow down is a good solution to get rid of the lift off issue all together.

Without seeing the whole picture though, none of us can give you a complete answer. I'd recommend reviewing with someone locally to make sure you've covered all of the bases.
_________________________
Paul


I recently worked a project with a designer who was adamant that pipe should NEVER lift off supports. I could never explain the following to him:

1. CAESAR II is an approximation to the real world. Among the things that it does not account for UNLESS YOU TELL IT TO are:

a. The stiffness of the supporting elements in your pipe rack.

b. The ovalization of large-bore pipe at support locations.

c. Imperfections in fit-up and assembly of piping systems.

2. CAESAR II does a marvelous job of checking the state of each support and correcting your model "on the fly" for support nonlinearities such as lift-off.

3. When Spielvogel wrote the first real textbook on how to perform pipe stress analysis, nonlinear support effects were, in practice, impossible to analyze. But the Code committees of the era were well aware of the fact that pipe does, in fact, lift off of supports in many cases (and that guides with gaps often exert no forces on the pipe). So they built conservatism into the Code to account for things they couldn't analyze. That conservatism is, for the most part, still there.

So, when you have a 1800 mm pipe that lifts off a support at the top of a riser when hot, CAESAR II recognizes that fact and, in the lift-off cases, computes stresses and deflections without accounting for any supporting force at the lifted-off support locations. The stresses and deflections that it calculates for those load cases are consistent with a model where there are no support forces at the lift-off locations. So, when CAESAR II says that your model passes the Code checks, it means what it says. You may create instabilities due to wind or seismic loads at lifted-off supports, but you will not experience sustained or thermal fatigue failures.

Still, if you want to have an analysis where all the i's are dotted and all the t's crossed, you can calculate a stiffness for each structural member supporting your pipe near the lifting supports. And you can go into Roark and calculate a stiffness for the surface of the pipe resting on each support, accounting for ovalization due to the point load. If you include these things, you will have a much more accurate model, and it's very likely that your supports will no longer be lifting. But you will have blown your budget beyond repair.

My designer eventually gave up trying to arrange a support system with no lift-offs. He wasn't happy about it, but in addition to blowing the stress budget (me), he was starting to blow his own budget. He wasn't much worried about me blowing my budget, but when his started to go he became much less fussy. I'm not sure he is convinced, though, that lift-off isn't really an issue.

It's good to see young engineers worrying about everything. But when you see something that bothers you, the first thing you should ask yourself is, "Am I the first to observe this?" If you answer "yes" to this question, you are probably wrong.

Once you convince yourself that the problem that is bothering you has been seen many times before, the next step is to consider the consequences of your worst fears. As I noted above, CAESAR II calculates stresses and forces vary accurately in lifted-off support cases. But if you allow a support at the top of the riser to lift off, you now have a situation where lateral loads may cause unacceptable deflections and other nasty things. So you might want to analyze a wind or seismic case to see what happens.

As one of the earlier posters mentioned, it's usually a good idea to support long risers so that thermal growth occurs at the bottom rather than the top. This is easy to accomplish if you place the upper support much nearer (do some hand calcs to determine the value of "much!") to the riser than the lower support.

When you're designing a support system, your only true friends are gravity and leverage. Friction is a fickle ally, ready to run away at a moment's notice when dynamic loads appear. But gravity and leverage remain true friends. If you have a problem that is nagging at you, try to think how you can use gravity and leverage to help you. You'll find a way, most likely.
_________________________
CraigB

Icopy this answer from other thread to close this discussion, which from those answer and after study this case i think it i ok to let the pipe lift off as long as we check:
1. As long as the lift off still below 3 mm or below construction tolerance than it is ok, if it is doesn't effect the support next to it or equipment near to it.
2. Check the Occasional case such as seismic and wind, whether is it ok in those case if the pipe lift off from the support?
3. You may check the Operational Case by toggle on the Appendix P option to see your Operation stress, whether it is still acceptable or not.

Me personally does not agree with term to remove the pipe support because sometimes you it will effect in the expansion case or occasional case.
Like Paul said, the worst case is the Operator at site will call you....:)

Thanks


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