1. I am in the process of analysing large diameter pipe (1800Dia), the temperature is 270 deg. C, the pipe is getting lift off at the changes in vertical direction, i know it is unavoidable, the allowable span for support is 20Metre, but it's taking 33M to sit in the structure, code stress are passed even at the 33M support.

2. For large diametre pipe, what is the maximum displacement allowed ?

Kindly advice me for my above clarifications.

I think the maximum displacement allowed would be that below which you don't see a stress exceeding allowable. Of course, for very large displacements it might become unsightly or there may be clearance issues that you must address. But to my knowledge the piping codes don't actually compare or compute an allowable displacement.

Many span charts are "suggested spacing". These distances may be exceeded if the line is (properly) analyzed. (Kind of like "design by rule" vs. "design by analysis".) You may want to keep an eye on mid-span sag by adding midspan nodes in your model.

There is only a vertical upward lift, no sag up to 33M, is it not a problem to allow vertical pipe lift.

-If you want to get rid of it, review the configuration and change the support location(if possible) and try to coordinate with the design group.

-Or, Put a spring hanger for lifting(this is expensive).

Hi Moorthi,

Good day!
My analysis of your line is that, it bend up due to restriction of expansion... you can check the restrain acting parallel to the pipe if you found out that the force present in that location is extremely big then the statement above i said is true. about the allowable displacement, i agree w/ Sir Loren explanation.
Correct me if im wrong...
Regards!

Your change in elevation could be 3D expansion loops. If yes check if you can have the vertical leg in 45 degr.

Just remember to put correct guiding. Even big lines walk away.

Thanks for the replies.

It is not possible to provide the 45 deg. because of the obstructions in the existing plant.

Is it the correct procedure to provide the hinged joint and after the hinged joint one -Y restraint to restrict the upward vertical movement...Pls advice me.

Some would say that if a pipe lifts off a support, the support should be relocated. Moving the support should have a higher priority than adding "exotic" hardware to alter the response.

I assume you mean hinge compensator. One doesn't help. Two may help. Three in 3 pin always help but expensive.

If your lifts are couple mm I would not worry too much. In reality it will not happen. You could possibly try to put support at 20 m and use Y and see what happens.

Not couple mm sir...it is resting at 33 Metre instead of 20 Metres, Providing Y restraint may be the solution to arrest the vertical upward lift, i tried out, but it's giving some abnormal result of 49 Tons upward vertical load.

If it is correct to allow the 33 Metres at operating condition, so than we can avoid some other alternatives(Hanger, Bellows & -Y restraint. Pls. advice me.

Whether or not a 33 meter span is acceptable is going to be determined by the code you are using; you shouldn't take someone's word for it. I believe the December 2004 Newsletter addresses support lift off, maybe you should give it a read.

Piping code didn't cover about the support span, also i red the News letter, i couldn't find the solution in that. Pls. experts help me.

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.

If your applicable code is ASME B 31.3 Ed 2004 or later, you can use App-P - toggle on it's implementation in Caesar-II 5.00 or 5.10 config in 'SIF & stresses' heading. Calculate operating stress & compare with allowables & defend your design.

But, be careful, loads on your supports will be high & the same need to be properly designed with co-ordination with civil/structural.

Support spans & allowable displacements are suggestives & can be violated without violating allicable code. With high mid-span sag, there may be need for high slope for draining.

regards,

sam

Moorthi,
I fully agree with Sir Dave comments. Using springs or Y restraint can give better results but they may not be the best solution. Altering the support location many a times provides the simple answer to the complicated question.

you try to modify the adjacent support..

If it is easier/economical to design your concrete/steel support below then place your shoe near the bend below. Allow the pipe to lift above. Guide the pipe above as pointed out by Sir Jouko(the height of the guide should off-course be higher than the calculated lift-off).

Although your stresses are Ok even with longer spans, try to determine the support span which will make you and your structural friend happy.

Good Luck.

Maybe if you will show your ISOMETRIC DRAWING it will be easy for us to understand...

Regards!

You have misunderstood Jouko's question. He asked how big is the lift-off (in mm) in the middle of 20m span, not the difference in spans from 20-33m. If lift-off is less then say 5mm I would not worry about it on such a large diameter duct. I would add a thin "imaginary" packing under +y to see what happens.
Have you created a node in the middle of the span to check the highest stress?
Is your duct D/t>100?
Local stress has to be considered in any case, 20 or 33m span.
Regards
Ranka

I second the request for a quick ISO....

dear moorthi

your question is really important , am been involved with this kind of big lines from begining

when u have 1800 ID pipe with 270 c , its evident that as a stress engineer you dont have much elbows to kompnsate

but as apreliminary check

1) please check the lift amount ( how much mm)
2) if its not much then analyse your piping at operating temeperature and mention that it is never going to lift off
3) or perform separate CAESAR II run to check the stresses and nozzle loads after removing that support , if no problem then allow it to lift off but mark clealry on isometric

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.