Hi,erveybody!

I have a doubt on the Load cases comb method of "Hot sustained stress" mentioned in the 6 Chapter of Technical Reference Manual:
L1 = W + T + P （OPE）
L2 = W + P (SUS)
L3 = T (EXP)
L4 = L1-L2 (EXP)
L5 = L1-L3 (SUS)

Above,They use L5 to calculate the "hot sustained stress",But,I do not think this is a suitable method.
When the piping system is a nonlinear system，This approach will cause the stress which is generating by the nonlinear(which is secondary) to be included in the primary stress.
Many previous analysis of the pipeline, is now running very well for several years , but after analysis using this method, the primary stress is always not pass ,but even beyond 300%。

In my opinion, weight is also a limiting factor on the temperature，the virtual weightless Load case will lead the force of pipeline system (which one end is fixed,the other end is free) to be zero,In this state，all +Y support will lift off,and then,if you add the weight and pressure,the pipeline will sag back to the support,but the stress by the lift off gap will be included primary stress in this case. it will casuse a lot of pipeline system to be failure, even a lot of pipeline system which don't have support to lift off also don't pass and exceed a lot than the code stress .

in fact ,the weight is always earlier to exist in the piping system than the temperature，and later to unload.

if the fact is temperature is always earlier to exist in the piping system than weight,and later to unload,so the L5 is right,but it is not the fact,so i think it is not suitable to calculate the hot sus stress using this method.

this is why we can use (w+P+T+wind)-(w+p+T) to get wind ,because the wind is Generally to load after Terperatue.
For nonlinear systems, the sequence of loading is critical。

My English is very bad, I could not express what I mean。

But I eagerly want to know, how should we do, if you must do it according to this method, I think a lot of piping systems will not pass，You did not encounter the same problem？

In the Technical Reference Manual, if coade recommend to use this method, or not recommend, because i am bad in english , I do not know what they mean.

i have read some topics about this question in this forum,but different people have different views, I want to know coade's view

anyboby on line?

For this type of "automated" hot sus check, you MUST change the +Y restraints to Y for SUPPORTS that DO NOT LIFT OFF for the OPE or SUS raw cases.

This has been discussed many times on this forum and opinions vary greatly on this subject. Do some searches on HOT SUS etc. and hopefully the posts still exist since many posts have been lost during forum Host/IP changes over the years.

I have to disagree with Bob. The (Operating - Thermal) case will calculate a sustained stress based on the hot position of the pipe without changing those +Y's to Y's. Maybe this model change would eliminate questions but if you're going to go throught that work you might as well just remove the +Y's that are not active in the operating case and run the sustained case alone. But as Bob says, a search should uncover several of these conversations.

(Operating - Thermal) will account for the incremental change in the sustained stress (or really, the longitudinal stress due to sustained loads) as the load from thermal strain is incrementally added to the system (that is, as the system heats up).

I have no comment on occasional loads.

To "lanxuedream", I had the same comments and even more.
I think that we all have some doubts, the thing is that you will have to judge on case by case basis and not on one or other opinion that are in conflict.

My opinion is that as a general rule, try to avoid the lift off, if it is not possible (and some time it is not) try to judge on the nature of this lift off. If it is small lift off (about 2..3 mm)and if you think that after a while the pipe will sag down at that location (your call) this is ok. But do not allow that two consecutive supports will lift off.
In addition:
Remember that you are always on your own.

Regards

hi,dave

i want to know the method of the L5 is suitable?
i don't think it is right.
if there is not lift off for all support in the model in operate state,using this method to calculate the primary stress will be different with the default method,and always cause to not pass.

Hi,dan:
I understand what you said.
If there is no piping support to lift off in my pipe system, using traditional methods to check the primary stress and pass, but if you use L5 method to check the primary stess and not pass, it is entirely possible. then Do I need to focus on the problems caused by L5 method?
I think using the L5 method is not correct, because it will no only be a span question which cause the primary failure, and sometimes that is linear and nonlinear , flexible and not Flexible question which also cause primary stress using L5 method.
as BOB said, the + Y to Y, and sometimes will make the calculation by L5 to pass.
So I think L5 approach is not desirable,
do you know what I want to do say?
please give me some comments.

Hi,Coade‘s Engineer
I have read your company's Technical Reference Manual about Hot sus and non linear restraint.
From a technical manual, you have the view seems to consider french petrochemical code's view is suitable, using the L5 method is not your intention, but you could not guarantee that everyone agree with your view, so you Allow engineers to define the case According to their company's specification.
I understand it right?
If what i understand is right ,i agree with the french petrochemical code's view,the vary value from cold to hot state is an expansion range,not a primary stress

Firstly, we do not offer engineering consultation - we interpret Codes just like you do and implement our interpretations and those of our user base as changes (imporvements?) to CAESAR II and our other programs. So, simply because we show how the program can be used does not remove your obligation to exercise good engineering judgement. For after all, you are the engineer providing the evaluation of the piping system, not the program authors.
Yes, an older edition of the French code stated (I recollect) that changes in weight distribution is a secondary effect. This endorses your L5. So I agree with you but I am not the one you must convince. That French phrase has since been removed and I do not know why.
Standards come about from general agreement - when most people (engineers) perform a certain task in the same manner, a standard can emerge. Certainly, treatment of nonlinear response has not reached that point of general agreement.

Hello all!.

Let me please re-activate this topic...,

I always use the non-linear capabilities of caesar with the +y supports, gaps (where it should be..), friction of course.. and also use the "hot sustained case". But after an intense-debating with a work mate a question travel inside my head...,

He's always using "Y" supports instead of "+Y", ( also in truly +y resting points.., for example racks..), just because he claims that with this manner, he jump over the well knowed convergence issue..., etc..., he's introducing "linearity", but still uses non-linearity such friction, gaps, etc...; He's trying to change my mind over my way of modeling ( using "Y" only when it's realy a "Y" and "+Y" when it must be "+Y") but both of us haven´t the right argument to persuade each other..,

Here's my question: if the system really don't have lift-off issues, all the supports have neg. vertical forces.., both of us are using friction, maybe gaps, and other non-linearities except the "+y" ( in my case..) or always "Y" in my work mate case??, does any of the two system have any vantages / benefits ????, i truly think that using, as far as we can, the non-linearities, we always are near the real world. But in that specific case where +Y of one analysis and Y of the other same analysis are supporting negatives forces..., what are the benefits?? Maybe is more a "good practice matter" to help preventing problems or understandig in any other pipe systems with lift-off issues, etc???.

Thank you in advance.

Pls, any comment/opinion??

Thanks

My opinion is that is better to use +Y and not Y. If you pass the calculation to another person that will have to design the supports, maybe it will design hold down supports instead rest support. In addition, it will be more difficult to spot the supports that lift off.

Regards,

Unlike Autopipe, CII has no the function of defining sequence of loading,so I think it's better practice to substitute +Y with Y to aviod some unpredictable results.

Rds

Gino

Arysthos,
Both you and your friend method is correct, as long as both of you define a correct load cases sequence.
You can use +Y modelling method for the support as long as you set up the load case in non-linear load case.
Several experiences stress engineer that i know is more prefered to use Y method. Experience also needed to use this method as you will have to intrepret the result to see any of support lift off ( this also can be done by create a specific load case for it).
Some of Client specification that i know also required the analysis performef in linier system with double acting restraint method.
So, none of you is wrong, it's back to your skills and knowledge.

Regards

Apologies to Bump an Old Thread. I have a couple of related questions

1.Using load cases from the first post, for a stresses at a given node should be same for L2 and L5. In my analysis I get different values, even though there is no lift off supports, due to the frictional effects in operating case, there are horizontal forces for L5 load case, which I feel is not the intention of the build up of the load cases.

2. Also when the load cases are built-up generically for a project and used for negative design temperatures, there are -Y displacements at rest supports (+Y restraints) for L5 case, which is logically incorrect.

Any thoughts?

1. Even though there is no lift off, there is load redistribution. Think of it like jacking up your car, the four tires remain in contact with the ground for the first few cranks but you know their loads are changing. Friction is discussed below.
2. If you would analyze sustained loads in a model where all +Y's that lift off (in operation) are removed from the analysis, these points will sag. That negative displacement should match the your L5 deflections.

Now friction...
In my opinion, this sustained equals operating minus expansion approach works quite well for lift off. If you gain confidence in this approach and you wish to evaluate sustained stress for all support configurations, it will speed your analysis as you will not have to separately re-analyze sustained stresses for the various operating cases where supports lift off. But friction complicates things. The expansion that you are subtracting was created without deadweight and without deadweight, the friction components in the expansion set are probably underdeveloped. So those (larger) friction effects found in the operating state are not properly negated by subtracting the punier effects of the expansion set.
Again, my opinion.

It is also my opinion.

Dave, thank you for your response

I accept the -Y displacements. this can be ignored

But the Hot sustained cases sometimes fails due to the frictional effects and the difference "no weight movement" and "weighted movements" . There are big horizontal displacements for the hot sustained cases - along with corresponding forces in horizontal plane which results in increased stresses.
The original aim is to account for gravity loads in operating condition, but we end up with different set of loads/stresses

Again, I believe your difference is causesd by friction. You can confirm by rerunning your model without friction (simply set the friction multiplier to zero for all load cases in Load Case Options). I expect the hot sustained will then will more closely match a sustained load case with restraints that are inactive in the operating case removed (I'll call this Case2). They won't necessarily match exactly since Case2 does not have the bias associated with thermal change. (Remember that car jack?)

I will not account for Hot Sustained Check where Piping system will likely having a vibration phenomena. In term of load distribution and stress maybe it is acceptable when we check using hot sustained case, but when dealing with system that have Pulsation, slugging, FIV,etc i'm not recommended to have any pipe support lift off.

Regards