Hi Everybody,

Can any one throw some light on the purpose of preparing no friction case ( co-efficient of friction is deleted from all restraints) file in Caesar for analysis? Which modulus of elasticity is to be used for no friction case and why? What parameters should be checked for this case?

Thanks in advance

Hi chandras
In my understanding friction on piping come into effect when it start heating up to operating temperature, shutting down cold ambient temperature or some external load like wind, seismic come into picture that try to push pipe . Normal operating case when there is no temp, pressure variation or external load change friction load will not be there.
Friction will produce axial horizontal load ( mu*vertical load maximum) depends on which direction pipe is moving. Even friction co efficient is also variable value ( practical case).
With friction & without friction cover all possible case range may happen in piping as far analysis is concern. So we go for both the situation.
Modulus of elasticity has no relation with frictional load.
Expert please correct me if I miss something or there is something wrong in my understanding on friction.

The other reason to run a no friction case is a u of .3 or .4 is a best guess of what the friction will be. I am of the mindset that friction should never be used to reduce nozzle loads or stresses. In my opinion you shouldn't rely on friction to make your system safe. So in critical applications checking the system with both friction and no friction may be advisable.

Thanks Shr for your reply...

All the parameters should be checked for no friction case also?

In some projects i found no friction case is not considered at all.. so for those system we are taking the advantage of friction for nozzle loads evaluation and stress check but it is not the real case..Do you thick those analysis are complete and correct from prctical point of view?????

Hi chandras
In stress analysis we do approximation calculation. It may slightly defer with practical, like stress analysis report show 12 mm displacement of pipe at some node ,in site plant it may be 10mm or even less.
What we have to ensure is safety of the plant.
For general stress calculation if no friction case is not considered there will not be much issue.
But say you are doing stress analysis of very sensitive equipment like compressor, turbine it is always better to check both with friction and without friction.

In some company standard both the case is must for all stress calculation.

Regards
Habib

Thanks Habib for your quick reply..

I know Caesar give us apporoximate result by FEA method... Do you mean to say that analysis of Caesar is that much conseravative that even if we dont consider no friction case in our analysis the system is practically safe???

Hi chandras
Caesar use structural beam theory to calculate stress, load.
I do not generalized anything. Normally we try to restrict sustain stress below 50% expansion stress range below 80% nozzle load also check with cold modulus of elasticity. So we generally keep some margin.
If there is any critical system you identify you again must check with friction & without friction if your company standard do not require all stress calculation to check like this.


Regards
Habib

Chandras,

Correctness of an analysis is dependent on

1)The accuracy of the mathematical model of the physical system

2) The inherent limitation that the mathematical model has.

The first parameter depends on the analyst.

FEM is an approximate solution of a physical problem with sound mathematical basis.There is a book by Babouska and Straboulis on the reliability of FEM.To state in a single sentence, FEM's reliability is proved by mathematicians, hence it is beyond question.Hence , we can evade this issue.

The real issue should be point (1). The analyst has to always learn from his/her seniors continously because pipe stress is not only about understanding the theory behind M/Z but rather on finding out the real M acting on the system.

Regarding friction, my one line advise is: do not use it for advantage iei reducing loads.Friction is atransient phenomenon and cannot be accurately modelled.

Also , an analyst has to understand for which systems friction will play an important role.

Regards

Hi Chandras,

Analysis without friction may increase moment in such direction rather than with friction. Someone did analysis w/ and without friction to check the load. I think that is a safety wise.

Here is a good link:
http://www.pipestress.com/papers/Friction.pdf

Read also table 3...

Hi Anindya,

Thanks for your advice... But it is still not clear to me for which system friction should be considered and for which it should not be?/ Is there any clear cut guideline for it? My observation is that, the decision to check no friction case is not left on the stress analyst, rather it is client requirement/ company practice. I worked earlier with a Oil/ Gas EPC company (Europe Based) where I never done this kind of analysis...


_________________
Chandrasekhar

It cannot be quantified. It depends a lot on the project specification and if not explicitely mentioned there, on part of the experience of the analyst and the system in question.

Why don't you make the analysis with and w/o friction and use the conservative results?

Regards

I agree with your proposal.. to analyse the systems in both the cases and cosidering the conservative result is best option for any stress analyst..But if it is not asked to consider the no friction case in the project spec then there may be manhour constraints to do so...

Anyway, thanks a lot for your valuable guidance..


________________________
-- Chandrasekhar Chowdhury

Friction can increase or decrease the nozzle loads depending on the configuration. A no friction case is an unrealistic condition. That is why we use Teflon pads and corresponding friction factor. So it is not that what we use in the analysis. But what we are going to do in order to achieve that condition in reality. With steel to steel contact friction coefficient is 0.3. If a Teflon pad is used below the sliding saddle of the equipment or at a support location, the friction factor can be 0.1 at that location. But we need to actually place a Teflon pad below the saddle or the support at site.

Hi shiny
I think you have diverted to another topic.
And do not draw conclusion "A no friction case is an unrealistic condition"

Hi chandras
Making a no friction case is not a big task. You do not have to make another caesar file & delete friction from all support.
Just go to "load case option"-"friction Multiplier" put 1 means friction is active, put zero means friction is deactive.
Just your number of load case will increase.
This activity is also not required (Normal stress packages ) if your project specification don't ask you to do so.

Regards

Habib