We add flexibility in a piping system by change in direction of line routine, using spring support instead of rigid support, using expansion joint etc, i.e. all related to line routine.

My question is how piping flexibility is related to mechanical property of materials. Is it 1/elastic modulus ?? Has there any relation of expansion coefficient with piping flexibility ??
With same displacement stress range if expansion is more then expansion stress is high means system is less flexible.

Thanks is advance for any comments /guidance .

Regards

Sk Habibur Rahaman

Habib,
Please do not be offended but I am really surprised at your questions as they appear to be coming from an 'Engineer', I take it you are a mechanical engineer. These are fundamental engineering questions that any technical student should be able to answer. Perhaps you are a bit rusty and need to revisit your old college text books for the answers.

Regards
A

Though many querys in this forum seeem innocent, most of those come from less experienced young engingers, They may began their careers just one or two years ago.

In this forum, many experienced old birds have their knowledge,kindness and patience, they are happy to answers detaily and timely, This is a very very good feature of this fourm. Also Coade give help to it's new user,just like me, Their market share will increase. because people like to buy more if they can enjoy technical help.

The direction this thread seems to be going prompts me to address something that has bothered me for a long time. That is, "Where are the mentors for these young engineers?"

Since many of the questions posted on this board by young engineers can be answered in minutes by a more experienced engineer down the hall, I have to believe that many of these folks don't have anybody in their own company willing to help bring them up.

While this bulletin board is a great tool, nothing can replace one-on-one interaction on a personal level.

To all of you experienced engineers, you have a moral obligation to help coach the young engineers in your department.

With that said, I also want to commend those of you who continually go above and beyond the call of duty with your numerous detailed posts, even on evenings and weekends, sharing your experience and knowledge.

Thanks everybody
I am relatively new stress analyst. I shall be very pleased if some body can provide me some authentic documents regarding my previous query of ''natural frequency of piping system ''
dated 01.10.05

Best Regards

Sk Habibur Rahaman

Kevin,

Hopefully, the mentors will be here at this forum when other sources elude our younger colleagues.

Hello habib,

Yes flexibility is related to the elastic modulus (i.e., "Young's modulus"). The B31 Code rules require the use of the "cold" modulus of elasticity for calculating stresses in the pipe (we often use the "installation temperature" as the cold temperature). That is conservative.

We are also conservative when in calculating our displacement stresses, we consider "points of termination" (e.g., vessel nozzles) as "anchors" (six degrees of fixity). This would not be a conservative assumption in dynamic analyses.

When we evaluate the piping system's loadings on a vessel nozzle we often use the pipe material's modulus of elasticity at the operating temperature (the pipe will be more flexible) and we sometimes include the effect of the flexibility of the nozzle and the adjacent vessel shell. Just a small rotation (deflection) at the nozzle will result in very much reduced moments at the nozzle. However, pipe stresses calculated in such an analysis are invalid for comparison to the allowable stress range. You will also find that using the modulus of elasticity at the operating temperature will often affect the distribution of forces and moments over a pipe support system.

Please look into the possibility of obtaining Dr. Chuck Becht's book on the B31.3 Code from ASME.

http://www.asmenews.org/archives/backissues/july04/features/704apress.html

Also, look for the book by Glynn Woods and Roy Baguly from CASTI Publishing.


Please come back to this forum with your questions and remember that the only "dumb question" is the one that is not asked.

Regards, John.

Dear John,

You mentioned that "When we evaluate the piping system's loadings on a vessel nozzle we often use the pipe material's modulus of elasticity at the operating temperature".

Does CII do like that?I once heard that CII
use "cold" modulus of elasticity for calculating nozzle loading.

Best regard,

The B31 codes generally require the use of the cold modulus when evaluating displacement stresses, hence the CAESAR II method.

Check the model input sheet for details...

Dear Whm,

We all wish you, young ones, to grow up properly. Please learn self-respect from the old birds.

You may value speed over performance, but need to be careful, too. With todays' softwares like Caesar-II, we can't rely on 'hearing', we need to study the software manuals,available online.

I feel, some protectionism allows the young engineers with inadequade value system to prosper & defame the Youth in the interesting times we are living in now. Nowhere in the World,there is any shortage of books, resources or teachers or time to learn.

Dear Whm, In the global marketplace, You are like the 'Red Queen' in Lewis Carroll's 'Alice in Wonderland' : 'In this place it takes all the running you can do to keep in the same place'.
You must show the World that You represent not only the best prepared & well educated Youth of today but in the years to come too , only by continuous learning.

regards,

sam

A question for John Breen:
Your reply seems to say that the flexibility of a nozzle should not be considered when calculating pipe stress; that it can be considered only when calculating nozzle loads (and support loads I assume). Have I understood your reply correctly?

Regards,
Bruce Hebb

Dear sam,

I can understand what you mean, yes, self-help learning and continuous learning are the soul of personal progress,need to be treated seriously and i will treat it seriously.

come back, to the "modulus of elasticity", I found CII manual don't mention which one it use in OPE case(it may have mentioned ,just i have not found out which page it hide:)),i "heard" from a paper that ....., but i can not do any test or something else to know which mudulus is using by my self unless i have the code of the software.

The value for elastic modulus used by CAESAR II is clearly shown on the input sheet directly below the material box.

look at the value in your model an compare it to the code value for the same material. Check the values for "room temperature" and at the temperature that your analysis is set at.

Until recently, all flexibility analysis defaulted to using the "cold modulus of elasticity". You can check this by calculating the tip deflection of a cantilever: delta = P * L^3 / (3 * E * I).

"Until recently"... EN-13480, and CODETI want the hot modulus used for flexibility analysis. This will be addressed in CAESAR II Version 5.00.

WHM,

You mentioned that "When we evaluate the piping system's loadings on a vessel nozzle we often use the pipe material's modulus of elasticity at the operating temperature".

As John Luf noted, the B31 Codes require the use of the pipe material’s modulus of elasticity at the installation temperature (i.e., “cold modulus”) for calculating the displacement stress range. The Codes also disallow including “cold springing” in the calculation of the displacement stress range. So, to assure the validity of the stress range calculations these requirements must be complied with. The calculated piping stress range will be conservative BUT, with all the vagaries of fabrication and erection, I would not want to undermine that conservatism. Richard Ay added further explanation.

But, when you look at the printed tabular data from your piping stress analyses, the indicated loadings on the “anchors” (e.g., nozzles) will be unrealistically high. If the so calculated vessel nozzle loadings are less than the established (perhaps by the vessel designer/builder) limits, you do not have a problem and you can take comfort in knowing that you have some additional margin in the piping design. However, if the analyses that you performed in accordance with the Code for the purpose of determining the displacement stress range (necessarily using the cold modulus of elasticity) indicates that the loadings transferred to the vessel nozzle by your piping EXCEEDS the established limits for forces and moments, you will have to “sharpen your pencil” and look for a solution to the problem. You might think about redesigning the piping system to provide additional flexibility (and include restraints to direct forces (“thrusts”) and moments away from the vessel and toward your “new” offsets and expansion loops). But before you do that, you might want to look at your design margin conservatism and see if you might “trade off” some of that conservatism to calculate more realistic forces and moments at the vessel nozzle. You could begin by using the pipe material modulus of elasticity at the operating temperature (just remember the so calculated stresses in the piping are invalid from the Code’s perspective). With the “hot modulus”, the piping system will be more flexible and there should be a reduction in the forces and moments at the vessel nozzle. Next, you might calculate the flexibility inherent in the structure that you are attaching your piping system to – the vessel. You can calculate the combined flexibility of the nozzle and the vessel shell immediately around the nozzle and use these flexibilities at the “anchor” that represents your vessel nozzle. This technique may further reduce the calculated forces and moments at the “anchor” (nozzle). Next you might try to use restraints (guides, line-stops, etc.) to direct the expansion thrusts away from the nozzle. If your loadings are still too high you might be tempted to specify “cold springing” (I wouldn’t do that but that is the topic for another thread). If all of these things do not reduce the calculated nozzle loadings to an acceptable level, then redesign is indicated. There is nothing wrong with trying various techniques to arrive at the desired resulting design.

Bruce Hebb,

A question for John Breen:

Your reply seems to say that the flexibility of a nozzle should not be considered when calculating pipe stress; that it can be considered only when calculating nozzle loads (and support loads I assume). Have I understood your reply correctly?


As I alluded to above, I like to allow some conservatism to enter the structural analyses of piping systems because a multitude of deviations from design have been known to occur during fabrication and erection. Also, there are going to be differences between the theoretical perfection of the analysis models and the real world “as built” piping system that it represents (the pipe is not “round”, the wall thickness is not uniform, the wall thickness at “Tees” and elbows varies greatly, even the most carefully erected piping system will have the residual cold stresses of inadvertent “cold springs”, and etc., etc.). So, when it is my purpose to calculate the stresses in the piping system (for comparison to the calculated allowable stress range, SA), I will choose to ignore the inherent flexibilities at vessel nozzles and other “terminal equipment”. The allowable stress range is based upon the concept of using all the range of strain between the hot and cold yield strengths of the material. There is a little margin built into the equation for SA but on the other hand there are some secondary stresses that the Code does not address (e.g., pressure cycling). In this case, it is the piping engineer’s decision as to how to model the system. I don’t think that by ignoring the nozzle flexibilities I will have a design that overly wastes material. Further to that, I will do as much redesign as needed to result in calculated stress ranges that are less than the allowable stress range with the "anchors" modeled as rigid - but again, that is just my choice.

Having said all of that, I will add the caveat that “anchor” flexibilities must be included in calculating pipe stresses due to dynamic loadings.

Regards, John.

Dear John Breen,

Thank you very much for your detailed reply!why i pay much attention to the modulus problem?
because the sofeware that i used before take the hot modulus as an element into consideration when calculating nozzle loading.SO when CAESAR II come, i should try to know the difference clearly.

In addition, I am reading the document "Seismic Design and Retrofit
of Piping Systems"you suggested in july 25,2005,
i think it's good!Thanks!

Best Regards,