Hello pals ,

I do not understand the concept behind zero friction effect. Even in the Flexibility Analysis Specifications of my project, it had been clearly mentioned that no credit should be taken for the friction and CAESAR report with no friction is mandatory.

Can anyone explain me the reason behind this?

First, use the [Search] option just above the Calendar (on the right) to review the previous posts on "Friction". You'll find quite a lot of useful information and discussion.

Second, as to your question ...


The first part here (that no credit should be taken for friction) is a consequence of the unknown nature of friction. Friction is non-conservative, non-linear, path dependent, and mu varies with time and the condition of the contacting surfaces. So, if in a certain load case, friction helps you out, you're not allowed to count on this (friction effect). Friction can only be used to worsen the situation, not as a beneficial effect.

The second part (a report with no friction is mandatory) is simply a necessity to "cover all the possibilities". While we may all agree that there will be some amount of friction at sliding supports, you won't know how much friction is helping or hurting without making a run with "no friction".

This is what is so nice about the "friction multiplier" on the "Load Case Options" tab. You can define your best guess at mu in the input, and then manipulate the "friction multiplier" at the load case level to address the points discussed above. For example, assume mu is defined as 0.3 in the input:

Case 1 - "friction multiplier" = 1.0: This case uses the specified value of mu, representing the expected situation.

Case 2 - "friction multiplier" = 0.5: This case represents the best possible situation with friction (perhaps when the system is new). The value of mu used here is 0.15 (0.5 * 0.3).

Case 3 - "friction multiplier" = 1.33: This case represents the worst possible situation with friction (perhaps after the system is 5 years old, there is some rust, dirt, trash, or paint on the surfaces). The value of mu used here is 0.4 (1.33 * .3).

Case 4 - "friction multiplier" = 0.0: This case representst he "no friction" situation. The value of mu here is 0.0 (0.0 * .3).

The real fun begins when you have multiple operating conditions with occasional loads - and you want to play the friction game.

Hello Sir,

Thanks a lot for explaining the significance of the no friction run. I had been struggling a lot to get answer for this question.Your explanation gave a clear picture for the reason behind it. Once again Thanks a lot.



Have a nice NEW YEAR



With Regards,

Vigneshalex

You're welcome. A few other points to note....

First, the values I used above for mu and the multipliers are values you should determine. Don't take my examples as gospel.

Second, after setting up all the load cases, you want to use the summary reports to obtain the worst situation; the highest stresses, the largest restraint/nozzle loads, etc. Then you have to be the engineer and get everything to work.

Thank you sir,

Surely i will have a keen look for the things that you have mentioned above. By the way i had read your article regarding Linear and Non linear Restraints, it gave me a good insight of the same.Im a fresher started piping stress analysis can you please suggest me some good books/ materials to read, because over here no one supports to know what is happening behind CAESAR instead they are simply insisting on completing Stress system reports.

Eagerly awaiting for your Suggestions ......


With Thanks and regards


Vigneshalex

Check the "References" page on this website:

http://www.coade.com/support_reference.asp?varflag=Reference#4

Thanks a lot

See John Breen's Reply for your question on books on:

http://www.eng-tips.com/viewthread.cfm?qid=189629&page=4
http://www.eng-tips.com/viewthread.cfm?qid=12824&page=236

As a minimum, have following books:
(1) Atleast one basic book on Pipe Stress like "Introduction to Pipe stress ananysis-Sam Kannappan"
(2) Design of Piping System-M W Kellog Co.
(3) One Code Guidebook - CASTI Guidebook to ASME B31.3 Process Piping OR
Complete Guide to ASME B31.3- Charles Becht
(4) Piping Handbook - Mohinder Nayyar

And, get Tube Turns book "Piping Enginnering" and read A.R.C. Markl's papers in it. Finally, Read..Read..Read...the Code