liberal allowable stress

Dear Sirs, In which cease we need to switch on the liberal allowable stress in CII ? What is the advantage for that one?

You should read the ASME B31.3 code. This is a fundamental question which is fully explained there.

the use of the word "liberal" I take exception to... that word is not defined in the code nor used by B31.3 it is simply another allowable value which takes into account the actual affect of Sl loads in the system...

you should read the explanation from Breen here:
http://www.coade.com/ubb/Forum1/HTML/000057.html

Dear Lingrajan,

I think the advantage is we will have higher allowable. We can see it in ASME B 31.3 Code.

Assume we have two simple "different" models, one takes SAL and other takes SA as their allowable. Then they have "same" stress ratio in many nodes case EXP. The model using SA insteaf of SAL as allowable will has better flexibility. And may be (my own opinion) will have more longer fatigue life.

In code I find no word for "liberal" they still say for both as Allowable displacement stress range SA. Caesar may just need to make them different as SA and SAL since different in formula. Sorry if you already know about it

Well, no.

Let me see if I have the hypothesis correct.

Two Identical Piping Models.
One Analyzed with SA
One Analyzed with SAL

THE SA MODEL has greater fatigue life???? Based on what????? The piping cares not whether SA or SAL is in effect.

Dear Luf,Breen,

I think you miss my overall assumtions:

Assume we have:
-Two simple "DIFFERENT" models.
-Model I takes SAL and Model II takes SA as their allowable.
-The two models have "SAME" stress ratio in many nodes case EXP.

Of course the model II using SA will has better flexibility insteaf of model I using SAL as allowable. But may be will need more pipe material which will increase the cost.

For ASME B31.3 the failure mechanism for secondary load (temperature) is fatigue. For primary (W, P) is yielding.

correct me if I'm wrong, there's nothing to do with pipe flexibility whether you want to use SA or SAL formula, the pipe still stay the same when using SA or SAL . It's all about material, isn't it?
but if you mean, "better flexibility" is less conservative approach/calculation when using SAL formula instead of SA one, then I agree

Dear Omdo,

Assume model I have ratio 99.9% (pass)in load case case EXP using SAL as allowable.

Still in model I, but the ratio 100.1 % (fail) if we use SA as allowable.

Let say we or our client in order to have 99.9% in ratio does't want to use SAL instead of SA as allowable. Then we create model II such as doing rerouting to have 99.9% in ratio.

I think model II has better flexibility. But may need more pipe which increase the cost.

Note:
Some of us may will select 80% as higher ratio for load cases EXP.

Dear Omdo,

Assume model I have ratio 99.9% (pass)in load case case EXP using SAL as allowable.

Still in model I, but the ratio 100.1 % (fail) if we use SA as allowable.

Let say we or our client in order to have 99.9% in ratio does't want to use SAL instead of SA as allowable. Then we create model II such as doing rerouting to have 99.9% in ratio.

I think model II has better flexibility. But may need more pipe which increase the cost.

Note:
Some of us may will select 80% as higher ratio for load cases EXP.

SA is function(Sycold+Syhot) = 1.5SC+1.5SH,
for safety reason it becomes (1.25SC+1.25SH) – Mean Stress
Mean stress usually is sustain loads, as per code is 1.0 SH
Hence,
SA=1.25SC+0.25SH

while
SAL ~ (1.25SC+1.25SH) – SL

as we know SL is always < SH (in order to meet ANSI B31.3 chapter II para 302.3.5), which means SAL will provide bigger allowable stress than SA.

then again, SA or SAL cares not to pipe flexibility because two of them are the approach to limit the Expansion Stress.

In my experience, they are used as tools for engineering judgment, eg; if the sustained stresses were calculated as 80 or 90 percent of Sh, I usually not to take SAL in my calculation or my design will be more conservative

Regards

Dear Mr.Samsul,



I want to know the reason why you said SAL will have longer fatigue life??
The pipe will remain the same no matter you use SA or SAL, it has nothing to do with fatigue life.

I think paper of Markl, about "Piping Flexibility" was a good basic for this discussion or book of Piping Stress Calculation simplified by Spielvogle.

Thank you

Dear Dylan,

Please don't take the conclusions before overall assumption complete:
Assume we have:

1. Two simple "DIFFERENT" models. So this not talking about the model stay the same like Omdo said but compasrison two diferent models.
2. Model I takes SAL and Model II takes SA as their allowable.
3. The both models have "SAME" stress ratio in many nodes case EXP.

Model II which using SA will have better flexibility insteaf of model I using SAL as allowable (assumtion no. 3 shall be fulfill at first). But model II may will need more pipe in order to make its ratio let say the same with model I.

When I said that Model II better in flexibility and will have more longer fatigue life is only my own opinion. My opinion based on that fatigue is failure mechanism in EXP case.

I also agree with Omdo:
better flexibility is less conservative approach/calculation when using SAL formula instead of SA one.

I think I mean the same by making comparison of the two models.

I realy dont know what your trying to do..... is that liberal you said is less conservative well.....

Dear bom,

Using SA is more conservative approach rather than using SAL.

Dear Mr. Samsul P.Manik,

Your Examples is only right to show about how to make a piping system flexibel but there is no further explanation from you and your example here why is SA more conservative....
If true SA more conservative so why CAESAR II use SAL as Default here not SA (Altough that will be back to engineer judgement for better analysis).
Sl here is included in SAL equation is in purpose to prevent from thermal Ratcheting that will lead to incremental plastic deformation. For more detailed about it you can see at Goodman papers and David Burgreen Books, and many2 discussion about it on Thermal Expansion Stress Thread and Problem to have more than one sustained case Thread if you use the Search function.....
Thanks


Best Regards

Tengku Syahdilan

Let me explain this one more time, slowly.

A106B, or any other piping material, doesn't give a rat's ass what your Code says the allowable stress is. It's going to, on average, have the same failure properties whether your Code assigns it an allowable stress of 12 ksi, or 20 ksi, or 24 ksi, or 36 ksi, or 3,000,000 ksi. (Of course, if your design Code assigns it the latter allowable, you're going to experience a lot of failures!)

So, since the real-life failure behavior of a large sample of A 106B pipe is going to have the same limits,

THE LOWER YOUR ALLOWABLE STRESS FOR A PARTICULAR DESIGN CASE THE MORE CONSERVATIVE THE DESIGN IS!!!!!!!

That wasn't too tough, now, was it?

The term Liberal in Liberal Allowable Stress - as presented in C2 - is misunderstood or even, may I say, misapplied. A better definition would be "Complicated" Allowable Stress versus the "Easy" Allowable Stress.

If I hand you a slide rule and ask you to check the expansion stress of a piping system, you'll use use the "Easy" limit every time.

Currently some web based training is going on from Paulin Research group which is FREE. The technical content is too good and one gets to learn from none other than Tony Paulin and Chris Hinnat and in my humble opinion attending these courses are must to properly understand the code rules. Many of the code related questions in the forum will have their answers in the different modules of these webinars.


Another excellent and must read document is NUREG CR3243.
Regards

SAL as default in Caesar II?
ah c'mon.. don't you have to tick on special execution parameter, the liberal allowable stress item?

SA use SH as Mean Stress & SAL use SL as mean stress (which usually resulting SA<SAL). the lower the code stress the longer life of pipe from fatigue failure is

just read sustained loads effect on fatigue in any kind of mechanical book, no need heavy read on it

SA use SH as Mean Stress & SAL use SL as mean stress (which usually resulting SA

I am sorry, can you expand upon that? Mean stress?? This is B31 is it not?

John

DEar Omdo,

I think what Sir Craig and Dave explain above has made everything clear.
And about SAL as default, well you can check and read at C2quick document. and in my opinion, CAESAR give this SAL as default just to make it the easy way to calculate expansion stress, but all the decision to use SAL or SA will back to us, don't you think so?

Dear everyone,
Regarding to above discussion, if i use material ASTM API 5L GrB, i have Sc=138 MPa
Sh=138 Mpa

refer to ASME B31.3 chapter II para 302.3.5, SA=f(1.25Sc+0.25Sh)
with f=1, allowable stress for expansion is
Se
when i model Caesar II, the result for expansion is not same with 207 Mpa
(result: allowable 302 or 297 or 310 depend on thermal case cheking)

I think there are somethings need to change in caesar II to meet the allowable stress for expansion is 207 Mpa but i don't know where to change this.I hase opened "configuration setup" but i didn't see the one related to Se
Please advice me to do this.
Thanks

By default c2 uses the "other" equation for the Expansion Stress Allowable:

Se = f(1.25Sc + 1.25Sh - Sl)

You're seeing different allowables through out your system because Sl is varying. If you don't want to use the above equation, uncheck the "liberal allowable" item on the "Special Execution Options" dialog.

This is the whole point of this thread.

Thank you
It worked well.