Analyzing a CS piping system at –40F. Per ASME B31.3 Fig 323.2.2B (Page 47 in 2004 edition) systems at temp less than –20F (minimum metal temp)can be analyzed without “impact testing”, by considering the stress (ratio) reduction from the graph on the same page.

But the notes defining the stress ratio are obscure. I Interpreted that, for temp @-40F the stress ratio is 0.8, so now the allowable becomes 0.8*20000psi=16000psi. But the general notes section on the same page implicitly defines that my approach is not correct and mandates a different approach.

How do we get the "S" value at minimum design temp as used in the notes to calculate the stress ratio?

Any thoughts!!!!

Thanks
Reub

I guess the thread(below)left more questions and confusion. Let me rephrase it for better understanding.

what are the Allowable stresses and the procedure to calculate the Allowable stresses for CS @-40F? Caesar-II uses default 20000psi for temp less than -20F.

Thanks
Reub

Hello Reub,
If you are looking for allowable stress @ -40 F for carbon steel material, it is 20KSi.

But if you want to use it below recommended temp for a thk as suggested by table(323.2.2A).Than you have to be careful about it and I would recommend not to use it if you are starting a fresh in the project.

And "Stress ratio" and it`s usage has been included and amended from ASME BPV, Apart from stress ratio, keep in mind that you have to satisy Note 2 of the taBLE 323.2.2.

Stress ratio can be only determined when you have stress analysis model, which is normaly not there at the begning of project(at the time of making piping material specification)

So my advise will keep away from it. But if you are into it , let me know,i will try to help it.
Cheers!

Having worked with this in the past, I'm with Alok and I'd recommend that low temp CS be used... though of course, it will depend on what is in the pipe and why you have your low temperature.

Shannon

Hello Alok,

Thank you for the information. Let me describe the project scope.

Its an existng system. Client is upgrading the system from -25F to -36F. One of the option is to avoid the "impact testing" on the existing sysem. So what's your thought process?

Also the table 323.2.2B: general notes uses "S" as stress for min design temp? Is S=20000psi in this case?

Correcr me if I am wrong, So using S=20000 psi, calulate the stress ratio ( max of the note 1, 2 and 3)and plot the temperature reduction from the graph. So as long as the "minimum design temp( ie is -36F in my case)" lies within the temp reduction, I am okay.

Thank you in advance!!!

Reub

Hi,
First of all, what is the material specification and thickness, I assume you are using A 671 CB 60 for pipe.

Going ahead with this. We will follow step by step to reach the conclusion
You can repeat the same steps if you have different material than from what I assumed.

1)First check the Fig 323.2.2A, This particular material is on Curve B and covered with Note 3. The Minimum design temperature is -20 F (i wonder why earlier your client has specified -25 F?) Note 3 is not applicable to this case.

2) now let us go to clause 323.2.2 c, which gives requirement to be met for any listed material to be used below it`s minumum design temperature as given in the figure 323.2.2A For our case it takes us to column B-3 for Type of material listed under 3.


So if you satisfy this requiremnt move to step 3



3)go to clause 323.2.2 d (1) which allows you to take advantage of stress ratio reduction factor. While calculating stress ratio you have to generate seperate load case (model)in stress analysis where you have to include axial stresses and exclude sif for components.

and then use it as you have mentioned earlier.

You can aslo refer to UCS -66 of BPV Sect VIII- Div 1.


I had dealt with similar sort of case, but my material was LTCS used in flare line, due to modicationin existing plant process parameter. tempetaure droped to -58 C.
Client was iclined to use same material, after running stress ratio we found it is 0.7 and advised that we cannot do so.
SS 316 L was chosen to replace the section.

Best of luck,
Cheers!

Alok,

we did find out that the Stress ration is 0.714 and go for "impact testing".

Thank you for help

-Reub

Hi reub,
of course you can do impact testing and be in safe side. A 106 Gr B materials to my knoweldge have even failed in temp above -20F during testing because of material composition control issue. So it is better to be safe rather than sorry.
All the best and thanks for keeping us posted.
Best Regards

Are you impact testing your existing pipe?

Yes

Second option: derate the system or replace with SS pipe.

Reub

Reub,
Why SS? you an use LTCS for temp mentioned by you -36 F
regards

Alok,

LTCS is also on the table. It is for the client to make the call, and also modify the existing pipe spec.

Reub