ASME- II - Part D - materials and max allowable stress values 2

[JadalbertWincent]

Hi everyone,
possibly the question, I want to ask, is a quite stupid - Im sorry.
Using ASME Code Part 1 section VIII (flanges) there are two values: Sa & Sb. These are maximum allowable stress value. And question: should (have to) I use the values from ASME -II-Part D materials instead Sa and Sb? Tha main problem is that, for ex. using A 182 F1 as flange material I have from table 1A S value when tensile strength is 70 ksi = 482 MPa, yield strength is 40 ksi = 276 MPa, but (!)(?) the max value in 200F = 93oC is only 20 ksi = 138 MPa. Very, very low value. And now a lot of calculation results dont met the requirements of the standard. May be I should use other values (Sy, Su...). What is the right way to select and use maechanical material values for ASME flange (and bolts) calculation? I'm from Europe and sometimes have a problem to understand US standards.
Thanks in advance.

 

[XL83NL]

Best thing to do for you now is to step out of this and ask an expert to help you. With all due respect, I hope your question remains unanswered regarding the tehcnical content. With the type of question you made and posting provided, you do not even seem to be closely qualified to work on this. Giving you detailed advice on such Code questions may unintentionally give you the idea you should continue to work on this problem.
This may sounds hard and/or rude, bit its meant in the best possible way.

A similar thing happened here, on a somewhat different topic, to give you an idea how members here on eng-tips usually think on such questions.

 

[JadalbertWincent]

XL83NL thank You for reply.. I've read your link very carefully and in my opinion:
1. With all respect to you! There is a saying in my country: There are no stupid questions, there are only stupid answers. I wrote, that my question propably is quite stupid. It is the firs sentence of my ask.

2. I need help because I have a problem and doubt and hope the forum is the place when I can get the advice. I will use a citation for discussion:
"There is too much of this going on on this forum now
a - Poster doesn't have a copy of the code/standard they are supposedly working to.
I have a copy.
b - Poster doesn't understand the code/standard they are supposedly working to.
- I have doubts and I'm surprize that the valus of max allowable stresses are so different between yield point and max value in RT. But I understand the rules how this value is determine.
c - Poster is too lazy to even open the code/standard they are supposedly working to.
- I read ASME Code in fragments about the selection of materials Div. 1 and part D and wathever I found. I found the values in tables, changed the units, wrote them in my question. Am I lazy? And I still have a doubst and this is not a denial!

"Back to this post.
Good old Google would have told MAbdo that you cannot weld Tantalum to Stainless Steel so then he would not have been on here asking what welding wire to use."

- I dont think I'm leazy, just have doubts, so I try to find the help on this forum. What is the "not stupid" question to ask in them in this group and get the answer? In my opinion wise people should try to help others. That is the one of reason existing the place / forum like that. Don't you think, do you? Of course, there is a lot of specific knowledge that specialists have and do not want to share this knowledge on the forum. I understand that. My question was quite specific and you could answer it so well or you do not do it badly. Look for other solutions.
Best regards, and I hope you will write an answer.

 

[EdStainless]

I think what you need is some basic background on how the Code is structured.
With regards to materials properties; there are material specifications and the minimum required properties, and then there are the stress levels that you are allowed to use in designs.
The ratio between them is the Safety Factor. In general the SF is 3.5.
There are other allowed stress values for various design cases, and of course they are temperature dependent.
If a material has a minimum allowed UTS of 70ksi you would never use it in a design at a stress higher than 20ksi, and many times it will be much less than that.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[JadalbertWincent]

Thank you very much EdStainless!
That is an answer I expected. So I understand that, do flange calculation, I HAVE TO use the value from tables of ASME Part D. I asked, because I had a doubt. If the project of existing flange connection dont met the requrements then a designer have to change the gasket (its coefficients) for example.

 

[XL83NL]

Sorry Ed, but it is not a safety factor of 3.5. It’s a design margin against a certain failure mode. Which one? That hasn’t been specified, so technically 3.5 is not correct, as we don’t know for sure what the OP is dealing with and what his application is.
The answers are in the code and that’s what the OP should start reading.
I don’t want to be rude, but providing him such answers puts him on the wrong track. Furthermore I’m hesitant to believe he understand basic mechanics. For me, the OP is just too far off from given a basic answer like Ed (unfortunately incorrectly) tried to. With all due respect Ed, since I always appreciate your input here.

 

[JadalbertWincent]

I just only want to be sure if I use ASME standard Div. 1 to calculate a gasket load and a flange, should I use the values of maximum allowable stress value from tables in ASME - part D. Thats all. I dont need any lecture or trening about mechanics. In the code there are entries about it. But if I use our local standard for calculate flange connection, the rules specifying the allowable stresses for materials in assembly and work condition are different and the determine values ​​are higher than the ASME values.

 

[JadalbertWincent]

And thank you EdStainless for help, and XL83NL for showing me where, in his opinion, my place is.

 

[metengr]

XL83NL is spot on with their responses.

 

[DriveMeNuts]

The design margin of any specific failure mode is made up of a series of factors multiplied together.

For the specific failure mode of plastic collapse, the design margin is made up of a single factor which is the "safety factor".

The magnitude of this safety factor is 3.5 for ASME VIII Div 1.

This safety factor for this specific failure mode is in turn made up of many sub-elements.

 

[TGS4]

I have been watching this thread with bemusement. And it is time to be pedantic.

There is no such thing as a safety factor. If there were, it would be some function of all of the failure modes and all of the design margins against those failure modes. And given the multitude of failure modes present in pressure vessel design, I'm not sure that such a beast is possible to calculate:

For VIII-1 (VIII-2 Class 1 and Class 2 also listed parenthetically), the margins are as follows:
1) Plastic collapse - 3.5 (3.0/2.4) against engineer ultimate stress OR 1.5 (1.5/1.5) against engineering yield
2) Buckling collapse - 2.0 (2.0/2.0 for DBR, but 3.0/2.4 for DBA - but we're fixing DBA rules)
3) Ratcheting - not explicitly listed, but implicitly 1.0 (1.0/1.0)
4) Fatigue - not listed (20 on mean life or 2 on stress - really just a lower-bound of the vast data scatter, although for welds the structural stress method does provide some statistical treatment, but it doesn't really translate well into a design margin, per se)
5) Creep - complicated to explain for all Codes. Certainly not a single value like for plastic collapse.
6) Leakage at flanges - dunno - something greater than 1.0, see Appendix S.

 

[metengr]

Nicely stated, TGS4. See you in DC.

 

[DriveMeNuts]

TGS4, are you sure you shouldn't have started your post with "In my opinion...."? Many literature sources contradict your contribution.

With respect to your first paragraph relating to a single all encompassing Safety factor to cover all failure modes simultaneously, I agree. I doubt it is possible or worthwhile trying to calculate such a thing. The generic "Design Margin" term is more appropriate to use in this general design context.

With respect to your second paragraph relating to each specific failure mode, I disagree. When referring to a specific failure mode using the generic "Design Margin" term is acceptable if you choose to do so, however referring to the specific factors that contribute to that failure mode, including "safety factor" is more useful.
And the factors of UTS/3.5 and Yield/1.5 are for different failure modes.
1) Plastic collapse is prevented by applying a safety factor of 3.5 to the UTS. (Ref. WRC 429 - 3D STRESS CRITERIA for ASME VIII Div 2)
2) Excessive Plastic Deformation is prevented by applying a safety factor of 1.5 to the Yield stress. (WRC 429 and Pressure Vessel Design : Concepts and Principles - Spence, J.; Tooth, A. S.)
3) For buckling in accordance with ASME VIII Div 2 Section 4.4 two types of factors are applied:
a) Capacity reduction factors which take account of the effects of imperfections, boundary conditions, nonlinearity of material properties and residual stresses. Applying these factors calculates the lower bound buckling stress.
b) A "design factor", FS with a value of 2.0 (design margin as suggested by TGS4 is not mentioned anywhere). Section 4.4.2 specifies that the predicted buckling stress is determined by setting FS=1.0. This means that FS=2 is a safety factor of 2. Any guesses for what the S stands for in FS? PD5500 and EN 13445 use a similar methodology and call FS a "Safety factor".
4) I wouldn't refer to fatigue as having a Safety factor and even design margin doesn't seem right. Survival Probability is more appropriate. The PD5500 method has a statistical calculation which predicts survival probability. The ASME VIII Smooth bar fatigue method doesn't provide any measurement of margin or certainty, It is a Yes/No guestimate based on decades of experience (The factors mentioned by TGS4 are made up numbers that have been proven with experience to do the trick).

EN 13445-3 uses the term "Safety Factor" rather than "Design Margin" for UTS/2.4 and Yield/1.5.

 

[TGS4]

Everything on here is "in my opinion". If I was speaking authoritatively, I would writing in the Code.

BTW, the term FS that you describe in 4.4.2 is something that I working to remove/modify. Did I mention that I detest the term Factor of Safety?

I completely agree with you, though, with respect to your note 4) on fatigue. Probability of Survival is a good term.