No ASTM A420-WPL6 or ASTM A234-WPB in CAESAR II Materials Database 1

[SNORGY]

I first drew reference to this issue in another forum, but I consider it important enough to raise it again here and ask for some advice.

Recently, on a project, when inputting ASTM A-234 for elbows via the CII drop-down menu and running subsequent analyses, everything was fine. Then a change was made in the same piping to low temperature (ASTM A333-Grade 6 / ASTM A-420-WPL6) materials, whereupon it was discovered that the default materials database in CII does not include ASTM A-420-WPL6. The stress engineer input the appropriate values from ASME Section II Part A, and suddenly a warning message was produced in the CII runs that stated that the elbow wall thicknesses were less than the minimum required by B31.3 Code. Hand calculations in accordance with the Code verified this to be true, so we looked into the differences between ASTM A-234-WPB and ASTM A-420-WPL6. Confused by the fact that we could not find a difference that would give rise to the error, we looked at the CII database for ASTM A-234. It turns out that the values therein are the higher ones for ASTM A-234-WPC, not ASTM A-234-WPB. So, in some cases, ASTM A-234-WPB elbows might not meet the B31.3 Code minimum thickness for the design conditions if the piping specification design pressure is set at full flange rating. I suppose it would be best then to specify WPC where applicable and try to figure out the best way to deal with the WPL6 issue - which in a number of cases also would be short of Code requirements.

One can add the specific materials to the CII database, as the software has provisions to do that, but for such common materials I just didn't expect to find this.

Has anyone else come across / had trouble with this and am I going about things the right way? Am I reading the Code right?

Regards,

SNORGY.

 

[SNORGY]

I should have mentioned that the service was such that a corrosion allowance was specified...one can always reduce the corrosion allowance to get around this problem but that's not my preferred approach.

Regards,

SNORGY.

 

[NozzleTwister]

If your Pipe Material and your Fitting Material have the same allowable stresses and the same physical properties, why do you switch the materials back and forth in your computer model?

If you quit doing this, that will eliminate your problem.

NozzleTwister
Houston, Texas

 

[BigInch]

What's so unusual about that?

The important thing is that CII told you that you had overstresses.

ASTM A234 WPC won't work to full flange pressure ratings with a whole lot of wall thicknesses.

Direction change fittings are where the bulk of moments get resisted, so bending stresses are higher than in straight pipe while pressure stress are and axial stresses can be the same, hence total stress is normally higher at fittings.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[SNORGY]

NozzleTwister:

It was actually the practice of *not* changing materials for every elbow that identified the problem in the first place. Carrying A-106-B or A-333-6 straight through the user input produced the under-thickness warning whenever an elbow or bend was specified, and the message suggested it was a hoop stress problem, not a bending stress problem. Initially, we figured, well, if the material is in fact in the database, let's use it, so we input (for example) "A-234" at the elbows and the error message went away. So I (perhaps stupidly) assumed that forged elbows - like forged tees - might have slightly greater wall thicknesses than the corresponding pipe, explaining why the problem "went away". (Stupidly oblivious to the Code equations for bend wall thicknesses...).

The problem then arose when looking at A-333-6 (low temperature CS) piping systems, since A-420-WPL6 was not in the database. So, we used the same properties for "elbows" as for straight pipe, and the error message came back. It was at that point that we discovered that we could select A-234 for elbows and A-333-6 for straight pipe and the problem would again go away. That was the point when we discovered that the properties (allowable stresses) in the CII database were for A-234-WPC, not A-234-WPB.

With respect to the error message, it had nothing to do with bending. It was simply the minimum thickness required for pressure.

BigInch:

"ASTM A234 WPC won't work to full flange pressure ratings with a whole lot of wall thicknesses."

That is certainly true for WPB - and possibly WPC as well, although I have not checked that thoroughly. While the practice of specifying "Piping Line Class Specification Design Pressures" as being "..equal to the flange rating at temperature..." is perhaps not technically correct, it is, nonetheless, commonplace for a surprisingly large number of specifications that exist for various upstream oil and gas producers - and, unfortunately, EPC firms. Then, typically, ASTM A234-WPB is specified for an elbow, under the assumption that "...what works for straight pipe works for the elbow...", when indeed that may not be the case.

Accordingly, there is a bigger picture to address, namely, it's not always the flange that limits the design pressure in a specification in accordance with the governing Code calculations.


Regards,

SNORGY.

 

[NozzleTwister]

SNORGY,

As a CAESAR user, I've never incounterd a wall thickness warning on just elbows when the same properties (including wall thickness, C.A., mill tolerance, etc.) are the same.

Do you have rolled plate pipe with a 0.01" M.T. and 12.5% on your fittings? I've had problems with that scenario.

If not, I would send an e-mail to COADE Tech Support or post on their Forum to get the problem addressed/resolved.

Cheers,

NozzleTwister
Houston, Texas

 

[SNORGY]

NozzleTwister:

I'll have to play with this again when I return from vacation. (I am currently on the other side of the country at my "other" job.) It seems odd to me as well. I do far less of the actual model building and analysis now than I did years ago, but I never (until recently) had this problem surface either. To my understanding / recollection, it comes up as a warning when specifying DP=flange rating, MT=12.5% and CA=3.2 mm (the same as the pipe). It is not an "error" (i.e., it allows the analysis to proceed) but it is flagged as a warning during the model error-checking and at the user input preprocessor.

If we are certain there is an issue, we will mention it to COADE (CodeCad) but it's not really hurting us.

Regards,

SNORGY.

 

[MBlackman]

Are you sure the warning wasn't preceeded by 'if the bend is bent pipe' ?...as we all know listed components such as elbows to B16.9 don't need to follow the code equation for bends, and you don't really know what thickness they are other than at the ends anyway.

 

[SNORGY]

MBlackman:

No, it's pretty general in nature. I can't quote it verbatim here since I am currently in a hotel room across the country with no access to CII. However, it simply says, paraphrasing:

"...the thickness of the bend specified at Node X is less than the minimum required Code thickness..."

Paragraph 303 in ASME B31.3 - 2006 (once one goes through the various other paragraphs and tables referenced) and B16.9 do seem to suggest that one can treat elbows as straight pipe for pressure design, and that would solve all of our problems. Unfortunately, CAESAR II returns this somewhat vague warning message, and then when one goes to B31.3 2006 Paragraph 304.2.1 and Equations (3c), (3d) and (3e), one comes across the rather unfortunate choice of words in the definition of R1:

"R1 = bend radius of welding elbow or pipe bend".

I will be able to look into this in more detail next week when I get back, but meanwhile, it is creating some confusion for us. The confusion is then compounded by the fact that CII has a material database that, for A234, defaults to the higher stress values for WPC instead of those for WPB. Because of that, when we were specifying A234 elbows with A-106-B pipe, we never got this warning message. We only started getting the message when we looked for A420 in the CII database and, when then we did not find it, we used the stress values for A-333-6.

Regards,

SNORGY.

 

[SNORGY]

MBlackman:

I have returned from vacation (for a day or two at least).

The offending message in CAESAR II is, indeed, preceeded by "if the bend is bent pipe". COADE confirmed that Lorenz factors do not need to be applied for ASTM A234 or A420 elbows, as you stated.

This is also discussed on Page 46 (and 105) of LC Peng's book, a copy of which I recently purchased but didn't have with me in Ottawa.

Thank you for answering that, and please accept my apology for the delayed response and acknowledgement.

Regards,

SNORGY.