May as-measured thickness be used in Code calcs? 4

[BNEAL]

I have a section of (seamless) pipe which I want to use as a nozzle in a Div. I, Code vessel.

May I use its actual, as-measured wall thickness in my calculations?

My Code calculation software says that my pipe's schedule is "inadequate" because its wall is "too thin" when the nominal thickness is reduced by 12.5% for mill tolerance.

However, I can demonstrate that the pipe's thickness is not below that required in this service.

 

[roca]

I would say yes – but may depend on your client or where you are in the world.
You would need (obviously) the back-up documentation – UT thickness inspection reports, material cert, etc

It’s the same when you do a re-rate of an existing vessel. Wall thickness inspections should be done and the lowest found thickness in the particular componenet used in the calculations.

 

[BNEAL]

roca - Your's is the reasonable, "common sense" answer...But most of those that I've consulted with think otherwise. Do you (or anybody) know of a particular passage in the Code or a favorable "Code Case"?

 

[gr2vessels]

Refer to UG-16(d);- 'except for nozzle wall reinforcement area requirements in accordance with UG-37 and UG-40.
Also, your 'code' software should have the 12.5% undertolerance optional, you should turn off that option. As roca said, you need to document the actual wall thickness, material certs, since you are not selecting a theoretical catalogue component but using an existing component allowed by the code.
Sorry, I don't know of any helpful 'Code Case'.
Cheers,
gr2vessels

 

[gr2vessels]

Just for clarification, the definition of the tn on page 43 excludes the mill tolerance of 12.5% on pipe, if calculating the reinforcement area.
Cheers,
gr2vessels

 

[carthago]

..suggest to use API 579,..you can use the measured thickness and even used averaged values..but keep in mind the remaining thickness

 

[davefitz]

for a short nozzle, one can probably use the results of a UT survey, where many data points are read around the circumfererence. One can apply statistics to these data points and use the average thickness minus 2 standard deviations as the min wall thickness.

For a long pipe, though , espescially if several pipe lengths from different mill batches are welded together, a more sophisticated and involved sampling would be required, assuming the Code permits such a rating.

 

[jamesl]

No, you cannot use actual thickness in your calc if you are manufacturing a new vessel per ASME Div.1 code. Except for area reinforcement calculations, you have to include the 12.5% mill undertolerance when you use pipe as pressure components. See code interpretation, VIII-1-86-11.

 

[CodeRef]

jamesl is correct.
For ASME (new construction) UG-16(d) requires that the nominal thickness less the manufacturing undertolerance be equal to or greater than the minimum thickness required. The undertolerance is only a consideration when selecting the schedule of pipe to be ordered after determining the required thickness.
See jamesl referenced interpretation VIII-1-86-11 as well as interpretation VIII-1-89-164

FAQ731-376

 

[racookpe1978]

Consider very, very carefully whether you have actually "measured" all the places on the pipe, and know - beyond any doubt - that you know that NO place on that pipe (soon to be a nozzle) where it is less than the where you measured it the first time.

Can't you buy a short chuck of thicker pipe - a nozzle is seldom more than 24 - 39 inches long - for less than you can spend measuring the wall thickness of every sq inch of the thing?

 

[gr2vessels]

The Interpretation VIII-1-89-164 is correct, as follows:
"Reply: No, the undertolerance is only a consideration when selecting the schedule of pipe to be ordered after determining the required thickness."
That is, the undertolerance is not any longer a consideration after correctly selecting, purchasing and receiving certified and inspected nozzle neck pipe.
The OP is enquiring about the use of an existing pipe, not a theoretical selection of a suitable componet for the nozzle design. It is obvious that you won't ignore the actual pipe thickness for only reason of pleasing a vague re-interpretation of the formal Int.VIII-1-86-11 or Int VIII-1-98-164. By the way, the VIII-1-86-11 is also irrelevant to the fact of using the existing thickness of the pipe for the As Built documentation.
The Code is not intended for blind use of it's letters, but be used wisely through proper engineering judgement.
Yes, you have to be sure that the thickness used is the actual one, measured in sufficient places to give you the confidence that the nozzle won't fail. Otherwise you'll be a correctly reciting parrot, not engineer.
Cheers,
gr2vessels

 

[roca]

I agree
Again though - client or inspection authority will have final say

 

[CodeRef]

The OP has selected the existing pipe schedule in the design calculation for which the program has rejected for the correct reasons. If you were to present this package in an ASME Joint Review what would you expect to be the result? From my understanding this is new construction.
Per the last sentence in UG-16(d) .....After the minimum wall thickness is determined, it shall be increased by an amount sufficient to provide the manufacturing undertolerance allowed in the pipe or tube specification....
With all of that said, I would be willing to accept my AIA's approval, only if in writing.

FAQ731-376

 

[BNEAL]

Thanks to everyone for your well-considered responses.
CodeJackal...You cite UG-16(d) but base your position (only) on "...it shall be increased...". However, the passage goes-on to reveal it's intent with "...to provide (for) manufacturing undertolerance..." If, defacto, there is no undertolerance, then (I would contend) the "increase" is not required. As you said, however, the AI and Customer have to be "on-board".

 

[CodeRef]

BNEAL, I recognize and appreciate your (and the others) common sense position. If no undertolerance, does it still meet the paragraphs intent? ....that would be up to those above me to decide. I have to go with the literal sense of the words.
UG-16(c) for plate materials allows variables helpful in determining the paragraphs "intent"...but, this discussion is not provided in UG-16(d) I don't know why, it was a committee that voted on it. I respect your opinion nonetheless.

FAQ731-376

 

[jamesl]

There is a reason that Div.1 code is called "design by rules". The rules in the code provide minimum requirements, and engineers use engineering judgment to enforce additional requirements on materials, design, fabrication and testing. Engineers do not use engineering judgment to override code rules.

You cannot use as-tested mechanical properties to decide allowable stresses; you cannot use fracture mechanics when the material does not pass impact test, you cannot use FEA when nozzle don't have enough area reinforcement... I believe the code committee set up the rules for a good reason-quality control. If everything is up to engineering judgment, then the code will not have mandatory rules, but all guidelines or recommended practices.

 

[rneill]

Further to the discussion of engineering judgement, this is a controversial area and the code committee has attempted to address it with some comments in the forward to ASME VIII Div 1 where they say ...

"The Code is not a handbook and cannot replace education, experience and the use of engineering judgment. The phrase engineering judgment refers to technical judgements made by knowledgeable designers experienced in the application of the Code. Engineering judgments must be consistent with Code philosophy and such judgements must never be used to overrule mandatory requirements or specific prohibitions of the Code."

I would concur with jamesl and my interpretation of this is that where the code specifies a requirement, it must at a minimum be complied with. However, where the code is silent, or where a designer might want to go beyond the minimum code requirement, they can use engineering judgement to determine the requirements consistent with the basic code philosophies.

In that regard, short of submitting a formal request for interpretation, I believe we are bound to follow specific rules or instructions provided in the Code.