Pipe Wall Tolerance vs Design Min Wall

[qaqceng]

Hello,
Question: We're designing an ASME Section I boiler component (replacement part for existing unit), and one of the drain pipes has a calculated min wall of 0.811". It's ASME SA335 P91 Type 2 we need, and the only pipe we can find in stock is 6 inch Sch XXS (0.864" nominal thickness). With the 12.5% under tolerance from the SA335 spec, it could be 0.756" thick which is below the calculated min wall.

Is there any basis in ASME Section I or II whereby you could measure the actual thickness to confirm it's above 0.811" and comply with all the requirements of the Code?

Thanks in advance for any advice!

 

[EdStainless]

You can specify it as MinWall, but good luck finding it.
Were you planning on having pieced UT scanned to assure that there is no location less than 0.811"?
Pipe is notorious for having 15% wall variation in each piece (or even just side to side).
I guess that you could write the spec as 0.811" MW, but in the future people will curse you.

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P.E. Metallurgy, consulting work welcomed

 

[qaqceng]

Haha, yeah that's what I figured and what I was afraid of. I thought about trying to order it as min. wall pipe like you said but it's an availability issue.

To check the actual thickness, we thought of taking UT readings at the 0, 90, 180, 270 position, and every 2 feet or so down the length (or something similar). A full body scan would be best, but would probably involve shipping it to another sub somewhere. The distributor's warehouse and the equipment fabricator would not have that capability.

Appreciate the help!

 

[EdStainless]

Just about anyone who can do a conventional UT could set sensors to 90deg and do a "Z" 100% wall thickness scan. Might be worth it.

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P.E. Metallurgy, consulting work welcomed

 

[KevinNZ]

0.811-0.756 is 0.055". (1.4mm) Do you have a corrosion allowance? how does that compare? The max mil tolerance may only reduce the time to use up the corrosion allowance by a small amount.

 

[qaqceng]

We've got zero corrosion allowance.

 

[LittleInch]

To get back to the original question, I don't think there is anything specific for this rather odd situation.

All the code needs you to do is have a minimum wall thickness. It doesn't care how you satisfy yourself that that min wall is achieved.

The easy way is to take the spec driven tolerance and job done.

The harder way is the way you want to do it.

How long is this piece of pipe?

I would do 6 or 8 UT points at no more than 300mm intervals and probably 250mm or less to be able to have sufficient confidence to sign it off.
Are you signing it off??

Or find some thicker bits from somewhere and fly it in.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[david339933]

PG16.5 seems to prohibit this. "the ordered material shall include provision for the allowed manufacturing undertolerance..."
There is no interpretation for PG16.5, but UG-16 is much the same:

Standard Designation: BPV Section VIII Div 1
Edition/Addenda:
Para./Fig./Table No:
Subject Description: Section VIII-1, UG-16(d), Actual Pipe Thickness
Date Issued: 09/26/1985
Record Number: BC85-287
Interpretation Number : VIII-1-86-11
Question(s) and Reply(ies):
Question (1): Does UG-16(d) allow the selection of nominal pipe size material which has an actual minimum thickness which is equal to or greater than the calculated minimum wall thickness, regardless of the fact that the undertolerances provide material which is less than that required?

Reply (1): No.

Question (2): Does UG-16(d) require that the nominal thickness less the manufacturing undertolerance be equal to or greater than the minimum thickness required?

Reply (2): Yes.

 

[LittleInch]

OK, that's clear, but what if the supplier has guaranteed that that the "allowed manufacturing undertolerance" for this particular pipe, is different to the "standard" pipe?

Or is that stretching things too far?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

We used to get pipe and tube orders with odd min wall tolerances regularly. As long as they were still in the bottom 5% of the standard range we didn't care, but these were mill quantity orders. If they were heavier than that we would usually treat them as minwall unless there was some other reason not to.

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P.E. Metallurgy, consulting work welcomed

 

[weldstan]

Whereas this is drain pipe, can the pressure conditions as originally specified be changed, with owner approval, to permit use of thinner replacement pipe?