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ASME B31.3 Mill Tolerance

ASME B31.3 Mill Tolerance

Is there any way to avoid using the full 12.5% mill tolerance in pipe wall thickness calculations and still be ASME B31.3 code compliant?

Can the wall thickness be measured and applied in the calculations without the mill tolerance? Or with a reduced mill tolerance?

I have an existing pipe spool in which the 12.5% mill tolerance is just causing my wall thickness to be inadequate. Reducing down to 8% mill tolerance would allow my wall thickness to pass.

Is there any case where you can reduce the mill tolerance if the exact wall thickness is known / verified?

Thank you in advance.

RE: ASME B31.3 Mill Tolerance

Quote (engpes)

Reducing down to 8% mill tolerance would allow my wall thickness to pass.

I don't understand that part. What do you mean by reducing down to 8%?

RE: ASME B31.3 Mill Tolerance

Can you apply a tolerance Leas than 12.5%?

RE: ASME B31.3 Mill Tolerance

Maybe I'm missing something...let me give you an example of how I understand this:

6" STD Pipe
= 0.280" wall * 12.5% = 0.245" minimum wall thickness required.
= 0.280" wall * 8% = 0.257" minimum wall thickness required.

RE: ASME B31.3 Mill Tolerance

So your design requires 0.257" min wall?
Then you would need to order material that meets that, can you do it?
There are some sources of min wall pipe, that would be 0.280" min wall.
Most pipe specs let you invoke that option, though finding such material may be very difficult.
You may just want to go to sch80.

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

RE: ASME B31.3 Mill Tolerance

I think when you say required, you mean available right?

Long story short, my pipe thickness is not enough after subtracting the 12.5% mill tolerance. If I only subtract 8%, my pipe wall is adequate.

Does ASME B31.3 allow for any mill tolerances other than 12.5%?

RE: ASME B31.3 Mill Tolerance

These are several existing spools than I cannot change. If I can verify my existing spools are adequate, than this would save quite a bit of time and money.

Is there anyway to verify the pipe wall with UT thickness gauge and apply these thicknesses while still being code compliant?

RE: ASME B31.3 Mill Tolerance

No, I dong think you're allowed to do that when you use 'standard' pipe acc. an ASTM standard which has a specified mill tolerance, like A312 or A106. I believe there's an interpretation for that. But maybe someone can back me up on that, I'm on holiday so don't have any papers with me.

The only option you have is what EdS mentioned. DVWE's reply is incorrect, thinking the wrong way around.

RE: ASME B31.3 Mill Tolerance

When you say "existing pipe spools", that means they are built, correct?

Then yes, you can verify the actual wall thickness of the spool(s) with UTT and as long as they are not less than the minimum required wall thickness (after subtracting 12.5%) they are fine.

RE: ASME B31.3 Mill Tolerance

When it's existing, does it still fall (or can it be put) under the scope of B31.3, which is for new construction only?

RE: ASME B31.3 Mill Tolerance

Yes these spools are built, but the nominal wall thicknesses are less than what I need after subtracting the 12.5%. The actual thickness that we measured with UT is sufficient because we did not subtract the 12.5%.

If we subtract the 12.5%, we do not meet the thickness requirements in B31.3 equation 304.1.2.

RE: ASME B31.3 Mill Tolerance

So you have done 100% wall thickness inspections?
I am not sure that you can get away with saying "we bought X, and it came in above the minimum so it works", that is just luck.
You would have to make sure that all of the drawings for the unit make clear that these are not regular sch40 pipe and cannot be replaced with sch40 pipe.
I think that you need to show that you bought material that meets the requirements, and you clearly did not.

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

RE: ASME B31.3 Mill Tolerance

You need to review 304.1.1 and ensure that T, t, and tm are all adequate.

If they are not, then I agree with Ed.

RE: ASME B31.3 Mill Tolerance

I think you can but you need a good report on actual thickness and then use this to create a design basis for your system where you specify the thickness in terms of mm rather than a percent.

It only works for that pipe or set of pipes.

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

RE: ASME B31.3 Mill Tolerance

Better pull the original paper on the spools, see what's going on...



The problem with sloppy work is that the supply FAR EXCEEDS the demand

RE: ASME B31.3 Mill Tolerance

I agree with a number of posters. You should be able to use the pipe you have if you carry out 100% thickness measurement.

But your drawings will have to be updated so when the spools are replaced in the future the pipe used has the same checks.

The 12% is there to allow for seamless pipe to have the bore/hole off centre. If was 12.% under everywhere it would not meet weight/length tolerance.

We have for projects specified pipe with 5% tolerance, which is easy for pipe made from plate.

RE: ASME B31.3 Mill Tolerance

12,5% mill under tolerance for wall thickness of pipes is only applied during original design calculation, not during further analysis after construction.


RE: ASME B31.3 Mill Tolerance

Instead of focusing on the mill tolerance, you may reconsider the corrosion allowance in your piping system, which may help to reduce the pipe wall thickness requirement.

RE: ASME B31.3 Mill Tolerance

If the pipe spools are existing then get the thickness measured and use this as the basis of your assessment. Adopting the Code mill tolerance is only applicable before you actually know what the thickness is (before you have the pipe).

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