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B31.3 Req'd Thickness

B31.3 Req'd Thickness

I would like some help clarifying the required thickness calculation in ASME B31.3.  Someone pointed out to me that the Pipe Undertolerance (12.5%) must be added to the required thickness calculation in B31.3 .  After reading B31.3 para 304.1.1 (a), I think that the code does require this undertolerance be added.  Can someone please verify if this is true?



RE: B31.3 Req'd Thickness

Hello Chris,

When we are determining the required pipe wall thickness for pressure, we refer to B31.3, paragraph 304.  If you look at paragraph 304.1.1, you will see equation (2) is given as tm = t + c.  In 304.1.1(b) we see:

tm = minimum required thickness

tm = minimum required thickness, INCLUDING MECHANICAL, corrosion, and erosion allowances.

t = pressure design thickness, as calculated in accordance with paragraph 304.1.2 for internal pressure or as determined in accordance with paragraph 304.1.3 for external pressure.

The (lengthy) definition for "c" also includes mention of "mechanical allowances for threading or grooving". This is not the same as the mention of "mechanical allowances" in the definition of "tm".

I believe that in the mention of mechanical allowances under the definition of "tm" the Committee intends to include the mill tolerances or "permissible thickness variations" that ae included in the ASTM pipe specifications.  When you look at the ASTM specifications for seamless and  seam welded pipe you find permissible thickness variations of plus or minus 12.5 percent for seamless pipe and plus or minus 0.01 inch for rolled plates that would be used for making seam welded pipe. Of course it is the minus variation that must be added to the calculated required thickness.

So, we add 12.5 percent to the required wall thickness for mill "under-tolerance" when computing "tm" and for seam welded pipe we add 0.01 inch when computing "tm".  Then we all more thickness for "c".  Then we have to "round-up" to the nearest largest commercial pipe wall thickness (schedule).

Note that in paragraph 319.3.5, B31.3 says "NOMINAL THICKNESSES, and outside diameters of pipe and fittings SHALL be used in flexibility calculateion".  So this is different from the wall thickness calculation.

This is all covered in the CASTI book, "Practical guide to ASME B31.3", by Woods and Baguley.

All the above is just my opinion.

Best regards, John.  

RE: B31.3 Req'd Thickness

Thanks John!

I've debated on whether the mill tolerance is what is being referred to under the tm definition where it indicates "Mechanical Allowances".  My confusion stems from the following statement in B31.3:

Para 304.1.1 (a)
"The minimum thickness T for the pipe selected, considering manufacturer's minus tolerance, shall not be less than tm."

Based upon your interpretation, it looks like you also believe the mill undertolerance is added to the min thickness req'd for pressure.

Thanks Again,


RE: B31.3 Req'd Thickness

Hi Chris,

As they say in Texas, "If it were a snake, it woulda bit me".

I should have quoted directly from B31.3, Paragraph 304.1.1 General:

(a) The required thickness of straight sections of pipe shall be determined in accordance with Equation (2):

tm = t + c

The minimum thickness T for the pipe selected, considering the MANUFACTURER'S MINUS TOLERANCE, shall be not less than tm.  (upper case letters are mine).

The manufacturer's minus tolerance is the reference to the the mill tolerances or "permissible thickness variations" in the ASTM specification.  So, there it is.

Oh by the way, I did not provide you with an interpretation.  Only the full B31.3 Committee can do that and it would have to go up through the approval process to the B31 Main Committee. I can only voice my opinion which is no better than the next guy's.  Glad it helped.

Best regards, John.

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