31 May 02 11:36
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.