The following
article is some of my research results of studying pipeline to be a pipeline
engineer :). of course i would list the sources of those articles so you can
check and we could discuss or learn together about this field of study.
“Where does mill tolerance come
from?” Does the mill tolerance affects weight, Fy and sustained stress for
B31.3?”, “What should be taken as a tolerance on plate? “, “What is the
significance of “-12.5% Mill Tolerance”. “What if I check “all case corroded”
in Caesar?”. These are samples of questions about mill tolerance found on
forums.
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| Seamless Pipe Production Process |
First answers
are contained in the above picture, from www.tmk-group.com,
because the best way to understand mill tolerance is to understand where it
comes from. Steel pipes are produced by two main manifacturing methods.
Basically welded pipes are produced by joining together two ends of steel
strips. Seamless pipes are produced by piercing a billet (step 8 of above
picture).
Mill tolerance comes from the
manufacturing process of seamless pipe. Welded pipes have a tolerance as well,
but usually is not a mill tolerance and it is much lower because it’s more easy
to get high accurancy when producing steel plates. On the other hand welded
pipes need to consider the weld joint efficiency factor, to take in account the
weakness in the welding.
The B31 Codes include the 12.5 percent wall thickness
variation in their calculation for minimum required wall thickness for SEAMLESS
pipe. This requirement adjusts the calculation to account for the
method of seamless pipe manufacture. When seamless pipe is made, a
mandrel is pushed through a hot billet (or "bloom") of metal to
create the "hole" that is the inside diameter. In this
process the mandrel may "wander" (slightly off-course) as it is
pushed through the billet. The result of this slight side-to-side
movement of the mandrel is that the pipe wall may be 12.5 percent thicker in
the wall on one side and 12.5 percent thinner in the wall directly across the
diameter. To be sure that the thinnest wall permitted by this
manufacturing methodology the B31 Codes include in the "mill
tolerance" in the minimum required wall thickness calculation.
Pipe that is made of seam welded plate has much less thickness variation. However, unless the pipe's longitudinal seam weld is 100 percent radiographed (NDE), the required pipe wall thickness calculation must include the weld efficiency coefficient for the type of weld used in the seam. Of course, there is also much more "out-of-roundness" in seam welded pipe.
Pipe that is made of seam welded plate has much less thickness variation. However, unless the pipe's longitudinal seam weld is 100 percent radiographed (NDE), the required pipe wall thickness calculation must include the weld efficiency coefficient for the type of weld used in the seam. Of course, there is also much more "out-of-roundness" in seam welded pipe.
