Mill tolerance in Seamless Pipe

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.

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.

sources :
http://www.vannella.com/tolerance-mill-tolerance-corrosion-and-piping-stress-analysis/

https://www.eng-tips.com/viewthread.cfm?qid=157136