B31.8 and Seamless Mill Tolerance
B31.8 and Seamless Mill Tolerance
(OP)
Hi guys. For a B31.8 line that uses semaless pipe, do I add the mill tolerance into the calculation for wall thickness?
I think I know the answer, but I want to hear what you all think.
Thanks guys! Pete
I think I know the answer, but I want to hear what you all think.
Thanks guys! Pete





RE: B31.8 and Seamless Mill Tolerance
§804.5 Dimensional Tolerances and Definitions
nominal wall thickness, t: the wall thickness computed... (blah blah) Under this Code, pipe may be ordered to this computed wall thickness without adding allowance to compensate for the underthickness tolerance permitted in approved specifications.
To me, that says I do not have to add the 12.5% mill tolerance. What say ye?
RE: B31.8 and Seamless Mill Tolerance
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
All answers are personal opinions only and are in no way connected with any employer.
RE: B31.8 and Seamless Mill Tolerance
Example for calculating allowable Pressure based on maximum permitted hoop stress, (assuming no temperature or joint factor reduction)
P = 2 * wt * SMYS * DF / OD, for a DF area of 0.72
P = 2 * 0.375" * 60,000 psi * 0.72 / 12.75" = 2541 psig
You can ask the pipe mills to produce pipe to criteria other than what is stated in the standard (API 5L, or other) specs that might include asking for a perticular custom tolerance, but be prepared to pay for it.
Independent events are seldomly independent.
RE: B31.8 and Seamless Mill Tolerance
What would be missed - I think! - is corrosion allowance, right?
That is, your calculation of "required wall thickness" must include the corrosion allowance (if any) applicable to your given plant and circumstances BEFORE you go look up the available seamless pipe wall tables.
RE: B31.8 and Seamless Mill Tolerance
the vendor can sell standard wall pipe that is .328" thick and still call it standard wall pipe, but it will not meet your requirement.
you can specify min wall pipe, but that brings with it other aspects that are not all obvious too.
RE: B31.8 and Seamless Mill Tolerance
If you arrived at a 0.360 wall thickness, you may order any standard wall with a thickness greater than that, ie. 0.375" and use it as if it was 0.375 The wall thickness tolerance would permit some wall thickness measurements to be as low as 0.328, however because of the additional WEIGHT TOLERANCE (typically +10 to -3.5%, in some cases even less, SEE API 5L TABLE 10) it would not allow the wall thickness of the whole pipe to be furnished at 0.328". A weight tolerance of -3.5% applied to a 12.75" OD x 0.375" wall thickness pipe would only allow the average pipe wall thickness measurement to drop to as low as 0.361", not nearly as low as your 0.328"
Independent events are seldomly independent.
RE: B31.8 and Seamless Mill Tolerance