specifying mill tolerance
specifying mill tolerance
(OP)
I've read a few posts in this forum that in the manufacturing of seamless pipe that one side of the pipe will be thin, while the opposite side will be thick (w/ respect to nominal wt). I understand that this thickness variation will happen, but is it true with respect to nominal? That implies that there will always be under tolerance except where the thickness is exactly nominal. Is that the case, or is it possible to find pipe that is say from nominal to +12.5% (i.e. no under tolerance)?
I ask because it has been suggested that we order our pipe in this manner (i.e. min wall = nominal) for the purpose of getting extra life in operation. My concernw are 1) added cost, although this may be made up for in extra life and 2) availability. Is it readily available or will we be limiting ourselves?
I ask because it has been suggested that we order our pipe in this manner (i.e. min wall = nominal) for the purpose of getting extra life in operation. My concernw are 1) added cost, although this may be made up for in extra life and 2) availability. Is it readily available or will we be limiting ourselves?





RE: specifying mill tolerance
2) Talk to your current seamless pipe vender. The 12.5% undertolerance has been the same value since the 1950's. You can ask if the vendor can live to tighter tolerances. I have heard on instaces where they could easily produce to 5% underthickness tolerance with the process improvements that have occured to date (this is somewhat dependant upon the pipe size and wall thickness), and this involved no greater expence as they were already producing the pipe in this fasion. Remember that the undertolerance is a maximum allowance, it is in the vendors best interest to stay the hell away from this value to avoid rejected pipe.
Just my two cents worth.
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RE: specifying mill tolerance
Concur with the recommendation to get a quote from a supplier.
Note: if a manufacturer did have the ability to more closely regulate wall thickness, I suspect they would typically be targeting the thinnest wall they could while still falling within the tolerance band of the specification unless otherwise dictated by the customer. Logically, why would you want to supply more steel than you were required to ?
RE: specifying mill tolerance
RE: specifying mill tolerance
The answer to the question of "why don't the pipe manufacturers target a pipe wall thickness all around the pipe of say 10%?" lies elsewhere. They cannot save 10% of the weight by doing this simply because they are also held to a weight tolerance – presumably to avoid just this type of thinking.
Interestingly, SA-53 is more tolerant of weights while maintaining the thickness tolerance: This specification allows plus or minus 10% - so in theory a fabricator might save a bit of steel by targeting a lower weight/thickness.
All the more reason to specify -106 over -53!
Alloy and Stainless will likely refer to SA-999 for general requirements and that spec has the same +10% -3.5% weight tolerance as SA-106. However, when ordered on a minimum wall basis, the requirements change – read the spec for details.
jt
RE: specifying mill tolerance
RE: specifying mill tolerance
I think the difference of weight tolerances between SA 106 B and SA-53 is due to the fact that SA-53 could be welded or seamless. The weight tolerance for SA 999 is for seamless pipe.
RE: specifying mill tolerance
good point, I hadn't thought of the weight tolerance issue.
Also a good reminder that you should always check the applicable materials specification to confirm the wall thickness tolerance that is specified. Pipe grades such as A671 or A672 (made from plate) have significantly tighter tolerances than do the more common A106 materials. Also, line pipe grades such as API 5L material may have different tolerances as well. Specified tolerance may also vary by nominal diameter.
RE: specifying mill tolerance