Mill Tolerance 1

[AdilJassem]

Good Morning everyone,

My question is, why do we use the 12.5% mill tolerance for seamless pipes in ASME B31.3 only while it is absent in B31.4 & B31.8 for seamless pipes ?

 

[LittleInch]

The rationale is that the manufacturing tolerance when pipes are supplied to the normative code listed (API 5L) is taken account of in the design factor in 31.4 and 31.8

Thus while all other pipeline codes and many piping ones as well add the tolerance to the calculated wall thickness, in some cases they allow a higher design factor than 32.4 or 31.8.

How this came to pass is not clear to me, but has been in existence for a long time so it has in effect passed into the realms of proven by longevity not to be a significant failure issue.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MickMc]

I would also add that mill tolerance should be as per the ASTM spec being used, typically 12.5% for seamless with e.g. A672 Welded Pipe 0.3mm.

 

[bimr]

The mill tolerance is included in the design factor (for B31.4 & B31.8), and so doesn't need to be included. The reference is a book Piping and Pipeline Engineering Design, Construction, Maintenance, Integrity, and Repair By George A. Antaki.

 

[MickMc]

bimr,

That may be true for Pipeline codes such as ASME B31.4 but mill tolerance has to be included in ASME B31.3.

 

[bimr]

The question: why is mill tolerance absent in B31.4 & B31.8 for seamless pipes.

The answer: The mill tolerance is included in the design factor and so doesn't need to be included

 

[BigInch]

Why the difference?
Ans: B31.3, 4 & 8 codes were each written by different engineering committes. B31.4 and 8 committes believed in keeping things as simple as possible.