Machining Tolerances and Tubular MAWP, UG-16? 1

[croy]

Normally the machined tubular parts we use have +/- .003" on all dimensions. To calculate MAWP per B31.3, B31.1, etc. we use the minimum material (wall thickness) condition, given the machining tolerances. Although it is not really relevant to the discussion, UG-16 of Section VIII allows the lesser of .01" or 6% undertolerance on plate while allowing full ordered thickness for MAWP. Is there any Code Case or Interpretation that will allow me a larger undertolerance so I can just use the nominal dimensions of precision machined tubular parts for MAWP calculation?

 

[SnTMan]

Three thousandths makes a practical difference?

Regards,

Mike

 

[croy]

Normally this would not make a difference. However, sometimes we make machined tube to tube adapters where one end could be .875 OD X .049 wall and then it does make a difference. 6% of the wall thickness in this case is only .0029. With OD/ID machining tolerances and concentricity issues, this does make a difference. It would be great to have some sort of "official" interpretation that would put this matter to rest and be something to point to when customers pose the question to us, which they have.

 

[JohnBreen]

Croy,

We (ASME B31.3 Code Committee) will be meeting in Puerto Rico next week. You could write an inquiry (see B31.3, Appendix Z) and send it to our secretary, Joe Wendler, PE (WendlerJ@asme.org) and ask him to present it to the Committee for consideration. That might result in an interpretation that you could use.

Regards, John.

 

[croy]

John,

I was thinking of an interpretation request such as this:

"May piping components with an undertolerance of not more than the smaller value of 0.01 in. or 6% of the required design thickness be used at the full design pressure for the specified design thickness?"

This borrows heavily off Sec. VIII UG-16(c). This problem actually comes up more often than you might think.

I would greatly appreciate it if you could provide feedback regarding this request and any tips on how to effectively present it.

Thanks,

Chris