Reconciliation of dimensional variance from design

Hello all,

One of the frequently asked (and not always completely answered) questions in this forum concerns how much variation from the designed (and analyzed) system is allowed in the "as built" system. It would be good if a recognized agency would make a statement on how much variation from the design is allowed.

My answer has always be to reference the WRC document "Technical Position on Piping Installation Tolerances", Welding Research Council Bulletin 316, July 1986 and also the EPRI document "Guidance for Piping System Reconciliation", NCIG-05, Electric Power Research Institute, EPRI-5639, May 1988. I think one of these documents even has guidance on how many changes in direction there must be between existing piping and the system you analyze to assure that you can "declare" a "virtual anchor".

A colleague pointed out to me today that some of the information from one of these two documents has made it into the ASME B&PV Code and resides in Section III, Nonmandatory Appendix T, Article T-1000, "Recommended Tolerances for Reconciliation of Piping Systems". Sadly there is no mention of "virtual anchors".

I hope this information is useful to someone.

Regards, John.

Dear Breen,

Can we say that virtual anchor is quite close with "point of no movement" like mentioned in book "Introduction to Pipe stress Analysis" by Sam Kannapan Chapter IX - Special topic for buried pipeline? I mean at least according to your own opinion.

Samsul,

Ahhhh, buried pipelines. I was thinking of above ground piping systems in my mentions of "virtual anchors". Well, of course if you have a few changes in direction in a buried system it is a little easier to make "educated assumptions" about locating points of zero movement. In long lengths of straight pipe there are more uncertainties. Too many variables to be certain of very much. The friction of the piping interface with the soil is problematical at best (assumptions about soil compaction are no better). The best you can do is try to learn something about the mechanics of the soil-to-pipe interface by observation of the movements of existing systems at the same site. Then it becomes a matter of engineering judgment.

That answer does not please me but I must say if there is a better answer it has evaded me for 45 years. Sorry.

Regards, John