piper_78
It is not just a matter of the length of the piping. The popular opinion is that there must also be at least three changes in direction (e.g., bends) between the "virtual anchor" and the point where your analysis model begins. The use of a "virtual anchor" is not codified but you can get some useful information if you 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 (note that these two documents ARE NOT CODES).
ASME B&PV Code, Section III, Nonmandatory Appendix "T" might also interest you but it does not answer your question.
John.
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John Breen