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B31.8 Appendix N5 Clarification

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mjc0303

Mechanical
Joined
Feb 18, 2016
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I have a situation where an operator wants to use Appendix N5 of B31.8 to verify the minimum yield strength of an existing DOT-regulated line for which there is no documentation. They have a target value in mind and would like to hydro test to an appropriate value to prove that the pipeline remains in the elastic range during pressurization (i.e., Pressure vs. Volume remains linear). The way the Appendix is written, it seems to require pressurizing to the onset of yield as detemined by the deviation of P vs. V from the originally established linear relationship. Is there a specific reason that the code requires pressurization to yield? Testing to a lower stress level and confirming linearity of P-V would seem to me to be conservative. At worst, one could argue the operator is not going to be getting "the most" out of their line because they are using a lower yield strength in their calculations. Am I missing something? Thanks in advance for your help on this.
 
I don't read section N-5 it that way.

It doesn't "require" the pressure to go to yield, but it defines what "yield" is, i.e. double the number of stokes per unit increase in pressure from the elastic phase.

Therefore if your operator has guessed conservatively and you get to your test pressure and it's all elastic, then simply stop there.

Only if the operator is actually trying to find "yield" or certify his pipe for a higher MAOP do you need to go to the point where you are getting double strokes for every increase in pressure.



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Also: If you get a response it's polite to respond to it.
 
Thanks for the reply. I just wanted to make sure I wasn’t missing something.
 
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