ASME B31.3 302.2.4 Temperature Variation and piping flexibility

[DBreyer]

We have an existing pipe for which a new, rare set of process conditions was identified, Exceeding design temperature in the case of power outage (well below the creep range).



For the circumferential stresses we can assess this with the provisions in 302.2.4 as a temperature variation.

Sustained longitudinal stresses also shouldn’t be significantly changed, as this would only be the case if the pipe lifts of supports.

Now we ask ourselves, how to we assess the increased elongation due to this and the resultant secondary stresses? 302.2.4 only mentions primary stresses and makes no mention of stress ranges and paragraph 319 makes no mention of temperature variations.

My take on it would be, since the flexibility assessment is based on a fatigue analyses, this is covered by the provision in 302.2.4 that no more than 1000 load cycles are allowed for the variation.

This significant reduction from the typical 7000 cycles would also allow a higher allowable stress range.

Since the higher temperature will also only result in secondary stresses, which are self limiting, some yielding is expected but no catastrophical failure.



Do you agree or would you see the need for the temperature exceedance to be assessed as the new design temperature (including flexibility analysis)?

 

[RVAmeche]

I would say the proper thing to do, especially if this is a newer system where you have the original pipe stress analysis, is to update the pipe stress analysis and confirm its okay, support loads are still within acceptable ranges, etc. I would guess you're realistically correct that a very rare event probably won't cause a massive failure, but its hard to say in this hypothetical. If its a 40 year old system and you're going from 400F to 900F that may break some things.

If this is an older system that was only designed with flexibility analysis and not pipe stress, I think it'd be prudent to develop an analysis.