Hi Ken,
First John will do although I admit to being a cranky 54 year old guy....
The components which your analysis shows are thermally overstressed I assume are elbows, tees, etc.
One way to inspect for fatigue damage is UT looking for microcracking remember fatigue failures usually are leak before fail modes. Another way of evaluating fatigue damage might be abnormally thin sections where localized yielding has significantly thinned the metal.
Before you elect to trash it also a fatigue evaluation if you can get the field data would be good... I would cut off such that the lowest number of cycles would be N= 100. If you notice in the 2004 Edition (302.3.5) for ferritics we gave people an increase of 1.2 to their allowed stresses at N=1000. Which brings another point to my mind what your doing no longer really falls under B31.3 fitness for service in-situ surveys of existing systems is not B31.3 stuff.
Finally if its just carbon or low alloy stuff... just redesign the system. Show the client the magnitude of changes required and he /she will probably be glad to swap things out..... and don't forget dead legs as possible moment reversals set up your load cases to capture the stress range.
Phew lots of stuff....
If you want a sanity discussion you can call me....
216-521-2182 office
or
330-204-4739 mobile
Ken Nisly-Nagele <knislynagele@applied-e-s.com> wrote: John -
Thank you for replying and in particular for pointing out the added uncertainty at N below 100. In looking at some of Markl's (and associates) papers I see that N=110 and N=500 were the lower limits of some of their work.
In reading up on API RP 579 it appears it address primarily the structural integrity of piping and PVs containing a flaw (metal loss, weld misalignment/distortions, cracks). The $600 question is: Is it Appendix B that covers a stress analysis overview that you were thinking would be particularly helpful?
Due to the extent of piping calculated as overstressed, and the fact that I can't put together conceptually how it would be acceptable to introduce new strains on the piping, I'm thinking either to replace the piping or get some help with this one. Thankfully, I think the client is beyond the "...it must be okay as it has worked for 10 years" thinking.
Ken A. Nisly-Nagele, P.E.
Project Engineer, Mechanical
Applied Engineering Services, Inc.
7999 Knue Road
Indianapolis, IN 46250
317-585-8920
-----Original Message-----
From: PipingDesign@yahoogroups.com [mailto:PipingDesign@yahoogroups.com]
On Behalf Of John Luf
Sent: Friday, February 24, 2006 8:23 PM
To: PipingDesign@yahoogroups.com
Subject: Re: [PipingDesign] Modifications to B31.3 Piping w/ Overstress
This level of overstress is good cause for concern. A full fatigue evaluation would be good but as pointed out getting sound meaningfull data is difficult at times.
B31.3 & .1 uses a base load of cycles of 7000. This is a very high figure for most systems and we have speculated that many a marginal design / installation has been saved because the N the system actually sees is lower.
However low cycles at high stresses gets tough because the scatter of the data can get you in trouble so many people try not to go N=100.
If you have the $$$$ API 579 is helpful.... you may want to drag some API 579 experts into the fray.
The problem with modifications at this point is the modifications will reset the clock on the thermal strain energy in the system and if things had settled down they will have to undergo a new shakedown.
Or if you get the old "Its worked OK for 10 years it must be OK" you make your recomendations and walk away....
Best Regards,
John C. Luf
Cleveland Ohio U.S.A when I'm not in Austria
Member B31.3, Piping Engineer - Stress Analysis, Pipe Supports, Component Design, and Surge Analysis... and according to my daughters master of unimportant trivia
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Best Regards,
John C. Luf
Cleveland Ohio U.S.A when I'm not in Austria
Member B31.3, Piping Engineer - Stress Analysis, Pipe Supports, Component Design, and Surge Analysis... and according to my daughters master of unimportant trivia
[Non-text portions of this message have been removed] Received on Mon Feb 27 20:24:00 2006
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