Elastic follow-up in high temperature piping.
Elastic follow-up in high temperature piping.
(OP)
I am interested in obtaining a design method for analysing the problem of "elastic follow-up" in high temperature piping. An elastic analysis shows the thermal displacement stresses and sustained stresses to be well within the B31.3 allowable. However, cracking has occured in certain regions and metallugical examination does not indicate evidence of the usual suspects.
Is there a simplified approach that does not involve non-linear fe analysis.
Is there a simplified approach that does not involve non-linear fe analysis.





RE: Elastic follow-up in high temperature piping.
Where are the cracks located? Could it be due to poor PWHT? (or lack of it) Also maybe the SCF's are the cause!!. There are a number of reasons why cracks could be occuring. What's the material? Temperature?
RE: Elastic follow-up in high temperature piping.
RE: Elastic follow-up in high temperature piping.
1 1/4 Cr 1/2 Mo material has become famous for creep-fatigue interaction. That is why the allowable stress was reduced some years ago. Cracking usually occurs at branch connections. 900+ degrees F is a lot of temperature for this stuff - probably there will be some carbon migration to the grain boundaries and resulting embrittlement. Use A-335-P22 next time.
I think Dr. Charles Becht covers the elastic follo-up issue in his book but I do not have it here in Virginia to check.
Regards, John.
RE: Elastic follow-up in high temperature piping.
rmw
RE: Elastic follow-up in high temperature piping.
Also, the P11 header is likely a lower alloy than the tubes, so there may be a dissimilar metal weld at the tube to header weld ( bad design if true).
Some headers have a geometry such that all tube to header welds occur on one side of the header; this implies the header will bow or hump during startups and cause excessive loadings at some parts of the header. If the loading is greater than yield ( locally) there will be generated residual stresses which are then creep-relieved at steady elevated temperatures .
Several design methods to avoid these problems are :
a) stub to header weld to be of same alloy
b) stub to hdr welds to be aligned on 2 opposite rows for symetrical heating of header, or else support header such that bowing or warping will not result in increased stresses. Examples may be to have the stubs weld into the side centerline since horiziontal warping can be easily accomodated by vertical support straps, or else use constant load spring supports.
c) use a higher grade alloy which has much higher allwoabel stresses at elevated temperaures such that secondary stresss do not significantly contribute to creep damage.
RE: Elastic follow-up in high temperature piping.
Thankyou gentlemen for the information.
RE: Elastic follow-up in high temperature piping.
Run the analysis, and then rerun it with the local area you think is subject to elastic followup with a reduced value of elastic modulus. This roughly simulate accumulation of creep strain. Then look at the change in stress relative to the change in strain. If there is no change in stress, it is 100% elastic followup. If the change in stress reduced in accordance with the simulated creep strain increase, then there is no elastic followup. The simulated creep strain is the the stress divided by the difference in the elastic moduli used in the first and second analysis. The analysis can be repeated with various values of elastic modulus.
RE: Elastic follow-up in high temperature piping.