Bulging or increased in tube/pipe diameter in superheater

[replica]

Hi all ..

Is bulging or increased in tube/pipe diameter in superheater tubes operating at creep loading (temperature and pressure) always related to creep? Or is there any other possibility? Comments are highly appreciated.

 

[metengr]

Typically, yes, and it can be related to short term overheat versus long term overheat. Bulging or tube swell is permanent deformation and associated with creep deformation in service.

 

[replica]

metengr
Thank you very much for a comment. So bulging is always the manifestation of creep. From my experience while performing in situ replication on the bulged region or even burst tube , I occasionally could no find significant creep cavities or aligned cavities on the bulged region. This sometimes puzzled me because as the tube bulged or burst due to creep, there must be significant amount of cavities/isolated/aligned etc. Is it either 1) my technique is not good enough? or 2) I am doing surface replication while creep propagates from inside tube diameter as such that the cavities could be observed or 3) The cause of the bulging is not due to creep. In short term overheating is creep cavities always evidence?

 

[chicopee]

There are some excellent articles on creep in the bulletins published by the National Board.

 

[metengr]

The bulging can be by short term overheat. Here stress rupture can play a role in local deformation and final failure. The microstructure for stress rupture would be elongated grains.

 

[replica]

chicopee
Thank you for posting
metengr
So short term overheating can cause bulging and eventually fails by stress rupture. It can happen at low stress (API RP571) and evidence by elongated grains as we always see in fish mouth like appearance on the burst tube. The question, is stress rupture or short term overheating creep or they are two different mechanism? If so is it possible that the reason I could not see the creep cavities in the bulged/burst region is because the bulging is due to short term overheating ?. Sorry if I may look dazed and confused here....correct me if i am wrong.

 

[metengr]

Stress rupture is the result of exceeding the elevated temperature tensile strength of the material. Typically no creep voids are seen but it is a subset of creep deformation -Deformation under load at temperature.

 

[replica]

metengr....well explained...thank you very much

 

[racookpe1978]

The same amount of metal makes up the circumference before and after the pipe creeps to a larger diameter?

So, if I understand Meteng properly, he is saying that the pipe creeped (to a larger diameter) at the bulge and thus it has a smaller wall thickness at the bulged point.

The smaller wall thickness means the stress (pressure/cross-section area of wall) in the wall is higher at the bulged pipe, and so - if temperatures or pressures remain high - the bulged section will see higher pipe wall stress, creep more, thin the wall more at the bulge, and potentially turn into a failure "by creep" at the bulge.