Allowable Stress during PWHT (after repair) 1

[Guest102023]

I am calculating the crane force needed to permit safe 360° PWHT of a medium height, thin-walled vertical steel vessel. I can do the calcs (wind, dead weight), but I am having trouble finding an allowable stress for A285-C material at 1100°F, since that is beyond its permitted max. temp in section II-D.

And are there any other loads I need to consider?

 

[metengr]

brimstoner;
The concern is permanent buckling while at PWHT temperature for a vertical vessel. I would suggest using a percentage of the YS at temperature as a means to evaluate buckling forces. For similar material, the data I have seen at 1100 deg F indicates a knock down factor of 0.4 of RT yield strength.

 

[Guest102023]

metengr,

I'm not sure how to interpret 'knock down factor'. Does this mean I use an allowable of 0.4 x RT YS? Sounds a bit high, but I will consult my EPRI handbook for carbon steels ...

There is no doubt the vessel will be crane-supported, perhaps by two. Is wind still a consideration once you have decided on crane support? I have not yet compared the significance of wind-induced stresses to that of vessel dead weight.

Thanks for helping at this late hour.

 

[TGS4]

This is a short-term creep problem - specially creep-buckling. There is published creep data at typical PWHT temperatures for most materials. Develop a isochronous stress-strain curve (based on the duration of the PWHT), and then you can base an "allowable" on that.

 

[Guest102023]

TGS4,
Do you have that 1974 paper by Griffin?
My research has led me to API 579, Annex F. If I understand it correctly, allowables are calculated for external pressure in the creep range, and extrapolated into the PWHT range for that short-term situation.

 

[TGS4]

I may back in my office. I am currently attending Boiler and Pressure vessel Code Week in Boston. I can also ask fellow Code Committee members if they have that paper.

You're definitely on the right track.

 

[metengr]

TGS4;
I am also attending B&PV Code week in Boston. Perhaps I will see you at the hotel bar......

 

[metengr]

brimstoner;
You might find this paper interesting;

http://www.ultragen.com/doc/tech-articles/en/Article1_V3N2_JI_IRI_PWHT.pdf

 

[Guest102023]

Thanks gents. The paper describes my situation very well. I am doing this without software.

I'm still trying to come to grips with the concept of creep buckling; wouldn't that depend very much on the geometry as well as temperature and stress? For example, I expect a longer heated band would increase vulnerability to compressive buckling of the shell.

p.s., while in Boston please don't add any more non-essential variables to Section IX

 

[Duwe6]

AMEN to no more variables being added to ASME IX.