In-Situ L:ocal PWHT of Column 1

[thermmech]

We want to repair by a weld build-up a 2 meter bend of a column that has suffered a loss of wall due to corrosion. The bend is in the lower third of the column. Column info:
SA-516-65N
OD = 3m
L = 22 m
column is supporting another vessel above
design pressure is low, ext pressure is governing
Twall = 5/8"
PWHT is required (service, not thickness)

We would like to build-up by welding the thickness of the wall of the column so we can run safely until the next shutdown. We need to PWHT in-situ a bend of about 2m at T = 621°C.

The loads during in-situ PWHT would be strictly mechanical (compressive due to weight and bending due to wind).

We have rationalized the allowable loads as follows:
Sall = {Sy from Table Y-1 (ASME II-D) extrapolated to 1150°F } / {SF = 4.0} = 17 ksi / 4 = 4,200 psi
The calculated stresses are Sm+Sb = 2,500 psi

Have we missed anything?
Do we have to evaluate for creep per API 579? (heat exposure = 2h max.)
Is it a good practice to PWHT while component is under a 2,500 psi stress ?

Any comments would be appreciated.

 

[metengr]

thermmech;
Based on your stated approach above I see no cause for immediate concern. I would re-check the allowables against creep deformation. If the time dependent allowables are well below your calculated dead weight stresses, this should be acceptable for the rather short duration for local PWHT. Your real time at PWHT temperature should be less than 1 hour excluding heating and cooling rates.

I would perform a wet fluorescent MT of the PWHT bend surfaces to ensure no problems after PWHT.

 

[EdStainless]

Would it be easier to install a helper plate?
If there is sound material toward at least one end then you could do this with much less welding (and much less PWHT)

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[metengr]

Not permitted by code. You either need to weld build-up or flush patch.

 

[thermmech]

Gents, thanks very much on your comments.

In the meantime I have gone through API 579 Level 1 assessment: Column would be OK @ calc'd stress level and 1200°F close to 250h (STEP 4) and the total accumulated damage Dc = 0.001 << 0.25 max.

Thanks again as this helps a lot to my comfort level when facing issues for the first time.