Vertical Vessel Overlay Patch

[280474]

Dear All,
I have a corroded skirt problem at my vertical vessel.It looks like window patching is not applicable since the corroded parts are almost along the weld joint between skirt and vessel. So, I try to install overlay patch with 60cm of wide at along circumferencial of the vessel. The overlay patch will be same thickness with existing skirt and fillet welded to the vessel shell (upper part) and to the skirt shell (lower part). The size of the weld will be similar with the existing weld size between skirt and vessel.
The height of vessel is 30m, diameter = 1540mm and skirt height is 3m.
Is the installed overlay patch adequate to compensate the corroded parts..?

 

[metengr]

If the vessel is under the National Board Inspection Code (NBIC), a "patch plate" repair is not permitted. The main reason is that fillet welds used for establishing the new pressure boundary have poor fatigue strength in comparison to full penetration weld joints. As you stated, a flush patch or window repair is acceptable.

Have you considered weld overlay? I would evaluate using a weld overlay to restore the vessel wall thickness.

 

[jte]

289474-

Just want to verify that the repair is in the skirt, that is, a non-pressure retaining chunk of steel. If so, you do not fall under the NBIC.

I would not allow your proposed repair at my facility if the quesiton came across my desk. As metengr stated, a weld buildup or flush patch would be preferred. For short term reinforcement I'd consider simply adding some ribs.

The concern I would have with your proposed method (a fillet welded lap patch) is that although your fillet welds are the same size as those attaching the skirt to the shell (not head, right - those would be in effect a butt weld), that weld is designed for the top of the skirt. If your skirt is governed by wind or seismic you will have higher loading at the patch area due to the higher moment.

I'm also concerned about the fact that you already have a corrosion problem which may be related to existing heat affected zones. You will not be solving the symptom, not the problem. Hopefully you will also call for proper coating of the skirt to keep it from corroding again.

jt

 

[280474]

Thanks for the inputs.
The reason for not applying the flush patch is the corroded parts are located at more than half of vessel circumference. According to my calculation, 15 deg of circumferencial area will be the maximum length of cutting for window repair method. But having the heavy corroded area on the other side of the window, makes me worried. Installing the temporary support is not an option since there is no space available due to large size of support required (based on calculation).
With wind pressure of 195 kg/m2 and 0.375 of seismic factor, what makes the fillet welded lap patch not strong enough ? Even it has compensated the high moment due to wind and seismic based on proper calculation..

 

[jte]

Over 1/2 the circumference is heavily corroded? I'd strongly recommend paying for a crane and replacing the skirt at the next turnaround. Unless you have large diameter piping there to hold the vessel up! Just kidding!

You could most likely add some temporary legs and replace the skirt in say 90° or 120° sections with a properly sized support. This might actually be able to be done online, depending on specifics. What you may consider is what are your typical actual winds for the repair period, not the design winds. Lay out a procedure where you will simply stop work when the wind hits say 20 mph if the calculated remaining section is good for 30 mph, even though the long term design is for 70 mph (for example).

jt