API 650 / 653 - Tank Shell to Annular Ring Joint Fitness for Service

[BlueEngineer]

All,

I have a 50' x 36' carbon steel tank in water service that was built to API 650. The tank has experienced severe cooling tower overspray corrosion. The area that has seen the most damage is the critical shell to annular ring (chime) weld. Not only is the shell and weld severely pitted, but the annular ring is severely corroded with the typical "phone book" appearance. I'm not sure how much good metal is remaining in the annular ring. I can assess whether the thickness of the shell is adequate per simple calculation and can evaluate if the chime is thick enough per API provided table, but I am more concerned with the interaction of the two defects at the highly stressed weld.

If you click on the attachement I have on this post you will see my issue. The red area is the corroded area I am talking about. I assumed that the annular ring exhibits the typical "knife edge" type corrosion that I am used to seeing for annular rings.

Does anyone have any suggestions on how to evaluate that critical weld for fitness for service?

Your input will be greatly appreciated. Thanks in advance for any help you can provide.

-Blue
Refinery Fixed Equipment
Coker Unit
4 Years Experience

 

[TGS4]

For local corrosion of that nature in such a critical location, I can only think that an API-579 Level 3 assessment would work. I would be particularly concerned about the remaining length of the outside fillet weld. Of course, providing adequate data to generate the FEA would be a challenge...

 

[Duwe6]

With a 50-ft tank, it may be just as cheap to fix it as to do a properly detailed FEA. If the upper 'leg' of the corrosion band does not extend much above the corner weld, use a weld build-up. FCAW can be pretty fast.

If the metal loss extends up too far for a reasonable weld build-up, go inside the tank and apply a 3 x 3 x 1/4" rolled angle to the corner area. This method adds metal, reinforces the crital chime area, and causes your installation welding to occur outsied the API-653 defined "Critical Zone". Again, FCAW would be my preferred weld process -- wire for productivity, plus flux for cleaning of the existing basemetal.

 

[BlueEngineer]

I like your reinforcement method; however, I am really unsure of the soundness of the floor plate. The chime is a wreck and I'm sure the plate exhibits the typical "knife-edge" look if the scale was removed. This tank is a critical one to the operation of the unit and I would be in a tough place if we didn't have sound metal to weld to when we got into the tank to install the rolled angle. Also, are there any crevice corrosion concerns under the newly installed angle?

Regards,
-Blue

-Blue
Refinery Fixed Equipment
Coker Unit
4 Years Experience

 

[Duwe6]

RE: soundness of floor plates; hire an NDE/API Inspx firm to do a "Floor Scan" to determine where and how big/deep the underside pitting is.

RE: crevice corrosion of Interior Angle -- if the welds do not have through & through porosity there will be no corrosion. Rust takes replenishment of oxygen.