API653/API650 as it applies to corroded shell and scope of repair 1

[Andrew63411]

Hello everyone. I have a 800,000 gal storage tank that has experienced significant external corrosion for about 30 ft of the circumference (soil build up on tank, poor housekeeping).

I have also found that the floor to shell weld has started to corrode for a wider range of 61ft give or take with minor external corrosion of the shell for the bottom 3in in those areas. I have suggested replacing all 61ft of shell as well as the bottom (24in in from tank shell and weld to old floor) at this location to ensure all areas are covered. However, I am getting a lot of pushback from my management that the duration is fairly long due to cleaning/welding etc. and the tank is critical in the process. (hasn't been empty in 30+years to my knowledge).

Can i just replace the 30ft of bad corrosion (shell+floor) 2ft up and then grind out the weld for the remaining and lay down a new bead?

Any other suggestions on code section to look at?

Another issue since we haven't emptied the tank in forever is no one knows what the bottom will look like (bottom plates) My interpretation is if we need to replace the bottom fully, the shell needs to be slotted and the shell need to come back under the shell to allow a double sided fillet weld of proper size to be welded. We cant just overlay and do a full penetration weld to the inside of the shell?

The other option we have is to just repair the bottom if its not bad by using welded on patch plates.

Any advice appreciated. tank dimensions are 55ftx46ft high with a fixed dome roof and lap welded bottom.

 

[Andrew63411]

Wanted to add, bottom plate thickness now is 5/16, and the bottom shell course thickness was at design (1978) .609+1/16 CA.

 

[JStephen]

My advice is to go ahead and schedule a complete inspection inside and out to see what you're actually dealing with.
Based on the shell thickness, it sounds like this was either designed to low stresses (AWWA tank, for example), or to store molasses, fertilizer, acid, or other heavy products, in which case, interior corrosion may be an issue as well.
If the tank is not actually storing petroleum, there may not be any regulatory requirement to follow API-653, which would allow a little more freedom in how things are done.

 

[Andrew63411]

JStephen,

That would be helpful, but the criticality of this tank does not allow easy removal from service to allow for an inspection inside. I have been asked to collect more external UT data to develop the scope of the repair. It was design to store black liquor Sg=1.3. We don't have regulatory requirements to follow this standard, just good engineering practice. I am just trying to understand the scope of these repairs as it applies to the design standard of this tank (API 650)

 

[JStephen]

“If you don't make time for your wellness, you will be forced to make time for your illness.”- A common quote that might relate to the topic.