Hi every one
Recently we inspected a internal floating roof tank and found that at 6 Feet from Bottom of the shell height all around the circumference a band of 36 inch width on shell was found pitted and corroded, Upon sand blasting several  holes were found in this locations.
 This tank was out of service for several years.The question is to bring back this tank into operation. Our maintenance team proposes to weld patch plates around the circumference ( internaly) of the tank in this corroded band. They touch base with the floating roof designer and he informed that there is no problem for the movement of the floating roof. But the API653( 2009 4th edition, section 9.3.1.7)  limits the maximum size of the patch 48"X72". Is it acceptable to have multiple patch around the tank to cover the full circumference ???? or anyother type of solution is required to adhere to the code regulations. your quick response will be appreciated  

You can have a new shell course welded in the tank at the height where the plates are corroded.  It would be better than welding a bunch of patches on the inside.

There is a limit on lap patch size, not butt-weld patches

Interestingly there's a limit on the patch size but no limit on the number of patches around the tank nor that the patches can't be adjacent to each other.

Maybe this is should be submitted as an enquiry case to the API?

Adam Potter MEng CEng MIMechE
www.ax-ea.co.uk