API 653 "Belly Bands"

[clkimbal]

Hi,

I have been searching high and low regarding the repair methods using large lap bands, otherwise called “belly bands,” on tanks. I am seeking for an engineered or technical explanation as to why large lap bands and or belly bands are not an acceptable repair method as per API 653 Sections 9.3.1.1 – 9.3.1.7?

For example, if there is a corroded and or thin walled area between the 50-70% liquid level all around the circumference of a tank, what is best practice to repair this section of the tank? Since a belly band is not supported by API 653, would I completely remove the affected course and replace with new or is there a means of patching the circumferential area that is supported by API 653? I have also been told that "although belly bands are generally discouraged, there are some cases where they can be used safely and effectively. It depends on the specific circumstances for the tank."

Thanks in advance!

 

[Duwe6]

I would presume that the reason is that when the shell finally corrodes to "zero effective strength", the belly-band becomes a single-sided lap-welded shell plate. Horrible joint efficiency. The 'traditional' method* is to cut out the corroded areas and insert new plates there. Now the existing-to-new welds start out at 100% joint efficiency and stay there as they corrode.

*people who try to reinvent the wheel generally have bad results. There are a myriad of reasons that one method becomes 'standard', and it's tough [fi not impossible] to figure out all of them. As with pressure vessels, there is essentially nothing left to innovate on in tank erection and repair. Our predecessors were at least as smart as we are. *ALL* the 'low-hanging fruit' was picked before the 1980's.

 

[weldstan]

I believe that if you read all of 9.3, you will ascertain the reasons. Joint Efficiencies and see Duwe6. If through wall corrosion occurs, the effect of corrosion on the patch and accelerated crevice corrosion is somewhat indeterminate.