By Vincent A. Carucci
Welded-on plates may be added to an aboveground storage tank bottom for reasons other than to repair a corroded area. Examples of this include under floating roof support legs, or under the supports for internal heating pipes. Addendum I of API-653 requires that these added plates meet the same design, installation, and inspection requirements as for repair plates (see Figure 9-5 of API-653).
In addition to acceptable design details, Figure 9-5 of API-653 contains minimum weld spacing requirements for welded-on patch plates (e.g., distance to bottom plate lap welds or the bottom-to-shell junction weld). API-653 permits a relaxation in these weld spacing requirements for welded-on plates that are not being used for bottom repair. In this case, if the minimum weld spacing requirement cannot be met, all exposed welds that do not meet the weld spacing criteria must be Magnetic Partial or Dye Penetrant examined. However, any plates that are located within the critical zone must meet all bottom patch plate requirements, including weld spacing.