An external welded box can be used to reinforce or prevent leakage of a component. The box shape can be cylindrical, rectangular, with flat sections and shaped tops, following the geometry of the component to be repaired. This method is also used to enclose flanged connections, valves, nozzles, drains and vents. They are normally defined with a specific design and use of double gutters, caps and plates. The annular space between the box and the component can be filled with epoxy, sealant, fiber or refractory material.

The box design can be developed just to contain the leak or to resist the internal (structural) pressure.