Repair, Alteration, and Reconstruction
Three different animals
A repair restores the tank to its original condition - same thickness or thicker, same configuration, same service. An alteration changes configuration or service: raising or shortening the shell, changing diameter, replacing the bottom or roof type, adding a nozzle beyond the allowed limits, or switching to a product with a higher specific gravity. A reconstruction is dismantling and re-erecting a tank, typically at a new location.
The consequence matters: an alteration requires the changed portion to meet current API 650 design, weld quality, and testing; a repair only has to restore the original design basis. Calling an alteration a repair is the most common trap.
Repair methods: weld build-up for shallow pitting; fillet-welded lap patches with rounded corners on shell or bottom (restricted near the shell-to-bottom junction, and never a substitute for full-penetration replacement); insert plates with full-penetration butt welds, a minimum plan dimension around 12 in. (300 mm), thickness equal to or greater than the original, and radiography of the new joints.
Welding controls: qualified WPS and welders (ASME Section IX), API 650 joint details, preheat where required, postweld heat treatment for thick shell (above roughly 1.5 in./38 mm), and hot work only after cleaning, gas-freeing, and combustible-gas testing.
Testing: major shell or bottom work normally requires a hydrostatic test; only minor repairs are accepted on NDE alone.
Reconstruction: plate marking and numbering, material identification and brittle-fracture screening, all new welds to API 650, a new hydro test, an updated nameplate and updated records.
Rule of thumb: moving from G = 0.85 to G = 1.0 is an alteration, and t_min rises by about 18% - recheck the shell before you take the product.