API Standard 653, "Tank Inspection, Repair, Alteration, and Reconstruction," was first published by the American Petroleum Institute (API) in January 1991. Its scope encompasses the maintenance, inspection, repair, alteration, and reconstruction of existing above ground, atmospheric storage tanks constructed to API 650, and its predecessor, API 12C. Since original publication, API 653 has undergone many changes. In December 1995 API published the Second Edition of API 653, the first major revision to API 653 since publication in 1991. We reported on significant changes in the Second Edition in a previous newsletter. This newsletter summarizes significant changes made in Addendum 1 (December 1996) and Addendum 2 (December 1997) to the second edition of API 653.

2.2.2 Fixed Roofs| 97

Addendum 2 adds the requirement that distorted roof support members be evaluated and repaired or replaced, in addition to corroded or damaged members

2.3.3 Minimum Thickness Calculation For Welded Tank Shell | 97

Addendum 2 makes a major change to the minimum shell thickness formula by substituting H for H-1 in the formula: 2.6 x DHG/SE. Why they made this change is not clear. The term H-1 originated during the early days of tank design when it was recognized that the greatest hoop stress in a tank shell course was approximately 1-foot above the horizontal butt weld joining two shell courses or 1-foot above the shell- to-bottom joint. This is due to the restraining effect of the joints on hoop strains. As the shell thickness formula in API 653 locates H at the bottom of a locally corroded area, rather than at a shell course butt weld, API may be recognizing this difference. However, this change is not consistent with t min calculations for existing tanks containing no locally corroded areas where the H-1 term should apply as it does to new tank calculations. The effect of substituting H for H-1 is to make the t_min calculation more conservative for uncorroded tanks.

Also added to addendum 2 is a formula for calculating a maximum hydrostatic test height, based on the minimum thickness in the locally corroded area, using greater allowable stresses than that permitted for calculating product t_min.

2.4.7 Bottom Thickness Measurements | 96

Addendum 1 adds three new paragraphs (2.4.7.5, 2.4.7.6 and 2.4.7.7) clarifying requirements for tank floor pit repairs and specifying minimum dimensional requirements for the floor extension on the outside of the tank. Pit repairs are limited to pit welding, overlay welding and lap patching. Welding within the "critical zone" is still limited as before. The minimum thickness of the floor projection shall be at least 0.1" and shall extend a minimum of 3/8" from the outside toe of the floor-to-shell weld.

2.4.8 Minimum Thickness For Annular Plate Ring | 96

The minimum outside dimensional requirements specified in 2.4.7.7 for lap welded floor extensions also apply to annular rings.

4.3.2 External Inspection | 96

Besides changing the title of the paragraph from Scheduled Inspection to External Inspection, paragraph 4.3.2.1 formalizes the requirement that the external inspection be conducted by an "Authorized Inspector", replacing paragraph 4.10, which defined inspector qualifications. API 653 now defines an Authorized Inspector as ".. an employee of an "Authorized Inspection Agency" who is qualified and certified to perform tank inspections under this standard." Implicit in this requirement is that the Authorized Inspector hold API 653 certification.

4.4 Internal Inspection | 96

Paragraph 4.4.1.2 now requires the Authorized Inspector to review the NDE results, in addition to conducting the visual inspections.

7.3 Shell Repairs Using Lap Welded Patch Plates | 96

Section 7 significantly departs from the original second edition in that it now permits lap welded shell patch plates, within certain restrictions. The first and second Editions of API 653 were overly conservative with respect to shell lap patches. Lap patch repairs were not originally permitted to shell plates of any tank, based on brittle fracture considerations. However, Section 3 - Brittle Fracture Considerations, explicitly stated that brittle fracture of tanks with shell plates less than 1/2" in thickness was not a concern. Addendum 1 to the second edition of API 653 remedies that conundrum.

Lap patch repair plates can now be used to repair tank shells less than 1/2" thick with certain restrictions on size, placement and dimensions. Specifically: (7.3.1.3) - the patch plates shall be the smaller of 1/2" thick or the thickness of the shell plate, but not less than 3/16", (7.3.1.4) - corners shall be rounded to a minimum 2" radius, except at the shell-to-bottom joint, (7.3.1.5) - lap patches may cross flush ground shell butt welds, provided that they extend at least 6" beyond the butt weld, (7.3.1.6) - they may extend to, and intersect with, the external shell-to-bottom joint at a 90 degree angle, (7.3.1.7)- maximum dimensions are 48" x 72" and minimum dimensions are 6" x 6", (7.3.1.8) - shell openings cannot be positioned within a lap patch, (7.3.1.9) - shell plates under lap patches have to ultrasonically inspected for plates defects and remaining thickness and, (7.3.1.10) - they cannot be placed over lap-welded or riveted shell seams, other lap patch plates, distorted areas or areas containing shell defects.

In addition, lap welded patch plates now can be used to close holes in shell plates due to corrosion or removal of shell penetrations, provided that they meet a host of requirements, defined in paragraphs 7.3.2.1 through 7.3.4.6, including the maximum 1/2" existing shell plate thickness criteria.

7.9 Alteration of Existing Shell Penetrations| 97

Revised old paragraph 7.8.2.2 (new 7.9.2.2) to restrict removal of an existing reinforcing plate and addition of a new one to unstress relieved assemblies.

7.10 Repair of Tank Bottoms | 97

Revised paragraph 7.9.2.3 (new 7.10.2.3) to add spacing requirements (3" or 5t) between floor plate or annular ring welds and tank shell vertical welds for shell plates of unknown toughness.

7.14 Hot Taps | 96

The hot tap requirements have been updated and tightened to reflect and clarify brittle fracture considerations. Specifically, the new requirements add additional restrictions when hot tapping tanks containing shell plates of unknown toughness greater than 1/2" that did not exist in the original second edition.

8.3.2 Bottoms| 97

Paragraph 8.3.2.2, covering reuse of existing tank bottoms, has been revised to effectively prevent reuse of the portion of the tank containing the floor-to-shell weld and the floor extension. The original second edition allowed the entire floor to be reused intact by removing the shell 12" above the shell-to-floor weld.

8.3.3 Shells | 97

Shell plates 1/2" or thinner can now be cut, leaving part of the weld and all of the heat affected zone. The original second edition may no allowance for thin shell plates.

10.1.4 Temporary and Permanent Attachments To Shell Plates | 96

Paragraph 10.1.4.2 clarifies the inspection requirements for new, permanent attachments and attachments that have been removed on tanks with shell materials of Groups IV, IVA, V, or VI. The previous paragraph included no inspection requirements for removed attachments. Also, the revised paragraph only permits the purchaser to substitute magnetic particle inspection with liquid penetrant inspection, presumably acknowledge the increased sensitivity of the magnetic particle method to detect defects in carbon steel material, compared with liquid penetrants.

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