The Hendrix Group Reporter©
February 26, 2000 (Vol. X No. 2)
API 653 Update No. 3: Addenda 3 Changes to the Second Edition of API 653.
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 1998 API published the Third Addendum to the Second Edition of API 653, a major revision to API 653 since the second addendum, published in December 1997. We reported on significant changes in the Second Edition in a previous newsletter.
Below is a summary significant changes made in Addendum 3 (December 1998) to the second edition of API 653. For complete details, see Addendum 3.
- Section 1.55 adds several new definitions, including:
Corrosion Rate - The total metal loss divided by the period of time over which the metal loss occurred.
Critical Zone - That portion of the bottom or annular plate within three inches of the inside edge of the shell, measured radially inward. This is a significant change with respect to earlier editions of API 653. Previously, the critical zone was defined as within 12 inches from the shell, within the annular ring, or within 12 inches of the inside edge of the annular plate ring. This definition represents a significant relaxation of the requirements for bottom plate repair.
- With this addendum, API has embraced the use of risk-based inspection (RBI) methods as an alternative to stated absolute inspection interval requirements or minimum thicknesses. For example, Section 126.96.36.199 allows the use of RBI as an alternative to the mandated minimum bottom plate thicknesses at next inspection. The proper use of RBI technology can now extend the here-to-fore maximum 20-years between next internal inspection intervals. Explicitly stated by API is the proper experience, knowledge and training that personnel conducting RBI assessments must possess.
- Several ease-of-use changes have been made to the brittle fracture exemption curve in Figure 3-2. The figure makes it clearer that shell plates less than 0.5" should not be susceptible to brittle fracture. Also, more detailed grid lines have been added to allow more accurate interpolation of shell thicknesses vs. minimum safe allowable temperatures.
- Section 4.4 - Internal Inspection, now clarifies that the Authorized Inspector must also assure the quality and completeness of the NDE inspection, in addition to conducting the visual inspection. The internal inspection can not be done with the tank in service, as long as the intent of the inspection is only to verify the condition and integrity of the bottom. This is in recognition of several advanced NDE tools coming into commercial use that use robotics to inspect the tank floor. The use of robotics will probably be limited to tanks storing relatively clean liquids and those with little floor accessories, such as heating coils, swing arms, etc.
- A new Appendix F summarizes the NDE inspection requirements for tank undergoing repairs or reconstruction, including acceptable standards, inspection qualifications and procedure requirements. This appendix is provided only as a guide and is not intended to replace specific requirements stated in the body of the standard.
- Section 7.10 - Repair of Tank Bottoms, includes several significant changes, many of them relaxations of previous requirements or clarifications, including:
- Design, use and limitation of welded on patch plates for floor repairs, including significantly, the use of welded on patch plates with the critical zone with certain restrictions.
- Table 7-1 lists a new hot tap nozzle size break of <= 6" for shell plates 3/16" thick.
- Section 8 - Dismantling and Reconstruction. Several clarifications have been made to this section regarding where to cut shell plates, particularly with respect to cutting shell plates through welds and weld heat affected zones (HAZ). Also added is a requirement for the minimum offset for mew vertical joints in adjacent shell courses.
- Many changes/additions were made with respect to inspection of welds, including:
- Distinguished between inspection requirements for welds attached existing shell plates to existing shell plates versus those for welding new shell plates to each other;
- Added and specified inspection requirements for existing shell-to-bottom welds when adding a patch plate under or within 6" of the plate; - Clarified criteria for testing newly welded bottom joints using a vacuum box or tracer gas; - Specifies inspection criteria for welded on patch plates within the critical zone.
- Section 10.3 Hydrostatic Testing. Significant changes have been made, including:
- A hydrotest must be conducted when an engineering evaluation has indicated the need for one due to an increase in severity of service, such as when the specific gravity of the stored product is increased or the service temperature is lowered;
- A hydrotest is not required after adding a new floor if, (a) the foundation is not disturbed and/or, (b) the annular ring remains intact or repairs do not result in welding in the critical zone.
- Section 10.3.2, for when a hydrostatic test is not required, has been completely rewritten. See Addendum 3 for complete details.
- Significant changes were made to Appendix B, Evaluation of Tank Foundation Settlement. Included are expanded details on acceptable procedures for evaluating differential shell settlement. The section on edge settlement has been completely rewritten and a distinction is made between internal settlement and edge settlement.