The Hendrix Group Reporter©

July 27, 1996 (Vol. VI No. 1)

Update: Has The New Edition of API 653 Affected Aboveground Storage Tank Inspection Requirements?

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, or its predecessor, API 12C. Since publication, tank owners, operators, and inspection companies have struggled to interpret and meet its requirements. In December 1995 API published the first major revision to API 653 in almost five years. Does this revision rewrite the rules for inspecting above ground storage tanks? This paper describes the significant changes made to API 653 and how they may influence tank inspections.

Section 1- Introduction.

API has added two definitions to the standard, Authorized Inspection Agency, and Inspector. With these two new definitions API has addressed the increasing incorporation of API 653 requirements into state or local jurisdictions. In this respect API Standard 653 has formalized the meaning of "inspection agencies" and "inspectors," similar to The National Board Inspection Code and API Standard 510, "Pressure Vessel Inspection," governing ASME Section VIII pressure vessels. This addition, depending on the jurisdiction, could potentially impose regulations or restrictions to independent tank inspection companies.

Section 2- Suitability For Service

Tank Shell Evaluation- A welcome revision to the tank shell evaluation section is the inclusion of Table 2-1- Joint Efficiencies for Welded Joints. The table lists the various joints efficiencies that API 650 and API 12C used for new tank design through the years. This table should facilitate maximum product height calculations by permitting joint efficiencies greater than the API 653 default 0.7. API 653 specifies a default 0.7 joint efficiency for product height calculations when the original joint efficiency is not known. Based on Table 2-1, a joint efficiency as low as 0.7 has not been specified since API Standard 12C (except for API 650 Appendix A construction with no radiographic inspection). Using 0.7 for product height calculations was artificially restricting maximum product heights for tanks designed with a greater efficiency. The primary reason for this was that most people did not have access to the original joint efficiency (it required having all of the previous standards).

Tank Bottom Evaluation- The wording "General" in paragraph 2.4.1 has been revised to include language that describes what are considered acceptable methods for satisfying periodic leak assessments between internal inspections. The first standard implied that leak assessments should be conducted at intervals less than or equal to the required internal tank inspections but gave no guidance as to what was an acceptable leak detection method. The new edition specifies leak detection tests or monitoring systems (double bottoms) as examples of acceptable methods.

Foundation Analysis- Some may be disappointed to know that the acceptance criteria for tank out-of-plane and edge settlement was not revised. It is this writer's opinion that many people thought the foundation settlement acceptance criteria too conservative.

Bottom Leak Detection- API has added a footnote to paragraph 2.4.5 stating its support for Release Prevention Systems (RPS) and Release Prevention Barriers (RPB). This paragraph potentially affects tank owners in that API is reinforcing its position that owners should consider equipping tanks with a leak prevention system or barrier, i.e., internal liners, cathodic protection, double bottoms, etc., when replacing an existing tank bottom.

Minimum Thickness For Tank Bottom Plate- Paragraph is a new paragraph that defines minimum required thicknesses for the "critical zone" of a tank bottom. The minimum "critical zone" thickness is now based on the thickness of the original floor (1/2 of original thickness, not including any corrosion allowance) and the first-course shell thickness (1/2 of t(min.)). A absolute minimum thickness of 0.1" still applies. This could conceivably result in the "critical zone" minimum required thickness being greater than the rest of the floor and greater than that required by the first edition (0.1").

Section 3- Brittle Fracture Considerations

This section has been rewritten to increase clarity; however, its main provisions have not changed.

Section 4- Inspection

Inspection Frequency Considerations- "Change in service" has been added to the list of factors that must be considered when determining tank inspection intervals.

In-service Ultrasonic Thickness Measurements of the Shell- Paragraph has been revised to eliminate the requirement to inspect new tanks within five years of commissioning. This revision could lengthen the required first inspection interval for newly constructed tanks if corrosion rates can be established by using experience gained from "similar service" tanks.

Internal Inspection- A new paragraph ( has been added to require that internal inspections must be performed by the "Authorized Inspector." In certain jurisdictions this requirement could place restrictions on those personnel allowed to conduct internal inspections.

Inspector Qualifications- API has added an item to the required inspector qualification options to allow five years of experience in the inspection of aboveground storage tanks in the petroleum or chemical industries, besides the combination of education and experience requirements. Paragraph 4.10.2 has been revised to include inspector certification as a requirement for Authorized Inspectors. Paragraph 4.10.3 has been revised in state that personnel conducting nondestructive inspections do not have to be API Certified Aboveground Storage Tank Inspectors.

Appendix D- Authorized Inspector Certification- Paragraph D.3.2 implies that the "grandfather" clause originally permitted with the initial certification requirement in the January 1992 addendum to API 653 is being reinstated in the second edition. This paragraph is not correct and should have been omitted in the 1995 edition.

Section 6- Design Considerations For Reconstructed Tanks

Shell Design- Paragraph 6.4.3 in the first edition, detailing the maximum liquid height for hydrostatic testing, was eliminated from the second edition. It is not clear to this writer why the paragraph was eliminated except that it may have conflicted with the "full" hydrostatic test required for reconstructed tanks in Section 10- Examination and Testing.

Section 7- Tank Repair and Alteration

Repair of Tank Bottoms- Paragraph has been revised to provide specific requirements for removing and replacing a tank floor. The first edition had no requirements. Paragraph has been added, stating that replacement tank bottoms shall meet all requirements of API 650, including shell weld spacing.

Section 10- Testing and Examination

When Hydrostatic Testing Not Required- Paragraph was revised to reference paragraph 7.2, Removal and Replacement of Shell Plate Material, for weld spacing requirements. This has the effect of increasing the minimum weld spacing between the bottom of the doorsheet and the shell-to-bottom weld from three to six inches.

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