Significant Technical Changes Made to API 653
By Vince Carucci
Significant technical changes are included in Addendum 3 of API 653. Many of these changes relax requirements that were included in prior editions of API 653, and compensate for this by increasing inspection and/or engineering review requirements. Other changes provide additional clarification for what is required by this standard.
The following summarizes several of the significant technical changes that were made. See Addendum 3 for complete details.
- Several new or revised definitions are included.
- Corrosion rate. Total metal loss divided by the period of time over which the metal loss occurred.
- Critical zone. That portion of the tank bottom or annular plate within 3 in. of the inside edge of the shell, measured radially inward. This is a significant change from earlier editions of API 653. The critical zone was previously defined as within the annular plate ring, within 12 in. of the shell, or within 12 in. of the inside edge of the annular plate ring. This also significantly affects what bottom repair options are acceptable.
- The use of Risk Based Inspection (RBI) procedures as a valid alternative means to determine whether tank condition is acceptable is given more prominence in several areas as a means to determine whether a storage tank has acceptable integrity. More detailed guidelines are also provided for using RBI procedures.
- Some modifications and clarifications were made to the brittle fracture exemption curve (Figure 3-2).
- The figure now makes it clear that there is no brittle fracture concern for shell plate thicknesses less than ½ in.
- The figure is larger and a grid work is added to make it easier to locate shell thicknesses and read the corresponding acceptable shell metal temperatures.
- The curve was shifted slightly for thicknesses over ½ in. and less than 0.9 in. such that the shell metal temperatures must be slightly higher in order to be exempt from brittle fracture concerns.
- Internal inspection may be done with the tank in service provided that the inspection is only being done to determine the condition and integrity of the bottom. Such onstream inspections may be done using robotic ultrasonic thickness measurement methods. Electromagnetic methods may be used to supplement the ultrasonic thickness inspection that is done.
- A new Appendix F summarizes NDE requirements. This Appendix is not intended to be used alone or to replace inspection requirements that are specified within the body of the standard.
- There are several significant changes to the section on repairing a portion of the tank bottom. These include:
- Design details and limitations are specified for welded patch plates.
- Minimum distance requirements are specified between patch plate welds and other bottom plate welds.
- Welded patch plates are now permitted within the critical zone of the bottom provided specified requirements are met. For example:
- The patch plate cannot be more than ¼ in. thick.
- If the patch plate is within 6 in. of the shell, it must be "tombstone" shaped and have its sides intersect the shell-to-bottom joint at a 90° angle.
- Patch plate material must meet the toughness requirements of API 650.
- Patch plates cannot be used in the critical zone if the tank operating temperature exceeds 200°F for carbon steel or 100°F for stainless steel.
See Addendum 3 for additional requirements and limitations.
- An additional nozzle size break point (£ NPS 6 with a minimum required shell thickness of 3/16 in.) has been added to the table that gives minimum shell plate thickness requirements for hot tapped nozzles.
- For reconstructed tanks:
- Some modifications/clarifications were made in where the shell could be cut.
- Requirements were added that define the required offset of vertical shell joints.
- Several changes were made in weld inspection requirements. For example:
- In specifying radiographic requirements for shell plate welds, a distinction is made between welding between new plates or to existing plates.
- The shell-to-bottom weld must be inspected if it will be under (or within 6 in.) of a patch plate.
- Bottom plate weld inspection requirements have been clarified.
- Inspection is specified for patch plates located within the critical zone.
- Some hydrotest requirements were modified. For example:
- A hydrotest is required if the tank service has changed to one of increased severity (e.g., higher specific gravity or lower service temperature).
- A new bottom installation is not considered to be a major alteration for purposes of hydrotesting if the foundation is not disturbed and either the annular ring remains intact (for tanks with annular rings), or there is no welding on the bottom within the critical zone.
- The section providing criteria for when a hydrotest can be waived has been completely revised and expanded.
- Significant changes were made in Appendix B covering tank settlement. These include the following:
- Much more detail is provided on acceptable procedures for evaluating differential shell settlement. A basis is also provided for using finite element analysis for settlement evaluation.
- The section on edge settlement evaluation was also completely rewritten and additional details provided.
- A distinction is now made between internal bottom settlement or bulges and edge settlement, and different acceptance criteria are specified for each. Edge settlement definition, measurement, acceptance criteria, and repair requirements are provided in detail.
The changes to API 653 made in Addendum 3 are, in many cases, welcome relaxations of previous requirements that were unnecessarily conservative. They also more clearly permit experienced engineers to use their judgement, knowledge, and alternative methodologies to evaluate storage tank integrity. Companies with established storage tank inspection, maintenance, and repair programs should review them in light of these revised API 653 requirements.