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Design standards

THE AMERICAN Petroleum Institute (API) Subcommittee on Pressure Vessels and Tanks is taking a hard look at three standards related to design and construction of large, welded, and low-pressure storage tanks, as well as welded tanks for oil storage. Another standard being reviewed addresses tank inspection, repair, alteration, and reconstruction.

Roland Goodman, API senior standards associate, briefed members of the Independent Liquid Terminals Association at its 2005 annual International Operating Conference in Houston, Texas.

The revision to API 650 applies to welded steel tanks for oil storage and adds ASTM A 841 Class 2 material to the list of acceptable plate. One revision would define load combinations for the design of tanks and tank components. Another provides consistent terminology and definitions for the temperatures to be used for design.

API 653 addresses changes for inspection, repair, alteration, and reconstruction. The definition of alteration is changed to read “any work on a tank that changes its physical dimensions or configuration.”

Repair is defined as work necessary to maintain or restore a tank to a condition suitable for safe operation.

“Repairs include both major repairs or repairs that are not major repairs,” he emphasized. He listed as examples removing and replacing material (such as roof, shell, or bottom material, including weld metal) to maintain tank integrity; re-leveling or jacking a tank shell, bottom, or roof; adding or replacing reinforcing plates (or portions thereof) to existing shell penetrations; and repairing flaws, such as tears or gouges, by grinding and/or gauging followed by welding.

As for standards addressing design and construction of large, welded, low-pressure storage tanks, the committee examined API 620. Some of the subjects addressed include (1) plates: clarify thickness limitations and notch toughness for basic API 620 tanks and (2) radiographic/ultrasonic examination: provide additional guidance for radiographic requirements for thick compression bars.

Under exemptions in this section, the revision states: spot or full radiographic examinations are not mandatory on tank bottoms that are uniformly supported throughout (for example, concrete slab or compacted sand) or on components that have a design thickness controlled by compressive stresses only, and are not subject to tension stresses greater than 0.1 times the minimum tensile strength of the material.

Other changes to API 620 apply to piping, tubing, and forgings, as well as production weld tests.

More information on all these changes to API standards 650, 653, and 620 can be obtained by e-mailing Goodman at [email protected].

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