Title: Radiography of Compression bar on api 620 Tanks




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Agenda Item: 620-269
Title: Radiography of Compression bar on API 620 Tanks
Contact: Rob Annett, Alyeska Pipeline Service Company

907.787.8167



AnnettRC@alyesk-pipeline.com
Source: Hiner email of 9/12/2003
Purpose: Provide additional guidance for radiographic requirements for API 620 thick compression bars
Background: API 620 paragraph 5.26.2 "Wall Thickness" -states, "Complete radiographic examination is required for all double-welded butt joints wherever the thinner of the plates or the tank wall thickness at the joint exceed 1 1/4" and the joint is subjected to tension stresses greater than 0.1 times the specified minimum tensile strength of the material."

API 620 paragraph 5.26.4 "Exemptions" - states, "Spot or full radiography examination is 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 stress only.

Table 5-2 which allows "roofs above liquid level" to be single-welded butt joints with backing strip.
Discussion: Compression bar design thickness is almost always controlled by the compression stress, however, under dead load or dead load plus external pressure the bar will go into tension and this stress could be more than 0.1 times the specified minimum tensile strength of the material.

A statement should be made clearly addressing which paragraph 5.26.2 or 5.26.4 controls in this case.

Table 5-2 allows “roofs above liquid level” to be single-welded butt joints with a backing strip. There is no thickness limitation provided. A compression bar could possibly be considered “Roof”, not double-welded as noted in 5.26.2, and radiography would not be required.

Justification: The compression bar can be subject to tension under certain loading conditions. As such complete radiography is required per paragraph 5.26.2. (See following proposed wording change)

A compression bar is not considered “Roof”. Clarification of what is and is not included in the “roof” is prohibitve and unnecessary. (No proposed wording change)



Current Wording

Proposed Wording

5.26.4 Exemptions

Spot or full radiographic examinations is 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.



5.26.4 Exemptions

Spot or full radiographic examinations is 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 are not subject to tension stresses greater than 0.1 times the minimum tensile strength of the material.




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