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Fracture mechanics

MOST storage tank steels behave like taffy — they have a brittle aspect at low temperature, but become ductile at higher temperatures. That's something to keep in mind when addressing fracture mechanics, said Phil Myers of Chevron.

While discussing a brittle fracture exemptions study for small storage tanks, Myers pointed out that current rules exempt “small tanks” from the assessment and the current studies underway will confirm that.

“This work may also yield results that indicate when it might be valuable to conduct fracture assessments using fracture mechanics,” he said. “Most likely, this will be under very low temperature conditions where there are significant flaws, such as lack of weld penetration. Risk acceptability and tolerance will be implicit in these cases.”

The Environmental Protection Agency would have to explicitly recognize in guidance documents that brittle fracture assessments are not required for small tanks, he said.

Meanwhile, more validation cases must be provided and the limits of the exemptions will have to be more exactly defined — as well as establishing when brittle fracture assessments should be conducted.

Myers believes that brittle fracture can be exempted in all cases except when a new tank bottom is installed. However, the exemptions that require fracture mechanics approaches must be technically sound, involve material toughness testing, and be documented.

“These exemptions are particularly valuable when water is costly to either fill or empty the tank, when time considerations are important,” Myers said.

He pointed out that tank failures have occurred during an initial hydrotest, on the first filling in cold weather, after a change to a lower service temperature, and after a repair or modification.

“It's rare when this happens, but when it does occur, these are the likely conditions,” he said.

He expects recommendations that will exempt small tanks as within the scope of the Steel Tank Institute SP 0001 from brittle fracture; rigorously follow American Petroleum Institute Standard 653 or 579 for tanks with greater than 1/2 inch thickness plates; and seek exemptions using API 579 FFS.

More information on this issue can be obtained from Myers at [email protected].

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