Testing and Recommended Practices to Improve Nurse Tank Safety: Phase III

Testing and Recommended Practices to Improve Nurse Tank Safety: Phase III

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  • Alternative Title:
    Testing and Recommended Practices to Improve Nurse Tank Safety: Phase 3
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    Final Report, May 2015-March 2016
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  • Abstract:
    This report summarizes findings from a re-examination of 411 tanks that were relocated of 532 previously examined anhydrous ammonia (NH3) nurse tanks using the single-beam side-angle ultrasonic nondestructive evaluation method. There were 532 tanks examined previously in phase II of this research (conducted in 2012). In comparison to the earlier study, the Phase III effort (conducted in 2015) provided better training to its undergraduate student examiners majoring in material science and deliberately attempted to discriminate between ultrasonic indications that resulted from stress corrosion, almost exclusively perpendicular to the weld seam, versus those more likely caused by issues in the geometry of the weld seam. Important findings include: Growth rate of many existing perpendicular indications found in the Phase II and Phase III studies were slower than observed by the constant stress laboratory tests conducted as part of Phase II. Even though indication growth of existing perpendicular cracks was less than found in the constant stress laboratory tests, there were numerous new perpendicular indications found to have developed in the 3 years since the Phase II survey. Perpendicular cracks were the only ones single-beam ultrasound could reliably distinguish. A phased-array approach would be necessary to try seeing into the weld seams for distinguishing parallel cracks. Growth of existing indications and development of new indications were both disproportionately greater in newer tanks. Possible reasons why newer tanks are having more problems are discussed in the body of this report. There was a decrease in shell wall thickness following the 1997 American Society of Mechanical Engineers (ASME) change in guidance, which may be associated with the increase in the number of indications in newer tanks. Tank wall thickness measurements showed 5 percent of tanks inspected were near or below thickness standards that would be applied if the tanks were missing their ASME data plates, which would make them subject to 5-year periodic testing. Post-weld heat treatment (PWHT, annealing) reduces residual stresses remaining in steel after welding and is associated with dramatic reductions in both initiation of indications and their subsequent growth.
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