Goal
To refine the non-destructive metallurgical testing method and protocol for detecting cracks in nurse tanks used to support guidelines (via a rulemaking) for when to require repair or order a nurse tank out-of-service, and to examine possible relationship of crack initiation and propagation to thickness of the steel used to fabricate the tank. Results are to support a National Transportation Safety Board (NTSB) recommended rulemaking by the Pipeline and Hazardous Materials Safety Administration (PHMSA) to address non-destructive testing of nurse tanks.
Background
Pressurized nurse tanks are used as the containers to provide the anhydrous ammonia for injection as a nitrogen fertilizer into the soil of fields for crops needing such a fertilizer. Tank failure reports indicate the greatest challenges to maintaining tank integrity are stress corrosion cracking (SCC) that tend to initiate around unannealed welds, and locally created high stress areas caused by tank damage incurred during service. An unknown number of tanks manufactured in the 1950s through the 1970s that are still in use today were manufactured with higher strength steel, which is more prone to SCC. Then, beginning in 1999 with the new American Society of Mechanical Engineers (ASME) standards, nurse tanks started being manufactured with thinner steel, the welds are not annealed to relieve the very high residual stresses in the welded joints where SCCs could initiate and then propagate, and more recently manufacturers have begun using the thinnest possible steel stock meeting the ASME thickness requirement, which when fabricated has locations that do not meet the minimum thickness requirement.
Summary
Refine the recommendations for non-destructive testing of nurse tanks to enable PHMSA to undertake the rulemaking recommended by NTSB. Revisit previously measured tanks to better determine the rate of propagation of the detected cracks over time, and the initiation of new ones.
Outcomes
Refine the recommended testing protocol developed under Phases I and II, and make a refined estimate of crack propagation rate.
Milestones
Summer 2015: Retest the sample of nurse tanks initially tested in 2013
February 2016: Final report for Phase III due
Funding
FY 2015: $100,000
Current Status
Data analysis in progress.
Contractor
MaineWay Services with Iowa State University
Updated: Thursday, March 3, 2016