Recommended Non-Destructive Testing to Improve Nurse Tank Safety, Phases I and II

Goal

To determine a recommended, non-destructive metallurgical testing method and protocol for detecting cracks in a nurse tank, and to develop guidelines for when to require repair or order a nurse tank out-of-service. Research results are intended 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

Any nurse tank testing procedure is likely to impose additional costs on agricultural operations that use nurse tanks to inject nitrogen fertilizer in the form of pressurized anhydrous ammonia into the soil of their fields. Tank failure reports indicate the greatest challenges to maintaining tank integrity are stress corrosion cracking (SCC), defective welds, and tank damage incurred during service. An unknown number of tanks manufactured in the 1950s through the 1970s still in use today were manufactured with higher strength steel, which is more prone to SCC. Beginning in 1999 with the new American Society of Mechanical Engineers standards, nurse tanks are now manufactured with thinner steel, and the welds are not annealed to relieve the very high residual stresses in the welded joints where SCCs could initiate and then propagate.

Summary

This study developed recommendations for non-destructive testing of nurse tanks, supported by sufficient justification and cost-benefit information to enable PHMSA to undertake the rulemaking recommended by NTSB. The study also determined whether a manufacturing process that allowed pinholes in welds caused a safety problem.

Outcomes

Recommendations for a testing protocol were developed under Phase I. Under Phase II, we are verifying whether Phase I model parameters need further refinement, assessing safety of pinhole leaks, examining tanks to determine whether testing can be limited to the vapor area of the heads, and providing technical details about testing for possible inclusion in a PHMSA rulemaking.

Milestones

Summer of 2012: Test a statistical sample of nurse tanks in the Midwest.
August 2013: Final report for Phase II due. Final report from Phase I awaiting approval for publication.

Funding

$892,000

Status

Final reports were published and can be accessed at:

Phase I:  https://rosap.ntl.bts.gov/view/dot/168

Phase II: https://rosap.ntl.bts.gov/view/dot/163

Contractor

Virginia Technical Transportation Institute