To refine the non-destructive metallurgical testing method and protocol for detecting cracks in nurse tanks and to determine possible relationships to causal factors not previously considered in detail.

Pressurized nurse tanks are used as the containers to transport anhydrous ammonia (a hazardous material) to agricultural fields for injection in the soil as a nitrogen fertilizer. Tank failure reports indicate the greatest challenges to maintaining tank integrity are stress corrosion cracks (SCC), which tend to initiate around high-stress areas created by 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. Beginning in 1999, with the new American Society of Mechanical Engineers (ASME) standards, manufacturers began producing nurse tanks with the newly reduced thinner steel stock (meeting the thinner ASME thickness requirement). No remaining manufacturer of nurse tanks anneals them to relieve the very high residual stresses in the welded joints where SCCs could initiate and then propagate. Furthermore, when nurse tanks are fabricated with the thinnest steel allowed, there can be locations on the nurse tank that do not meet the minimum thickness requirement.

This research supported a National Transportation Safety Board (NTSB) recommended rulemaking by PHMSA to address non-destructive testing of nurse tanks. It sought to refine the recommendations for 1) non-destructive testing of nurse tanks, and 2) nurse tank manufacturing requirements. The research team revisited previously measured nurse tanks to better determine the rate of propagation of the detected cracks over time and to identify the initiation of new cracks (in relation to the causal factors associated with SCC).

Refine the recommended testing protocol developed under Phases I and II, and make a refined estimate of crack propagation rate; provide a concise summary of all work accomplished in all phases, and recommendations for rulemaking.

Summer 2015: Re-tested the sample of nurse tanks tested in 2013
February 2016: Draft final report due
May 2016: Final report submitted for publication
Spring 2017: Publish final report

FY 2015: $100,000

Complete—Report published and available at: https://doi.org/10.21949/1502552.

MaineWay with subcontract to Iowa State University