Ammonia Release | Nurse Tank Weld Failure

Summary

In August 2023, the catastrophic failure of a nurse tank containing anhydrous ammonia resulted in the release of all the ammonia contained within the tank. 

Incident Description

The nurse tank was sitting stationary in a farm co-op lot when the tank failure suddenly occurred. According to the report, “The failure caused the tank shell to ‘rocket’ over 300 feet from its original location.” While no injuries were reported from the flying tank shell, six (6) people were treated at the hospital for ammonia exposure. Anhydrous ammonia nurse tanks are typically sized to hold 1,000-1,500 gallons of ammonia, but it is unknown how much ammonia was present in the nurse tank at the time of failure.

Investigation

An investigation was conducted to determine the root cause of the failure, and whether any similar nurse tanks were at risk. The failed nurse tank was manufactured in 2009 by American Welding and Tank (AWT). The nurse tank owner contracted a third-party testing company to examine eight (8) similar nurse tanks that were manufactured between 2008 and 2012 by AWT. Only one (1) out of eight (8) nurse tanks passed the inspection and was considered suitable for continued use. In the other seven (7) tanks, radiographic testing revealed stress corrosion cracking, porosity, inclusions, and voids in the welds that join the nurse tank heads to the shells. If left in service, these nurse tanks would most likely fail due to the weld defects.

Stress Corrosion Cracking        

Porosity

Slag Inclusions                                                            

Voids

These results prompted further testing of additional nurse tanks. Radiographic testing was performed on 142 nurse tanks manufactured by AWT between 2007 and 2012. All of the nurse tanks manufactured in 2012 passed the inspection, but 100 tanks that were manufactured between 2007 and 2011 failed the test. This correlates to a passing rate of only 30%.

Significance

A safety advisory has been issued by the Federal Motor Carrier Safety Administration (FMCSA) and the Pipeline and Hazardous Materials Safety Administration (PHMSA). The safety advisory strongly recommends periodic testing (once every five years) of nurse tanks manufactured from January 1, 2007, through December 31, 2011 by AWT at its Fremont, Ohio plant. 

The inspection/testing should include “periodic visual inspection in accordance with 49 CFR §173.315(m)(2)(i); thickness testing in accordance with 49 CFR §173.315(m)(2)(ii), and pressure testing in accordance with 49 CFR §173.315(m)(2)(iii).” If owners are unable to perform pressure testing, it is recommended that either radiographic or ultrasonic testing be conducted instead.