Testing and Recommended Practices to Improve Nurse Tank Safety, Phase I

Abstract

This is the final report for Phase II in a multi-year research project examining options for, and practicality of, non-destructive examination (NDE) of nurse tanks, which are used to transport anhydrous ammonia (NH3) locally from retailers over public roadways and farm fields. Historically the tanks were approximately 1,000 or 1,500 gallons; however, some individuals (more recently) are using dual and tri-mounted tank running gear. On the road, nurse tanks can be pulled by pickup trucks or tractors. They are sometimes involved in incidents such as overturning, which can damage their valves and cause them to vent ammonia. Nurse tanks hold NH3 at multiple times the atmospheric pressure. NH3 is a hazardous material that can cause chemical burns, frostbite, and suffocation. Tank failures can release this pressure with catastrophic force, posing the risk of impact injury to workers and bystanders. This report presents findings from the following: 1. Examination of causes for pinhole leaks and risk of failure. 2. Extended testing of stress corrosion crack (SCC) growth measurement. 3. Comparison of residual stress in annealed versus unannealed nurse tanks. 4. Statistical findings of indications in a sample of 532 in-service nurse tanks both with and without annealing. This project addressed four topics: (1) Pinhole leaks in nurse tanks were studied by radiography, serial milling, and side-angle ultrasound. These measurements indicated that welding surfaces contaminated by water, mill scale, rust, or other contaminants caused weld porosity that caused greatly accelerated metal fatigue of remaining non-porous wall thicknesses, which rapidly led to complete tank wall penetration, causing slow leakage of NH3. (2) Stress corrosion crack (SCC) growth rates were measured in three different ammonia solutions. These tests showed that neither nitrogen purging of a tank’s vapor space during filling nor adding N-Serve to ammonia changed crack growth rates. (3) Neutron diffraction analysis on a nurse tank given an American Society of Mechanical Engineers (ASME) protocol post-weld heat treatment (PWHT) lowered residual hoop stress near welds by two-thirds and lowered residual axial stress near welds by one-third. Since residual stress drives initiation and propagation of stress corrosion cracking, these findings suggest that SCC should be greatly reduced by PWHT in tank manufacture. (4) Side-angle ultrasound measurements of welds in 532 in-service nurse tanks showed that 40 percent of the tanks had no observable cracks, and the remaining 60 percent of tanks contained a total of 3,326 indications. Cracks are most common in tank heads near the circumferential welds in the vapor space near the tanks’ tops. Newer tanks manufactured using thinner steel have substantially more indications, and tanks with greater porosity in their welds appear to have more indications.