The National Transportation Safety Board today determined that the probable cause of an accident last year involving an agricultural nurse tank was inadequate welding and insufficient radiographic inspection during the tank's manufacture, and lack of periodic testing during its service life.
On April 15, 2003 two loaders filled a cargo tank with anhydrous ammonia at a tank filling facility near Calamus, Iowa. Nurse tanks are used exclusively for injecting anhydrous ammonia into the soil to increase the nitrogen content. As the loaders were preparing to connect the tank to the truck, a 53.5 inch long split opened near the right center at the tank bottom. Approximately 1,300 gallons of poisonous and corrosive gas escaped, injuring two loaders. Nine days after the accident, one of the injured loaders died.
The accident nurse tank, a non-U.S. Department of Transportation specification cargo tank, was made of SA-455 steel and built in 1976 according to the requirements of the American Society of Mechanical Engineers (ASME). The ASME Boiler and Pressure Vessel Code permits the use of spot radiography to check the welds for all pressure vessels constructed to the code. Spot radiography consists of performing a 6-inch-long radiograph for every 50 feet of welds. When manufactured, the accident nurse tank's longitudinal welds were subject to spot radiography, in accordance with the ASME code's acceptance spot radiography procedures. Additionally, the accident nurse tank was hydrostatically pressure tested. Since the mid 1980s, manufactures use radioscopy to conduct full radiographic examinations of the longitudinal welds on nurse tanks.
Tank shells are formed by two pieces of plate steel rolled into a cylinder or tube until the opposite edges of the plate meet, creating a longitudinal seam. The longitudinal seam is closed by welding inside and outside the shell. Post accident metallurgical examination of the tank indicate that, when manufactured, a portion of the nurse tank's interior longitudinal weld was not centered on the shell seam where the inner weld was offset to one side of the seam. At this location, an unfused region was found.
Although River Valley Cooperative had no records of inspections of its nurse tanks, investigators were informed that the operator conducts external visual inspections of all of its nurse tanks annually. There are no Federal or State requirements that periodic inspections be conducted on nurse tanks.
As a result of the accident, the Safety Board made the following recommendations:
To the Research and Special Programs Administration: Require periodic nondestructive testing be conducted on nurse tanks to identify material flaws that could develop and grow during a tank's service and result in tank failure.
To the River Valley Cooperation: Review manufacture's material safety data sheets for anhydrous ammonia, the National Institute for Occupational Safety and Health's HAZOP of Anhydrous Ammonia Use in Agriculture, and the Emergency Response Guidebook and establish written emergency procedures for employees to follow when anhydrous ammonia release poses an inhalation hazard.
A summary of the accident investigation report, including the findings, probable cause and safety recommendations, can be found on the Publications page of the Board's web site, www.ntsb.gov. The complete report will be available in about a month.