Abstract
In an accident in southwest Iowa, USA in 2012, an anhydrous ammonia nurse tank vented its entire cargo of 5500L (1500gallons) of liquid ammonia to the atmosphere. Follow-up study of the failed tank revealed a through-crack along a weld used to connect the tank to its running gear. Side-angle ultrasound examinations were performed on 532 used anhydrous ammonia nurse tanks to measure the locations, sizes, and orientations of flaw indications. The tanks examined had manufacture dates ranging from 1952 to 2011. A total of 83 indications were found in or near the leg welds of 50 of these 532 tanks. Several factors suggest that these indications are fatigue cracks, not the stress corrosion cracks more commonly detected in nurse tanks. These findings suggest that roughly 9% of the 200,000 nurse tanks in the U.S. nurse tank fleet may contain leg-weld fatigue cracks. Nurse tanks are the only large, pressurized packages for hazardous cargo that do not contain manways; thus, their interior walls cannot be inspected for flaws with magnetic particle or fluorescent dye penetrant methods. Since the tank interior is inaccessible, side-angle ultrasound is the only detection method capable of detecting cracks in nurse tanks initiating at both interior and exterior tank surfaces. For this reason, the authors recommend that side-angle ultrasound be considered for use in periodic nurse tank inspections.
Highlights
Nurse tanks (Fig. 1) are welded steel pressure vessels used to transport anhydrous ammonia fertilizer from vendor sites to farm fields
More numerous leg weld indications more nearly correlate to the transition from 3800-L to 5500-L tanks that occurred in the mid-1980s
Side-angle ultrasound is not routinely used to inspect nurse tanks, but these findings suggest that it can be a useful tool to identify cracks before they become through cracks that leak NH3 or critical-sized cracks that cause tank explosions
Summary
Nurse tanks (Fig. 1) are welded steel pressure vessels used to transport anhydrous ammonia fertilizer from vendor sites to farm fields. In one incident [7], an exploding tank rocketed across a farmyard and struck a tractor, severing the rear wheels and cab from the engine and front wheels In another incident [6], a weld failed while the tank was being filled, killing one worker and inflicting permanently disabling lung injuries on another. Ultrasound examination of the nurse tanks was generally performed in accordance with the ASME Boiler and Pressure Vessel Code 2011a Section V Article 4: Ultrasonic Examination Methods for Welds. The details of these methods are described elsewhere [9]. Since the tanks were in active commercial service (and contained pressurized ammonia during the inspections), cutting tanks to obtain access to their interior walls was not feasible
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