1 2.3b Potchefstroom, SA, Ammonia Tank Failure, 13 July 73 Presented to ES-317y at UWO in 1999 Dick Hawrelak

2 The Incident A large crack developed on one head of a horizontal pressure vessel. The leaking ammonia chilled the steel near the crack (from ambient to -28 deg F). The crack immediately spread into a 4 ft. diameter gaping hole (Titanic Stl.??). 38 te of anhydrous ammonia were lost instantaneously from the vessel system.

3 Plot Plan

4 The Consequences 18 people died from a lethal toxic dose of anhydrous ammonia. 14 of 350 plant workers died. 30 workers were within 70m of the vessel. 4 fatalities in a nearby residential area 150m to 200m from the failed vessel. 65 workers required medical treatment.

5 Some Observations NH3 flowed horizontally and impinged on nearby tanks. The NH3 lost momentum and there was considerable flashing (17%). No liquid appeared to be pooling on the ground (no rain-out). The initial cloud was 150m in diameter by 20m high.

6 Reasons For Failure Hot work had been performed on the vessel prior to placing it back into service. The vessel was not stress relieved after the welding repairs. The vessel had not been stress relieved during initial fabrication. The vessel materials were not suitable for -28 deg F service. A small crack easily spreads to a much larger crack.

7 Lessons Learned Most pressure vessels require stress relieving. Vessel should be stress relieved after hot work. Select materials for worst case temperatures (hot & cold). Residential area too close to fence line.

8 Lessons Learned Cont’d Emergency planning with community is required. Emergency directional horn would have warned residents to shut windows and stay indoors. Consider option of atmospheric storage with suitable containment dyke. Store hazardous chemicals in a remote area.

9 Possible Exam Questions After hot work has been conducted on a pressure vessel, what must be done to insure that the vessel is suitable for its intended use? What was the problem with the unsuitable material selected for anhydrous liquid ammonia storage? What was the problem with the location of the residential area?