Metallurgical examination of the 24" pipe showed that a fatigue failure occurred, induced by thermal cycling. Examination of the fracture faces of the ruptured seam weld showed a significant amount of fatigue cracking initiating at the bore of the pipe along a length of about 3 meters of the weld downstream the water injection point. At the upstream end, the rupture propagated circumferentially around the pipe as a shear failure. At the other end, the rupture arrested at a circumferential weld and then propagated as a circumferential tear in the pipe material. The spray pattern from the nozzle was such that water at 80C was intermittently hitting the inner surface of the pipe and quickly cooling the surface. The process gas stream at a temperature of 230C quickly heated up the pipe wall again only for it to be quickly cooled by water impingement at the start of the next cycle. This phenomenon which affected a number of areas in the pipe wall downstream of the water injection nozzle caused a number of small fatigue cracks which ultimately developed so that the material near the longitudinal weld failed. Subsequent examination of the spray nozzle showed that this may have been distorted for some time prior to the accident causing preferential impingement possibly enhancing the failure mechanism although it is not concluded that this distortion was the main cause of the failure. There was a two year history of previous leaks in welds further downstream on this section of pipe which it is now recognized were due to the same cause. At the time the company wrongly diagnosed the cause as external stress corrosion cracking.