Explosion in a SMR unit of an ammonia plant

A series of failures occurred on a 24-in. (61-cm) synthesis gas line connecting primary and secondary reformers. The line contained hydrogen and carbon monoxide at a pressure of about 400 psi (28 bar) and a temperature of about 930°C. The affected section of this pipe had a high-temperature alloy steel outer wall, a refractory liner, and a stainless-steel inter-liner.

Two explosions occurred, the first due to weld failure, the second due to overstress in elbow.

The first explosion was located on a 42-in. long welded section of the pipe which suddenly blew out. Plant employees heard a rumble and observed a flame above the ruptured pipe. Before the torch fire at the rupture site could be extinguished, it damaged the secondary reformer and most of the remaining pipe. A shower of catalyst pellets from the secondary reformer was also emitted at the rupture site. The hot catalyst ignited several secondary fires in oil deposits in the area.

18 days later, the repairs of the pipe were completed, and the ammonia plant was brought up to production temperature and pressure. After startup, a flame was noticed coming from the same synthesis gas line that that had been repaired. When the reformers were shut down and the flame extinguished, a 10-in. (25-cm) crack was discovered in the inside radius of an elbow in the expansion loop. The elbow had apparently cracked on startup and allowed synthesis gas to escape and autoignite.

[Zalosh and Short, 1978]

The first rupture was caused by a weld failure, but a specific failure mechanism is not reported. The second fracture was probably caused by excess stress on the elbow because of the damage caused by the first explosion, in particularly a 1-in. displacement of the expansion loop during the first incident.
The ROOT CAUSE(S) are not clearly identified. Welds can fail due to several different mechanisms, which often could be brought back to inadequacy of the original design. This is confirmed by the history of repairs which had to be regularly performed during the previous years. Additional evidence of shortcoming in design is provided by the fact that it turned out that the gas expansion loop was not necessary.

The refractory lining had been repaired several times before (the latest time one year before) because of localised deterioration and hot spots. The repair procedure consisted of cutting a section of pipe, repouring the refractory liner, and patch welding the outer wall.

The first explosion eroded the refractory lining in the secondary reformer and most of the remaining pipe. Instrumentation in the secondary reformer was damaged as well as piping and vessel insulation in the vicinity of the rupture. The expansion loop between the primary and secondary reformers dropped about 1 in. (2.5 cm) and cracks were found in two elbows on the loop.

Zalosh and Short derived an important lesson from this and other the ammonia plant rupture incidents: the need to carefully engineer and test all repairs and modifications performed on to high pressure process equipment. Although the consequences of this incident were relatively limited and did not cause damage to humans, an incident with similar characteristics, involving a cyclohexane vapor cloud explosion, had much more devastating consequences (Flixborough, England1 June ,1974). The explosion was caused by the rupture of an improperly designed and installed temporary bypass pipe for the repair of the reactor.

The plant managemen decided to eliminate the fractured synthesis gas expansion loop because it wasn't necessary in the first place.