Explosion from the hydrogen recycle line at a refinery

The event occurred at a FCCU (Fluid Catalytic Cracking Unit), on a section of the recycle hydrogen line. The section was a by-pass around the hydrogen pre-heat exchanger. It ruptured due to localised corrosion, releasing hydrogen and bringing to an explosion and fire.

The failed part of the line had been identified by inspection as a dead leg. After investigation it was found that the mechanism of corrosion was ammonium chloride under deposit corrosion. The source of chloride has not been traced, but hydrogen from the catalytic reformer was strongly suspected.
No injury. There was damage to equipment, material loss and product loss.

The INITIATING CAUSE was the loss of confinement of a pipe carrying recycled hydrogen, due to internal corrosion. The report of this event is not clear on the possible causes of corrosion. Corrosion by ammonium chloride is suspected, although the source of this substance remains unknown. Moreover, the role of hydrogen is unclear.
The ROOT CAUSE was identified in an inadequate inspection of the dead leg.

recycle hydrogen line, pre-heat exchanger

There was damage to equipment, material loss and product loss.

The root cause of this hydrogen release was identified in an inadequate inspection, which was unable to identify internal of the pipe carrying hydrogen.
However, localised corrosion mechanisms are difficult to detect by means of the usual detection technique based on fixed point ultra-sound testing (UT). Moreover, the prevention of internal corrosion of dead leg corrosion is difficult, because there are several different corrosion mechanisms to consider.