Hydrogen Explosion in Hydrodesulfurization Reactor Outlet Piping

Overview: A pipe end containing fuel oil corroded at the outlet of a heat exchanger on the outlet side of a desulfurization reactor. The corroded pipe end leaked hydrogen gas, which exploded, causing oil to leak from the heat exchanger. The leaking oil developed into an oil fire, and the damage spread. The causes of the pipe end corrosion include the following:

• There was a high concentration of corrosive substances in the process injection water.
• The concentration of corrosive substances increased due to re-molding the heat exchangers.
• The shape of the pipe cap was dead end piping.

Incident: During normal operations at a fuel oil refinery, a pipe end in a desulfurization unit developed a hydrogen leak, which led to an explosion. The pipe end was located on an outlet header of a reactant condenser (a fin-fan cooler with eight runs), which was the furthest downstream in a series of heat exchangers on the reactor outlet side of the plant. Scale deposited and built up on the cap section of outlet piping at a heat exchanger. Corrosion caused by ammonium hydroxide and chlorine in the process injection water had progressed over time. The build up of these corrosive substances decreased the thickness of the cap section. In addition, a dummy support was welded to the cap section, which might have caused thermal stress. At this plant, the concentration of corrosive substances was controlled at a higher level than it would be at similar plants. In 1985, the addition of a heat exchanger and the rearrangement of heat exchangers at the reactor outlet were carried out to realize additional energy recovery. Therefore, the concentration of corrosive substances at the heat exchanger outlet increased. According to general opinion at that time, no one believed the wall thickness at the part would be reduced by corrosion, so the part was not selected for wall thickness measurements. Therefore, it was not possible to discover the decreasing thickness at an early stage. The cap section was at the end of an outlet header of eight heat exchangers, and the exit of the collecting pipe of heat exchangers had a dead end. In the dead end section, fluid replacement is difficult, and concentrations of corrosive substances would increase locally. Thus, the piping structure is an ancillary cause of the incident. At the time of the incident, the leaking hydrogen gas was ignited by static electricity and exploded. Other heat exchangers were also damaged by the blast. In addition, fuel oil escaped and led to an oil fire, which damaged near-by equipment and structures.

Response: An operator shut down the plant immediately after the accident. The fire brigade arrived and cooling water was sprayed on peripheral equipment.