Explosion at outlet piping of a hydrodesulphurisation reactor

The incident occurred at an indirect oil desulphurisation unit, and originated on a piping at the outlet of a heat exchanger. The exchanger was located in the furthermost downstream of the reactor. The dead-end cap of the piping failed due to corrosion and opened, releasing high-pressure hydrogen which leaked, ignited and exploded. Due to the explosion, the heat exchanger was damaged and oil leaked out contributing to spread the damage.
Approximately 1200 kg of hydrogen and 30000 l of oil were burned. No injuries.

The INITIATING CAUSE was corrosion at the dead-end of a piping, decreasing its thickness. The corrosion was caused by corrosive substances such as ammonium hydroxide and chlorine which was included in the injection water. In addition, a dummy support had been welded to the cap, so that thermal stress might could have also contributed to the loss of containment.

The ROOT CAUSE was a non-optimal risk assessment and management of changes.
Four years before the incident, a heat exchanger had been added and a rearrangement of heat exchangers at the outlet of the reactor was carried out to rationalize energy recovery. This caused an increase of the concentration of corrosive substances at the heat exchanger outlet.
Despite that fact that the plan was controlling the concentration of corrosive substances, no one believed the wall thickness at the piping dead-end part would be reduced by corrosion. Therefore it was not so the part of the plan for wall thickness measurements.

dead-end piping, heat exchanger

Four years before the incident, a heat exchanger had been added and a rearrangement of heat exchangers at the outlet of the reactor was carried out to rationalize energy recovery.

The piping, a fin-fan cooler, instrumentation devices and a structure etc., installed near the high-pressure separator burned out.
At the factory, windows of 57 buildings in a 1 km radius were damaged.
Outside the factory, windows of six buildings within a 2.5 km radius were damaged, and 14,000 cubic meters of hydrogen and 30 kL of vacuum gas oil burned out.

The modification to the unit brought four years before had changed the operative parameters and caused an acceleration of corrosion. The risk assessment and the safety provisions must be reviewed and reinforced when modifications are introduced to the system and when installing new facilities. This is a case where a proper management of change would have prevented the incident.

Moreover, the possibility of a failure due to corrosion had not been considered when performing a risk assessment. In this case, corrosion was accelerated by (i) iron sulphide scale deposits, (ii) by chlorides concentrated in the scale, (iii) tensile stress. It is however unknown if these degradation phenomena fully understood in the period of this event.
In dead-end piping, concentration of corrosive substances increases quicker than in part characterised by high flows of liquid or gases. Therefore, specific safety provisions are required for these areas.

There are also two more general lessons still valid several years after the occurrence, regarding the safety management system:
1. Inspection and maintenance must be properly designed and managed, to identify and quantify early enough abnormal conditions, such as localised corrosion.
2. Plants with same processes are not necessarily characterised by exactly the same hazards and risks. A full understanding of each plan is crucial to guarantee its safety.

1. The management system of operations and facilities was strengthened to be able to monitor abnormal phenomena such as local corrosion and to respond properly.
2. The safety review system for modifying new facilities was improved.
3. The risk assessment of existing facilities was re-evaluated.
4. All refineries with the same kind of desulphurisation unit were required to inspect dead-end piping in the areas similar to those where the incident occurred.
5. They were also recommended to perform a safety review of third operational and facilities management system.