Hydrogen release an explosion in a refinery

The event occurred in a desulphurisation unit for heavy oil (hydro-cracking, Isomax process).
The hydrogen released from one of the heat exchangers where the feed gases are pre-heated by the reactor products. It happend during the restart of the unit, following a shutdown needed for replacing the catalyst.
When the restart process was nearing normal operating conditions, the lock ring, the channel cover and a few other parts of one of the heat exchangers burst apart, projecting to more than a hundred meters away. There was a simultaneous explosion of released hydrogen and fire developing near the exchangers. A few minutes before the explosion, a major emission of hydrogen, probably accompanied by heavy oil, noisily arose from vent and drain holes and other locations. The explosion took place while plant operators and site workers were taking measures to stop the emission.

The metallic ring which failed in first instance (part of a called breech-lock closure, 1,42 m diameter), was responsible for the tightness of one of the heat exchanger channel. Its gasket had been damaged by thermal cycling before, which was replaced during maintenance by a wrong type of gasket. Moreover, some source report as additional error the removal of the thermal insulation, which induced thermal deformations.
The accidents caused ten fatalities seven injuries, among the workers which were trying to intervene in the original leak.

The INITIATING cause was the failure of a metallic component, damaged by maintenance and thermal cycling.

The sources mentions errors occurred during maintenance and repair. Therefore, the ROOT CAUSE relates to erroneous executions of maintenance procedures, and/or to lack of instructions/training.

heat exchanger

The event took place when resuming normal operation, restarting after a shutdown to replace the catalyst.
During the shutdown, the feed/reactor effluent exchangers (including the failed one) were not opened for inspection.

The accident resulted in 10 fatalities and 7 injured.
Part of the refinery facilities were lost, as well as part of the storage tanks and lubricant manufacturing plant located next door.
The total amount of direct property loss is 2.4 billion yen (about £15 million, or $22 million, 1992).

GENERAL
This case highlights the importance of sound and validated procedures for both preventive and corrective maintenance. The procedures must be based on a risk assessment and updated according the return of experience from operation. They have also be documented, and communicated and explained (training). in case contractors are in charge of the interventions, a coordination between plant operators and the external workers has to be in place, including supervision.